Weiqing Huang *,Lirong Cai ,Haifeng Dang ,Zhe Jiao ,Hongbo Fan,* ,Faliang Cheng

1 School of Environment and Civil Engineering,Guangdong Engineering and Technology Research Center for Advanced Nanomaterials,Dongguan University of Technology,Dongguan 523808,China

2 Guangdong Provincial Key Lab of Green Chemical Product Technology,South China University of Technology,Guangzhou 510640,China

ABSTRACT Due to the rapid urbanization and industrialization,and excessive consumption of fossil fuel,haze weather characterized by PM2.5 has become a severe pollution problem in major Chinese cities recently,which has harmful effect on the air quality,visibility,clime system and human health.To indicate suitable directions for the prevention and control of haze pollution,the pollutant source apportionment and formation mechanism of urban haze should be figured out firstly.In this work,we briefly review the frequently-used methods for PM2.5 source apportionment and formation mechanism analysis of urban haze based on normal perspective in recent years.Furthermore,based on the new perspective of systematic methodology,the utilization of fault tree approach for the causation mechanism analysis of urban haze is significantly introduced and discussed.Finally,the recent progress on controlling strategies of urban haze in China is also synoptically introduced and discussed.It is expected that more effective tool/method can be found,developed and employed for the causation mechanism analysis and risk management of urban haze in China.©2018 The Chemical Industry and Engineering Society of China,and Chemical Industry Press Co.,Ltd.All rights reserved.

Keywords:Urban haze Fossil fuel Formation Sustainability

1.Introduction

Due to the rapid urbanization and industrialization,and excessive consumption of fossil fuel,haze weather characterized by PM2.5has become a severe pollution problem in major Chinese cities recently,which has harmful effect on the air quality,visibility,clime system,human health and economic development[1-4].For its obvious harmful effect that can be felt directly in the public’s normal living,haze weather and PM2.5received unprecedented attention from both the Chinese government and public,which may open a door to public participation addressing environmental challenges in China [5].According to the 2017 China Ecological Environment Situation Report [6],only 29.3% of the 338 prefecture-level cities in China reached the new ambient air quality standard.PM2.5and PM10were the most important pollutants leading to the severe air pollution events in those cities.So the air quality control in Chinese cities is still a big challenge.

Some positive efforts for the prevention and control of haze pollution were carried out by the Chinese central government.In February 2012,the China’s Ministry of Environmental Protection enacted the new National Ambient Air Quality Standards(GB3095-2012),which sets the limits for the first time on PM2.5and PM10of 35 μg·m-3and 75 μg·m-3for annual and daily average concentrations,respectively [7].In September 2013,China’s State Council released the “Atmospheric Pollution Prevention and Control Action Plan”primarily aiming to reduce inhalable PM by at least 10%by 2017 against 2012 levels,and reduce PM2.5concentrations by 25%,20%and 15%in the Beijing-Tianjin-Hebei area,the Yangtze River Delta region and the Pearl River Delta region,respectively[8].To improve the air quality of Chinese cities,a “Clean Air Research Plan”was also issued by the China’s Ministry of Environment Protection in September 2013 [9].In February 2014,the China Premier made a statement of “Reinforcing the haze treatment,to eliminate the lung hazard and heart afraid for Chinese citizen”in the executive meetings of China’s State Council.Those plan and statement indicated the determination on the haze pollution control from the Chinese government.

However,according to the statistical data from the National Bureau of Statistics of China,the total energy consumption was up to 435.8 million tons standard coal in 2016,and coal and oil were still the most important and the second important energy consumed in China,respectively [10].The energy consumption structure of China in 2016 is illustrated in Fig.1,which indicated that the ratio of clean energy in China was very low.So,due to the excessive consumption of fossil fuel and the unreasonable energy structure in China,the prevention and control of haze pollution in China will be a long,difficult and complicated challenge[11].

Fig.1.Energy consumption structure of China in 2016.

