Aerosol Optical Properties Inferred from Simultaneous Lidar, Aerosol-Counter, and Sunphotometer Measurements

摘要

Simultaneous measurements of aerosols by use of an airborne optical particle counter (OPC) and a ground-based lidar were performed during January, 1986, over the Tsukuba area. The measurements were taken over the middle and lower troposphere in order to obtain both spatial and size distributions, to infer an extinction-to-backscatter ratio (S1) and a complex index of refraction (m).Bimodal distributions were commonly found in the aerosol volume size distributions from the OPC measurements over the flight range (310-4270 m). Although the spectral shape of the accumulation mode exhibited a slight difference between altitudes, it was reasonable to use a constant S1 estimated from the aerosol size distribution near the surface in analyzing the lidar signal. This was true except for the cases when a heavy dust layer was found in the higher troposphere, which may have originated from a Kosa (Asian dust) event.Assuming spatial homogeneity of aerosol optical properties, the mean value of S1 and its range of variation was inferred from the lidar signal along with additional information on optical thickness. Moreover, the tropospheric aerosol refractive index was estimated from the relationship between S1 and m based on the OPC data.In this experiment, two heavy-dust layers were found, one aloft near a height of 5000 m and the other just above the ground surface. In order to determine the ranges of optical parameters for the aerosols in both layers, it was necessary to treat each level separately. Since no in situ data was available for the upper dust layer, S1 for the upper layer was assumed while the data was analyzed for the lower layer to estimate the range of S1 there.The mean extinction-to-backscatter ratio and the imaginary part of the complex refractive index for the total air column of the lower dust layer (less than 4300 m) were estimated respectively as ranging from 32 to 66, and less than 0.04, assuming the real part as 1.55±0.03.