Feasibility of Using Multiwavelength Lidar Measurements to Measure Cloud Condensation Nuclei

Abstract

This paper addresses the feasibility of using mulliwavelength lidar measurements to differentiate both qualitatively and quantitatively between the relative concentrations of hygroscopic and nonhygroscopic aerosol particles. The proposed technique utilizes the fact that hygroscopic 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 µm and the 9?11.5-µm range. It is shown that under certain conditions, a judicious choice of lidar wavelengths can provide a differential backscatter, sufficient to provide information on the size and percentage number concentration of the hygroscopic aerosol and, consequently, cloud condensation nuclei concentration. The presence of a mode of coarse particles (median radius greater than 0.3 µm) produces ambiguous results and limits application of the technique to regions sufficiently distant from coarse mode sources (e.g., in the free troposphere). The authors have identified a pair of wavelengths in the infrared region that provides a clear indication of the existence of these particles. The potential benefits of distinguishing hygroscopic particle concentration from nonhygroscopic particle concentration are great since remote measurement can provide good temporal and spatial coverage of these properties and valuable information for climate monitoring.