In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution episodes were observed ...In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution episodes were observed by the ＂Forming Mechanism and Con- trol Strategies of Haze in China＂ group using an intensive aerosol and trace gases campaign that simultaneously obtained data at 11 ground-based observing sites in the CARE-China network. The characteristics and formation mechanism of haze pollu- tion episodes were discussed. Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei （Jing-Jin-Ji） area; the two most severe episodes occurred during 9-15 January and 25-31 January. During these two haze pollution episodes, the maximum hourly PMz5 mass concentrations in Beijing were 680 and 530 ~tg m-3, respectively. The process and evolution of haze pollution episodes in other major cities in the Jing-Jin-Ji area, such as Shijiazhuang and Tianjin were almost the same as those observed in Beijing. The external cause of the severe haze episodes was the unusual atmospheric circulation, the depres- sion of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions. How- ever, the internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols, which contributed to the ＂explosive growth＂ and ＂sustained growth＂ of PM2.5. Particularly, the abnormally high amount of nitric ox- ide （NOx） in the haze episodes, produced by fossil fuel combustion and vehicle emissions, played a direct or indirect role in the quick secondary transformation of coal-burning sulphur dioxide （SO2） to sulphate aerosols. Furthermore, gaseous pollutants were transformed into secondary aerosols through heterogeneous reactions on the surface of fine particles, which can change the particle＇s size and chemical composition. Consequently, the proportion of secondary inorganic ions, such as sulphate and nitrate, gradually incr展开更多
为探讨以细颗粒物(PM2.5)持续高浓度暴露为特征的大气污染事件对于人群因病入院的急性影响,收集2013—2018年北京市大气PM2.5浓度、气象因素数据和人群因病入院数据,以PM2.5浓度〔75.0μg m 3、150.0μg m 3、第95百分位数(205.8μg m 3...为探讨以细颗粒物(PM2.5)持续高浓度暴露为特征的大气污染事件对于人群因病入院的急性影响,收集2013—2018年北京市大气PM2.5浓度、气象因素数据和人群因病入院数据,以PM2.5浓度〔75.0μg m 3、150.0μg m 3、第95百分位数(205.8μg m 3)〕和持续时间(≥2 d、≥3 d)联合定义6种持续高暴露情景,采用基于quasi-Poisson回归的广义相加模型开展时间序列分析,获得不同情景下PM2.5对人群因病入院的急性影响.结果表明:①对于全人群研究发现,相较于非持续高暴露情景,持续2 d及以上的重污染(PM2.5浓度>150.0μg m 3)情景引起非意外总疾病和心脑血管系统疾病的发病风险均显著增加,增幅分别为5.0%(95%CI,1.2%~9.0%)和5.6%(95%CI,1.8%~9.5%).②亚组人群分层分析发现,持续2 d及以上的重污染情景下,男性、女性、0~64岁、65~74岁、75岁及以上等亚组人群的非意外总疾病和心脑血管系统疾病的发病风险均显著增加;当出现极端PM2.5暴露浓度(>205.8μg m 3)且持续3 d以及上时,女性和0~64岁、65~74岁人群的非意外总疾病和心脑血管系统疾病的发病风险均显著增加.③因呼吸系统疾病入院分析发现,仅在PM2.5浓度>150μg m 3且持续时间≥2 d的情景下,观察到0~64岁人群的发病风险显著增加,增幅为3.4%(95%CI,0.2%~6.6%).研究显示,以PM2.5持续高暴露为特征的大气污染事件对人群因病入院有显著影响.展开更多
A hygroscopic tandem differentialmobility analyser(H-TDMA)was used to observe the sizeresolved hygroscopic characteristics of submicron particles in January and April 2018 in urban Beijing.The probability distribution...A hygroscopic tandem differentialmobility analyser(H-TDMA)was used to observe the sizeresolved hygroscopic characteristics of submicron particles in January and April 2018 in urban Beijing.The probability distribution of the hygroscopic growth factor(HGF-PDF)in winter and spring usually showed a bimodal pattern,with more hygroscopic mode(MH)being more dominant.The seasonal variation in particle hygroscopicity was related to the origin of air mass,which received polluted southerly air masses in spring and clean northwesterly air masses in winter.Particles showed stronger hygroscopic behaviour during heavy pollution episodes(HPEs)with elevated concentrations of secondary aerosols,especially higher mass fraction of nitrate,which were indicated using the PM2.5(particulate matter with diameter below 2.5μm)mass concentration normalised by CO mass concentration.The hygroscopic parameter(κ)values were calculated using H-TDMA(κhtdma)and chemical composition(κchem).The closure study showed thatκchem was overestimated in winter afternoon when compared withκhtdma,because the organic particle hygroscopic parameter(κorg)was overestimated in the calculations.It was influenced by the presence of a high concentration of hydrocarbon-like organic aerosol(HOA)with a weak water uptake ability.A positive relationship was observed betweenκorg and the ratio of oxygenated organic aerosol(OOA)and HOA,thereby indicating that the strong oxidation state enhanced the hygroscopicity of the particles.This study revealed the effect of local emission sources and secondary aerosol formation processes on particle hygroscopicity,which is of great significance for understanding the pollution formation mechanism in the North China Plain.展开更多
