FEASIBILITY OF USING MULTIWAVELENGTH LIDAR MEASUREMENTS TO MEASURE CLOUD CONDENSATION NUCLEI

This paper addresses the feasibility of using multiwavelength lidar me asurements to differentiate both qualitatively and quantitatively betw een the relative concentrations of hygroscopic and nonhygroscopic aero sol particles. The proposed technique utilizes the fact that hygroscop ic particles undergo a size increase and refractive-index change with increasing relative humidity and that different wavelengths respond to these changes in different ways. The lidar wavelengths considered are 0.289, 0.355, 0.532, 0.694, 1.064, and 2.02 mu m and the 9-11.5-mu m range. It is shown that under certain conditions, a judicious choice o f lidar wavelengths can provide a differential backscatter, sufficient to provide information on the size and percentage number Concentratio n of the hygroscopic aerosol and, consequently, cloud condensation nuc lei concentration. The presence of a mode of coarse particles (median radius greater than 0.3 mu m) produces ambiguous results and limits ap plication of the technique to regions sufficiently distant from coarse mode sources( e.g., in the free troposphere). The authors have identi fied a pair of wavelengths in the infrared region that provides a clea r indication of the existence of these particles. The potential benefi ts of distinguishing hygroscopic particle concentration from nonhygros copic particle concentration are great since remote measurement can pr ovide good temporal and spatial coverage of these properties and valua ble information for climate monitoring.