Long-Term Trends Of PM_2.5 And O₃ Comprehensive Air Pollution And Its Responses To Precursors Emission Reduction In The Pearl River Delta

In recent years,the problems of comprehensive air pollution in China represented by PM_2.5 and O₃ have become increasingly serious.The Pearl River Delta（PRD）region is one of the fastest-growing regions in China,in which air pollution incidents are often both complex and regional.The contributions of the primary emissions of PM_2.5 have been significantly reduced,but the contributions of the secondary species generated by the interaction of atmospheric pollutants have begun to become prominent,causing its concentration to drop further into a bottleneck period.The difficulties in O₃ control are mainly due to determining the relative contributions of local formation and regional transport,and the conversion mechanism of precursors NO_x and VOCs have not yet been fully clarified.Therefore,it is particularly important for cities in PRD to combine meteorological conditions and precursor emission characteristics to control comprehensive air pollution.In response to the above problems,this study investigates the long-term trends of comprehensive pollutants and complex nonattainment pollution(O_x+PM_2.5),explores the effects of precursor emission changes and meteorological conditions,and proposes synergistic air pollution control schemes.First,the long-term trends of PM_2.5 and its chemical compositions from 2000 to 2019 are analyzed by Meta-analysis which collects the observation data of PM_2.5 compositions from published literature in PRD,and the trends of secondary species changes in PM_2.5 are corroborated with the changes in emissions and concentrations of each gaseous precursor.Secondly,the case libraries of pollution processes and complex nonattainment pollution are established to analyze the persistent and regional characteristics of the comprehensive pollution in PRD,and then the dominant weather situations and meteorological factors are identified.Finally,focusing on Dongguan,a typical heavily polluted city in PRD,after judging different transport conditions by CO tracer method,dividing O₃ formation sensibility by the Empirical Kinetic Modeling Approach（EKMA）and identifying potential source areas of O₃ by the FLEXible PARTicle dispersion model（FLEXPART）,multi-gradient sensitive tests of emission source reductions were designed to analyze the responses of O₃ concentration by the Weather Research and Forecasting model coupled with Chemistry（WRF-Chem）,and then establish effective methods for the comprehensive pollution control under different transport conditions.The main results of this study are as follows:（1）The concentrations of PM_2.5 and its main chemical components(OC,EC,SO₄^2-,NO₃^-,and NH₄⁺)in PRD from 2000 to 2019 show an increasing trend followed by a decreasing trend.Three significant transitions in secondary species percentage in PM_2.5were observed:first declined in 2000-2005 before rose in 2005-2008 and followed by a stable period in 2008-2019.The percentage of secondary species stabilized at around80%,of which secondary organic aerosol（SOA）and secondary inorganic aerosol(SIA,including SO₄^2-,NO₃^-,and NH₄⁺)had identical contributions to PM_2.5 over the past decade.There was no significant upward trend in the share of SOA in organic aerosols due to decreasing O_x concentrations and slowing growth of VOCs emissions.SO₄^2-and SO₂ have declined with significant reductions in SO₂ emissions.NO₃^-and NH₄⁺were positively and negatively correlated with the atmospheric concentration of NO₂ and NH₃,respectively.In addition to the current efforts on SO₂ and NO_x controls,further measures are needed to control NH₃ and VOCs emissions over a larger area for PRD to effectively improve the air quality.（2）From 2013 to 2019,the annual average concentration of PM_2.5 in PRD decreased from（44±7）μg·m^-3 to（29±3）μg·m^-3 which met the annual standard for five consecutive years.The annual average concentration of O_x decreased from（127±14）μg·m^-3 in 2013 to（114±12）μg·m^-3 in 2016,and then showed a general trend of rebound to（145±16）μg·m^-3 in 2019,dominated by rising O₃ concentrations.The proportion of pollution processes with O₃ as the primary pollutant have increased from 34%in 2013to 88%in 2019,and the regional characteristics of simultaneous pollution in multiple cities have been highlighted.The complex nonattainment pollution of O_x and PM_2.5occurred 79 times during the study period,and mainly in urban sites and suburban sites.The largest number of days of complex nonattainment pollution occurred in autumn（44%）,because strong solar radiation was conducive to ozone formation,and then the high oxidization of the atmosphere promoted the second generation of PM_2.5.The weather conditions that caused the complex nonattainment pollution in the PRD mainly include outflow-high-pressure type（43%）,subtropical-high-pressure type（30%）,and tropical-depression type（27%）.In terms of specific meteorological conditions,when the temperature was in the range of 20-25℃and relative humidity was in the range of60%-75%,the proportion of complex nonattainment pollution was the highest（22%）.In the process of O₃ heavy pollution,the high relative humidity and low wind speed at nighttime caused the concentration of NO₂ and PM_2.5 to rise significantly,and then the high temperature at daytime aggravated the complex nonattainment pollution.（3）During the local formation period,O₃ formation was a transitional regime and the reduction of VOCs emissions was the most effective.The key area and VOCs emission sources in Dongguan were identified as the urban center and the source of solvent use.The reduction efficiency of solvent use source was 1.1 times of all VOCs emissions,and the reduction efficiency of the key area was 1.4 times of the whole Dongguan.During the regional transport period,50%of NO_x emissions in upwind areas（parts of Guangzhou and Huizhou）were reduced,the daily maximum 8-h average mass concentration（MDA8）of O₃ in Dongguan dropped significantly（17%）,and among them,only controlling NO_x emissions in parts of Huizhou was the most effective.But at this period,local VOCs emission reduction can hardly reduce O₃ concentration.The studies of emission reduction response for different pollutant transport conditions provide reliable ideas for O₃ control in the PRD.