Atmospheric oxidizing capacity(AOC)is the fundamental driving factors of chemistry process(e.g.,the formation of ozone(O_(3))and secondary organic aerosols(SOA))in the troposphere.However,accurate quantification of AO...Atmospheric oxidizing capacity(AOC)is the fundamental driving factors of chemistry process(e.g.,the formation of ozone(O_(3))and secondary organic aerosols(SOA))in the troposphere.However,accurate quantification of AOC still remains uncertainty.In this study,a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing,where O_(3) and PM_(2.5) episodes had been experienced successively.The observation-based model(OBM)is used to quantify the AOC at O_(3) and PM_(2.5) episodes.The strong intensity of AOC is found at O_(3) and PM2.5 episodes,and hydroxyl radical(OH)is the dominating daytime oxidant for both episodes.The photolysis of O_(3) is main source of OH at O_(3) episode;the photolysis of nitrous acid(HONO)and formaldehyde(HCHO)plays important role in OH formation at PM_(2.5) episode.The radicals loss routines vary according to precursor pollutants,resulting in different types of air pollution.O_(3) budgets and sensitivity analysis indicates that O_(3) production is transition regime(both VOC and NOx-limited)at O3 episode.The heterogeneous reaction of hydroperoxy radicals(HO_(2))on aerosol surfaces has significant influence on OH and O_(3) production rates.The HO_(2) uptake coefficient(γHO_(2))is the determining factor and required accurate measurement in real atmospheric environment.Our findings could provide the important bases for coordinated control of PM_(2.5) and O_(3) pollution.展开更多
Episodes of fine-particulate matter(PM_(2.5))pollution are a widespread and common occurrence in China,and have potentially serious implications for human health.Meteorological conditions play an important role in air...Episodes of fine-particulate matter(PM_(2.5))pollution are a widespread and common occurrence in China,and have potentially serious implications for human health.Meteorological conditions play an important role in air quality and influence the formation of regional air pollution episodes.This study applied a new classification method and daily PM_(2.5)concentration data to(a)evaluate different levels of air pollution in the Sichuan-Chongqing region between 2015 and 2017,and(b)investigate their relationships with meteorological parameters including atmospheric boundary layer height,wind speed,temperature inversion,weather type,and atmospheric transport.We identified a total of 40 air pollution episodes,the most severe of which occurred in winter when the atmospheric layer height was low.These heavy pollution events also coincided with low surface(10 m)wind speeds and temperature inversions,weather conditions that generally result from low pressure over the northwestern Sichuan Basin(90-102°E,28-36°N)and southerly atmospheric transport.展开更多
Amines are important nitrogen-containing compounds in fine particles(PM2.5)in the atmosphere.Observations are necessary for in-depth understanding on the characteristics,sources and atmospheric processes of aminiums.I...Amines are important nitrogen-containing compounds in fine particles(PM2.5)in the atmosphere.Observations are necessary for in-depth understanding on the characteristics,sources and atmospheric processes of aminiums.In this study,the observation of ten C_(1)–C_(4) aminiums in PM_(2.5) was conducted in January and March of 2021 in suburban Guangzhou.The concentration and composition of aminiums showed significant differences between the pollution episodes and non-episode periods.Seasonal difference was also observed between winter and spring.The influence of meteorological factors(i.e.,wind speed,atmospheric pressure,temperature and relative humidity)was investigated.The variations of aminiums were also affected by different sources.Anthropogenic sources were suggested to be major contributor to aminiums in the pollution episodes,while biological sources were important sources to aminiums in the non-episode periods,especially in spring.Positive matrix factorization receptor model was applied to investigate the source contributions,and four major sources were identified.The results show that vehicular emission,industrial production,biological emission and soil/dust were the major sources of aminiums.This study emphasizes the importance of source contribution and meteorological conditions on the variations of aminiums,which provides further understanding of organic nitrogen in the atmosphere.展开更多
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
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展开更多
