1 大气成分及相关特性变化的观测研究

1.1 2016年12月红色预警的北京冬季重污染事件中边界层内气象要素对PM2.5爆发性增长的相对作用

2016年12月至2017年1月PM2.5重污染事件（HPEs）频发，但其中PM2.5质量浓度爆发性增长的成因仍不确定。本研究利用地面PM2.5质量浓度以及风、温、湿等垂直分布的气象要素及ECMWF再分析资料，着重分析边界层内气象要素对此爆发性增长的相对作用。北京HPEs 前期以输送为主，后期以累积为主。输送阶段 （TS） 地面高压位于北京以南，较强偏南风将北京南部的污染物输送至北京促使污染形成。累积阶段 （CS） 中以弱偏南风为主，以小风静风、异常逆温和近地层水汽聚集为特征的稳定层结触发了PM2.5爆发性增长 （质量浓度在几至十几小时内加倍）。累积阶段弱南风抑制污染物水平扩散,有利于输送而来的污染物累积。较高浓度的PM2.5会使更多的太阳辐射反-散射回空间，使到达地面的辐射减少，近地面温度降低，进而引起异常逆温。逆温抑制垂直方向的湍流扩散，降低边界层高度，进而使得污染物和水汽进一步累积；同时近地面冷却会降低饱和水汽压，使得相对湿度大幅增加。近地面水汽聚集加强了气溶胶的吸湿性增长并加速其液相反应和非均相化学反应，其中未完全量化的反应过程需要进一步研究。以上气象要素的正反馈过程对PM2.5累积阶段中的爆发性增长过程的贡献超过80%。（张小曵）

1.2 北京春节及国庆假期期间气溶胶光学特性观测及其与气象和排放条件的关系

国庆期间减少的城区车流量和春节期间燃放的烟花爆竹为研究北京地区人为因素对气溶胶光学特性的影响提供了一个独特的机会。将2014、2015年的国庆假期和2015年、2016年的春节假期选为研究时期，发现在2015年国庆期间和2015、2016年春节期间，440 nm波段的气溶胶光学厚度呈现出增长的趋势，并且分别超过了背景水平的81%、21%和36%，而2014年国庆假期间的气溶胶光学厚度可能由于降水的原因而相对较低。440 nm波段的吸收性光学厚度与光学厚度变化较为一致，分别超过了背景水平的75%、19%和23%。春节期间的单次散射反照率较背景水平偏高，而国庆期间可能由于主城区机动车流量减少而偏低。在假期间的污染过程中，细模态和粗模态粒子的体积浓度较背景水平分别高出了0.04～0.25 µm3和0.03～0.15 µm3。潜在源区分析和浓度权重分析发现北京以南区域对北京地区偏高浓度的PM2.5有着极大贡献。这些发现有利于深入了解北京地区人为活动对气溶胶光学特性的影响并且有助于区域空气质量的提高。（车慧正）

1.3 2014—2015年中国主要城市大气污染特征及其与气象条件的关系

2013年1月开始，环境监测总站向公众发布全国主要城市实时的6项污染物（CO、NO2、O3、PM10、PM2.5和SO2）小时浓度，该数据为研究我国大气污染时空变化特征提供了基础数据。尽管少量研究分析了我国大气污染的时空变化特征，但多尺度气象条件与我国大气污染的关系及空间差异依然认识不清。本文利用2014—2015年全国31个省会城市空气质量观测数据，分析了我国大气污染的时空变化特征以及多尺度气象条件对大气污染的影响。2014年，31个省会城市6项污染物（CO、NO2、O3、PM10、PM2.5和SO2）年均浓度分别是1.2、42.4、49.0、109.8、63.7和32.6 µg/m3。2015年CO、NO2、PM10、PM2.5和SO2年均浓度分别下降了5.3%、4.9%、11.4%、12.0%和21.5%，O3年均浓度上升了7.4%。首要污染物比率最高的是PM2.5，其次依次为PM10、O3、NO2、SO2和CO。气象条件是决定污染物浓度逐日变化的首要因子，其解释方差超过了70%。与2014年相比，2015年气象条件更加不利，间接说明排放管控措施取得显著成效。（何建军）

1.4 云凝结核（CCN）数浓度的分粒径预报及其活化特征的长期观测

大气气溶胶粒子可作为云凝结核影响大气水循环和气候变化。为了提高对云凝结活化特征的理解并准确预报其浓度，在中国长三角地区开展了长期野外观测试验。结果表明，云凝结核在受到污染的农村站点比在清洁站点（如亚马逊地区）和多尘站点（如春季的坎布尔）更容易活化，但在污染较严重的地区（如北京）更难以活化。使用包括最大活化率和异质程度2个额外参数的新方法被提出以便精准预报CCN的分粒径浓度。评估了这些参数的取值范围以及预测不确定。在内混的假设下，使用该新方法并结合分粒径的气溶胶化学组分，预报的CCN与长期野外观测具有较好的一致性。（张小曵）

1.5 中国5个大气本底站卤代化合物的浓度特征

2011年1月至2012年12月对中国5个本底站大气中的3种含氯氟烃（CFCs）、3种氢氟氯碳化合物（HCFCs）、6种氢氟碳化合物（HFCs）、3种氟碳化合物（PFCs）以及六氟化硫（SF6）开展了高精度的观测。CFC-11、CFC-12、CFC-113、HCFC-22、HCFC-141b、HCFC-142b、HFC-125、HFC-134a、HFC-143a、HFC-152a、HFC-23、HFC-32、PFC-116、PFC-218、PFC-318以及SF6在5站的平均浓度分别为（239.5±0.69）×10-12、（536.5±1.49）×10-12、（74.66±0.09）×10-12、（232.1±4.77）×10-12、（23.78±0.29）×10-12、（22.92±0.42）×10-12、（11.75±0.43）×10-12、（71.32±1.35）×10-12、（13.62±0.43）×10-12、（9.10±1.26）×10-12、（25.45±0.1）×10-12、（7.28±0.48）×10-12、（4.32±0.03）×10-12、（0.63±0.04）×10-12、（1.36±0.01）×10-12、（7.67±0.03）×10-12，这些浓度和北半球2个AGAGE本底站：爱尔兰MHD站和美国加利福利亚THD站相近。与前期在北京上甸子的在线观测结果对比可知大气CFCs浓度在下降而HCFCs、HFCs、PFCs和SF6均呈上升趋势。观测到的CFCs、HCFCs和HFCs污染事件占总观测数的比例呈现出临安站＞上甸子站＞龙凤山站＞香格里拉站＞瓦里关站的特征；浓度抬升值呈现出HCFCs（（40.7 ～ 175.4）×10-12）＞HFCs（（15.8 ～ 66.3）×10-12）＞CFCs（（15.8 ～ 33.8）×10-12）＞PFCs（（0.1～0.9）×10-12）的趋势，其中HCFC-22的抬升范围为36.9×10-12到138.2×10-12。经分子量换算后，本研究的结果表明，HCFCs的排放量在上述站点的区域排放量最大，HFCs和CFCs次之，PFCs最小，这与我国逐渐以HCFCs替代CFCs的特点相吻合。此外，在我国长三角地区这些卤代化合物的相对排放强度由强到弱依次为： HCFC-22＞HCFC-141b＞HFC-134a＞HCFC-142b，而在华北地区则为HCFC-22＞HCFC-142b＞HCFC-141b≈HFC-134a。（张根）

1.6 中国兰州城市环境下颗粒物数浓度粒径分布和新粒子生成事件特征

利用扫描电迁移粒径谱仪从2014年8月14日到11月18日，在我国西北地区兰州开展颗粒物数浓度变化特征以及新粒子生成事件特征研究工作，研究颗粒物粒径范围在14.6 nm 到661.2 nm之间。研究表明，研究时段颗粒物核模态、爱根模态、积聚模态和全模态平均数浓度分别为2833、12898、6210、21940/cm3。平均数浓度粒径分布呈现明显单峰模式，峰值粒径在60 nm左右，粒子数浓度的日变化特征主要受新粒子生成事件的影响。爱根模态、积聚模态和全模态的日变化规律均表现出了早晚2个时段的峰值，这2个峰值与交通和餐馆排放有关。由于人为活动的增强，数浓度和质量浓度在工作日的浓度水平均高于非工作日的浓度水平，数浓度水平白天高于夜间。亚微米气溶胶以及硫酸盐、硝酸盐和铵盐的质量浓度白天高于夜间。而有机物浓度夜间高于白天，主要是由于夜间取暖和晚高峰排放叠加所致。观测期间，新粒子生成事件发生频率为34%，核模态粒子生成和增长速率分别为1.71/（cm3/s）and 6.10 nm/h，典型的新粒子生成事件有2级增长，并对粒子增长和演变过程起到了至关重要的作用。后径向轨迹分析结果显示，来自西部的气团颗粒物的数浓度和质量浓度均偏高，主要受西部局地工业排放的影响，而来自北部和西北部的气团代表着相对洁净的空气。（张养梅）

1.7 2015年秋冬季北京北部地区气溶胶重污染事件中化学组分的特征及气象要素的作用

重污染事件通常起于深秋，进入冬季后在北京及周边地区更为严重。本文着眼于北京及周边地区北部的重污染事件形成和变化的原因。其中主要关注2015年秋冬季重污染事件中，PM1的分粒径化学组分及气象条件变化。研究表明，区域气团稳定和与弱南风相关的较高的大气凝结过程导致了重污染事件形成。重污染事件开始后，细颗粒物浓度在数小时内增加超过2倍。其中80%的PM爆发性增长被认为由来源于更稳定大气和更高水汽凝结率的气象因子正反馈引起，这源于形成的污染与边界层内气象条件的相互作用。硝酸盐是秋季PM1的最大组成部分，也是相对于秋冬季清洁阶段的重污染过程中增长最为显著的部分。在秋季有机气溶胶的比例与硝酸盐类似，但是重污染事件中上升幅度要小很多，这主要因为清洁阶段有机气溶胶浓度较高。与增长幅度最大的硝酸盐相比，铵盐也出现相似的上升，表明大量铵盐主要与硝酸根结合生成，而不是硫酸根。除了弱偏南风输送北京及周边地区南部的污染物外，偏西风也输送内蒙古中部和山西北部由于燃煤引起的污染物，导致冬季北京及周边地区北部的重污染形成，并引起较高浓度的硫酸盐、硝酸盐和有机气溶胶（张小曵）

1.8 长三角背景地区夏季PM2.5化学组成特征

随着城市化和工业化的进程加快，长三角地区大气化学的研究越来越受到关注。本研究对长三角背景地区临安大气本底站夏季PM2.5进行连续采集，并对其进行OC、EC、左旋葡聚糖和硫酸盐、硝酸盐和氨盐等水溶性离子测试分析。临安夏季PM2.5平均浓度为30.19±8.86 µg/m3，低于以前的研究报告，说明通过逐步排放控制措施，中国的空气污染得到了一定的改善。硫酸盐、硝酸盐和铵盐之间的关系研究表明，主要以的形式存在，而则可能是以NaNO3和KNO3的形式存在。基于分子示踪，天气数据以及气团轨迹分析，发现静稳天气和区域传输都可以影响临安地区PM2.5的化学组成特征。通过对临安霾和清洁天气PM2.5化学组成特征比较发现，二次有机气溶胶和二次无机气溶胶呈现了不同的变化特征：二次有机气溶胶在洁净晴朗天气时更为重要，而二次无机气溶胶则是在霾污染天气更容易生成，从而变得更重要。（梁林林）

1.9 利用气溶胶质谱仪对我国西北城市冬季亚微米气溶胶化学特征的研究

2014年10月27日到12月3日期间利用气溶胶质谱仪测量了兰州城区亚微米气溶胶主要化学组分质量谱。观测期间，冬季亚微米气溶胶PM1平均质量浓度为37.3 µg/m3（从2.9 变化到128.2 µg/m3），其中主要化学组分有机物（48.4%）、硫酸盐（17.8%）、硝酸盐（14.6%）、铵盐（13.7%）和氯化物（5.7%）是主要成分。利用正交矩阵因子分析法（PMF）和多线性工程（ME-2）手段，解析出了6类有机气溶胶，分别为与机动车排放有关的有机气溶胶（HOA）、燃煤排放类有机气溶胶（CCOA）、餐馆排放类有机气溶胶（COA）、生物质燃烧排放类有机气溶胶（BBOA）和2种氧化类有机气溶胶（OOAI和OOAII）分别占有机气溶胶的8.5%、20.2%、18.6%、12.4%、17.8%和22.5%。一次排放的有机气溶胶在细颗粒物中占有相当大的比重，而且在兰州冬季重污染事件中起着非常重要的作用。后径向轨迹分析结果表明，来自西部区域传输过程载带的污染气团是导致兰州冬季亚微米气溶胶污染的主要来源。（张养梅）

1.10 北京大气颗粒物中真菌孢子对有机碳贡献

真菌孢子是大气气溶胶的重要组成部分，也是气溶胶中有机碳（organic carbon，OC） 的主要贡献者。本研究在北京城区连续一整年每天同时采集PM10和PM2.5，并在晚春和夏季在北京郊区进行PM10样品对比采集。研究结果发现，在北京市区PM2.5和PM10中，真菌孢子OC质量浓度的年均值分别为0.3±0.2µg C/m3和0.8±0.7 µg C/m3，其对气溶胶中OC的贡献分别为（1.2±0.7）%和（3.5±3.7）%。在PM2.5和PM10中，真菌孢子对OC的贡献呈现相似的季节变化规律，从高到低的顺序为夏、秋、冬、春。在夏季北京市区和郊区的对比采样中，发现北京市区和郊区真菌孢子对OC的贡献均处于较高水平，分别达到了（7.3±3.3）%和（14.1±10.5）%，说明即使在北京城市地区，真菌也是碳质气溶胶中不可忽略的重要组分。因此，需要更多的研究来关注真菌气溶胶对OC贡献的时空变化特征和粒径分布特征。（梁林林）

