ON THE RETRIEVAL OF URBAN AEROSOL MASS CONCENTRATION BY A 532 AND 1064 nm LIDAR

Based on the hypothesis of a monomodal, lognormal size distribution, the uncertainty affecting the humid-mass retrieval from LIDAR data was estimated by considering our ignorance of the distribution width to be a source of error. The mass to backscatter ratio and its uncertainty were computed for six accumulation-mode aerosol models as a function of the backscatter angstrom coefficient ((alpha)) and of the relative humidity (RH). A mass to backscatter uncertainty of less than ±30% was obtained for all six models. We computed the mass and simulated the expected LIDAR backscatter at 532 and 1064nm for a test data set of 14 "real-world" multimodal size distributions obtained from the literature. The possible presence of 0-20%-50% water-insoluble compounds in each aerosol mode was assumed. An urban-type accumulation mode and 10 different coarse mode compositions were considered, including dust-like aerosols. The aerosol mass concentration was derived by fitting the simulated LIDAR data at 532 and 1064nm with a monomodal distribution of urban aerosols of "unknown" width. The relative over- or underestimation of the mass with respect to the real aerosol mass was expressed in terms of (alpha)and RH for the 10 coarse aerosol types. The LIDARderived mass turned out to be underestimated by 0 - 15% in the case of (NH4)2SO4, NaCl, maritime, and H2SO4 coarse aerosols. In the case coarse dust aerosols, the range of underestimation was wider (0-30%). Absorbing aerosols showed a maximum underestimation of 40-50%.