3-Dimentional Simulations Of Urban Haze And Precision Air Pollution Control Approach For Severe Haze

Severe and persistent haze pollution involving fine particulate matter concentrations reaching unprecedentedly high levels across many cities in China poses a serious threat to human health.Although mandatory temporary cessation of most urban and surrounding emission sources is an effective,but costly,short-term measure to abate air pollution,development of long-term crisis response measures remains a challenge,especially for curbing severe urban haze events on a regular basis when the smog red alert is issued.For example,the good air quality for the large international events such as 2008 Beijing Olympics,2010 Shanghai Expo,2014 Beijing Asia-Pacific Economic Cooperation(APEC)Summit,2015 Beijing Grand Military Parade and 2016 G20 Hangzhou Summit are the results of stringent emission control measures in the city and the regional joint control over the surroundings.This proves that the short-term crisis response measures did fix the air pollution problems in China with the extreme cost by a forced temporary closure of most of factories in the host city and its surrounding areas.On the other hand,a red alert for smog pollution,the highest pollution level,was issued for Beijing on December 8,2015 However,these mandatory and proposed emergency responses forced by the first-ever smog red alert issued by Beijing government on December 7 of 2015 only helped to low the PM2.5 concentrations by 10%.The ineffective results are probably because the real pollution sources which caused the severe haze episodes had not been controlled precisely by the mandatory and proposed emergency response measures.Therefore,the government desperately needs a new air pollution control approach that can be used to curb severe urban haze on the regular basis effectively,practicably and economically.In the first part of this study,we presented a case study about effects of stringent emission controls implemented over the Yangtze River Delta region on air quality inHangzhou during the 2016 G20 Hangzhou Summit.To prepare for the G-20 Hangzhou summit,held from September 3 to 5,2016 in Hangzhou,China,governments of Hangzhou and its surrounding provinces(Shanghai,Jiangsu,and Anhui)enforced a series of air pollutant emission reductions.Observations of PM2.5 and O3 at 8 monitoring sites in Hangzhou were used to evaluate simulations from the WRF-CMAQ model and assess the impact of emission controls on air quality in Hangzhou.The results indicate that the air quality in Hangzhou during the 2016 G20 Hangzhou summit on the basis of ground-based observations was considerably improved,most likely due to efficient emission controls across the Yangtze River Delta region.Simulated results under the targeted emission controls are much closer to the observations of PM2.5(R = 0.67,NMB =-8.7%)and O3(R = 0.73,NMB =4.6%)than those without emission controls.During the G20 Summit period,O3 and PM2.5 concentrations were reduced by 20.1 ?g/m3(or 25.4%)and 20.5 ?g/m3(or 56.1%),respectively,in Hangzhou,on the basis of the comparison of the model simulations without and with the emission controls.This demonstrates that the emission control strategy in Hangzhou and surrounding provinces is effective to ensure good air quality in Hangzhou during the 2016 G20 Hangzhou summit and provides support for use of the WRF-CMAQ model for assessing the impact of emission controls.In the second part of this study,we introduce and evaluate a novel precision air pollution control approach(PAPCA)to mitigate severe urban haze events.The approach involves combination of high PM2.5 concentrations(either observed or forecast),with a hybrid trajectory-receptor model and a comprehensive 3-D atmospheric model,to pinpoint the origins of emissions leading to such events and to optimize emission controls.The PAPCA involves three steps.The first step is to identify an impending period with high PM2.5 concentrations and time to start emission control schemes.Here,we have considered hourly mean PM2.5concentration ?150 g m-3 to define a severe urban haze event for the city.The second step is to calculate the concentration weighted trajectory(CWT)values for PM2.5 using the hybrid receptor model with 48-h back trajectories and PM2.5 concentrations to pinpoint source areas leading to high PM2.5 levels.Note that the PM2.5 trigger concentrations can arise from either observations or model forecasts at the receptor sites when the CWT values are calculated.The third step is to employ a comprehensive atmospheric chemical transport model(CTM)(here the two-way coupled WRF-CMAQ)to optimize emission controls using the CWT values as a weighting function as shown in equation.This last step involves a series of CTM simulations to determine the optimal emission control scenarios.The model performance evaluation for the five severe haze episodes in major urban areas in China suggest that the model can reproduce the urban PM2.5,O3,SO2,NO2 and CO concentrations for the five severe urban haze outbreaks and has reasonable performance for PM2.5 chemical composition for different heavy haze episodes in different cases.Results of the PAPCA application to five severe haze episodes in major urban areas in China suggest that this strategy has the potential to significantly mitigate severe urban haze by decreasing PM2.5 peak concentrations by more than 60%from above 300 ?g/m3 to below 100 ?g/m3,while requiring?30%to 70%less emission controls as compared to complete emission reductions.The PAPCA takes advantage of the predictive power of comprehensive atmospheric chemical transport models,offering effectiveness,practicality,and economic efficiency for significantly mitigating impending severe urban haze pollution.To the best of our knowledge,this paper is first to combine all three components(high PM2.5 concentrations(either observed or forecast),a hybrid trajectory-receptor model and a comprehensive 3-D air quality model)together to pinpoint the origins of emissions leading to heavy haze events and to optimize emission controls.We also prove that uncertainties of this PAPCA approach will not affect its effectiveness,practicality,and economic efficiency in its real application.In summary,the PAPCA has the potential to be used in the real emergency control and long-term governance of urban haze,and can be a pivotal scientific tool for policymakers and scientists.In addition,the PAPCA proposed in this work can not only be used to deal with urban severe haze,but also can be used to deal with other urban severe air pollution(such as ozone pollution).