1.11 递减的中国东部午后局地降水频次：气溶胶污染的可能影响

利用中国东部高密度（500余个）雨量站长时间序列（1970—2010年）的小时降水数据，针对夏季午后热对流空间覆盖小、与气溶胶效应关系密切等特点，定义了“午后局地尺度降水（LSP）”这一新概念。在此基础之上，发现中国东部LSP事件发生频次呈显著下降趋势，降水强度呈显著增加。同期的气溶胶污染排放也呈增加趋势，二者的相关性分析进一步揭示了气溶胶可能是导致这种局地降水事件减少、夏季午后极端降水强度增加的主因。由于本研究既在空间上给这些降水事件做了约束（局地尺度），时间上也仅限午后到傍晚，而非以往研究所采用的日降水这一统计尺度，因此，本成果修正了以往基于日累计降水的“气溶胶减少毛毛雨，增强强降水”观点。（郭建平）

1.12 气溶胶-边界层相互作用及其对空气质量影响

空气质量关系到气态和液／固相的大气污染物。后者被认为是气溶胶颗粒，也是通过多种机制影响空气质量好坏、天气和气候的重要介质。与气态污染物相比，在边界层内存在强烈的气溶胶—气象条件相互作用。作为气溶胶排放和存在的主要场所，边界层的发展易受气溶胶辐射效应影响。气溶胶的散射和吸收可有效减少到达地面的辐射，进而减少驱动边界层日变化的主要因子—感热通量。此外，气溶胶通过散射和吸收太阳辐射导致低层大气出现逆温层结，进而增加大气稳定度。这种稳定的大气层结将有效抑制污染物的扩散，反过来进一步促进污染物浓度的累积和升高。这种气溶胶—边界层之间的正反馈机制在重污染期间尤其明显。因此，人们对边界层过程的准确理解对于深刻理解气溶胶—气象条件相互作用机制具有重要意义。其中，一个非常关键的因素是气溶胶如何影响边界层，尤其是垂直方向上的的日变化过程以及空气质量的影响。本文主要综述了气溶胶观测新技术、边界层物理化学过程及其相互作用，特别对他们之间存在的复杂反馈机制进行全面阐述，并对未来气溶胶—边界层相互作用这一领域的发展，分别从观测和模式模拟的角度展开了全面系统的讨论。（郭建平）

1.13 中国环境中PM2.5对健康的影响

中国大部分人都暴露在高浓度的PM2.5中，造成较高的健康风险。利用2014—2016年全国367个城市共计1382个监测点的PM2.5浓度观测数据，评估了我国PM2.5的健康分担率。利用一种暴露—响应模式评估各种疾病和死亡率的健康风险，根据国家死亡率基线调整各省疾病和死亡率基线。研究发现，2015年因PM2.5污染成脑死亡比率占脑死亡总数的40.3%，下呼吸道感染死亡比率是33.1%，缺血性心脏病死亡比率是26.8%，肺癌死亡比率是23.9%，肺慢性阻塞性疾病死亡比率是18.7%，占总死亡人数比率是15.5%。考虑10年滞后期影响，PM2.5污染造成的死亡率要增加12%。人口权重平均PM2.5浓度满足WHO IT-1、IT-2、IT-3和AQG标准时，PM2.5污染导致的死亡人口占现状的24.0%、44.8%、70.8%和85.2%。（何建军）

1.14 华北农村饶阳站低对流层O3和气溶胶分布

随着数十年的经济快速增长，华北平原已经成为我国最大的污染区域。在华北平原的城市和农村站点都观测到了高浓度O3和气溶胶。但是大多数空气污染物的现场观测都局限于近地面，因此对O3和气溶胶的垂直分布的了解很不充分。本项研究于2014年夏天在华北平原一个农村站点上空利用携带微型设备的无人飞机开展了2.5 km以下O3和分粒径气溶胶数浓度的观测，此外还借助激光雷达和系留汽艇手段分别测量了低对流层气溶胶散射特性的垂直分布和1 km以下的O3垂直分布。根据激光雷达测量的粒子消光和无人飞机测量的气溶胶数谱垂直梯度定出了混合层和夜间残留层厚度，获得了每次无人机飞行中混合层及残留层的O3浓度和气溶胶数浓度平均值。结果表明，大多数飞行过程中混合层顶以上的O3浓度均高于混合层内的浓度。这种上高下低的O3垂直分布特征使残留层成为混合层O3的重要来源，尤其是在早晨混合层顶迅速抬升的时段。与O3分布相反，气溶胶数浓度在混合层内通常高于残留层，特别是在清晨时段。气溶胶粒子基本都分布在1 µm以下，混合层和残留层的谱分布只有微小差异。本次观测证实华北平原农村的低对流层已很大程度上受到当地人为排放和城市地区输送的影响。将本次夏季观测的O3垂直分布与来自MOZAIC民航飞机项目在北京及周边取得的O3垂直分布历史数据进行了比较，发现2004—2014年间2.5 km以下所有高度层的夏季O3浓度均增大了许多，其中最大增幅为41.6×10-9。这表明，华北平原北部地区经历了迅速的光化学污染加重的过程。未来应该更多关注这种光化学污染加重态势。（徐晓斌）

1.15 2016年12月一次华北平原重度雾霾期间气溶胶光学与辐射特性的综合分析

为了深入了解2016年12月15—22日华北平原重度雾霾期间的气溶胶光学与辐射特性，通过地面观测并结合卫星资料、气象观测资料和大气环境监测数据，在北京、石家庄和焦作开展多源综合分析。主要结论为，当雾霾污染发生时，3个观测站点的500 nm 波段气溶胶光学厚度从0.30以下突增至1.4以上。波长指数在雾霾污染期间几乎都大于0.80。单次散射反照率的日均值在雾霾污染较为严重的阶段都大于0.85，个别时期甚至超过了0.97。雾霾污染期间的细、粗模态粒子体积浓度分别较无霾时期增大0.05～0.21 µm3和 0.01～0.43 µm3。北京、石家庄和焦作的吸收性气溶胶光学厚度日均值变化分别为0.01～0.11、0.01～0.13和0.01～0.04。雾霾污染期间的吸收性波长指数在0.6～2.0的范围内波动。这3个站点的气溶胶对地面辐射强迫分别为-23～-227 W/m2，-34～-199 W/m2和-29～-191 W/m2；对大气层顶的辐射强迫分别为-4～-98 W/m2，-10～-51 W/m2和-21～-143 W/m2。卫星资料显示，在雾霾污染阶段边界层内主要存在以烟尘、污染沙尘气溶胶和污染大陆型气溶胶所组成的混合气溶胶。通过潜在源贡献分析及浓度权重轨迹分析得出局地排放和上游地区输送所造成的PM2.5浓度贡献值分别为190 ～ 450 µg/m3和100 ～ 410 µg/m3。（车慧正）

1.16 北京城区夏季大气氨特征及对铵盐气溶胶的影响

为提高对北京城区大气氨变化特征的了解，对2009年夏季高分辨率NH3数据进行了分析。研究表明，与其他一次污染气体不同，降雨后NH3分钟平均浓度总体呈上升趋势。这主要由于降雨增加了植物和土壤中微生物反应过程从而释放氨，导致大气中NH3浓度明显增加。NH3和NOx分钟平均数据在8月存在弱但显著的线性相关关系，说明夏季NH3浓度与交通流量有关。NH3浓度在早晨出现峰值与汽车流量早高峰相对应也支持了这一发现。NH3日均浓度与PM2.5中的浓度呈现弱相关，说明在形成过程中NH3起到了重要的前体物作用。NH3/NHx质量比平均值为0.76±0.13，表明夏季NHx主要受当地排放源的影响，NH3干沉降有助于NHx的沉积。夏季高温、高湿和高氧化能力增强了大气光化学反应和液相过程，对SO2向转化以及NH3向转化有促进作用。后向轨迹分析显示，来自华北地区的气团输送对北京城区大气NH3和的浓度变化有重要的影响。（孟昭阳）

1.17 我国临安和龙凤山区域大气本底站黑碳浓度变化特征以及源区识别

黑碳（BC）是大气中细粒子成分之一，与气候、天气、空气质量以及人类健康密切相关。但是，中国本底站长时间序列大气BC浓度的变化特征和潜在源区的研究是非常有限的。对2年中国大气本底站临安和龙凤山的BC观测数据的分析结果表明，临安站BC浓度的日变化有2个明显的峰值，但是龙凤山站日变化特征与其不同，可能是由于不同排放源的贡献差异。临安站和龙凤山站BC月中值浓度的季节变化分别呈双峰和单峰分布。2站的2010年年均浓度相对于2009年分别下降1.63 g/m3和0.26 g/m3。临安站年均背景浓度高于龙凤山站相应浓度的2倍以上。临安和龙凤山两站的主要排放源分别是工业和民用排放。通过后向7天的传输计算，临安站和龙凤山站分别对东南到西北的带状区域和近似圆形区域的表面BC排放敏感。敏感区域会随着大气环流而变化，主要受夏季风或者我国西北部陆地冷气流的影响。另外，通过BC观测数据和FLEXPART模式模拟计算的排放灵敏度进行了BC源区的统计分析。夏季，BC源区主要分布在临安站的西北部和南部、龙凤山站的西南部。低BC浓度与来自海上的气团密切相关。冬季，BC源区集中在临安站的西部和南部以及龙凤山站印痕stot分布区域的东北部。西北部的冷气流对净化大气中BC具有重要作用。在年尺度上，BC源区大约是临安站西北和东南的5个省以及龙凤山的西部。这些研究结果对于BC减排和空气污染控制具有重要作用。（程巳阳）

1.18 雾霾期间中国东北城市群颗粒物质量浓度与能见度及混合层厚度的相关性

分析了2009—2012年我国东北辽宁中部主要工业城市沈阳、鞍山、本溪、抚顺地区大气能见度、颗粒物质量浓度和混合层高度（MLH）之间的相关性及其对该地区大气污染的潜在影响。结果表明，沈阳、鞍山、本溪和抚顺地区的年平均能见度分别为13.7±7.8、13.5±6.5、12.8±6.1、11.5±6.8 km。污染物负荷量 （PM× MLH） 指出鞍山地区污染物的垂直扩散较弱，近地面污染物质量浓度较高。较高质量浓度的气溶胶细颗粒物部分来源于9月开始的我国东北区域辽宁及周边地区的生物质燃烧排放，以及在混合层高度较低条件下，冬季采暖的燃煤排放。非雾霾天气条件下，水平能见度大约是雾霾天气条件下能见度的2.5～3.0倍。在雾霾期间，PM2.5和PM1细颗粒物质量浓度分别约是非雾霾期间的1.8～1.9和1.5倍。与霾污染相比，在雾发生期间混合层厚度降低的更快。本研究结果有助于更好地认识我国东北地区辽宁中部城市群污染物垂直扩散对该地区空气质量的影响。（车慧正）

1.19 中国地区大气含水量变化趋势及其与气溶胶变化的潜在联系

利用探空数据、地面气象站数据和多源卫星数据分析了1979—2015年中国地区大气含水量（PWV）和地表温度年和季节长期演变趋势及大气含水量与地表温度、气溶胶变化之间的潜在联系。结果表明，1979—1999年期间，中国地区大气含水量呈现明显上升趋势；相反，2000—2015年期间，呈现明显的下降趋势。3种类型站点在不同时间段内（1979—1999年和2000—2015年）大气含水量多年平均变化率均表现为，主要城市站点＞省会城市站点＞郊区站点，分别反映出受到不同强度人为活动的影响。2005—2015年期间，中国大部分地区吸收性气溶胶光学厚度（AAOD）呈现明显的增长趋势。运用空间相关系数分析方法发现，中国大部分地区MODIS大气含水量与地表温度之间存在着高的相关性，年平均相关系数达到了0.596。此外，MODIS大气含水量与AAOD空间相关系数呈明显的负相关，在中国中东部和中国西北地区相关系数达到最大，分别为-0.76和-0.71，表明2000年以后AAOD的增加可能是造成中国大部分区域PWV减少的原因之一。（车慧正）

1.20 不同季节气象条件对北京城区高黑碳浓度变化的影响

利用2013—2015年北京城区黑碳气溶胶（BC）和PM2.5观测资料，结合地面气象观测资料、ECMWF边界层高度再分析资料和FNL/NCEP不同高度风速再分析资料，讨论了BC质量浓度及其在PM2.5质量浓度中所占比例（下文统称“黑碳占比”）的季节、月、日变化特征，并通过计算北京城区BC浓度与不同高度风速的相关矢量，分析了气象条件和外来输送对北京城区BC浓度变化的影响。结果发现，研究时段内北京城区BC浓度平均值为4.77±4.49 ug/m3；黑碳占比为（8.23±5.47）%。BC浓度和黑碳占比在春、夏季低，秋、冬季高，其日变化特征在4个季节均为“白天低夜间高”的单峰型特征。随着PM2.5浓度的升高，BC浓度增大，黑碳占比减小。当北京地区风向为东北、东北偏东、东南和西南偏西（主风向）时，BC浓度与风速和边界层高度均呈反向变化，即随风速和边界层高度的增大而减小。另外，不同季节BC浓度随风速变化的临界值及其变化速率不同。冬季高BC浓度时段，北京城区BC浓度在低层大气的关键影响区分别位于河北南部与山东交界地区以及河北西北部与山西内蒙交界地区；高空关键影响区主要位于北京以西的河北西部、山西北部和内蒙古地区。（程兴宏）