To provide theoretical basis and indicate effective direction for the prevention and control of urban haze in China,the pollutant source apportionment and formation mechanism of urban haze should be figured out firstly.In this work,to bring different viewpoint on the haze formation analysis,firstly we briefly review the frequently-used methods for PM2.5source apportionment and formation mechanism analysis of urban haze based on normal perspective.And then,based on the new perspective of systematic methodology,the utilization of fault tree approach for the causation mechanism analysis of urban haze is significantly introduced and discussed.Finally,the recent progress on controlling strategies of urban haze in China is also synoptically introduced and discussed.It is expected that more effective tool/method can be found,employed and developed for the causation mechanism analysis and risk management of urban haze in China.

2.Frequently-used Methods for Haze Formation Mechanism Analysis in China

To develop an effective pollution control method and relevant prevention policy of urban haze in China,the pollutant source apportionment and formation mechanism of urban haze should be figured out firstly.Some valuable works had been done on the source apportionment and formation mechanism of urban haze,which are briefly introduced and discussed in this section.

2.1.Source apportionment of PM2.5

PM2.5was believed to be the most important contributor of haze pollution in China.To provide suggestion for source apportionment of particulate matter(PM)in China,a“Guidance for particulate matter source apportionment”was released by the Ministry of Environmental Protection of China [12].There are three frequently-used methods used widely recently in China for PM2.5source apportionment,including emission inventory,source-oriented methods(Transport and dispersion simulation models),and receptor models[13].Emission inventory method relies on assessing the emission value of different sources to identify the major sources,but firstly the mission inventory database needs to be built based on the activity level and emission factor model of different pollutant sources.However,due to the difficulty of obtaining the activity level information and the uncertainty of emission factors,the statistic and investigation will be difficult [14,15].Source-oriented model method is a numerical method to calculate the contribution of different pollutant sources on the receptor pollutant concentration,which is based on the simulation of pollutant transport,diffusion,chemical conversion and settling.By combining emission inventory,meteorology and atmospheric physical and chemical processes,source-oriented models can be very powerful in forecasting and simulating air quality in different temporal and spatial scales[16].However,the limitation of computing capabilities and the uncertainties in emission inventory,meteorology and atmospheric chemical mechanism can impact the accuracy of results from sourceoriented models.Based on measurement from ambient air by receptor,receptor model is an important and widely-used PM source apportionment technique by combining chemical tracer information and mathematic statistical method to obtain accurate physical and chemical information.Major types of receptor models include chemical mass balance(CMB),principle component analysis(PCA),positive matrix factorization (PMF),and multivariate receptor model (UNMIX) [17].When using receptor models for PM source apportionment,the analysis results will still show some uncertainty due to the measuring uncertainty of chemical components and the sampling uncertainty of PM2.5from the air.So the integration and combination of multiple techniques were encouraged for PM source apportionment[18-23].

2.2.Formation mechanism analysis of urban haze

Generally,primary emissions from fossil fuel consumption and industrial wastes,secondary aerosol formation,adverse meteorological conditions,regional transport of particles and dust were considered as main factors contributing to the formation of haze weather[24-27].Yang et al.[28]used atmospheric environmental monitoring data in Beijing to investigate the formation mechanism of urban haze,the tracking of Cl-and K+indicated that coal combustion and biomass burning have taken a major contribution of haze formation in Beijing.Tie et al.[29]used a chemical/dynamical regional model and a box model to study the formation mechanism of Beijing haze.Zhang et al.[30]studied the secondary inorganic aerosol formation during haze episodes in Beijing.Huang et al.[31]used a comprehensive set of novel and state-of-the-art offline analytical approaches and statistical techniques to investigate the haze formation mechanism in Beijing,Guangzhou,Shanghai and Xi’an during January 2013,their analysis results showed that the secondary aerosol formation,the fossil fuel combustion and biomass burning had an important contribution for haze formation.However,the formation mechanisms leading to severe haze episodes with exceedingly high PM2.5levels in China remain highly uncertain[32],and uncertainty should not be ignored to guarantee the system safety[33-36].Due to the technique complexity and the professional perspective of the haze formation mechanism analysis method mentioned above,its mechanism would not be easily understood by the public and government.The development of more effective and easily understandable analysis method is still required.

