| Based on Raman-Mie lidar and CE-318 sun photometer,the extinction coefficient?optical depth and direct radiative forcing et al in Beijing from autumn 2017 to summer 2018 are continuously observed,and the monthly and seasonal variations of aerosols are obtained.feature.Combined with the PM quality concentration observation data of the National Environmental Monitoring Center,the reanalysis data of the European Center and the backward trajectory model,case analysis of several pollution processes occurring during this period is carried out:Overall,from the frequency,extent and duration of pollution:spring>summer,autumn>winter.The mass concentration of PM in spring is more prominent than that in other months.Due to the influence of northwest wind and north wind,the upstream dust is transported to Beijing through high altitude;the peak concentration of coarse mode particles in the particle spectrum is much higher than fine mode particles,Angstrom is also lower than other months;at the same time,the extinction of mineral aerosols is very effective in cooling the surface and the top of the atmosphere.In the summer with the hydrophilic fine particles such as nitrate and sulfate dominate,the abundant water vapor conditions increase the scattering ability of the particles,and the single scattering albedo can reach 0.98,although the summer PM mass concentration is lower than the spring,however,the aerosol optical depth has increased slightly;unlike the spring-absorbing coarse dust particles,the presence of strong scattering particles such as sulfate and nitrate in summer also has a significant cooling effect on the surface and the top of the atmosphere.Ground winds in spring and turbulence instability caused by enhanced solar radiation in summer makes the overall height of the boundary layer from April to September higher.In autumn,the relatively stable meteorological conditions cause the boundary layer to continue to be pushed down,which increases the pollution process;the mixed absorption particles and non-absorbent fine particles are dominant,and the absorbent dust particles can pass the strong northwest wind from high altitude.It is transported from Inner Mongolia;combined with the low wind speed at the lower level,it provides favorable conditions for high-altitude transmission and land subsidence of dust aerosols;the aerosol height can be extended to 3.5 km during dust and dust,and the air pollution caused by it is also not to be underestimated.The haze process,which also occurs in autumn,is significantly different from the process.The extinction coefficient in the process can reach 0.99 km-1 or more,and the depolarization ratio is significantly smaller than the dust process.The aerosol boundary layer is relatively low in height and does not exceed the overall 1 km.The long-term stability of meteorological conditions is not conducive to the spread of aerosols.At the same time,the discharge of pollutants such as local automobile exhausts causes the accumulation of particulate matter,resulting in increasing pollution.The study on clean days shows that meteorological conditions such as strong winds have a good diffusion effect on local aerosols.The significant reduction in solar radiation in winter and the diminishing diurnal variation of the boundary layer make it the lowest season in the four seasons.However,the cold air transit from time to time hinders the accumulation of aerosols such as coal combustion,automobile emissions and dust during the heating period.Therefore,there is no frequent large-scale pollution process in this winter.It can correspond to both the PM mass concentration and the aerosol optical thickness.The increase of absorption aerosols such as black carbon makes the single-scattering albedo significantly lower than other seasons.Although the cooling effect on the ground and the top of the atmosphere is far less than that in spring and summer,the heating effect on the atmosphere is second only to spring. |
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