Dominant Factors Of Pm_2.5explosive Rise In Stable Pollution Emission Period In Beijing Area

With a population of 1.4 billion people and large emissions of air pollutants and high economic growth for more than 40 years,persistent aerosol pollution events(HPEs)in Beijing and its surrounding areas(BIV)have been increasing in autumn and winter,peaking in 2013.Will HPEs still occur after 2017,especially after the Chinese government implemented the Air Pollution Prevention and Control Action Plan(known as the “Ten statements of Atmosphere”)from 2013 to 2017 and significantly reduced pollutant emissions by 2017,and will there be further HPEs in the middle stage of the HPE period? And the PM2.5 explosive rise(ER)phenomenon? What are the main control mechanisms of the ER phenomenon? Does it require further understanding? In particular,two HPEs and several ERs occurred in BIV in January and February of 2020,during the Chinese New Year and the COVID-19 outbreak.These events have particularly raised public thoughts and discussions on whether the control measures related to the “Ten statements of Atmosphere” are implemented.To answer these questions,this work firstly selects November as a typical heavy pollution month in Beijing and its surrounding area(BIV)in autumn and winter of 2018 to analyze the contribution of atmospheric aerosol pollution and planetary boundary layer structure to the PM2.5 explosive rise(ER)phenomenon in Beijing in autumn and winter after the“Ten statements of Atmosphere.” The vertical structure analysis of aerosols and meteorological factors in the planetary boundary layer observed four specific persistent heavy aerosol pollution events(HPEs)as well as eight ER phenomena in November2018,and clear interaction between the structural changes of the BIV near-surface boundary layer and aerosol pollution can be seen.Combining the mass concentrations of the near-surface pollutant PM2.5 and its main precursor gases and the boundary layer height changes,the contribution of chemical transformations during ERs is limited.The correlation of the contribution of unfavorable meteorological conditions to the explosive growth phase of PM2.5 was also quantified in the range of 66% to 88% using PLAM indices that characterize the stability of regional air masses.It indicates that the bidirectional feedback between unfavorable meteorological conditions and PM2.5pollution accumulation is still the main mechanism to maintain HPEs.The dissipation phase of the heavy pollution process,the upper atmosphere of the boundary layer in Beijing in autumn and winter,is often transited by mineral aerosols from the northwest,as found by the LIDAR receding signal.By paving the way for the first part of the work,we extend the study object.All this was occurring during the COVID-19 outbreak from autumn-winter 2017 to 2020,and ER phenomena are studied and analyzed.It was found that after a significant decrease in pollution emission levels since 2017,especially in January and February of 2020 when the COVID-19 outbreak occurred,pollution emission levels decreased further due to anti-epidemic measures,but 12 HPEs and 20 ER phenomena still occurred in Beijing and surrounding areas in autumn and winter.To investigate the main control mechanism of the PM2.5 explosive growth phenomenon(ER)under the premise of significant pollution reduction,the vertical structure and aerosol properties of meteorological elements were investigated using L-band second sounding radar and aerosol optical lidar.The PM2.5 and its main precursor gases,as well as the seven-wavelength Aethalometer TMAE31-The hourly black carbon(BC)mass concentration data,were analyzed.Also,the planetary boundary layer height(BLH)is estimated,and the adverse meteorological index(PLAM)is analyzed to quantify the effect of adverse meteorological conditions on the variation of PM2.5 concentrations.The results indicate that the underlying cause of ER recurrence is that pollution emissions have not declined sufficiently to decouple HPEs from the effects of adverse meteorological conditions.During the ER period,BLH declined significantly.The mass concentrations of PM2.5 and its precursor gases and black carbon increase in almost similar proportions,suggesting that changes in the boundary layer structure due to aerosol accumulation are the dominant cause of the ER phenomenon compared to the contribution of chemical transformations.A two-way feedback effect between the further deterioration of meteorological conditions and accumulated PM2.5 explains 54%93% of the ER phenomena.During COVID-19,an HPE starting on February 8,2020,experienced the worst meteorological conditions in the BIV since 2013.The average PM2.5 concentration during the HPE during COVID-19 was found to be reduced by about 34% under conditions experiencing similar levels of meteorological adversity,indicating that there was still a significant reduction of emissions during COVID-19.Also,by comparing the pollution emission intensities of major regions worldwide,the BIV still has high levels of pollution emissions.Together,these results show that China’s continued emission reduction efforts are in the right direction and have achieved the expected results.However,even with the results,BIV emissions have not yet declined to the level of “decoupling” from unfavorable meteorological conditions,and emissions reductions should continue.