3.The Utilization of Fault Tree Approach for Haze Causation Mechanism Analysis

The formation mechanisms leading to severe haze episodes in China remain highly uncertain,which have not yet been understood quantitatively by the currently available methods [37].So it is significant and important to develop more suitable/effective analysis method to reveal the formation mechanism of urban haze in China.In this section,based on the previous works by Huang et al.[38-40],the utilization of fault tree approach for haze causation mechanism analysis is significantly introduced and discussed.

3.1.The procedure of fault tree analysis

Fig.2.The analysis procedure of FTA method.(Modified from ref.[38]).

Fault Tree Analysis(FTA) is a deductive method by using a graphic expression(Fault Tree)to show how an undesirable event can occur in different ways by basic events[41].It had been applied in energy and resource fields such as fuel cell degradation,biogas systems,power distribution systems and water supply systems[42-46].The analysis procedure of FTA method is demonstrated in Fig.2.There are several key steps that should be done by using the FTA method.Firstly,an efficient fault tree system should be built by system understanding,relevant technical information survey and the identification of top event and basic risk events.Secondly,based on the minimal cut set or the minimal path set of the fault tree system,qualitative assessment of the basic events can be figured out by using the structure importance degree analysis.If we can get/provide the precise probability data for all the basic risk events,the occurrence probability of the top event can be calculated.The quantitative assessment of the basic events can also be figured out by using the probability importance degree analysis and the critical importance degree analysis.Finally,according to the qualitative and quantitative analysis results,some proper control measures can be proposed and implemented to prevent and interdict the development of the undesirable event.

3.2.Qualitative analysis

When the precise probability data of basic risk events cannot be provided,structure importance degree analysis can be carried out to provide a qualitative risk assessment result of basic events only based on the fault tree structure.The approximate discriminant of the structure importance degree coefficient is described as below[41].

where I(i) is the structure degree coefficient of basic event Xi,niis the number of the basic events in the minimum cut set including basic event Xi;and Kris the number of the minimum cut sets including basic event Xi.A minimal cut set is the minimal collection of basic events that will cause the top event to occur,which can be determined by using the Boolean algebra.The larger the I(i) is,the more important of the basic event Xiis in the fault tree system.

3.3.Quantitative analysis

After the minimal cut set is determined,the occurrence probability of the top event based on OR gatecan be calculated by[41].

where qKjis the occurrence probability of the minimal cut set Kj;for j=1,2,3,...,m.

The occurrence probability of the minimal cut set Kjbased on AND gatecan be given by [41].

where qXiis the occurrence probability of input event Xi;for i=1,2,3,...,n.

When the precise occurrence probability of the basic events can be provided,the importance assessment of basic events can be carried out by the probability importance degree analysis and critical importance degree analysis,which is demonstrated below [41].

where Ig(i) is the probability importance degree of the basic event Xi,is the critical importance degree coefficient of basic event Xi;P(T)is the probability of top event;and qiis the occurrence probability of the basic event Xi.

3.4.Illustrated examples for causation mechanism analysis of urban haze

In common sense,the excessive consumption of fossil fuel is the biggest contributor leading to haze pollution.Some illustrated examples by employing FTA method for causation mechanism analysis of urban haze related to fossil fuel consuming in China had been investigated in the previous works by Huang et al.[38-40].A fault tree system was built to carry out the haze causation mechanism analysis related to coal combustion for Beijing and Tianjin city in China,as shown in Fig.3.After the qualitative and quantitative analyses of the haze causation fault tree system related to coal combustion,it was found that the unreasonable energy structure (X3),lack of substitutable clean energy (X4),equipment defect (X1),and without exhaust treatment unit (X2)showed more important effect to the top event of haze weather—coal combustion explosive emission (T1) in the target city [38,39].