1.21 秋冬季节华北背景地区PM1污染特征及来源解析

利用高分辨率飞行时间气溶胶质谱仪（HR-ToF-AMS）在华北背景地区上甸子区域大气本底站开展亚微米气溶胶（NR-PM1）化学组分及粒径分布的连续观测试验，观测时段为2015年10月17日至2016年1月27日，涵盖了秋、冬2季。结果表明，整个观测期间NR-PM1平均质量浓度为25.2 µg/m3，PM1中有机物占绝对优势，硝酸盐占的比例高于硫酸盐。各化学组分平均粒径分布以积聚模态为主，其中有机物峰形最宽，峰值粒径最小，硝酸盐峰值粒径最大，表明有机物在颗粒物形成、增长初期及老化阶段均有贡献，硝酸盐在气溶胶粒子老化过程中更易于增长为大粒子。有机物种元素特性分析结果显示，秋、冬季有机气溶胶平均氧碳比（O/C）和氢碳比（H/C）为0.58和1.58，OM/OC达1.91，有机气溶胶的氧化程度高于城市站点平均水平。在华北地区污染环境下，有机气溶胶演变途径Van krevelen 拟合曲线斜率为-0.21，其老化潜质和速率较珠三角地区和欧美地区城市要慢。对比污染时段和清洁时段化学组成特征发现，在污染时段硝酸盐质量浓度及其对PM1的贡献率超过硫酸盐，有机物氧化程度明显高于清洁时段。后径向轨迹气团分析结果显示，污染时段气团来向较为复杂，来自西部、南部以及东北部气团均有贡献，清洁时段主要受来自西伯利亚洁净空气的影响，对站点污染物扩散作用明显。（张养梅）

1.22 雄安新区大气污染的气象特征分析

通过分析雄安新区2016年5月24日至2017年4月30日的空气质量数据和气象数据，对该地区的大气污染现状进行了研究，并揭示其与气象条件的关系，旨在为雄安新区的合理规划提供科学依据。研究发现，雄安新区的主要大气污染物为PM2.5和PM10，其重污染过程主要出现在秋、冬2季，污染程度与相对湿度呈正相关，与风速和温度呈负相关。尽管雄安新区目前的开发程度较低，但受本地排放和周边污染物输送的共同影响，大气污染问题不容忽视。在雄安新区的规划和建设过程中，需要考虑周边地区的协调发展,共同治理大气环境。（缪育聪）

1.23 表面羟基对NO2在TiO2上吸附反应的影响

羟基（OH）在氧化物表面反应过程中具有重要作用。本研究结合实验室原位红外光谱研究与密度泛函理论计算模拟，研究了室温、模拟空气气氛下NO2气体在不同晶型TiO2样品表面的反应特性。研究发现，NO2表面反应通道与TiO2表面羟基（OH）的种类密切相关。OH的存在将导致表面HNO3的生成，HNO3即可作为最终产物在无定形TiO2表面存在，又可作为锐钛矿TiO2上硝酸盐生成的中间体。已有研究中广泛认为亚硝酸是硝酸盐生成的重要中间体，本研究工作进一步提出了在具有丰富OH的表面上，HNO3也可能是硝酸盐生成的重要中间产物。（刘畅）

1.24 运用地基GPS遥感观测对比评估中国地区多源大气含水量产品

水汽是大气中最重要的温室气体之一，也是影响全球水循环、气溶胶微物理特性、气溶胶-云相互作用、气候变化和地气系统能量的一个关键因子。本文首先基于MODIS水汽产品分析了中国地区大气含水量的时刻分布特征，然后基于2011—2013年中国地区6个典型城市站点GPS、探空、MODIS和AERONET太阳光度计反演大气含水量数据，评估了4种产品的数据质量和精度。时空分布分析表明，中国地区大气含水量分布具有明显的地理差异，基本呈现由东南沿海向西北内陆逐渐递减的分布特征。受夏季风的影响，中国区域PWV分布具有明显的季节性差异，其中在夏季最多，在秋季和春季次之，而在冬季最小。评估结果表明：GPS与探空水汽含量之间的相关系数在白天（R2=0.98）比晚上 （R2=0.97）更高，白天平均的偏差（Bias）、标准差（STD）和均方根误差（RMSE）分别为 -0.03、2.36和2.60 mm，而晚上三者分别为 -0.23、2.76和2.95 mm，说明GPS水汽较探空水汽轻微偏低，晚上更为明显。MODIS水汽含量与GPS和探空水汽含量呈现了同样的相关性（R2=0.88）。此外，MODIS近红外水汽含量较GPS和探空水汽含量均偏高，MODIS近红外水汽含量与GPS水汽含量之间的偏差（Bias=1.50 mm，RMSE=5.76 mm）比MODIS近红外水汽含量与探空水汽含量之间的偏差更大（Bias=0.75 mm，RMSE=5.31 mm）。AERONET水汽含量与GPS、探空和MODIS水汽含量之间均呈现了高相关性，三者相关系数（R2）分别为0.970、0.963和0.923，RMSE分别为2.53、3.67和4.39 mm。在北京地区，AERONET水汽含量与探空、GPS和MODIS水汽含量之间的Bias分别为-0.09、-1.82和-1.54 mm，表明AERONET水汽含量在北京地区都较其他3种PWV产品偏低。（车慧正）

1.25 日变化及气象条件对PM2.5与大气光学厚度相关性的影响以及对PM2.5卫星遥感的影响

当前从卫星观测得到的大气光学厚度（AOD）数据，通过建立数学模型的方式来反演地面PM2.5浓度仍是一个巨大挑战。本文根据中国气象局大气观测站网已有的226个大气成分站2013年1月至2015年12月逐小时的PM2.5观测数据，结合10 km×10 km分辨率的MODIS/Aqua AOD产品，首先研究了中国地区的PM2.5浓度的日变化情况。结果显示，大约50%站点PM2.5浓度峰值出现在上午，PM2.5浓度峰值出现在下午的概率仅为5%，这可能与下午地表接收到更强的太阳辐射能量以及下午更加剧烈的大气湍流和对流活动有关。华北平原PM2.5峰值一般出现在凌晨时分，其浓度是珠三角地区的2～3倍；长三角地区PM2.5浓度峰值介于华北平原和珠三角地区之间。为了研究PM2.5日变化对二者相关性的可能影响，采用卫星过境时刻前后1、3、5、24 h的 PM2.5平均值，分别与MODIS AOD进行匹配。统计结果显示，气溶胶日变化对PM2.5和AOD的相关性具有显著影响。其次，不同区域和季节，PM2.5与AOD的相关系数存在显著差异。本文还研究了云和相对湿度对二者相关性的影响，结果表明高云量和高湿环境易使华北平原、长三角和珠三角地区PM2.5和MODIS/Aqua AOD之间的相关系数降低，可见在利用MODIS AOD反演地面PM2.5浓度过程中，气象条件在PM2.5卫星遥感反演过程中起到了重要作用。（郭建平）

2 大气成分模式的发展应用及大气成分的影响

2.1 我国瓦里关站大气CO2：气团传输特征、时间变化以及源汇区域代表性

为了探究影响瓦里关站CO2本底浓度的源汇区域，提出了一种统计计算方法。该方法的关键是找到最优印痕阈值。研究过程分成4个步骤：（1）气团传输特征通过FLEXPART模式模拟计算在后向7天时间尺度下瓦里关站的月总印痕表示。结果显示，欧亚大陆地区的CO2排放频繁传输到瓦里关站，瓦里关站主要受到冬季西风带影响，也受到东南亚夏季风的控制。（2）利用CT2015模式模拟的CO2浓度，经过严格的数据质量控制、筛分拟合处理并与观测浓度进行了比较。瓦里关全球大气本底站CO2浓度有明显的季节变化，最大最小值分别出现在4月和8月。观测和模拟之间的拟合本底浓度相关性为R2=0.91。瓦里关站CO2时间序列变化特征主要与生物圈和大气的碳交换、人类活动以及气团传输有关。（3）对于CT2015的月均CO2拟合本底浓度，结合潜在源区（印痕）分布和CO2排放数据，通过相关性分析和数值迭代方法确定最优印痕阈值。印痕值大于该最优阈值的网格单元就是最优阈值区域，即影响本底浓度的源汇区域。4月CO2浓度最大值代表的源汇区域主要位于青海省，但是8月CO2浓度最小值受到更大范围区域排放的影响。（4）统计分析了最优阈值区域内CO2源汇特征。由于较少的人类活动以及高海拔地区植被、气候影响，影响瓦里关站CO2本底浓度的源汇区域表现为碳汇，并且各源汇强度均相对较弱。夏季瓦里关站CO2浓度更多受到人类活动影响，因为此时传输到测站的气团经过许多城市区。因此，利用印痕和排放数据能够评价CO2本底浓度源汇区域代表性。（程巳阳）

2.2 北京夏季“污染型”大气环流的识别和边界层结构

近几十年以来，北京地区经济和城市化的迅速发展，带来严重的资源环境问题，尤其是空气污染问题。目前，北京是我国北方大气污染最为严重的城市之一，其污染的发生不仅与污染物的大量排放有关，还与不利的气象条件有关。为了厘清北京地区与污染有关的大尺度环流特征和局地边界层结构，本文利用再分析资料、夏季午后的探空观测、近地面气溶胶浓度观测和数值模式进行了较为系统的分析和研究。通过主成分分析，发现北京地区夏季“污染型”的大气环流（925 hPa气压场）主要有3种：（1）在渤海和黄海上存在较强的高压系统；（2）河北北部和内蒙地区受低压系统控制，且黄海上有弱高压；（3）华北平原受东南至西北方向的气压梯度的影响。在上述大气环流的作用下，北京地区边界层的结构会受到高低空冷暖平流、云量以及边界层内的气溶胶颗粒物的影响。（缪育聪）

2.3 东亚夏季风对散射和吸收性气溶胶强迫的快响应和慢响应

利用地球系统模式CESM1研究了东亚夏季风系统对散射性（硫酸盐）和吸收性（黑碳）气溶胶强迫的总响应中快响应和慢响应的作用。结果表明，东亚夏季风对气溶胶强迫的快响应和慢响应的机制明显不同。气溶胶引起的海表温度变化造成的慢响应在驱动季风变化中起到了关键作用。硫酸盐强迫的快响应主要通过减弱海陆表面热力对比减弱东亚夏季风；而慢响应则通过改变对流层热力和动力结构更强地减弱了东亚夏季风。最终，硫酸盐强迫的总响应造成东亚夏季风环流明显减弱，降水减少。黑碳强迫的快响应导致东亚夏季海陆表面热力性质对比增加和东亚副热带急流北移，表现为增强东亚夏季风。但是，黑碳强迫的慢响应通过对大气温度和环流的影响减弱了东亚夏季风。最终，黑碳强迫的总响应造成东亚夏季风略微增强，特别是30°N以北区域。黑碳强迫的总响应和慢响应造成东亚夏季降水变化的分布方式基本一致。该研究强调了海洋对气溶胶强迫的反馈在气溶胶强迫驱动东亚夏季风变化中的重要性。（王志立）

2.4 东亚夏季风对不同类型人为气溶胶强迫的平衡态响应

利用一个耦合了混合层海洋模式的气溶胶-气候在线耦合模式BCC_AGCM2.0.1_CUACE/Aero研究了东亚夏季风系统对1850—2000年不同类型人为气溶胶（硫酸盐、黑碳和有机碳）排放增加的平衡态响应。结果表明，不同类型人为气溶胶通过改变局地海陆表面热力对比和大气环流对东亚夏季风产生显著不同的影响。硫酸盐气溶胶排放增加造成海陆之间表面温压差减小和东亚副热带急流南移，中国东部及其周围海域出现明显的北风距平，明显减弱了东亚夏季风。然而，有机碳气溶胶增加造成30°N以北的洋面上明显降温，形成一个高压距平中心，导致海陆之间表面热力性质差异略有增加，且高压西部出现明显的南风距平，从而增强了东亚北部夏季风；而中国南部海陆之间表面热力性质差异减小并出现北风距平，从而东亚南部夏季风减弱。黑碳气溶胶增加对东亚夏季850 hPa风场和降水影响的分布和有机碳气溶胶的影响大体一致。黑碳气溶胶增加造成35°N以北东亚夏季风增强，而在35°N以南区域东亚夏季风略有减弱。同时还发现，显著增加黑碳气溶胶的排放并不改变模拟的东亚夏季风对黑碳气溶胶的响应，仅增强了其对季风影响信号的强度。总体来说，硫酸盐气溶胶的增加对东亚夏季风系统的减弱起了主要作用，而有机碳和黑碳气溶胶的增加在一定程度上减缓了东亚北部夏季风的减弱程度。（王志立）