基金supported by the Chinese Academy of Sciences Strategic Priority Research Program(Grant Nos.XDB05020000 and XDA05100100)the National Natural Science Foundation of China(Grant Nos.41230642 and 41021004)
文摘In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution episodes were observed by the ＂Forming Mechanism and Con- trol Strategies of Haze in China＂ group using an intensive aerosol and trace gases campaign that simultaneously obtained data at 11 ground-based observing sites in the CARE-China network. The characteristics and formation mechanism of haze pollu- tion episodes were discussed. Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei （Jing-Jin-Ji） area; the two most severe episodes occurred during 9-15 January and 25-31 January. During these two haze pollution episodes, the maximum hourly PMz5 mass concentrations in Beijing were 680 and 530 ~tg m-3, respectively. The process and evolution of haze pollution episodes in other major cities in the Jing-Jin-Ji area, such as Shijiazhuang and Tianjin were almost the same as those observed in Beijing. The external cause of the severe haze episodes was the unusual atmospheric circulation, the depres- sion of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions. How- ever, the internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols, which contributed to the ＂explosive growth＂ and ＂sustained growth＂ of PM2.5. Particularly, the abnormally high amount of nitric ox- ide （NOx） in the haze episodes, produced by fossil fuel combustion and vehicle emissions, played a direct or indirect role in the quick secondary transformation of coal-burning sulphur dioxide （SO2） to sulphate aerosols. Furthermore, gaseous pollutants were transformed into secondary aerosols through heterogeneous reactions on the surface of fine particles, which can change the particle＇s size and chemical composition. Consequently, the proportion of secondary inorganic ions, such as sulphate and nitrate, gradually incr
文摘为探讨以细颗粒物(PM2.5)持续高浓度暴露为特征的大气污染事件对于人群因病入院的急性影响,收集2013—2018年北京市大气PM2.5浓度、气象因素数据和人群因病入院数据,以PM2.5浓度〔75.0μg m 3、150.0μg m 3、第95百分位数(205.8μg m 3)〕和持续时间(≥2 d、≥3 d)联合定义6种持续高暴露情景,采用基于quasi-Poisson回归的广义相加模型开展时间序列分析,获得不同情景下PM2.5对人群因病入院的急性影响.结果表明:①对于全人群研究发现,相较于非持续高暴露情景,持续2 d及以上的重污染(PM2.5浓度>150.0μg m 3)情景引起非意外总疾病和心脑血管系统疾病的发病风险均显著增加,增幅分别为5.0%(95%CI,1.2%~9.0%)和5.6%(95%CI,1.8%~9.5%).②亚组人群分层分析发现,持续2 d及以上的重污染情景下,男性、女性、0~64岁、65~74岁、75岁及以上等亚组人群的非意外总疾病和心脑血管系统疾病的发病风险均显著增加;当出现极端PM2.5暴露浓度(>205.8μg m 3)且持续3 d以及上时,女性和0~64岁、65~74岁人群的非意外总疾病和心脑血管系统疾病的发病风险均显著增加.③因呼吸系统疾病入院分析发现,仅在PM2.5浓度>150μg m 3且持续时间≥2 d的情景下,观察到0~64岁人群的发病风险显著增加,增幅为3.4%(95%CI,0.2%~6.6%).研究显示,以PM2.5持续高暴露为特征的大气污染事件对人群因病入院有显著影响.
基金supported by the National Natural Science Foundation of China (Nos.42075082,41875147,42090031)the S&T Development Fund of CAMS (No.2020KJ001),Basic Research Fund of CAMS (No.2020Z002)the Innovation and Development Program of CMA (No.CXFZ2021Z089)
文摘A hygroscopic tandem differentialmobility analyser(H-TDMA)was used to observe the sizeresolved hygroscopic characteristics of submicron particles in January and April 2018 in urban Beijing.The probability distribution of the hygroscopic growth factor(HGF-PDF)in winter and spring usually showed a bimodal pattern,with more hygroscopic mode(MH)being more dominant.The seasonal variation in particle hygroscopicity was related to the origin of air mass,which received polluted southerly air masses in spring and clean northwesterly air masses in winter.Particles showed stronger hygroscopic behaviour during heavy pollution episodes(HPEs)with elevated concentrations of secondary aerosols,especially higher mass fraction of nitrate,which were indicated using the PM2.5(particulate matter with diameter below 2.5μm)mass concentration normalised by CO mass concentration.The hygroscopic parameter(κ)values were calculated using H-TDMA(κhtdma)and chemical composition(κchem).The closure study showed thatκchem was overestimated in winter afternoon when compared withκhtdma,because the organic particle hygroscopic parameter(κorg)was overestimated in the calculations.It was influenced by the presence of a high concentration of hydrocarbon-like organic aerosol(HOA)with a weak water uptake ability.A positive relationship was observed betweenκorg and the ratio of oxygenated organic aerosol(OOA)and HOA,thereby indicating that the strong oxidation state enhanced the hygroscopicity of the particles.This study revealed the effect of local emission sources and secondary aerosol formation processes on particle hygroscopicity,which is of great significance for understanding the pollution formation mechanism in the North China Plain.