Fig.3.The “Haze weather—Coal combustion exhausts explosive emission”fault tree.Note:Haze weather—Coal combustion explosive emission (T1),Insufficient coal combustion (E1),Equipment defect (X1),Without exhaust treatment unit (X2),Huge coal combustion (E3),Unreasonable energy structure (X3),Lack of substitutable clean energy(X4),Unqualified coal using(E2),Inferior coal(E4),Multi Sulfur(X5),Multi Nitrogen(X6),Multi Ash(X7),Without coal washing(X8),Interests driving(E5),Illegal selling(X9),Consumer buying (X10),Coal combustion laws execution defect (E6),Non-strict law enforce (X11),Law defect (X12).(Modified from Ref.[38]).

Another haze causation fault tree system related to vehicle emissions was also built,and Guangzhou was taken as an illustrated case,which is shown in Fig.4.After the qualitative and quantitative analyses,it was found that the excess vehicles (B7),severe traffic jam (B6),long service life of the high pollution vehicles(B1),and supervising defect of the high pollution vehicles(B2)showed more important impact on the top event of haze weather—vehicle exhaust explosive emission (T2) in Guangzhou [40].

Based on the qualitative and quantitative analysis result of the haze causation system shown in Figs.3 and 4,some useful control strategies can be developed for the prevention and risk management of urban haze in China.For example,adjust energy structure and exploit and use new cleaner energy;improve coal combustion technical and force the installation of the exhaust treatment equipment during the coal combustion process for the haze causation system as shown Fig.3.Also,reinforce the traffic management,optimize the urban function and planning,encourage public transport priority policy,control and reduce the vehicle possession,speed up the elimination of yellow label vehicles,and improve the supervision on high pollution vehicles for haze causation system as shown in Fig.4.

Fig.4.The fault tree of “Haze weather—Vehicle exhausts explosive emission”.Note:Haze weather—Vehicle exhaust explosive emission (T2),High pollution vehicles using(M1),Long service life (B1),Supervising defect (B2),Vehicle using with design defect (M2),Non-strict supervision (B3),Ignition device defect (B4),Lack of purification device(B5),Bad traffic(M4),Severe traffic jam(B6),Excess vehicles(B7),Unqualified oil using(M3),Inferior oil(M5),Multi Sulfur(B8),Improper octane number(B9),Lack of detergent(B10),Interests driving (E6),Illegal selling (B11),Consumer buying (B12),Absence of supervision on oil quality (B13).(Modified from ref.[40]).

3.5.Further application prospect of FTA method for urban haze in China

Until now,just very simple application studies of FTA method on the causation mechanism and risk management of urban haze in China had been carried out[38-40].Some further application directions of FTA method for urban haze in China are discussed below.

(1) When carrying out quantitative analysis by traditional FTA method,the precise probability data of all basic risk events is required.However,it is hard to get the precise occurrence probability of the risk events especially when the system is complicated or is a new one.So efficient data extraction approach still needs to be exploited and developed for the quantitative analysis of haze causation system by using FTA method.

(2) The data and detailed situation in different cities are different at different periods.More effective haze causation systems still need to be exploited and developed for other major Chinese cities.The comparing analysis results for different cities can enlarge and further advance the application of the FTA method for haze causation system in China.

(3) The integration and combination of FTA with other techniques are encouraged,which may enhance and improve the efficiency of the analysis results for causation mechanism and risk management of urban haze in China.

4.Control Strategies of Urban Haze in China

The aim of formation mechanism analysis is to provide theoretical basis and indicate effective direction for the prevention and control of urban haze in China.The recent progress on controlling strategies of urban haze in China is synoptically introduced and discussed in this section.

4.1.Control strategies basedon the policy andplan by Chinese government

Some effective and beneficial policies and plans had been made by the Chinese central government for the prevention and control of urban haze in China during 2012-2017.Such as the “Atmospheric Pollution Prevention and Control Action Plan”[8]and the “Clean Air Research Plan”[9]released in 2013.Since then,many similar action plans and clean air plans for specific Chinese cities and Provinces wereissued byrelevant Chineselocalgovernments.Somesimilar and comprehensive control strategies were proposed in those plans to mitigate the regional air pollution issue in China.The control strategies include implementation of synchronous control of multiple pollutants including PM,SO2,NOx,and VOC emissions,promotion of energy structure optimization,development of clean energy resources and promotion of clean and efficient coal use,adjustment and optimization of the industrial structure and promotion of industry upgrading,enhancement of vehicle pollution control,enhancement of the emission standard and supervision of air pollution,joint prevention and control of regional air pollution,etc.[8,9].

