| Aerosols are minute particles suspended in the atmosphere and can be natural or artificial, which are mainly generated from dust, sea salt particles, biomass burning and industrial activities etc. Aerosols interact both directly and indirectly with the Earth’s radiation budget and climate, they degrade air quality and pose a threat to human health and, the relevant issues become a focus of researches in recent days. Eastern China (28°N-40°N,110°N-122°E), with rapid urban growth and the development of the industry during the last decades, now suffers from significant environmental problems, which is due to the massive emissions of atmospheric pollutants from industries, transportation and so on.By using the integrated datasets of satellite remote sensing aerosol products, meteorological field data from reanalysis datasets and the back trajectory model of air mass, the air pollution in eastern China are analysed from the perspective of meteorological effects. The study present the definition of regional pollution episode and persistent circulation process at first, and then aming to the following two aspects based on the calculation of two different similarity measures:firstly, the climatological characteristics of aerosols and atmospheric circulation, and thereby analyzing the annual variation of aerosols and persistent circulation; secondly, taking the autumn as an example to reveal the impacts of large-scale circulation on the occurrence of regional pollution/clean episodes, and the long-term trend of different synoptic types. The main results are presented as follows:(1) The seasonal and interannual variation characteristics of AOD in China during the last 15 yearsHigh AOD mainly concentrated in economically developed plains, such as Eastern China, Sichuan Basin and Pearl River Delta region, and value mainly ranges from 0.8 to 1.0. However, it is relatively cleaner in plateau sections at high elevations and northeast region of China since the AOD is less than 0.3. Distribution pattern of AOD is similar in different seasons, but AOD in spring and summer is larger than that in autumn and winter. Besides, in eastern china, AOD shows an interannual decreasing trend in spring and summer, while for autumn and winter, both show an increasing trend with a value of about 0.1 decade-1.(2) The interannual variation and seasonal difference of persistent circulation process in eastern China during the last 65 yearsBased on the results calculated from two different similarity measures, it is found that the circulation in summer and autumn is less variable and can continue for many days, whereas the number of days with persistent circulation is relatively few in other two seasons, especially in spring. The occurrence frequency of persistent episode also shows a notable change in accordance with the decadal shift of climate at the end of the 70’s, which is significantly smaller after the time. During 1951-1978, decreasing trend in all seasons were found, and the total number of persistent days in descending sequence were:summer, autumn, winter and spring. While in 1979-2015, the number of days in autumn was slightly higher than summer. Both autumn and summer showed an interannual increasing trend, and the decreasing intensity of spring and winter were weaker than the previous three decades. In addition to the winter, the trend of other three seasons are clear and do not depend on the setting of the critical values. The duration of persistent episode are concentrated in the 2-4 days for various seasons and a few episodes can last for more than 5 days. By comparison, the long duration episodes are most likely to occur in summer. Furthermore, the duration of persistent episodes shortened during the past decades, the frequency of two-day episodes were increasing, while others were decreasing.(3) Typical circulation model of pollution and clean episode in autumn over eastern ChinaIn eastern china, autumn is mainly influenced by large-scale synoptic systems and uniform descending motion prevails over the area. The daily mean AOD value ranges from 0.3 to 0.9, the fluctuation is more volatile over the region where the mean AOD is higher. It is deduced that the variation of AOD is closed with the vertical and horizontal movements of air flows and the interannual variability of atmospheric circulations has an important contribution to the aerosol variance in the polluted area of the eastern china. The Six polluted types (AOD>0.6) and three clean types (AOD<0.4) are schematically illustrated in the manuscript. Generally speaking, weather patterns such as a uniform surface pressure field in eastern China or a steady straight westerly in the middle troposphere, particularly when being at the rear of the anticyclone at 850 hPa, are typically responsible for heavy pollution events. Meanwhile, clean episodes occur when strong southeastward cold air advection prevails below the middle troposphere or air masses are transported from sea to land. Moreover, the value of vertical velocity averaged from 1000 to 100 hPa and divergence of wind field in the lower troposphere are used in this study to quantify the diffusion conditions in each circulation type. The results reveal that it is often a clean episode when both the mean downward motion (larger than 2.56×10-2Pas-1)and the divergence of low level winds (larger than 1.79×10-6S-1) are strong, otherwise, it is more likely to be a polluted episode.(4) The interannual variation trend of pollution/clean episodes in autumn over eastern ChinaThe results show that number of polluted days are obviously more than that of clean days. Under three different critical values, the annual variations of the selected clean and polluted episodes are basically consistent, namely, both clean and polluted episodes show an apparent interannual increasing trend, however, the trend of the pollution type is stronger than that of clean type. It indicates that the regulating effect of large-scale circulation in autumn tends to promote the occurrence of pollution. |
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