In North China,the return flow(RF)refers to the airflow at the rear of an inshore high pressure,bringing southerly wind to the Beijing–Tianjin–Hebei(BTH)region when the high pressure pushes deeper from coast into th...In North China,the return flow(RF)refers to the airflow at the rear of an inshore high pressure,bringing southerly wind to the Beijing–Tianjin–Hebei(BTH)region when the high pressure pushes deeper from coast into the mainland.Many studies have pointed out the significant contribution of southerly anomalies to the transport and accumulation of pollutants in the BTH region.However,the relationship between RF and heavy pollution episodes(HPEs)in the BTH region requires more in-depth analysis,and this study will focus on this topic.By objectively identifying RFs and HPEs based on the ERA5 reanalysis data and observed hourly PM_(2.5)concentration data during 9 winters of 2012–2020,it is found that almost two-thirds of the HPEs in the BTH region coincide with the occurrence of RFs.The northward transport of warm and humid air is stronger in the HPEs under RF conditions,whereas the sinking motion and the decrease in boundary layer height dominate the HPEs without any RF.We then classify the RFs into north and south types by a demarcation line of 32°N.Both types of RFs are featured with a zonal circulation pattern,stable atmosphere,and southerly airflow favoring the development of HPEs,but the south type RFs bring warmer and wetter air masses to the BTH region,forming a more stable and thicker inversion layer and causing more severe HPEs.With occurrences of the RF,low-level winds are observed to accelerate,and the ageostrophic wind components contribute mainly to this acceleration.During the presence of RFs,the kinetic energy generation at the high level decreases,and the weakened downward transport results in weak low-level northerly winds,weak turbulence,and a shallow boundary layer,thus hindering the diffusion of atmospheric pollutants in the BTH region.展开更多
Air pollution in China is complex,and the formation mechanism of chemical components in particulate matter is still unclear.This study selected three consecutive heavy haze pollution episodes(HPEs)during winter in Bei...Air pollution in China is complex,and the formation mechanism of chemical components in particulate matter is still unclear.This study selected three consecutive heavy haze pollution episodes(HPEs)during winter in Beijing for continuous field observation,including an episode with heavy air pollution under red alert.Clean days during the observation period were selected for comparison.The HPE characteristics of Beijing in winter were:under the influence of adverse meteorological conditions such as high relative humidity,temperature inversion and low wind speed;and strengthening of secondary transformation reactions,which further intensified the accumulation of secondary aerosols and other pollutants,promoting the explosive growth of PM_(2.5).PM_(2.5)/CO values,as indicators of the contribution of secondary transformation in PM_(2.5),were approximately 2 times higher in the HPEs than the average PM_(2.5)/CO during the clean period.The secondary inorganic aerosols(sulfate nitrate and ammonium salt)were significantly enhanced during the HPEs,and the conversion coefficients were remarkably improved.In addition,it is interesting to observe that the production of sulfate tended to exceed that of nitrate in the late stage of all three HPEs.The existence of aqueous phase reactions led to the explosive growth sulfur oxidation ratio(SOR)and rapid generation of sulfate under high relative humidity(RH>70%).展开更多
Based on the Weather Research and Forecasting model and the Models-3 community multi-scale air quality model(WRF-CMAQ),this study analyzes the impacts of meteorological conditions and changes in air pollutant emission...Based on the Weather Research and Forecasting model and the Models-3 community multi-scale air quality model(WRF-CMAQ),this study analyzes the impacts of meteorological conditions and changes in air pollutant emissions on the heavy air pollution episode occurred over North China around the 2020 Spring Festival(January to Februray 2020).Regional reductions in air pollutant emissions required to eliminate the PM2.5 heavy pollution episode are also quantified.Our results found that meteorological conditions for the Beijing-Tianjin-Hebei and surrounding"2+26"cities are the worst during the heavy pollution episode around the 2020 Spring Festival as compared with two other typical heavy pollution episodes that occurred after 2015.However,because of the substantial reductions in air pollutant emissions in the"2+26"cities in recent years,and the32%extra reduction in emissions during January to February 2020 compared with the baseline emission levels of the autumn and winter of 2019 to 2020,the maximum PM2.5 level during this heavy pollution episode around the 2020 Spring Festival was much lower than that in the other two typical episodes.Yet,these emission reductions are still not enough to eliminate regional heavy pollution episodes.Compared with the actual emission levels during January to February 2020,a 20%extra reduction in air pollutant emissions in the"2+26"cities(or a 45%extra reduction compared with baseline emission levels of the autumn and winter of 2019 to 2020)could help to generally eliminate regionwide severe pollution episodes,and avoid heavy pollution episodes that last three or more consecutive days in Beijing;a 40%extra reduction in emissions(or a 60%extra reduction compared with baseline emission levels of the autumn and winter of 2019 to 2020)could help to generally eliminate regionwide and continuous heavy pollution episodes.Our analysis finds that during the clean period after the heavy pollution episode around the 2020 Spring Festival,the regionwide heavy pollution episode would only occur with at least a 10-fold increase in air pollutant emissions.展开更多
基金Supported by the National Natural Science Foundation of China(41790471).