According to the 2017 China Ecological Environment Situation Report [6],after the implementation of the comprehensive control strategies focusing on multiple pollutants and emission sources at both the local and regional levels during 2013-2017,the air pollution problems in China had been mitigated.Compared with the situation in 2013,the inhalable PM10in the 338 prefecture-level cities of China was reduced by 22.7% in 2017,and 39.6%,34.3% and 27.7% of PM2.5concentrations in 2017 were reduced in the Beijing-Tianjin-Hebei area,the Yangtze River Delta region and the Pearl River Delta region against the 2013 levels,respectively [6,8].Those achievements indicated that the control strategies and the mandatory target based on the action plans and policies by Chinese government were useful[47].

4.2.Control strategies discussed for urban haze based on scientific research

The haze protection and control also received a lot of attention and discussion by scientists and researchers recently[48-51].Fu and Chen[48]synoptically summarized the policies and controlling strategy enacted by the Chinese government,the challenges and future direction were also discussed.Liu et al.[49]studied the prevention and control policy for energy-related regional pollution management in Jing-Jin-Ji region of China,and the potential problems with the implementation of the control policy were also discussed.Zhang et al.[50]summarized and discussed the air pollution and control action in Beijing.Tao et al.[51]studied the control action of PM2.5in Guangzhou during the 16th Asian Games period for the hazy weather prevention.Yang et al.[52,53]studied the cleaner production technology for coal chemical industry to save energy and lower the emission.

Based on the studies mentioned above,clean coal technology[54]may be one of the important directions for air pollution control[55].Wu et al.[56]used the coal-fired power as a representative case to investigate the technology schemes and assess their cost effectiveness,so as to shed light on the future directions for pollutioncontrol strategies.The analysis results indicated that more pollution damage caused by particulate matter and mercury could be avoided at less cost if the current mainstream scheme can be improved at the beginning of air pollution regulation.Sun et al.[57]employed the dose-response function to evaluate health benefits resulting from coal control and air pollutants abatement in the Jing-Jin-Ji region.The analysis results could provide attractiveness for the government to set stringent pollutants reduction target and provide air pollution control policy guidance.Zhang et al.[58]studied the coal consumption control strategy for Shandong province.The analysis results indicated that improving air quality was the primary reason for coal consumption control.Yang and Teng [59]developed a bottom-up model to study the air quality co-benefit of coal control strategy in China.The analysis results indicated that the coal control strategy was of high importance for air pollutant reduction.

Although clean coal technology is very important for the energy saving and emission reduction,a more comprehensive control strategies focusing on multiple pollutants and emission sources at both the local and regional levels are still required to mitigate or solve the severe air pollution problem in China.

5.Conclusions

To indicate suitable directions for the prevention and control of haze pollution,the formation mechanism and pollutant source apportionment of urban haze should be figured out firstly.In this work,we briefly reviewed the frequently-used methods for formation mechanism analysis of urban haze based on normal perspective.And then,based on the new perspective of systematic methodology,the utilization of FTA method for the causation mechanism analysis of urban haze was significantly introduced and discussed.However,the reliability of quantitative risk assessment still needs to be improved when using the FTA method.It also should be noticed that the formation mechanisms leading to severe haze episodes in China remain highly uncertain,which have not yet been understood quantitatively by the currently available methods.So the integration and combination of multiple techniques still need to be developed for PM source apportionment and haze formation mechanism analysis.

The recent progress on controlling strategies of urban haze in China were also synoptically introduced and discussed in this work.A more comprehensive control strategies focusing on multiple pollutants and emission sources at both the local and regional levels are still required to mitigate or solve the severe air pollution problem in China.It is expected that more effective tool/method based on different viewpoints can be found,employed and developed for the causation mechanism analysis and risk management of urban haze in China in the future.