2.5 全球、东亚和非东亚人为气溶胶排放变化对东亚夏季风系统的影响

利用一个气溶胶—气候在线模式BCC_AGCM2.0.1_CUACE/Aero定量研究了全球、东亚和非东亚人为气溶胶排放变化对东亚夏季风系统的影响。结果显示，1850—2000年人为气溶胶排放增加造成东亚季风区夏季大气顶和地表全天平均净短波辐射通量分别减少了4.8 W/m2和5.0 W/m2。辐射通量的变化及其引起的热传输、水汽传输、云量等的变化共同造成了东亚季风区夏季地表冷却。总的人为气溶胶排放增加造成东亚季风区夏季平均地表气温降低了2.1 K，表面气压升高了0.4 hPa，其中东亚之外的气溶胶排放变化对二者变化的贡献更大。陆地上表面气温和气压的变化明显高于海洋上，从而减弱了海陆温压差对比，造成夏季中国东部、南部以及周围海域出现明显的北风和东北风距平，进而减弱了东亚夏季风，造成东亚季风区夏季平均降水减少12%。东亚外气溶胶排放变化在造成东亚局地气温和气压变化中起了更为重要的作用，因此明显加剧了局地气溶胶变化对东亚夏季风的减弱。二者对季风环流减弱的贡献程度相当，个别区域贡献甚至更大。局地和非局地气溶胶变化对洋面上降水减少的贡献相当，但却造成了中国东部降水相反的变化。本文的结果强调了非局地气溶胶变化在人为气溶胶变化对东亚季风系统影响中的重要性。（王志立）

2.6 陆面资料不确定性对近地面气温和降水模拟的影响

本文利用WRF模式降尺度模拟2006年中国区域高分辨率（10 km）气象场，近地面气温和降水站点观测资料评估WRF模拟结果。将高分辨率的陆面资料（包括SRTM地形资料、MODIS土地利用和覆被资料、HWSD土壤类型资料）应用到WRF模式中，并通过敏感性试验模拟研究了高分辨率陆面资料对模拟结果的影响。WRF模式较好地模拟了近地面气温和降水的时空变化特征，模拟性能存在显著的区域和季节差异。在云贵高原、青藏高原、东北、东南沿海地区，WRF模式模拟近地面气温偏低；在华北、黄土高原、四川盆地、新疆西部等地区，WRF模式模拟近地面气温偏高。在大部分湿润地区，WRF模式模拟降水偏多，在大部分干旱区，WRF模拟降水偏少。从季节来看，夏季WRF模式模拟降水偏多，冬季模拟降水偏少。采用高分辨率的陆面资料后，WRF模拟气温和降水的均值和极值都有所改进，日均气温和日累积降水的RMSE分别减小了7%和2.3%。从统计上来看，气温模拟改进效果显著，但降水模拟改进效果不显著。陆面信息不确定对气温的影响比降水的影响显著。这些发现对天气预报以及东亚气候变化研究具有重要的意义。（何建军）

2.7 北京颗粒物污染区域输送数值模拟研究

区域大气污染频繁发生，给大气污染防治工作带来严峻挑战。利用空气质量数值模式CUACE和大气扩散模式FLEXPART，本文研究了北京不同季节颗粒物的区域输送、潜在源区以及输送路径。通过排放源敏感性试验得出2013年夏季（7月）和冬季（12月）北京近地面PM2.5区域输送比率分别为53.4%和36.1%，PM10区域输送比率分别为51.8%和 35.1%。夏季北京颗粒物区域输送较冬季更显著，气象条件季节差异和排放空间分布特征是造成区域输送季节差异两大主要因子。敏感性试验结果还显示二次气溶胶受区域输送影响更显著。北京区域输送的颗粒物主要来源于河北和天津。基于后向轨迹分析，北京夏季气团来源方向非常复杂，冬季气团来源主要是西北方向。大气污染水平、污染物区域输送比率与输送路径和输送距离紧密相关。（何建军）

2.8 多尺度环流“接力输送”对京津冀地区气溶胶污染的影响

本研究结合风廓线雷达资料、MODIS数据、WRF-Chem模式，以及近地面气象和气溶胶观测，对京津冀地区2013年6月的一次气溶胶污染过程进行了研究。结果发现，当黄海上存在较强的高压系统时，江苏和山东等地的污染物会被输送到渤海地区；随后，随着渤海地区海风的发生和发展，海上的大气污染物会被输送到北京地区，造成空气质量的恶化。此外，本研究首次利用逐小时的近地面气象观测资料，证实了渤海地区的海风可以影响到北京地区。（缪育聪）

2.9 1.5 ℃和2 ℃全球变暖下极端气候变化的情景依赖性

2015年《巴黎协定》的目标是较工业化前水平把全球平均表面气温升高控制在2 ℃之内，并努力把升温控制在1.5 ℃之内。决策者们需要关于这些变暖水平造成的影响的可靠信息来制定气候减缓和可适性政策。本文利用3组全球气候模式的集合模拟试验研究了1.5 ～2 ℃全球变暖下气候极端（往往与经济损失和人员伤亡紧密联系）的变化以及其情景依赖性。结果显示，额外的0.5 ℃增暖（1.5～2 ℃）导致大部分区域极端温度和极端降水的显著增加。但是，预估的这些变暖水平下气候极端的变化强烈地依赖排放情景的路径，即情景中有不同的温室气体和气溶胶强迫比值或者温室气体浓度水平。在一些重污染区域，不同的情景造成的额外的0.5 ℃（1.5 ～2 ℃）增暖导致的极端降水变化的差异可高达几倍。本研究结果阐明当评估1.5 ～2 ℃全球变暖的影响时，不仅仅需考虑总的辐射强迫及其导致的变暖水平，也必须考虑排放情景中的化学组成。（王志立）

2.10 2013年1月重霾污染过程中国地区气溶胶短期辐射影响及其地区差异

采用WRF-CMAQ-SES2离线耦合模式系统模拟了2013年1月重霾污染过程中国地区气溶胶对地面辐射的短期影响效应及其地区差异。首先采用WRF-CMAQ模式模拟了全国PM10和PM2.5浓度，并采用自适应偏最小二乘回归法和153个环境监测站的实测浓度对PM10和PM2.5模拟浓度以及气溶胶各主要组分的垂直廓线进行了订正；然后设计2种模拟试验方案，采用澳大利亚气象局辐射传输模式SES2以及订正后的气溶胶浓度垂直廓线、WRF模拟的云和水汽等气象要素垂直分布，分别模拟了2013年1月考虑和未考虑气溶胶影响的地面总辐射和直接辐射通量；最后采用全国78个辐射观测站资料评估了上述模拟结果，并分析了气溶胶对总辐射和直接辐射的短期影响效应及其地区差异。结果表明：（1）经过地面实测PM10和PM2.5浓度订正后，气溶胶模拟浓度较接近于实测值，显著减小了气溶胶模拟的不确定性；（2）采用上述不确定性较小的气溶胶垂直廓线资料模拟的总辐射和直接辐射更接近于实测辐射值；（3）2013年1月重污染过程气溶胶对总辐射的削减值介于100～500 W/m，其短期影响可达气溶胶长期气候效应的2个量级；（4）由于气溶胶化学组成及其垂直分布特征的地区差异导致气溶胶的短期辐射影响程度自北向南呈减小趋势，其中霾污染较重的京津冀地区气溶胶对辐射的影响最大，长三角地区次之，珠三角地区削减较小；（5）在地面相同的PM10和PM2.5浓度下，气溶胶对总辐射的影响亦存在显著的南北差异，其主要原因是不同地区高空气溶胶浓度的垂直分布差异较大。（程兴宏）

2.11 春季东亚沙尘气溶胶跨太平洋输送：观测与中尺度模式模拟研究

借助星地同步观测，系统揭示了发生于2015年3月12—22日的一次典型的起源于中国西北地区塔克拉玛干沙漠和隔壁的沙尘跨太平洋输送事件。CALIPSO卫星结果显示，在沙尘源区，沙尘分布的高度在1～7 km，其在向东传输过程中，经过中国华北平原和东北地区沙尘气溶胶发生了一系列混合，老化等过程，变成污染性沙尘气溶胶。受太平洋上空（其气溶胶分布高度范围介于2～9 km）的阿留申低压系统影响，此污染沙尘带分叉为2支，一支往北到达阿拉斯加地区，另一支继续往东到达加拿大西海岸。中尺度模式WRF模拟的云水含量结果表明，（污染）沙尘气溶胶传输过程易受云雨的湿沉降过程影响。本研究从观测和模式的角度，从沙尘源区的大气廓线观测、沙尘气溶胶立体分布及其传输过程中的云降水湿沉降影响等方面，系统阐述了气溶胶远距离输送的物理机制。（郭建平）

2.12 基于CMAQ源同化反演方法的京津冀局地污染源动态变化特征模拟研究

利用“Nudging”源同化技术反演了京津冀地区2014年1、3、7、11月SO2、NOx的局地动态污染源，分析其排放源强、特征及地理分布，对比其与初始源的差异，同时检验反演源的模拟效果。结果表明，SO2、NOx污染源存在明显的季节变化，冬季或采暖期排放强度最大。由唐山、北京、天津、廊坊、保定、石家庄、邢台、邯郸构成东北-西南走向的带状污染物高排放区，最高排放中心主要集中在太行山、燕山山前区域，且排放具有典型的“城市化”特征，即各个城市市区及附近强度最大，周边郊县稍弱。与初始源模拟结果相比，采用反演源更能反映出污染物的时空变化特征，模拟值与实测值较接近，而且对于重污染过程亦具有较好的模拟效果。（程兴宏）

Progress in Atmospheric Composition and Atmospheric Chemistry Research

1 Observational studies of atmospheric composition and related properties

1.1 Relative contributions of boundary-layer meteorological factors to the explosive growth of PM2.5 during the red-alert aerosol heavy pollution episode in Beijing in December 2016

Based on observations of urban mass concentration of fine particulate matter smaller than 2.5 μm (PM2.5)in diameter, ground meteorological data, vertical measurements of meteorological factors, including winds,temperature and relative humidity (RH), and the ECMWF reanalysis data, the major changes in the vertical structures of meteorological factors in the boundary layer (BL) during the heavy aerosol pollution episodes(HPEs) that occurred in winter 2016 in the urban Beijing area were analyzed.Beijing HPEs can generally be divided into two stages: the transport stage (TS) under prevailing southerly winds and the period of PM2.5explosion growth and pollution accumulation characterized by temperature inversion with lower winds and higher RH in the lower BL, known as the cumulative stage (CS).During the TS, a surface high lies south of Beijing, and pollutants transported from the south of Beijing are a particularly striking feature.Subsequently,a stable BL characterized by weak winds, temperature inversion and moisture congregation occurs, which corresponds to the CS.Stable atmospheric stratification, which is characterized by light or calm winds,anomalous inversion, and accumulated moisture (RH > 80%) below 250 meters at the beginning of the CS,might be closely associated with the inversion, strengthened by the considerable decrease in near-surface air temperature attributable to the interaction between aerosols and radiation after aerosol pollution occurs.This association is suggested by a significant increase in the PLAM index after pollution formation, which primarily reflects the stability of the air mass and the condensation rate of water vapor on aerosol particles and is linearly correlated with the PM mass change.The difference in the growth rate between the PM2.5mass and the PLAM value during the first 10 hours of the CS indicates that the feedback effects from the more stable BL contribute to 84% of the explosive growth of PM2.5mass in a range of 73%–93%.Additional accumulated near-surface moisture (RH > 80%) caused by the ground-temperature decrease, weaker turbulent diffusion,lower BL height and inhibited vertical mixing of water vapor are conducive to the secondary aerosol formation in chemical reactions, including liquid-phase and heterogeneous reactions, and further increase PM2.5levels.The contribution of these reaction mechanisms to the explosive growth of PM2.5mass during the early CS and subsequent pollution accumulation needs further investigation.(Zhang Xiaoye)

1.2 Aerosol optical properties: observation and its relationship with meteorological conditions and emission during the Chinese National Day and Spring Festival holiday in Beijing

The reduction of traffic flow in downtown areas during the Chinese National Day holiday and the fireworks during the Spring Festival provides a unique opportunity for investigating the impact of urban anthropogenic activities on aerosol optical properties during these important Chinese festivals in Beijing.The National Day in 2014 and 2015 and Spring Festival in 2015 and 2016 were selected as study periods.The aerosol optical depth (AOD) at 440 nm increased over all the holiday periods and the average AODs during the 2015 National Day, 2015 Spring Festival and 2016 Spring Festival were about 81%, 21% and 36% higher than the background levels, respectively.The average AOD in 2014 National Day holiday was lower than the background level partly influenced by a precipitation event.The absorption AOD (AAOD) at 440 nm showed consistent variations with the AOD and the average AAODs during the 2015 National Day, 2015 Spring Festival and 2016 Spring Festival holidays were about 75%, 19% and 23% higher than the background level,respectively.The mean values of single scattering albedo were greater than the background level during the Spring Festival holidays, whereas the values during the National Day holiday in 2015 were lower partly due to the reduction of vehicular emissions in downtown areas.Fine- and coarse-mode particle volumes during pollution periods in holidays were 0.04–0.25 μm3and 0.03–0.15 μm3larger than the background levels,respectively.The results of potential source contribution function and concentration-weighted trajectory analyses identified the areas south of Beijing as the main source regions of PM2.5and were responsible for the extremely high PM2.5concentrations in Beijing during the holiday periods.The findings of this study may aid in understanding the effects of human activities on aerosol optical properties in the Beijing area and contribute to improving regional air quality.(Che Huizheng)

1.3 Air pollution characteristics and their relationship to meteorological conditions during 2014–2015 in major cities in China

