The Optical And Microphysical Characteristics Of Dust Aerosol Over Semi-arid Region In Northwest China

Atmospheric aerosols directly or indirectly affect the climate,through the scattering and absorption of solar radiation,and as a cloud of nuclear condensation to change its albedo and life cycle,change the radiation budgets of the surface and atmosphere,thus affecting the regional and global climate.Due to the uncertainty of the aerosol spatiotemporal change,there is a lack of sufficient understanding of the physicochemical and optical characteristics of aerosols,and it is difficult to accurately calculate the radiative forcing of aerosols,which is one of the most uncertain factors in climate prediction.Therefore,it is very important to study the physical and chemical characteristics and optical characteristics of aerosols,and it is very important for deep understanding and accurate estimation of the aerosol radiation effect.In this paper,based on the July 2006 to August 2012 sun photometer data from the SACOL(Semi-Arid Climate and Environment Observatory of Lanzhou University),the optical characteristics of aerosols are analyzed and the optical and microphysical characteristics of dust,black carbon and background aerosols are analyzed;Dust aerosol is determined by using a strict judgment,and then dust aerosol optical and microphysical characteristics are analyzed;The spring dust event from March 29 to 31,2010 is researched with lidar observation,and compared with the summer dust event from July 30 to 31,2010.The main results are as follows.(1)The daily average of aerosol optical depth(AOD550)is 0.38,and the daily average of ?ngstr?m exponent is 0.88,which is opposite in phase to AOD550.Compared with the AOD550,the daily average of water vapor content is 0.84,which is consistent with the climatic characteristics of the four seasons.Dust aerosol coarse mode peak(1.03 ?m3/?m2)is very large,fine modal particles concentration is very small,meanwhile black carbon aerosol coarse mode peak(0.08 ?m3/?m2)and fine mode peak(0.03 ?m3/?m2)are very small,showing that dust aerosols are mostly large particle size particles,and black carbon aerosols are mostly small particle size particles.?ngstr?m exponent of black carbon aerosol is larger than that of dust aerosol,which is consistent with the effective radius of the aerosol.The scattering of dust aerosol is stronger than that of black carbon,meanwhile the absorption of black carbon aerosol is stronger than that of dust,and dust and black carbon aerosol have the strongest absorption at 440 nm.Forward scattering characteristic of dust aerosol is stronger than black carbon.(2)Spring has the highest(2.80)dust aerosol optical depth(AOD550),which is mainly distributed from 0.3 to 0.8(about 82%),with a daily average of 0.63±0.44,indicating that the largest number of dust days occur in spring.?ngstr?m exponent with maximum and minimum in spring(0.002)and autumn(0.525),which is opposite in phase to AOD550,ranges between 0.2-0.4(about 56%)and has a daily average of 0.27±0.12,showing that dust aerosols are mainly large particle size.The coarse mode dominates the size distribution of the dust aerosol,and the spring concentration is the largest.Both total and coarse mode volume concentrations have similar annual variation to AOD550 and reach their maximum in April with the fine mode volume concentration changing very little.The aerosol total and coarse mode effective radius ranges from its maximum in spring to its minimum in autumn,and the amplitude of the fine mode particles is small.The real part of the refractive index with the change of the effective radius is higher in spring and winter while it's lower in summer and autumn,which indicates that dust aerosol has strong scattering.In opposite phase to imaginary part of refraction index,the single scattering albedo is larger in spring,but has a minimum value in February,so dust aerosol absorption weaker than black carbon aerosol.The asymmetry factor reaches its minimum in spring,which indicates that dust aerosol has stronger forward scattering than other kinds of aerosols.(3)After the spring dust event occurs,depolarization rate increases above 0.3 and optical depth increases sharply more than 1.The coarse mode peak reaches a maximum of 1.12 ?m3/?m2,while the fine mode concentration is still small,and the visibility is reduced obviously.The impact of spring dust event high-altitude key system is strong wind zone and strong front area.The afternoon surface intense warming for the occurrence of dust event to provide a favorable thermal condition.The wind is the main driving force for the formation of dust emission.Spring dust event is mainly affected by the remote transportation of dust,while the summer dust event is mainly affected by local dust particles.Spring dust event in the process of reducing the pressure less than the summer,the wind speed before the dust is also larger than the summer.The relative humidity decreases with the occurrence of dust event in the spring,and the relative humidity continues to decrease after the end of dust.In summer,the relative humidity increases with the occurrence of dust event,and the relative humidity of the near-formation air in summer is much higher than that in spring.