Modulation Effect Of Sudden Stratospheric Warming On The Dispersion Of Atmospheric Particulate Matter In The Beijing-Tianjin-Hebei Regio

As the concentration of PM2.5 is a direct reflection of regional air pollution,abundant evidence has confirmed the possible impact of the tropospheric variability on air quality in the Beijing-Tianjin-Hebei(BTH)region.However,the possible impact of the stratospheric variability on the regional air particulate pollution is still not well explored in literature.This study firstly compares the possible impact of the downward-propagating sudden stratospheric warming(SSW)event in 2018 and the nondownward-propagating SSW event in 2019.Then we focused on a SSW event under the background of strong cold air outbreak in 2021 and its impact on the subseasonal variation of the PM2.5 concentration in the BTH region.Finally,using the historical data from 1981 to 2021,this paper comprehensively analyzes the physical parameters representing the air pollution status,such as aerosol optical thickness(AOD),the number of days of haze and fog and the daily minimum visibility during the SSW events,and deeply discusses the impact of two different types of SSW events(polar vortex displacement and polar vortex split)on the air particulate pollution in BTH region.The main conclusions are as follows.(1)The stratospheric polar vortex changed suddenly from a strong to a weak state during the SSW in 2018.About 20 days after the SSW onset,the negative northern annular mode(NAM)signal propagated downward to the near surface.The SSW event in 2019 lasted for a longer period,but the negative NAM signal did not propagate downward to the near surface.Before the two SSWs onset,the enhanced planetary wave activity led to more air exchange between mid-latitudes and high latitudes.The cold air outbreak in this period corresponded to the lower PM2.5 concentration in BTH region.During the onset period of the two SSWs,the PM2.5 concentration in BTH region increased rapidly,reaching a light pollution level on average.After onset of the two SSWs,the PM2.5 concentration distribution in BTH region was different in February 2018 and January 2019.The SSW in February 2018 showed a stronger downward propagation signal,and the East Asian Winter Monsoon(EAWM)in the troposphere was strengthened,which was conducive to the diffusion of pollutants.In contrast,there was almost no signal of SSW propagating downward to the near surface in January 2019,and the PM2.5 concentration in BTH region was still very high and diffused slowly.(2)The SSW event in January 2021 was caused by the alternating continuous upward of planetary Wavenumbers 1 and 2.Its favorable conditions included the loss of sea ice in the Barents-Kara(BK)Sea in the autumn of the previous year and the cold ENSO state in the tropical Pacific.Both of these conditions were conducive to the deepening of the high latitude North Pacific low pressure and the enhancement of the Ural Mountain high pressure ridge.The cold air outbreak in China during the New Year in 2021 was mainly related to the development of the blocking situation in the Ural Mountains.The cold air outbreak in February in the United States and the extreme cold wave in Texas were related to the downward propagation of SSW signal.Before the onset of 2021 SSW,the tropospheric planetary wave strengthened and propagated upward to affect the stratosphere.This change of tropospheric circulation directly leaded to the good period of air quality in BTH region.During the onset period of SSW,the upward propagation of tropospheric interference weakened the fluctuation of stratosphere,and an 8-days PM2.5 air pollution process occurred in BTH region.After the onset of SSW,the negative NAM/AO pulse signal weakened by the stratospheric polar vortex was propagated downward to the surface twice,and the possibility of cold air outbreak in East Asia increased.The resulting cold air activity helped to dilute PM2.5 particles and reduce PM2.5 concentration in BTH region.During the intermittent interval of two stratospheric downward signals,the concentration of PM2.5 in BTH region increased rapidly,and a serious pollution process lasting 5 days occurred in the 2020/21 winter.(3)From 1981-2021,17 SSW events occurred in the Arctic stratosphere,including 8 displacement SSW events and 9 split SSW events.The frequency of the two events is similar,but the duration of split SSW events is longer and the downward propagation is clearer and deeper.During both displacement SSWs and split SSWs,the minimum visibility,haze days,fog days,light fog days and aerosol optical thickness(AOD)representing air pollution in BTH region showed consistent subseasonal changes.Specifically,the pollution before the onset of SSW is light,and the pollution at the beginning of the SSW onset period is aggravated.After the onset of SSW,with the downward propagation of stratospheric negative NAM signal,the pollution in BTH region was reduced.Furthermore,the subseasonal variation of pollution in BTH region during the split SSW events were more obvious than that of the displacement SSW events.This study is helpful to apply the stratospheric variability to the extended range prediction of air particulate pollution in the BTH region,and provides theoretical implication to improve the maximum limit of the air particulate pollution prediction.