In January 2013, the real-time hourly average concentrations of six pollutants (CO, NO2, O3, PM10, PM2.5and SO2) based on data from air quality monitoring stations in major cities in China were released to the public.That report provided a good opportunity to publicize the nationwide temporal and spatial pollution characteristics.Although several studies have systematically investigated the temporal and spatial trends of pollutant concentrations, the relationship between air pollution and multi-scale meteorological conditions and their spatial variations on a nationwide scale remains unclear.This study has analyzed the air pollution characteristics and their relationship with multi-scale meteorological conditions during 2014–2015 in 31 provincial capital cities in China.The annual average concentrations of six pollutants for 31 provincial capital cities were 1.2 mg m–3, 42.4 μg m–3, 49.0 μg m–3, 109.8 μg m–3, 63.7 μg m–3, and 32.6 μg m–3, respectively, in 2014.The annual average concentrations decreased by 5.3%, 4.9%, 11.4%, 12.0% and 21.5% for CO, NO2,PM10, PM2.5and SO2, respectively, but increased by 7.4% for O3in 2015.The highest rate of a major pollutant over China was PM2.5followed by PM10, O3, NO2, SO2and CO.Meteorological conditions were the primary factor determining day-to-day variations in pollutant concentrations, explaining more than 70% of the variance of daily average pollutant concentrations over China.Meteorological conditions in 2015 were more adverse for pollutant dispersion than in 2014, indicating that the improvement in air quality was caused by emission controls.(He Jianjun)

1.4 Prediction of size-resolved number concentration of cloud condensation nuclei and long-term measurements of their activation characteristics

Atmospheric aerosol particles acting as cloud condensation nuclei (CCN) are key elements in the hydrological cycle and climate.To improve our understanding of the activation characteristics of CCN and to obtain accurate predictions of their concentrations, a long-term field campaign was carried out in the Yangtze River Delta, China.The results indicated that the CCN were easier to activate in this rural but still poluted regional station, than in clean (e.g., the Amazon region) or dusty (e.g., Kanpur-spring)locations, but were harder to activate than in more polluted areas (e.g., Beijing).A new method that uses two additional parameters—the maximum activation fraction and the degree of heterogeneity is proposed to predict the accurate, size-resolved concentration of CCN.The value ranges and prediction uncertainties of these parameters were evaluated.The CCN predicted using this new method with size-resolved chemical compositions under an assumption of internal mixing showed the best agreement with the long-term field measurements.(Zhang Xiaoye)

1.5 Ambient mixing ratios of atmospheric halogenated compounds at five background stations in China

High precision measurements of three chlorofluorocarbons (CFCs), three hydrochlorofluorocarbons(HCFCs), six hydrofluorocarbons (HFCs), three perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) were made at five Chinese background stations from January 2011 to December 2012.Their station means in the background air were 239.5 ± 0.69 parts-per-trillion dry-air mole fraction mixing ratios (×10–12) for CFC-11, (536.5 ± 1.49) for CFC-12, (74.66 ± 0.09) for CFC-113, (232.1 ± 4.77) for HCFC-22, (23.78 ± 0.29) for HCFC-141b, (22.92 ± 0.42) for HCFC-142b, (11.75 ± 0.43) for HFC-125, (71.32 ± 1.35) for HFC-134a, (13.62± 0.43) for HFC-143a, (9.10 ± 1.26) for HFC-152a, (25.45 ± 0.1) for HFC-23, (7.28 ± 0.48) for HFC-32,(4.32 ± 0.03) for PFC-116, (0.63 ± 0.04) for PFC-218, (1.36 ± 0.01) for PFC-318, and (7.67 ± 0.03) for SF6,respectively, which were comparable with those measured at the two Northern Hemisphere (NH) AGAGE stations: Mace Head, Ireland (MHD) and Trinidad Head, California, USA (THD).Compared with our results for earlier years from in-situ measurement at SDZ, background-air mixing ratios of CFCs are now declining,while those for HCFCs, HFCs, PFCs, and SF6 are still increasing.The ratios of the number of sampling events in which measured mixing ratios were elevated above background (pollution events) relative to the total sample frequency (POL/SUM) for CFCs, HCFCs, and HFCs were found to be station dependent, generally LAN ＞SDZ ＞ LFS ＞ XGL＞ WLG.The enhancement (Δ, polluted mixing ratios minus background mixing ratios)generally showed distinct patterns, with HCFCs ((40.7–175.4)×10–12) ＞ HFCs ((15.8–66.3)×10–12> CFCs((15.8–33.8)×10–12) ＞ PFCs ((0.1–0.9)×10–12) at five stations, especially for HCFC-22 ranging from 36.9×10–12to 138.2×10–12.Combining with the molecular weights, our findings imply the biggest emissions of HCFCs in the regions around these Chinese sites compared to HFCs and CFCs, while the smallest of PFCs, consistent with CFCs being phased out and replaced with HCFCs in China.In addition, relative emission strengths(emission was expressed by mole fractions) of these halocarbons in China were inferred as HCFC-22 ＞ HCFC-141b ＞ HFC-134a ＞ HCFC-142b for the Yangtze River Delta (YRD) and as HCFC-22 ＞ HCFC-142b ＞ HCFC-141b ≈ HFC-134a in the North China Plain (NCP).(Zhang Gen)

1.6 Characterization of particle number size distribution and new particle formation in an urban environment in Lanzhou, China

A scanning mobility particle sizer was utilized for the real-time measurement of particles in the size range of 14.6 to 661.2 nm from August 14 to November 18, 2014 to characterize the evolution of particle number size distribution and new particle formation (NPF) events in Lanzhou, China.The mean number concentrations of nucleation mode, Aitken mode, accumulation mode, and total particles were 2833, 12,898, 6210, and 21,940 cm−3, respectively.The mean number size distribution was obviously unimodal, with a peak at around 60 nm.The diurnal variation of nucleation mode particles was mainly influenced by NPF events.Two peaks at traffic rush hours were observed for Aitken mode, accumulation mode, and total particles; the peaks were related to traffic and cooking soot emissions.The particle number and mass concentrations were all higher on workdays than on weekends mainly because of the enhancement in human activities and traffic density.Daytime also had higher values of number concentrations than nighttime.The mass concentrations of total submicron particles,sulfate, nitrate, and ammonium had much higher values at daytime than at nighttime, however, organics had slightly higher mass concentrations at nighttime mainly because of heating activities at night and intensified evening peaks.During the measurement period, the frequency of NPF events was calculated to be 34%.The average apparent formation and growth rates of nucleation mode particles were 1.71 cm−3s−1and 6.10 nm h−1, respectively.A typical NPF event with two-stage particle growth was selected and analyzed to provide insight into the evolution of particle number size distributions.Back trajectory clustering analysis indicated that air masses that originated from the west had high values of both particle number and mass concentrations associated closely with local source emissions.Air masses from the north and northwest represented the relatively clean air mass arriving in Lanzhou.(Zhang Yangmei)

1.7 Characteristics of chemical composition and role of meteorological factors during heavy aerosol pollution episodes in northern Beijing area in autumn and winter of 2015

Heavy aerosol pollution episodes (HPEs) usually start from late autumn and become more serious in winter in Beijing and its vicinity (BIV).In this study, we examined the reasons for the formation and change of HPEs in the areas of northern BIV.The size-resolved chemical components of PM1and meteorological conditions were investigated during HPEs in autumn and winter of 2015.Stable regional atmosphere and higher atmospheric condensation processes associated with southerly and lower speed wind led to the formation of HPEs.After the start of these HPEs, the concentration of fine particles increased more than twice in several hours.80% of the “explosive” growth in PM mass can be considered as a positive feedback of meteorological factors that come from even more stable atmosphere and larger condensation rate of water vapor, which was derived from the interaction between formed aerosol pollution and the meteorological factors in the boundary layer.Nitrate was the largest fraction of PM1in autumn, and the most significantly increased component during HPEs relative to clean period during both autumn and winter.The proportion of organic aerosol (OA) was similar to that of nitrate in autumn, but its rise in HPE was much smaller, mainly because the high concentration of OA existed during clean periods.Compared with the largest increase of nitrate, the similar uplift was found for ammonium production, showing that a large amount of ammonium mainly formed by the combination ofin HPEs, rather than.In addition to the lower southerly wind carrying pollutants from southern part of BIV, westerly wind from central Inner Mongolia and north Shanxi can also bring air pollutants originating from coal combustion, contributing to the heavy pollution in the northern BIV area in winter, and resulting in higher sulfate, nitrate and organic aerosol masses.(Zhang Xiaoye)

1.8 Chemical characteristics of PM2.5 during summer at a background site of the Yangtze River Delta in China

With rapid economic development and urbanization, particular attention has been paid to atmospheric chemical studies in the Yangtze River Delta in China.PM2.5samples were collected by a MiniVol™ air sampler in summer time at a background site of the Yangtze River Delta in China.Carbonaceous components, i.e.,OC and EC, levoglucosan and water-soluble inorganic ions, including sulfate, nitrate, ammonium, etc., were quantified.The average concentration of PM2.5in summer at Lin’an was (30.19±8.86) µg m–3, lower than that previous studies reported, confirming that air pollution in China is improving, e.g., by emission control measures and subsequent reduction in PM emissions in China.Examination of the relationship among sulfate,nitrate and ammonium showed thatexisted aswhilemight have been present as NaNO3and KNO3.Based on molecular tracer, synoptic data as well as air mass back trajectory analysis, it was revealed that regional transport and stable synoptic conditions both played important roles in controlling the variations of aerosol chemical components.The comparison of aerosol chemical components between clean and hazy days showed that secondary organic and inorganic aerosols have different production processes.Secondary organic carbon (SOC) was much more important during clean days, while secondary inorganic aerosol species were readily produced and consequently became more important during polluted periods in Lin’an during summer time.(Liang Linlin)

1.9 Chemical characterization of submicron aerosol particles during wintertime in a northwest city of China using an Aerodyne aerosol mass spectrometry

An Aerodyne quadrupole aerosol mass spectrometry (Q-AMS) was utilized to measure the size-resolved chemical composition of non-refractory submicron particles (NR-PM1) from October 27 to December 3,2014 at an urban site in Lanzhou, Northwest China.The average NR-PM1mass concentration was 37.3 μg m–3(ranging from 2.9 to 128.2 μg m–3) under an AMS collection efficiency of unity and was composed of organics (48.4%), sulfate (17.8%), nitrate (14.6%), ammonium (13.7%), and chloride (5.7%).Positive matrix factorization (PMF) with the multi-linear engine (ME-2) solver identified six organic aerosol (OA) factors,including hydrocarbon-like OA (HOA), coal combustion OA (CCOA), cooking-related OA (COA), biomass burning OA (BBOA) and two oxygenated OA (OOA1 and OOA2), which accounted for 8.5%, 20.2%, 18.6%,12.4%, 17.8% and 22.5% of the total organics mass on average, respectively.Primary emissions were the major sources of fine particulate matter (PM) and played an important role in causing high chemically resolved PM pollution during wintertime in Lanzhou.Back trajectory analysis indicated that the long-range regional transport air mass from the westerly was the key factor that led to severe submicron aerosol pollution during wintertime in Lanzhou.(Zhang Yangmei)

1.10 Contribution of fungal spores to organic carbon in ambient aerosols in Beijing, China

Fungal spores are ubiquitous components of atmospheric aerosols and contributors to the organic carbon(OC) component in ambient aerosols.Daily PM10and PM2.5samples were collected concurrently at an urban site during an entire year, while PM10sampling was conducted at a rural site during late spring and summer.The annual average concentrations of spore-OC in PM2.5and PM10at the THU site were 0.3 ± 0.2 µg C m–3and 0.8 ± 0.7 µg C m–3, while the respective contributions of spore-OC to organic carbon were (1.2 ± 0.7)%and (3.5 ± 3.7)%, respectively.The contributions of fungal spores to OC in the two size fractions had the following seasonal trend (from highest to lowest levels): summer, autumn, winter and spring.During the summer sampling season, the contribution of fungal spores to OC was observed at a higher level at the rural site compared to the urban site, (14.1 ± 10.5)% and (7.3 ± 3.3)%, respectively.It can be concluded that fungi are a non-negligible source of carbonaceous aerosol even at urban locations such as Beijing, China.Thus, more studies are needed to better understand the spatial, temporal and size distributions of fungal OC contributions to atmospheric aerosols.(Liang Linlin)

1.11 Declining frequency of summertime local-scale precipitation over eastern China from 1970 to 2010 and its potential link to aerosols

Summer precipitation plays critical roles in the energy balance and the availability of fresh water over eastern China.However, little is known regarding the trend in local-scale precipitation (LSP).Here we developed a novel method to determine LSP events in the summer afternoon throughout eastern China from 1970 to 2010 based on hourly gauge measurements.The LSP occurrence hours decrease at an annual rate of 0.25%, which varies considerably by region, ranging from 0.14% over the Yangtze River Delta to 0.56% over the Pearl River Delta.This declining frequency of LSP is generally accompanied by an increase in rain rate of LSP but a decrease in visibility, whose linkage to LSP events was investigated.In particular, more LSP events tended to form when the atmosphere was slightly polluted.Afterward, LSP was suppressed.These findings have important implications for improving our understanding of the climatology of daytime precipitation at local scales.( Guo Jianping)