基金supported by the National Key Research and Development Program of China (No. 2017YFC0210001)the National Natural Science Foundation of China (Nos. 41830106, 42022039)+1 种基金Beijing National Laboratory for Molecular Sciences (No. BNLMS-CXXM-202011)the Youth Innovation Promotion Association CAS (No. 2017042)
文摘Atmospheric oxidizing capacity(AOC)is the fundamental driving factors of chemistry process(e.g.,the formation of ozone(O_(3))and secondary organic aerosols(SOA))in the troposphere.However,accurate quantification of AOC still remains uncertainty.In this study,a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing,where O_(3) and PM_(2.5) episodes had been experienced successively.The observation-based model(OBM)is used to quantify the AOC at O_(3) and PM_(2.5) episodes.The strong intensity of AOC is found at O_(3) and PM2.5 episodes,and hydroxyl radical(OH)is the dominating daytime oxidant for both episodes.The photolysis of O_(3) is main source of OH at O_(3) episode;the photolysis of nitrous acid(HONO)and formaldehyde(HCHO)plays important role in OH formation at PM_(2.5) episode.The radicals loss routines vary according to precursor pollutants,resulting in different types of air pollution.O_(3) budgets and sensitivity analysis indicates that O_(3) production is transition regime(both VOC and NOx-limited)at O3 episode.The heterogeneous reaction of hydroperoxy radicals(HO_(2))on aerosol surfaces has significant influence on OH and O_(3) production rates.The HO_(2) uptake coefficient(γHO_(2))is the determining factor and required accurate measurement in real atmospheric environment.Our findings could provide the important bases for coordinated control of PM_(2.5) and O_(3) pollution.
基金supported by the Innovative Research Groups of the National Natural Science Foundation of China(No.41521004)the National Natural Science Foundation of China(Nos.41875091 and 51979042)the Lanzhou City’s scientific research funding subsidy to Lanzhou University
文摘Episodes of fine-particulate matter(PM_(2.5))pollution are a widespread and common occurrence in China,and have potentially serious implications for human health.Meteorological conditions play an important role in air quality and influence the formation of regional air pollution episodes.This study applied a new classification method and daily PM_(2.5)concentration data to(a)evaluate different levels of air pollution in the Sichuan-Chongqing region between 2015 and 2017,and(b)investigate their relationships with meteorological parameters including atmospheric boundary layer height,wind speed,temperature inversion,weather type,and atmospheric transport.We identified a total of 40 air pollution episodes,the most severe of which occurred in winter when the atmospheric layer height was low.These heavy pollution events also coincided with low surface(10 m)wind speeds and temperature inversions,weather conditions that generally result from low pressure over the northwestern Sichuan Basin(90-102°E,28-36°N)and southerly atmospheric transport.
基金The authors thank the financial support of the National Natural Science Foundation of China(grant numbers 41975156,41675119)Fundamental Research Funds for the Central Universities(grant number 2014Zz0054).
文摘Amines are important nitrogen-containing compounds in fine particles(PM2.5)in the atmosphere.Observations are necessary for in-depth understanding on the characteristics,sources and atmospheric processes of aminiums.In this study,the observation of ten C_(1)–C_(4) aminiums in PM_(2.5) was conducted in January and March of 2021 in suburban Guangzhou.The concentration and composition of aminiums showed significant differences between the pollution episodes and non-episode periods.Seasonal difference was also observed between winter and spring.The influence of meteorological factors(i.e.,wind speed,atmospheric pressure,temperature and relative humidity)was investigated.The variations of aminiums were also affected by different sources.Anthropogenic sources were suggested to be major contributor to aminiums in the pollution episodes,while biological sources were important sources to aminiums in the non-episode periods,especially in spring.Positive matrix factorization receptor model was applied to investigate the source contributions,and four major sources were identified.The results show that vehicular emission,industrial production,biological emission and soil/dust were the major sources of aminiums.This study emphasizes the importance of source contribution and meteorological conditions on the variations of aminiums,which provides further understanding of organic nitrogen in the atmosphere.
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 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