文摘In North China,the return flow(RF)refers to the airflow at the rear of an inshore high pressure,bringing southerly wind to the Beijing–Tianjin–Hebei(BTH)region when the high pressure pushes deeper from coast into the mainland.Many studies have pointed out the significant contribution of southerly anomalies to the transport and accumulation of pollutants in the BTH region.However,the relationship between RF and heavy pollution episodes(HPEs)in the BTH region requires more in-depth analysis,and this study will focus on this topic.By objectively identifying RFs and HPEs based on the ERA5 reanalysis data and observed hourly PM_(2.5)concentration data during 9 winters of 2012–2020,it is found that almost two-thirds of the HPEs in the BTH region coincide with the occurrence of RFs.The northward transport of warm and humid air is stronger in the HPEs under RF conditions,whereas the sinking motion and the decrease in boundary layer height dominate the HPEs without any RF.We then classify the RFs into north and south types by a demarcation line of 32°N.Both types of RFs are featured with a zonal circulation pattern,stable atmosphere,and southerly airflow favoring the development of HPEs,but the south type RFs bring warmer and wetter air masses to the BTH region,forming a more stable and thicker inversion layer and causing more severe HPEs.With occurrences of the RF,low-level winds are observed to accelerate,and the ageostrophic wind components contribute mainly to this acceleration.During the presence of RFs,the kinetic energy generation at the high level decreases,and the weakened downward transport results in weak low-level northerly winds,weak turbulence,and a shallow boundary layer,thus hindering the diffusion of atmospheric pollutants in the BTH region.
基金supported by the Natural Science Foundation of Beijing Municipal(No.8202052)the National Natural Science Foundation of China(Nos.21777191,42075082 and 41875147)+2 种基金S&T Development Fund of Chinese Academy of Meteorological Sciences(No.2020KJ001)Basic Research Fund of Chinese Academy of Meteorological Sciences(No.2020Z002)supported by the Innovation Team for Haze-fog Observation and Forecasts of Ministry of Science and Technology of the People’s Republic of China and China Meteorological Administration
文摘Air pollution in China is complex,and the formation mechanism of chemical components in particulate matter is still unclear.This study selected three consecutive heavy haze pollution episodes(HPEs)during winter in Beijing for continuous field observation,including an episode with heavy air pollution under red alert.Clean days during the observation period were selected for comparison.The HPE characteristics of Beijing in winter were:under the influence of adverse meteorological conditions such as high relative humidity,temperature inversion and low wind speed;and strengthening of secondary transformation reactions,which further intensified the accumulation of secondary aerosols and other pollutants,promoting the explosive growth of PM_(2.5).PM_(2.5)/CO values,as indicators of the contribution of secondary transformation in PM_(2.5),were approximately 2 times higher in the HPEs than the average PM_(2.5)/CO during the clean period.The secondary inorganic aerosols(sulfate nitrate and ammonium salt)were significantly enhanced during the HPEs,and the conversion coefficients were remarkably improved.In addition,it is interesting to observe that the production of sulfate tended to exceed that of nitrate in the late stage of all three HPEs.The existence of aqueous phase reactions led to the explosive growth sulfur oxidation ratio(SOR)and rapid generation of sulfate under high relative humidity(RH>70%).
基金supported by the National Key Research and Development Program(Grant Nos.2016YFC0207502,2016YFC0208805)the National Research Program for Key Issues in Air Pollution Control(Grant No.DQGG0302)。
文摘Based on the Weather Research and Forecasting model and the Models-3 community multi-scale air quality model(WRF-CMAQ),this study analyzes the impacts of meteorological conditions and changes in air pollutant emissions on the heavy air pollution episode occurred over North China around the 2020 Spring Festival(January to Februray 2020).Regional reductions in air pollutant emissions required to eliminate the PM2.5 heavy pollution episode are also quantified.Our results found that meteorological conditions for the Beijing-Tianjin-Hebei and surrounding"2+26"cities are the worst during the heavy pollution episode around the 2020 Spring Festival as compared with two other typical heavy pollution episodes that occurred after 2015.However,because of the substantial reductions in air pollutant emissions in the"2+26"cities in recent years,and the32%extra reduction in emissions during January to February 2020 compared with the baseline emission levels of the autumn and winter of 2019 to 2020,the maximum PM2.5 level during this heavy pollution episode around the 2020 Spring Festival was much lower than that in the other two typical episodes.Yet,these emission reductions are still not enough to eliminate regional heavy pollution episodes.Compared with the actual emission levels during January to February 2020,a 20%extra reduction in air pollutant emissions in the"2+26"cities(or a 45%extra reduction compared with baseline emission levels of the autumn and winter of 2019 to 2020)could help to generally eliminate regionwide severe pollution episodes,and avoid heavy pollution episodes that last three or more consecutive days in Beijing;a 40%extra reduction in emissions(or a 60%extra reduction compared with baseline emission levels of the autumn and winter of 2019 to 2020)could help to generally eliminate regionwide and continuous heavy pollution episodes.Our analysis finds that during the clean period after the heavy pollution episode around the 2020 Spring Festival,the regionwide heavy pollution episode would only occur with at least a 10-fold increase in air pollutant emissions.