1.12 Aerosol and boundary-layer interactions and impact on air quality

Air quality is concerned with pollutants in both the gas phase and solid or liquid phases.The later are referred to as aerosols, which are multifaceted agents affecting air quality, weather and climate through many mechanisms.Unlike gas pollutants, aerosols interact strongly with meteorological variables with the strongest interactions taking place in the planetary boundary layer (PBL).The PBL hosting the bulk of aerosols in the lower atmosphere is affected by aerosol radiative effects.Both aerosol scattering and absorption reduce the amount of solar radiation reaching the ground and thus reduce the sensible heat fluxes that drive the diurnal evolution of the PBL.Moreover, aerosols can increase atmospheric stability by inducing a temperature inversion as a result of both scattering and absorption of solar radiation, which suppresses dispersion of pollutants and leads to further increases in aerosol concentration.Such positive feedback is especially strong during severe pollution events.Knowledge of the PBL is thus crucial for understanding the interactions between air pollution and meteorology.A key question is how the diurnal evolution of the PBL interacts with aerosols, especially in vertical directions, and affects air quality.We review the major advances in aerosol measurements, PBL processes and their interactions with each other through complex feedback mechanisms and highlight the priorities for future studies.(Guo Jianping)

1.13 Health burden attributable to ambient PM2.5 in China

In China, over 1.3 billion people have high health risks associated with exposure to ambient fine particulate matter (PM2.5) that exceeds the World Health Organization (WHO) Air Quality Guidelines (AQG).The PM2.5mass concentrations from 1382 national air quality monitoring stations in 367 cities, between January 2014 and December 2016, were analyzed to estimate the health burden attributable to ambient PM2.5across China.The integrated exposure-response model was applied to estimate the relative risks of diseasespecific mortality.Disease-specific mortality baselines in province-level administrative units were adjusted by the national mortality baseline to better reveal the spatial inequality of the health burden associated with PM2.5.Our study suggested that PM2.5in 2015 contributed as much as 40.3% to total stroke deaths, 33.1%to acute lower respiratory infection (ALRI, ＜ 5 years) deaths, 26.8% to ischemic heart disease (IHD) deaths,23.9% to lung cancer (LC) deaths, 18.7% to chronic obstructive pulmonary disease (COPD) deaths, 30.2%to total deaths combining IHD, stroke, COPD, and LC, 15.5% to all cause deaths.The population weighted average (PWA) attributable mortality rates (10-5yr-1) were 112.0 in current year analysis, and 124.3 in 10-year time lag analysis.The mortality attributable to PM2.5in 10-year time lag analysis (1.7 million) was 12% higher than the current year analysis (1.5 million).Our study also estimated site-specific annual PM2.5concentrations in scenarios of achieving WHO interim targets (ITs) and AQG.The mortality benef i ts will be 24.0%, 44.8%,70.8%, and 85.2% of the total current mortalities (1.5 million) when the PWA PM2.5concentrations in China meet the WHO IT-1, IT-2, IT-3, and AQG, respectively.We expect air quality modeling and cost-benefits analysis of emission reduction scenarios and corresponding health benef i ts in meeting the site-specific annual PM2.5concentrations (WHO IT-1, IT-2, IT-3, and AQG) this study raised.(He Jianjun)

1.14 Lower tropospheric distributions of O3 and aerosol over Raoyang, a rural site in the North China Plain

The North China Plain (NCP) has become one of the most polluted regions in China, with the rapidly increasing economic growth in the past decades.High concentrations of ambient O3and aerosol have been observed at urban as well as rural sites in the NCP.Most of the in-situ observations of air pollutants have been conducted near the ground so that current knowledge about the vertical distributions of tropospheric O3and aerosol over the NCP region is still limited.In this study, vertical profiles of O3and size-resolved aerosol concentrations below 2.5 km were measured in summer 2014 over a rural site in the NCP, using an unmanned aerial vehicle (UAV) equipped with miniature analyzers.In addition, vertical profiles of aerosol scattering property in the lower troposphere and vertical profiles of O3below 1 km were also observed at the site using a lidar and tethered balloon, respectively.The depths of the mixed layer and residual layer were determined according to the vertical gradients of lidar particle extinction and aerosol number concentration.Average O3and size-resolved aerosol number concentration in both the mixed and residual layer were obtained from the data observed in seven UAV fl ights.The results show that during most of the fl ights the O3levels above the top of the mixed layer were higher than those below.Such a positive gradient in the vertical distribution of O3makes the residual layer an important source of O3in the mixed layer, particularly in the morning when the top of the mixed layer is rapidly elevated.In contrast to O3, aerosol number concentration was normally higher in the mixed layer than in the residual layer, particularly in the early morning.Aerosol particles were overwhelmingly distributed in the size range ＜ 1 μm, showing slight differences between the mixed and residual layers.Our measurements confirm that the lower troposphere over the rural area of the NCP is largely impacted by anthropogenic pollutants locally emitted or transported from urban areas.Compared with the historic O3vertical profiles over Beijing from the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC), a strong increase in O3can be found at all heights below 2.5 km in the decade from 2004 to 2014, with the largest enhancement of about 41.6×10−9.This indicates that the lower troposphere over the northern part of the NCP has experienced rapidly worsening photochemical pollution.This worsening trend in photochemical pollution deserves more attention in the future.(Xu Xiaobin)

1.15 Optical and radiative properties of aerosols during a severe haze episode over the North China Plain in December 2016

The optical and radiative properties of aerosols during a severe haze episode from 15 to 22 December 2016 over Beijing, Shijiazhuang, and Jiaozuo in the North China Plain were analyzed based on the groundbased and satellite data, meteorological observations, and atmospheric environmental monitoring data.The aerosol optical depth at 500 nm was ＜ 0.30 and increased to ＞1.4 as the haze pollution developed.The Ångström exponent was ＞ 0.80 for most of the study period.The daily single-scattering albedo was ＞ 0.85 over all of the North China Plain on the most polluted days and was ＞ 0.97 on some particular days.The volumes of fine and coarse mode particles during the haze event were approximately 0.05–0.21 and 0.01–0.43 μm3,respectively—that is, larger than those in the time without haze.The daily absorption aerosol optical depth was about 0.01–0.11 in Beijing, 0.01–0.13 in Shijiazhuang, and 0.01–0.04 in Jiaozuo, and the average absorption Ångström exponent varied between 0.6 and 2.0.The aerosol radiative forcing at the bottom of the atmosphere varied from –23 to –227, –34 to –199, and –29 to –191 W m–2for the whole haze period, while the aerosol radiative forcing at the top of the atmosphere varied from –4 to –98, –10 to –51, and –21 to –143 W m–2in Beijing, Shijiazhuang, and Jiaozuo, respectively.Satellite observations showed that smoke, polluted dust, and polluted continental components of aerosols may aggravate air pollution during haze episodes.The analysis of the potential source contribution function and concentration-weighted trajectory showed that the contribution from local emissions and pollutants transport from upstream areas were 190–450 and 100–410 μg m–3,respectively.(Che Huizheng)

1.16 Summertime ambient ammonia and its effects on ammonium aerosol in urban Beijing, China

To improve the understanding of NH3variation in urban Beijing, high temporal resolution (1-min averaged) NH3data derived during the summer of 2009 were analyzed.Results indicated that after rain there was generally an increase in NH3concentration.The analysis of the 1-min data revealed that a weak but significant linear correlation existed between NH3and NOxin August, indicating a connection between NH3concentrations and traffic levels during summer.This was further supported by peak NH3levels approximately coinciding with morning rush hours.The daily NH3concentrations were weakly correlated withconcentrations, suggesting that NH3played an important precursor role inin PM2.5formation.The mean mass ratio of NH3/NHxwas 0.76±0.13.This revealed that NHxwas influenced by local sources during the summer and that NH3dry deposition could contribute substantially to NHxdeposition.A high temperature, relative humidity, and degree of oxidization could increase photochemical reactions and aqueous processing, having the important effect of SO2toand NH3toconversion in summer.The back trajectory analysis indicated that the transport of air masses from the North China Plain region contributed to the atmospheric NH3andaerosol variations in Beijing.(Meng Zhaoyang)

1.17 Temporal variation and source identification of black carbon at Lin’an and Longfengshan regional background stations in China

Black carbon (BC) is a component of fine particulate matter (PM2.5), associated with climate, weather,air quality, and human’s health.However, studies on temporal variation of atmospheric BC concentration at background stations in China and its source area identification are lacking.Here we used 2-yr BC observations from two background stations, Lin’an (LAN) and Longfengshan (LFS), to perform an investigation.Results show that the mean diurnal variation of BC has two significant peaks at LAN while different characteristics are found in the BC variation at LFS, which are probably caused by the difference in emission source contributions.Seasonal variation of monthly BC shows double peaks at LAN but a single peak at LFS.The annual mean concentrations of BC at LAN and LFS decrease by 1.63 and 0.26 μg m–3from 2009 to 2010,respectively.The annual background concentration of BC at LAN is twice higher than that at LFS.The major source of the LAN BC is industrial emission while the source of the LFS BC is residential emission.Based on transport climatology on a 7-day timescale, LAN and LFS stations are sensitive to surface emissions respectively in belt or approximately circular area, which are dominated by summer monsoon or colder land air flows in Northwest China.In addition, we statistically analyzed the BC source regions by using BC observation and FLEXible PARTicle dispersion model (FLEXPART) simulation.In summer, the source regions of BC are distributed in the northwest and south of LAN and the southwest of LFS.Low BC concentration is closely related to air mass from the sea.In winter, the source regions of BC are concentrated in the west and south of LAN and the northeast of the threshold areastotat LFS.The cold air mass in the northwest plays an important role in the purification of atmospheric BC.On a yearly scale, sources of BC are approximately from five provinces in the northwest/southeast of LAN and the west of LFS.These findings are helpful in reducing BC emission and controlling air pollution.(Cheng Siyang)

1.18 The relationship of PM variation with visibility and mixing-layer height under hazy/foggy conditions in the multi-cities of Northeast China

The variations of visibility, PM-mass concentration and mixing-layer height (MLH) in four major urban/industry regions (Shenyang, Anshan, Benxi and Fushun) of central Liaoning in Northeast China are evaluated from 2009 to 2012 to characterize their dynamic effect on air pollution.The annual mean visibilities are about 13.7 ± 7.8, 13.5 ± 6.5, 12.8 ± 6.1 and 11.5 ± 6.8 km in Shenyang, Anshan, Benxi and Fushun, respectively.The pollution load (PM× MLH) shows a weaker vertical diffusion in Anshan, with a higher PM concentration near the surface.High concentrations of fine-mode particles may be partially attributed to the biomass-burning emissions from September in Liaoning Province and surrounding regions in Northeast China as well as the coal burning during the heating period with lower MLH in winter.The visibility on non-hazy fog days is about 2.5–3.0 times higher than that on hazy and foggy days.The fine-particle concentrations of PM2.5and PM1on hazy and foggy days are 1.8–1.9 times and 1.5 times higher than those on non-hazy and non-foggy days.The MLH declined more severely during fog pollution than in haze pollution.The results of this study can provide useful information to better recognize the effects of vertical pollutant diffusion on air quality in the multi-cities of central Liaoning Province in Northeast China.(Che Huizheng)

1.19 Water vapor variation and the effect of aerosols in China

This study analyzed the annual and seasonal trends in precipitable water vapor (PWV) and surface temperature (Ts) over China from 1979 to 2015, and the relationships between PWV andTsand between PWV and aerosol absorption optical depth (AAOD), using data from radiosonde stations, weather stations,and multiple satellite observations.Results revealed a positive PWV trend from 1979 to 1999, and a negative PWV trend from 2000 to 2015.Analysis of the differences in the PWV trend among different station types showed that the magnitudes of the trends were in the order main urban stations ＞ provincial capital stations ＞suburb stations, suggesting that anthropogenic activities have a strong influence on the PWV trend.The AAOD exhibited a significant positive trend in most regions of China from 2005 to 2015 (at the conf i dence level 95%).Using spatial correlation analysis, we showed that PWV trend derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations is correlated withTs, with an annual correlation coefficient of 0.596.In addition, the spatial correlation between PWV and AAOD showed a negative correlation, with the highest correlation coefficients of –0.76 and –0.71 observed in mid-eastern China and central northwest China,respectively, suggesting that the increase in AAOD in recent years may be one of the reasons for the decrease in PWV since the 2000s in China.(Che Huizheng)

1.20 The influence of different seasonal weather conditions for high concentrations of black carbon in Beijing downtown

Based on the black carbon aerosol (BC) and PM2.5observational data in Beijing downtown from 2013 to 2015, according to the ground-based meteorological observational data, ECMWF boundary layer height reanalysis data and FNL/NCEP different heights of wind speed reanalysis data, the seasonal, lunar, diurnal changes of BC mass concentration and the ratio of BC in PM2.5were discussed, and the related vector of BC concentration in Beijing downtown and different heights of wind speed were studied, and the influence of meteorological conditions and remote transport for the change of BC concentration in Beijing downtown were analyzed.It was found that the average of Beijing downtown BC concentration is 4.77±4.49 ug m–3; the ratio of BC in PM2.5is (8.23±5.47)%.BC concentration and the ratio of BC in PM2.5in autumn and winter is higher than in spring and summer, the diurnal change of BC concentration and the ratio of BC in PM2.5in all season showed that they were lower in daytime than in night.When the concentration of PM2.5increased,BC concentration increased and the ratio of BC in PM2.5decreased.When NE, ENE, SE and WSW were the main wind direction over Beijing, BC concentration decreased with the increase of wind speed and boundary layer height.In different seasons, there were different critical values and change rates of BC concentration changing with wind speed.In winter, during the period of high BC concentration, the key influence area of BC concentration in Beijing downtown for low heights was located in two zones, one was the border region of southern Hebei and Shandong, another was the border region of northwestern Hebei, northern Shanxi and lnner Mongolia; the key influence area of BC concentration in Beijing downtown for high heights was located in western Hebei, northern Shanxi and lnner Mongolia.(Cheng Xinghong)

