| Since the implementation of the“Clean Air Action”in China in 2013,fine particulate matter(PM2.5)concentrations have decreased,whereas ozone（O3）concentrations have a trend of increasing.China is experiencing compound air pollution dominated by O3 and PM2.5,with serious pollution in typical pollution areas in eastern China,including the Yangtze River Delta（YRD）and the Beijing-Tianjin-Hebei（BTH）)regions.The double-high pollution by O3 and PM2.5(O3-PM2.5PDs,maximum daily 8 h average O3（MDA8 O3）>160μg m-3 and PM2.5>75μg m-3)occurred frequently during the warm seasons（April～October）,which are harmful to human health and ecological plants.However,it is still lacking comprehensive studies about the physical and chemical properties of O3-PM2.5PDs on the basis of long-term and large-scale observations.Therefore,in this work,the spatial and temporal distributions,the physical and chemical features,the dominant meteorological conditions,and the roles of anthropogenic emissions of O3-PM2.5PDs in typical polluted areas in eastern China during the warm seasons of 2013～2020 are investigated,by using observed data along with the global three-dimensional atmospheric chemical transport model（GEOS-Chem）,which provides scientific basis for understanding the mechanisms of O3-PM2.5PDs and for establishing effective emission reduction strategies.The main results are shown as follows:（1）Based on the ground-level observations of O3 and PM2.5 as well as the observed and reanalyzed meteorological parameters,the spatial and temporal variations of concentrations of O3 and PM2.5,the distribution of O3-PM2.5PDs,and the dominated meteorological conditions in the YRD during the warm seasons from 2013～2019 were analyzed.The results showed that the annual average concentrations of MDA8 O3(PM2.5)in the YRD increased（decreased）by36.8μg m-3(13.3μg m-3)during 2014～2019.The concentrations of MDA8 O3（Dev＿MDA8O3）and those of PM2.5(Dev＿PM2.5)have a strong positive correlation after removing the effects of annual and seasonal trends caused by emissions and meteorology.The correlation coefficient for the regionally averaged concentrations reached 0.44.As Dev＿MDA8 O3increased,the probability of Dev＿PM2.5 being positive（i.e.,higher than the monthly average）increased,and the probability of Dev＿PM2.5 being positive reached 86.4%when Dev＿MDA8 O3 was larger than 60μg m-3.The O3-PM2.5PDs events were severe in the eastern YRD,which occurred mainly in April（29.6%）,May（23.0%）,June（19.5%）and October（10.8%）.Compared to O3pollution days（O3SPDs）,O3-PM2.5PDs occurred under meteorological conditions with higher relative humidity,higher temperature,and lower wind speed;and the mean values of relative humidity,temperature,and wind speed anomalies during O3-PM2.5PDs（O3SPDs）were-6.2%（-7.3%）,1.84°C（0.46°C）and-0.40 m s-1(-0.17 m s-1),respectively.The dominant weather patterns in O3-PM2.5PDs were classified into four types,in which two types had high percentages.In the first type,YRD was influenced by stable westerlies at 500 h Pa without cold air intrusion and controlled by a high-pressure ridge at 850 h Pa,resulting in a hot,dry,and stable weather.In the second type,the subtropical high pressure at 500 h Pa reached southern YRD and the YRD was located at the center of the high-pressure system at 850 h Pa and was accompanied by strong sinking air flow.These two weather types accounted for 81.8%of the O3-PM2.5PDs.（2）By using the ground-level observations of O3 and PM2.5 along with the GEOS-Chem model,the physical and chemical characteristics of O3-PM2.5PDs in the BTH during the warm seasons of 2013-2020 were integrated and compared with those of O3SPDs and PM2.5single pollution days(PM2.5SPDs).The observational results showed that the occurrences of PM2.5pollution in the warm seasons over BTH were often accompanied by O3 pollution.The model results showed that the model could well reproduce the observed O3-PM2.5PDs.The O3-PM2.5PDs occurred with higher atmospheric oxidation and the highest surface-layer sulfate concentration compared to O3SPDs and PM2.5SPDs.The vertical distribution of PM2.5concentrations in O3-PM2.5PDs showed unique characteristics.PM2.5 concentrations held a more stable profile between 975 h Pa and 819 h Pa,and O3 concentrations were also higher at these altitudes compared to those in PM2.5SPDs.The process analyses showed that O3 and secondary aerosols（nitrate,ammonium,and sulfate）at 913～819 h Pa had large chemical formation together with the downward transport,resulting in an uniform and stable vertical distribution of PM2.5in O3-PM2.5PDs.The strong southerlies with high relative humidity at 850h Pa led to strong chemical formation for sulfate around 850 h Pa in O3-PM2.5PDs.（3）The GEOS-Chem model was used to quantify the contributions of meteorology and anthropogenic emissions to the frequency and intensity of O3-PM2.5PDs in the YRD and BTH during the warm seasons from 2013 to 2020.We also carried out emission reduction experiments for each precursor in 2020 to explore effective emission reduction strategies for controlling O3-PM2.5PDs.The results showed that in the BTH（YRD）region,the O3-PM2.5PDs in the baseline experiment decreased with a trend of 1.75（0.44）days yr-1 from 2013 to 2020,in which the meteorological-driven trend of O3-PM2.5PDs was-0.07（+1.54）days yr-1 and the anthropogenic emission-driven trend was 2.65（1.94）days yr-1,indicating that the dominant role of the changes in anthropogenic emissions.Meteorology mainly affected the interannual variations of O3-PM2.5PDs.Further,by reducing each precursor at different percentages,we found that the reductions in VOC（NOx）alone in the BTH（YRD）region reduced O3-PM2.5PDs more significantly than reductions in NOx（VOC）alone.A reduction of 50%in VOC（NOx）alone could lead to a 50.5%（49.4%）reduction in O3-PM2.5PDs.The reductions ins both NOxand VOC could result in more significant reductions in O3-PM2.5PDs;simultaneous reductions of 50%in NOx and VOC would lead to 74.0%and 66.6%decreases of O3-PM2.5PDs in BTH and YRD,respectively,and O3-PM2.5PDs can be effectively controlled. |
