Remote Sensing Of The Atmospheric Dust And Mixed Dust Aerosols

Atmospheric aerosols affect the radiative budget of the earth-atmosphere system by direct interaction with solar radiation through scattering and absorption.Also,by acting as cloud condensation nuclei(CCN)or ice nuclei(IN),aerosols alter cloud formation,albedo,lifetime,andprecipitation efficiency,indirectly influencing weather and climate.Currently,the representation of the aerosol physical and chemisty processes by atmospheric numerical models is difficult,leading to the high uncertainty in estimation of the aerosol radiative forcing,which is one of the main limitation factors in the predication ability of the atmospheric numeric models.Research on aerosol optical properties and radiative effects is of great scientific significance and application importance.We first improved the denoising method of the lidar signal based on the empirical mode decomposition(EMD)by using lidar observations at SACOL.Next,we studied the aerosol nonsphericity at SACOL and characterized the aerosol aspect ratio based on depolarization lidar observations and the T-matrix code.Then we analyzed aerosol vertical distribution,optical properties and mixing by dust and anthropogenic aerosols over China by using observations from CALIOP(Cloud-Aerosol Lidar with Orthogonal Polarization)lidar on board the CALIPSO(Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations)satellite and the ground based aerosol observation network of AERONET(Aerosol Robotic Network).Finally,we classified AERONET observations into dust dominated,black carbon dominated and mixed-type dust aerosols to better understand the optical properties and radiative effects of the mixed-type dust aerosols.The main findings and conclusions are as follows:(1)Based on the physical significance of intrinsic mode functions(IMF)and the noise component removed from the EMD method,the decomposition and reconstruction processes of the lidar signal were analyzed in detail.Two objective parameters,typical range(TR)and low-frequency fraction(LEE)were suggested as the reference principles to decide how many high-frequency IMFs should be removed as noise.An automatic algorithm was designed for batch processing of the lidar signals.Statistics results showed that our method was more efficient in noise reduction than the wavelet method.(2)SACOL showed the most significant aerosol nonsphericity in spring,followed by winter.No apparent aerosol nonsphericity was found in summer and autumn.The column-averaged lidar depolarization ratios were 0.13,0.09,0.08 and 0.10 for the spring,summer,autumn and winter,respectively.The aerosol depolarization ratio decreased with increasing height and the most nonspherical aerosols were found within the atmospheric boundary layer.The aerosol depolarization ratio decreased significantly with increasing column precipitable water vapor in summer,when there was sufficient water vapor.The column precipitable water vapor accounted 80.8% of the variation in depolarization ratio.Aerosol aspect ratios were derived by combining the lidar observed depolarization ratios and T-matrix numerical computations.The derived aerosol aspect ratio was more relevant to that of the ambient aerosols when compared with the scanning electron microscope method,which provides aspect ratio for dry particles because of the limitations induced by the observation principle.(3)The seasonal variations in the aerosol vertical distributions were well captured by the CALIOP observations,although the CALIOP aerosol extinctions represented aslight underestimation when compared with the ground-based lidar results at SACOL.The dust-dominant regions,Taklimakan Desert and Tibetan Plateau,exhibit the highest depolarization ratios and the highest color ratios;whereas the anthropogenic pollution-dominated regions,North China Plain,Sichuan Basin and Yangtze River Delta,show the lowest depolarization ratios and the lowest color ratios.The spring North China Plain and the winter Loess Plateau show intermediate depolarization and color ratios because of the mixed natural dust and anthropogenic pollution particles.In the Pearl River Delta region,the depolarization ratios are similar to those of the other polluted regions,but the color ratios are slightly higher because of the existence of coarse mode marine aerosols.(4)Long-range transport of dust in the middle and higher troposphere during the spring season is clearly evident in the CALIOP observed aerosol extinction coefficient profiles.The seasonal variations in aerosol vertical distributions indicate efficient transport of aerosols from the atmospheric boundary layer to the free troposphere because of summertime convective mixing,but stable meteorological conditions trap more aerosols within the boundary layer in autumn and winter.More than 80% of the column aerosols are distributed within 1.5 km above the ground in winter,when aerosol extinction lapse rate reaches a maximum seasonal average in all the study regions except for the Tibetan Plateau.For the polluted regions(i.e.,the Sichuan Basin,North China Plain,Yangtze River Delta,and Pearl River Delta),the aerosol extinction lapse rates in the planetary boundary layer are higher than those of the less polluted regions(the Taklimakan Desert,Tibetan Plateau,Loess Plateau,and Northeast China Plain).(5)The sunphotometer observations at SACOL were classified into spherical,nonspherical and mixed-type aerosols based on aerosol nonsphericity.The mixed-type aerosols showed strong scattering and absorption at SACOL.The mixed-type aerosols exhibited the highest BOA cooling efficiency(-139.8-2Wm),11.2% and 14.3% higher than that of the spherical and nonspherical aerosols(-125.7 and-122.3-2Wm),respectively.They also exhibited the highest ATM heating efficiency(+79.9-2Wm),20.3% and 20.9% higher than that of the spherical and nonspherical aerosols(66.4 and 66.0-2Wm),respectively.The TOA radiative efficiency showed the weakest dependence on the aerosol types,at-59.3,-59.9,and-56.3-2Wm for the spherical,mixed-type,and nonspherical aerosols,respectively.(6)The global AERONET observations were classified into dust dominated,black carbon dominated and mixed-type dust aerosols based on the wavelength dependence of aerosol single scattering albedo(SSA).The East Asia mixed-type aerosols exhibited the highest absorption,leading to the highest surface cooling and atmospheric heating effects.As a result,the mixed-type aerosols can inhibit the development of atmospheric boundary layer,thus enhance the air pollution in this region.To better estimating the radiative balance of the earth-atmosphere system,it's very necessary to carefully characterize dust mixing in global climate models.