1.21 Sources and characteristics of regional background PM1 in North China during the autumn and winter polluted period

The campaign of investigating the chemical compositions and particle size distributions of NR-PM1(nonrefractory PM1) was conducted by using a high resolution time of fl ight aerosol mass spectrometer (HR-TOF-AMS) at the Shangdianzi (SDZ) regional atmospheric background site (117.07°E, 40.39°N), northeast of Beijing, from October 17, 2015 to January 26, 2016.Results showed that organics was the main component of PM1, and the proportion of nitrate was higher than sulfate in autumn and winter.The mean mass-resolved size distributions for the main components displayed accumulation mode.The wider organic peak shape and larger nitrate peak size indicated that the organics contributed to both small and large particles at the beginning of the particle formation, growth and aging processes, while most of nitrate particles preferred to grow into large particles during the aging process.The ratios of elements between OM and OC, O/C and H/C were calculated as 1.91, 0.58 and 1.58, respectively.The oxidation state of organic aerosols was higher than that of other city sites.Nitrate was the major contributor of NR-PM1during the polluted period, while organics was significantly higher than that during clean period.The results of back trajectories analysis demonstrated that the air masses were complex during the pollution episode.The northwesterly wind from central Inner Mongolia and Siberia dominated the clean episodes, which was conducive to the spread of pollutants.(Zhang Yangmei)

1.22 Meteorological characteristics associated with air pollution in Xiong’an, China

In April 2017, the central government of China announced that it would establish the Xiong’an New Area in Hebei Province, as part of measures to advance the coordinated development of the Beijing-Tianjin-Hebei(BTH) region.The area is expected to become an innovative, market-driven green city; thus, it is urgent to address its environmental issues before urban construction begins.This study involved a systematic analysis of atmospheric pollutants PM2.5, PM10, CO, SO2, NO2, and O3, and their relationships with meteorological parameters in the Xiong’an area for a continuous period from May 2016 to April 2017.The study used observations of 2 m temperature, 2 m relative humidity, and 10 m wind speed along with measurements of the concentration of the six criteria pollutants in Anxin, Rongcheng, and Xiongxian taken on an hourly basis.Results revealed that the Xiong’an area experienced severe air pollution, with heavy aerosol loadings.The annual averaged concentrations of PM2.5and PM10were 101.3 and 144.2 μg m–3, respectively, significantly exceeding the Grade II standard of the Chinese Ambient Air Quality Standards.The maximum 24 h average concentrations of PM2.5and PM10were 540.1 and 642.1 μg m–3, respectively.Distinct seasonal trends were observed for PM2.5and PM10, with the maximum concentrations occurring in winter and the minimum in summer.Other trace gaseous pollutants, including CO, SO2, and NO2demonstrated similar seasonal variations.Contrarily, O3showed a reversed seasonal trend, with concentration peaking in summer, decreasing in spring and fall, and at its lowest in winter.Seasonal variations of atmospheric pollutants were modulated by the seasonal variations in pollutant emissions, but they were also strongly related to meteorological conditions.The relatively cool thermal conditions and low wind speeds in fall and winter can limit the development of the planetary boundary layer and the horizontal transport of pollutants.Such conditions may be partially responsible for the higher concentrations of the primary pollutants during these seasons.On the contrary,secondary pollutant O3exhibited positive correlations with temperature.(Miao Yucong)

1.23 Structure-activity relationship between surface hydroxyl groups during NO2 adsorption and transformation on TiO2 nanoparticles

The role of hydroxyl groups (OH) in understanding the surface reactions on oxides is important.Here we combined spectroscopic experiments and theoretical calculations to reveal the role of OH in the heterogeneous reactions of NO2on TiO2nanoparticles with various crystal structures.The interaction between OH and NO2was determined to be a critical step, giving rise to the formation of surface HNO3.HNO3was found to be either a stable product on amorphous TiO2, or an important intermediate of nitrate on anatase due to further reaction with nearby OH.Lack of surface OH on rutile limited its reactivity toward NO2.This study presents clear evidence that the reactivity of TiO2toward NO2greatly depends on the surface OH as well as the crystalline form.Considering the ubiquitous presence of TiO2nanoparticles in catalysts and in natural environments,our results could provide insights helpful to future research both in environmental catalysis and atmospheric chemistry.(Liu Chang)

1.24 Equilibrium climate response of the East Asian summer evaluation of radiosonde, MODIS-NIRClear, and AERONET precipitable water vapor using IGS ground-based GPS measurements over China

Water vapor is one of the major greenhouse gases in the atmosphere and also the key parameter affecting the hydrological cycle, aerosol properties, aerosol-cloud interactions, the energy budget, and the climate.This study analyzed the temporal and spatial distribution of precipitable water vapor (PWV) in China using MODerate resolution Imaging Spectroradiometer near-infrared (MODIS-NIR)-Clear PWV products from 2011 to 2013.We then compared the four PWV products (Global Positioning System PWV (GPS-PWV), radiosonde PWV (RS-PWV), MODIS-NIR-Clear PWV, and Aerosol Robotic Network sunphotometer PWV (AERONETPWV)) at six typical sites in China from 2011 to 2013.The analysis of the temporal and spatial distribution showed that the PWV distribution in China has clear geographical differences, and its basic distribution characteristics gradually changed from the coast in the southeast to inland in the northwest.Affected by the East Asian monsoon, the PWV over China showed clear seasonal distribution features, with highest values in summer, followed by autumn and spring, and the lowest values in winter.Intercomparison results showed that GPS-PWV and RS-PWV had a slightly higher correlation (R2=0.98) at 00:00 UTC than at 12:00 UTC(R2=0.97).The mean values of Bias, SD, and RMSE between GPS-PWV and RS-PWV (GPS-RS) were –0.03 mm, 2.36 mm, and 2.60 mm at 0000 UTC, and –0.23 mm, 2.76 mm, and 2.95 mm at 1200 UTC, respectively.This showed that GPS-PWV was slightly lower than RS-PWV, and this difference was more obvious during the nighttime.The MODIS-NIR-Clear PWV product showed a similar correlation coefficient (R2=0.88) with GPS-PWV to that with RS-PWV.In addition, MODIS-NIR-Clear PWV was greater than GPS-PWV and RSPWV.The MODIS-NIR-Clear PWV showed a larger deviation from GPS-PWV (MODIS-GPS Bias=1.50 mm,RMSE=5.76 mm) compared with RS PWV (MODIS-RS Bias =0.75 mm, RMSE=5.31 mm).The correlation coefficients between AERONET-PWV and the PWV from GPS, RS, and MODIS-NIR-Clear were 0.970, 0.963,and 0.923 (with RMSE of 2.53 mm, 3.67 mm, and 4.39 mm), respectively.In the Beijing area, the overall mean biases of the AERONET-PWV product with GPS-PWV, RS-PWV and MODIS-NIR-Clear PWV were –0.09 mm, –1.82 mm, and –1.54 mm, respectively, indicating that the AERONET-PWV product was lower than the other three PWV products.(Che Huizheng)

1.25 Impacts of diurnal variability and meteorological factors on the PM2.5-AOD relationship:Implications for PM2.5 remote sensing

PM2.5retrieval from space is still challenging due to the elusive relationship between PM2.5and aerosol optical depth (AOD), which is further complicated by meteorological factors.In this work, we investigated the diurnal cycle of PM2.5in China, using ground-based PM measurements obtained at 226 sites of China Atmosphere Watch Network during the period of January 2013 to December 2015.Results showed that nearly half of the sites witnessed a PM2.5maximum in the morning, in contrast to the least frequent occurrence (5%)in the afternoon when strong solar radiation received at the surface, resulting in rapid vertical diffusion of aerosols and thus lower mass concentration.PM2.5tends to peak equally in the morning and evening in North China Plain (NCP) with an amplitude of nearly twice or three times that in the Pearl River Delta (PRD),whereas the morning PM2.5peak dominated in Yangtze River Delta (YRD) with a magnitude lying between those of NCP and PRD.The gridded correlation maps revealed varying correlations around each PM2.5site,depending on the locations and seasons.Concerning the impact of aerosol diurnal variation on the correlation,the averaging schemes of PM2.5using 3 h, 5 h, and 24 h time windows tended to have largerRbiases,compared with the scheme of 1 h time window, indicating that diurnal variation of aerosols played a significant role in the establishment of explicit correlation between PM2.5and AOD.In addition, high cloud fraction and relative humidity tended to weaken the correlation, regardless of geographical location.Therefore, the impact of meteorological factors could be one of the most plausible alternatives in explaining the varyingRvalues observed, due to its non-negligible effect on MODIS AOD retrievals.Our findings have implications for PM2.5remote sensing, as long as the aerosol diurnal cycle, along with meteorology, are explicitly considered in the future.(Guo Jianping)

2 Model development and implementation, the impacts of atmospheric compositions on climate, weather, and human health

2.1 Atmospheric CO2 at Waliguan station in China: Transport climatology, temporal patterns and source-sink region representativeness

In order to explore where the source and sink have the greatest impact on CO2background concentration at Waliguan (WLG) station, here a statistical method is proposed to calculate the representative source-sink region.The key to this method is to find the best footprint threshold, and the study is carried out in four parts.Firstly, transport climatology, expressed by total monthly footprint, was simulated by FLEXPART on a 7-day time scale.Surface CO2emissions in Eurasia frequently transported to WLG station.WLG station was mainly influenced by the westerlies in winter and partly controlled by the Southeast Asian monsoon in summer.Secondly, CO2concentrations, simulated by CT2015, were processed and analyzed through data quality control, screening, fi tting and comparing.CO2concentrations displayed obvious seasonal variation, with the maximum and minimum concentrations appearing in April and August, respectively.The correlation of CO2fitting background concentrations wasR2=0.91 between simulation and observation.The temporal patterns were mainly correlated with CO2exchange of biosphere-atmosphere, human activities and air transport.Thirdly, for the monthly CO2fi tting background concentrations from CT2015, a best footprint threshold was found based on correlation analysis and numerical iteration using the data of footprints and emissions.The grid cells where monthly footprints were greater than the best footprint threshold were the best threshold area corresponding to representative source-sink region.The representative source-sink region of maximum CO2concentration in April was primarily located in Qinghai Province, but the minimum CO2concentration in August was mainly influenced by emissions in a wider region.Finally, we brief l y presented the CO2sourcesink characteristics in the best threshold area.Generally, the best threshold area was a carbon sink.The major source and sink were relatively weak owing to less human activities and vegetation types in this high altitude area.CO2concentrations were more influenced by human activities when air mass passed through many urban areas in summer.Therefore, the combination of footprints and emissions is an effective approach for assessing the source-sink region representativeness of CO2background concentration.(Cheng Siyang)

2.2 Classification of summertime synoptic patterns in Beijing and their associations with the boundary layer structure affecting aerosol pollution

Meteorological conditions within the planetary boundary layer (PBL) are closely governed by largescale synoptic patterns and play important roles in air quality by directly and indirectly affecting the emission,transport, formation, and deposition of air pollutants.Partly due to the lack of long-term fine-resolution observations of the PBL, the relationships between synoptic patterns, PBL structure, and aerosol pollution in Beijing have not been well understood.This study applied the obliquely rotated principal component analysis in T-mode to classify the summertime synoptic conditions over Beijing using the National Centers for Environmental Prediction reanalysis from 2011 to 2014, and investigated their relationships with the PBL structure and aerosol pollution by combining numerical simulations, measurements of surface meteorological variables, fine-resolution soundings, the concentration of particles with diameters less than or equal to 2.5 μm, total cloud cover (CLD), and reanalysis data.Among the seven identified synoptic patterns, three types accounted for 67% of the total number of cases studied and were associated with heavy aerosol pollution events.These particular synoptic patterns were characterized by high-pressure systems located east or southeast of Beijing at the 925 hPa level, which blocked the air flow seaward, and southerly PBL winds that brought in polluted air from the southern industrial zone.The horizontal transport of pollutants induced by the synoptic forcings may be the most important factor affecting the air quality of Beijing in summer.In the vertical dimension, these three synoptic patterns featured a relatively low boundary layer height (BLH) in the afternoon, accompanied by high CLD and southerly cold advection from the seas within the PBL.The high CLD reduced the solar radiation reaching the surface, and suppressed the thermal turbulence, leading to lower BLH.Besides, the numerical sensitivity experiments show that cold advection induced by the large-scale synoptic forcing may have cooled the PBL, leading to an increase in near-surface stability and a decrease in the BLH in the afternoon.Moreover, when warm advection appeared simultaneously above the top level of the PBL, the thermal inversion layer capping the PBL may have been strengthened, resulting in further suppression of the PBL and thus the deterioration of aerosol pollution levels.This study has important implications for understanding the crucial roles that meteorological factors (at both synoptic and local scales) play important roles in modulating and forecasting aerosol pollution in Beijing and its surrounding area.(Miao Yucong)

