| Persistent air pollution in the Beijing-Tianjin-Hebei region has become an extremely complex challenge due to the combined effects of industrial structure,regional characteristics,weather and climate,and development.Although China's air pollution levels have reduced significantly since the “Air Pollution Prevention and Control Action Plan” and the “Blue Sky Defense Plan” were implemented,the Beijing–Tianjin–Hebei region remains a sensitive and vulnerable area.From 2013 to 2018,Beijing–Tianjin–Hebei and surrounding areas still experienced several weather processes associated with air pollution,with fine particulate matter()as the principal air pollutant.Even two severe air pollution episodes occurred in succession around the 2020 Spring Festival in China from January to February 2020,i.e.the “COVID-19” epidemic prevention and control period.Despite emissions of air pollutants over North China has reduced substantially due to the reductions in socioeconomic activity levels during this period.The present study investigates the physical mechanisms of development of persistent heavy pollution weather during winter in the Beijing–Tianjin–Hebei region,and for the first time reveals the key modulation effect of tropospheric blocking on incessant heavy pollution weather in the Beijing–Tianjin–Hebei region during winter.Furthermore,the study analyzes the role and influence of a typical anomalous circulation pattern with a high–low level configuration in winter on persistent heavy pollution weather in the Beijing-Tianjin-Hebei region.The comprehensive effects of the tropospheric blocking and the leeward slope of a plateau terrain,atmospheric dynamics and thermal structure,and low–level abnormal water vapor transport structure on persistent heavy pollution weather in Beijing–Tianjin–Hebei region is elucidated.Finally,a comprehensive impact model of the atmospheric dynamics and thermodynamics of persistent heavy pollution weather in the Beijing–Tianjin–Hebei region during winter is proposed and developed,and the relationship between climate change and persistent pollution is inferred on an interdecadal scale.The key findings of the present article are as follows:(1)For the first time reveals the key modulation effect of tropospheric blocking on incessant heavy pollution weather in the Beijing–Tianjin–Hebei region during winter.Persistent and severely polluted weather in eastern China exhibits well-defined characteristics along the terrain.Winter is the season of persistent heavy pollution,which largely occurs in the Beijing–Tianjin–Hebei region and its surrounding areas.A total of 25 ‘ultra-long'(lasting6 days or more),wide-ranging pollution events occurred in the winters of 2012-2018 in the Beijing–Tianjin–Hebei region.Comparisons and analyses revealed that the circulation structure of the geopotential height anomalies at 500-h Pa in the 25 regional ultra-long pollution events(?6 days)in the Beijing-Tianjin-Hebei region and surrounding areas could be classified into two categories: blocking and non-blocking structures.The analyses results revealed that the atmospheric circulation anomalies pattern corresponding to 62.5% of the persistent ‘most serious' and ‘more serious' air pollution events in the Beijing–Tianjin–Hebei region were blocking structures,and that 43.75% of the 500-h Pa atmospheric circulation anomalies in the middle and high latitudes of Eurasia were ‘dual-blocking' pattern,and 18.75%of the atmospheric circulation patterns were ‘single-blocking' pattern.The mechanism of sustenance of the blocking system and the establishment,maintenance,and decline of the blocking condition are associated with the development,persistence,and dissipation of the ultra-long period of severely polluted weather in the Beijing–Tianjin–Hebei region.The blocking structure is a key atmospheric circulation pattern that causes ultra-long periods of winter in the Beijing-Tianjin-Hebei region,which prolongs the large-scale heavy pollution weather.The levels of air pollution and the intensity of blocking system exhibit the characteristics of positive feedback mechanism,especially the Okhotsk blocking.It is significantly positively correlated with concentration of the Beijing-Tianjin-Hebei region.(2)During the period of severely polluted weather in the Beijing-Tianjin-Hebei region,the upper and middle-high levels of the troposphere formed a unique three-dimensional circulation structure of “upper convergence-lower divergence”,which facilitated a "convergence and sinking" air flow effect over the Beijing–Tianjin–Hebei region.The convergent sinking dynamic effect of the upper atmosphere and the convergence of western and southern lower layers constitute the core dynamic mechanisms of the sustained development of heavily polluted weather in the Beijing-Tianjin-Hebei region,which demonstrates the role and influence of configurations of such abnormal high–low atmospheric circulation systems on incessant heavy pollution weather in the Beijing-Tianjin-Hebei region.(3)The local atmospheric dynamics and thermal structure are significantly modulated by the persistent large-scale abnormal circulation system.During the period of severely polluted weather in the Beijing-Tianjin-Hebei region,a downdraft occurred over most areas of the Loess Plateau and the eastern areas due to the blocking system and the terrain common influence.The vertical velocity anomaly structure of the entire atmosphere over the Beijing-Tianjin-Hebei region exhibits consistent downward characteristics of airflow on the eastern side of the Loess Plateau,which effectively constrains the development of a boundary layer.Similarly,the ‘subsidence(temperature)inversion' effect produced by the synergy between downward airflow of the eastern leeward slope of the Loess Plateau and a large-scale blocking system causes a continuous and stable ‘warm cover' structure in the middle of the troposphere on the eastern side of the Loess Plateau,which,in turn,weakens the vertical diffusion capacity of the atmosphere over the Beijing–Tianjin–Hebei region,facilitating water vapor accumulation in the lower atmosphere.Furthermore,the effect intensifies the radiation effects of aerosol pollutants in the atmosphere,as well as the ‘two-way feedback' mechanism between adverse meteorological conditions within the boundary layer and atmospheric pollutants,especially in the middle and latter stages of persistent pollution weather processes,in turn,causing ultra-long,heavy pollution weather in the Beijing–Tianjin–Hebei region.(4)The synergistic effect between the blocking system and abnormal structure of water vapor transmission resulted in persistent heavy air pollution occurred in the Beijing–Tianjin–Hebei region around the 2020 Spring Festival in China from January to February 2020,i.e.the “COVID-19” epidemic prevention and control period.Stable maintenance of low-level coastal high-pressure systems leads to relative humidity higher than normal year and blocking systems,which are two key meteorological factors that induce the persistent pollution weather in Beijing-Tianjin-Hebei.The eastward transportation flow pattern and southward transportation flow pattern in the lower atmosphere are the key water vapor flow patterns that drive the development of persistent air pollution weather.The interaction between the the dynamic structure of abnormal water vapor transmission structure and terrain effect facilitates the establishment of stable and high-humidity weather conducive for pollutant accumulation,in addition to driving persistent air pollution,especially in the background of blocking.These two factors can directly explain 46.8% of the meteorological causes of persistent heavy air pollution weather events in Beijing-Tianjin-Hebei around the2020 Spring Festival in China.(5)The circulation structure of middle-high atmospheric as well as the atmospheric dynamics and thermodynamic vertical structure exhibit notable interdecadal variation influenced,which exert a significant ‘climate regulation' effect on the interdecadal variations of air pollution in the Beijing–Tianjin–Hebei region and surrounding areas and also provide interdecadal ‘strong signals' for identification of frequently occurring regional air pollution.During winter,under the control of westerlies,the “reversion” phenomenon of the circulation structure of middle-high atmospheric as well as the atmospheric dynamics and thermodynamic vertical structure,which lead to forming a stable weather situation over the Beijing–Tianjin–Hebei region that is conducive for continuous pollutant accumulation.Although the variation trends of air pollution in the Beijing–Tianjin–Hebei region and surrounding areas depend on the levels of emissions from pollution sources,the variation of the vertical structure of local or regional atmosphere is also one of the climatic causes that has been driving frequent and sustained extreme air pollution events in the region over the last few years. |
PDF Full Text Request