2.3 Disentangling fast and slow responses of the East Asian summer monsoon to reflecting and absorbing aerosol forcings

We examined the roles of fast and slow responses in shaping the total equilibrium response of the East Asian summer monsoon (EASM) to reflecting (sulfate, SO4) and absorbing (black carbon, BC) aerosol forcings over the industrial era using the Community Earth System Model version 1 (CESM1).Our results show that there is a clear distinction between fast and slow responses of the EASM to aerosol forcings and the slow climate response due to aerosol-induced change in sea surface temperature plays an important role in the impacts of aerosols on the EASM.The EASM is weakened by a decrease in land-sea surface thermal contrast in the fast response (FR) component to SO4forcing, whereas the weakening is more intensive due to the changes in tropospheric thermodynamic and dynamic structures in the slow response (SR) component to SO4.The total climate adjustment caused by SO4is a significant weakening of the EASM and a decrease in precipitation.The BC-induced fast adjustment strengthens the EASM both by increasing the local surface landsea thermal contrast and by shifting the East Asian subtropical jet northwards.The BC-induced slow climate adjustment, however, weakens the EASM through altering the atmospheric temperature and circulation.Consequently, the EASM is slightly enhanced, especially in north of 30°N, in the total response (TR) to BC.The spatial patterns of precipitation change over East Asia due to BC are similar in the total response and slow response.This study highlights the importance of ocean response to aerosol forcings in driving the changes of the EASM.(Wang Zhili)

2.4 East Asian summer monsoon response to forcing of anthropogenic aerosol species

We used an online aerosol-climate model to study the equilibrium climate response of the East Asian summer monsoon (EASM) to increases in anthropogenic emissions of sulfate, organic carbon, and black carbon aerosols from 1850 to 2000.Our results show that each of these aerosol species has a different effect on the EASM as a result of changes in the local sea-land thermal contrast and atmospheric circulation.The increased emission of sulfate aerosol leads to a decrease in the thermal contrast between the land and ocean,a southward shift of the East Asian subtropical jet, and significant northerly wind anomalies at 850 hPa over eastern China and the ambient oceans, markedly dampening the EASM.An increase in organic carbon aerosol results in pronounced surface cooling and the formation of an anomalous anticyclone over the oceans north of 30°N.These effects cause a slight increase in the sea-land thermal contrast and southerly flow anomalies to the west of the anticyclonic center, strengthening the northern EASM.An increase in organic carbon emission decreases the sea-land thermal contrast over southern China, which weakens the southern EASM.The response of the summer 850 hPa winds and rainfall over the East Asian monsoon region to an increase in black carbon emission is generally consistent with the response to an increase in organic carbon.The increase in black carbon emission leads to a strengthening of the northern EASM north of 35°N and a slight weakening of the southern EASM south of 35°N.The simulated response of the EASM to the increase in black carbon emission is unchanged when the emission of black carbon is scaled up by five times its year 2000 levels, although the intensities of the response is enhanced.The increase in sulfate emission primarily weakens the EASM, whereas the increases in black carbon and organic carbon emissions mitigate weakening of the northern EASM.(Wang Zhili)

2.5 Impact of anthropogenic aerosols from global, East Asian, and non-East Asian sources on East Asian summer monsoon system

The impact of anthropogenic aerosols from global, East Asian, and non-East Asian sources on East Asian summer monsoon (EASM) system is studied using an aerosol-climate online model BCC_AGCM2.0.1_CUACE/Aero.The results show that the summer mean net all-sky shortwave fluxes averaged over East Asian monsoon region (EAMR) at the top of the atmosphere (TOA) and surface are reduced by 4.8 and 5.0 W m−2,respectively, due to the increases of global aerosol emissions in 2000 relative to 1850.Changes in radiations and their resulting changes in heat and water vapor transport and cloud fraction together contribute to the surface cooling over EAMR in summer.The increases in global anthropogenic aerosols lead to a decrease of 2.1 K in summer mean surface temperature and an increase of 0.4 hPa in summer mean surface pressure averaged over EAMR, respectively.It is shown that the changes in surface temperature and pressure are significantly larger over land than over ocean, thus reducing the contrast of land-sea surface temperature and pressure.This results in the marked anomalies of northward and northeastward winds over eastern and southern China and the surrounding oceans in summer, thereby weakening the EASM.The summer mean precipitation averaged over the EAMR is reduced by 12%.The changes in non-East Asian aerosol emissions play a more important role in inducing the changes of local temperature and pressure, and thus significantly exacerbate the weakening of the EASM circulation due to local aerosol changes.The weakening of circulation due to the sources is comparable,and the effect of nonlocal aerosols is even larger in individual regions.The changes of local and non-local aerosols contribute comparably to the reductions in precipitation over oceans, whereas cause opposite changes over eastern China.Our results highlight the importance of aerosol changes outside East Asia to the impact of the anthropogenic aerosol changes on the EASM.(Wang Zhili)

2.6 Impacts of uncertainty in land surface information on simulated surface temperature and precipitation over China

This study mainly focuses on the effects of uncertainty in land surface information on mesoscale numerical simulations.The Weather Research and Forecasting (WRF) model was used to simulate meteorological fields over China at a spatial resolution of 10 km during 2006.Near-surface air temperature and precipitation simulated in WRF were evaluated using site observations.The effects of accurate and updated land surface information, including Shuttle Radar Topography Mission (SRTM) data, Moderate resolution imaging spectroradiometer (MODIS) land use data, vegetation fraction derived from MODIS normalized difference vegetation index (NDVI) and Harmonized World Soil Database (HWSD) data (LAST simulation),on WRF’s performance were investigated by a comparison with the simulation using the default land surface information (BASE simulation).WRF reproduced the temporal and spatial variations of near-surface air temperature and precipitation over China quite well, although its performance varied significantly season by region.WRF underestimated near-surface air temperatures in most areas of the Yunnan-Guizhou Plateau, the Tibetan Plateau, the Northeast Plain, and the southeastern coastal regions, but temperatures were overestimated in most areas of the North China Plain, the Loess Plateau, Sichuan Basin, and western Xinjiang.WRF overestimated (underestimated) precipitation in most humid (arid) areas.A positive (negative) bias in simulated precipitation is found in summer (winter).With updated land surface information, WRF’s performance in terms of both daily average values and extremes is improved, and the root mean square error values for daily mean temperature and daily accumulated precipitation decreased by 7% and 2.3%, respectively.These improvements are significant for temperature, but not significant for precipitation.Therefore, the uncertainty in land surface information has a greater influence on temperature than on precipitation.These findings are very important for weather forecasting and studies involving climatological analyses over East Asia.(He Jianjun)

2.7 Investigation of particulate matter regional transport in Beijing based on numerical simulations

Based on source sensitivity research performed with the Chinese Unified Atmospheric Chemistry Environment (CUACE) model and dispersion simulations performed with the Flexible Particle dispersion model (FLEXPART), the regional transport of particulate matter (PM), potential source regions, and transport pathways were investigated for Beijing in summer (July) and winter (December) 2013.The mean near-surface trans-boundary contribution ratio (TBCR) of PM2.5in Beijing was 53.4% and 36.1% in summer and winter 2013, respectively, and 51.8% and 35.1% for PM10.Regional transport in summer was more significant than that in winter.Seasonal difference of meteorological condition combined with the distribution of emission was responsible for the seasonal difference of TBCR.The secondary aerosol was mostly contributed by regional transport.The transport of PM was mostly from Hebei Province and Tianjin municipality.Based on backward trajectories analysis, the air mass source occurred from different directions in summer, while occurred from northwest in winter.The pollution level and the TBCR were closely related to the transport pathways and distance, especially in summer.(He Jianjun)

2.8 Relay transport of aerosols to Beijing-Tianjin-Hebei region by multiscale atmospheric circulations

The Beijing-Tianjin-Hebei (BTH) region experiences heavy aerosol pollution, which is found to have close relationships with the synoptic- and local-scale atmospheric circulations.However, how and to what extent these multi-scale circulations interplay to modulate aerosol transport have not been fully understood.To this end, this study comprehensively investigated the impacts of these circulations on aerosol transport in BTH,focusing on an episode occurred on 1 June 2013, through combining both observations and three-dimensional numerical simulations.It was found that during this episode, the Bohai Sea acted as a transfer station, and the high-pressure system over the Yellow Sea and sea-breeze in BTH turned to affect the transport of aerosols.In the morning, influenced by the high-pressure system, lots of aerosols emitted from Shandong and Jiangsu provinces were first transported to the Bohai Sea.These aerosols were then brought to the BTH region in the afternoon through the inland penetration of sea-breeze, significantly exacerbating the air quality in BTH.The inland penetration of sea-breeze could be identified by the sharp changes in ground-based observed temperature, humidity, and wind when the sea-breeze front (SBF) passed by.Combining observations with model outputs, the SBF was found to be able to advance inland more than 150 km, reaching Beijing.This study has important implications for better understanding the aerosol transport in BTH and improving the forecast of such aerosol pollution.(Miao Yucong)

2.9 Scenario dependence of future changes in climate extremes under 1.5 °C and 2 °C global warming

The 2015 Paris Agreement aims to limit global warming below 2 °C and pursue efforts to even limit it to 1.5 °C relative to pre-industrial levels.Decision makers need reliable information on the impacts caused by these warming levels for climate mitigation and adaptation measures.We explore the changes in climate extremes, which are closely tied to economic losses and casualties, under 1.5 °C and 2 °C global warming and their scenario dependence using three sets of ensemble global climate model simulations.A warming of 0.5 °C(from 1.5 °C to 2 °C) leads to significant increases in temperature and precipitation extremes in most regions.However, the projected changes in climate extremes under both warming levels highly depend on the pathways of emissions scenarios, with different greenhouse gas (GHG)/aerosol forcing ratio and GHG levels.Moreover,there are multifold differences in several heavily polluted regions, among the scenarios, in the changes in precipitation extremes due to an additional 0.5 °C warming from 1.5 °C to 2 °C.Our results demonstrate that the chemical compositions of emissions scenarios, not just the total radiative forcing and resultant warming level, must be considered when assessing the impacts of global 1.5/2 °C warming.(Wang Zhili)

2.10 Short-term aerosol radiative effects and their regional difference during heavy haze episodes in January 2013 in China

Short-term direct effects of aerosols on surface shortwave radiation and its regional difference during heavy haze episodes in January 2013 in China were investigated using the offline Weather Research and Forecasting (WRF) the Community Multiscale Air Quality (CMAQ)- second Sun-Edward-Slingo radiative transfer (SES2) model system.The aerosol concentrations were first generated using the WRF-CMAQ model simulations and then corrected based on the observed concentrations of PM10and PM2.5.The atmospheric profile data produced by the WRF model and the corrected aerosol concentrations were used as inputs to the SES2 model to calculate the global horizontal irradiance (GHI) and direct solar irradiance (DIR) at the surface for a period of heavy haze episodes in January 2013 in China.The effects of aerosol on the GHI and DIR at the surface were then analyzed.The modeled radiation was evaluated against the observations first.The results show some improvement due to the correction for the aerosol concentrations.The aerosol shortwave radiative effects were determined by the difference between the model calculations with and without the inclusion of aerosols.It was found that the short-term aerosol radiative impacts during heavy haze days were very large,ranging between 100 and 500 W m–2.The aerosol concentrations had a large spatial variation with the highest concentration occurring in the areas of Beijing-Tianjin-Hebei, which caused a large difference in the radiative effect nationwide.In addition to the total concentration, the aerosol vertical distributions also varied from the north to the south in China and this led to a significant difference in radiative effect even when the PM10concentration was similar in the three regions.(Cheng Xinghong)

2.11 Trans-Pacific transport of dust aerosols from East Asia: Insights gained from multiple observations and modeling

East Asia is one of the world’s largest sources of dust and anthropogenic pollution.Dust particles originating from East Asia have been recognized to travel across the Pacific to North America and beyond,thereby affecting the radiation incident on the surface as well as clouds aloft in the atmosphere.In this study,integrated analyses were performed with the focus on one trans-Pacific dust episode during 12–22 March 2015, based on space-borne, ground-based observations, reanalysis data combined with Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT), and the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem).From the perspective of synoptic patterns, the location and strength of Aleutian low pressure system largely determined the eastward transport of dust plumes towards western North America.Multi-sensor satellite observations revealed that dust aerosols in this episode was originated from the Taklimakan and Gobi Deserts.Moreover, the satellite observations suggested that the dust particles could be transformed to polluted particles over the East Asian regions after encountering high concentration of anthropogenic pollutants.In terms of the vertical distribution of polluted dust particles, at the very beginning they were mainly located in the altitudes ranging from 1 km to 7 km over the source region, then ascended to 2–9 km over the Pacific Ocean.The simulations confirmed that these elevated dust particles in the lower free troposphere were largely transported along the prevailing westerly jet stream.Overall, observations and modeling demonstrated how a typical springtime dust episode developed and how the dust particles traveled over the North Pacific Ocean all the way to North America.(Guo Jianping)

2.12 Study on spatial-temporal variations of pollutant emission sources inversed by adaptive nudging scheme over the Beijing-Tianjin-Hebei region based on the CMAQ model

The pollutant source assimilation of nudging inversion method was applied to simulate the local sources of SO2and NOxin the Beijing-Tianjin-Hebei region during January, March, July and November in 2014.We then analyzed the strengths, patterns, geographical distribution of SO2and NOxsources by comparing their difference from the original sources and evaluating the inversed-source-based modeling performances.The results showed that the sources of SO2and NOxpresented the obvious seasonal variation, the strongest in winter or heating period.A high pollutant emission zone in the northeast-southwest direction consisted of Tangshan, Beijing, Tianjin, Langfang, Baoding, Shijiazhuang, Xingtai and Handan with a serious center in the front of the Taihang and Yan mountains.The pollutant emissions exhibited a typical “urbanization” effect,that is, the sources of SO2and NOxwere high in the urban area and its surroundings and weak in the rural area.Compared with the initial-emission-based modeling, the inversed-emission-based simulation could better capture the spatial-temporal variations of air pollutants against the observation, especially during high pollution periods.(Cheng Xinghong)