RAMAN LIDAR MEASUREMENTS OF AEROSOL EXTINCTION AND BACKSCATTERING - 1- METHODS AND COMPARISONS

This paper examines the aerosol backscattering and extinction profiles measured at night by the NASA Goddard Space Flight Center Scanning Ra man Lidar (SRL) during the remote cloud sensing (RCS) intensive operat ions period (IOP) at the Department of Energy Atmospheric Radiation Me asurement (ARM) southern Great Plains (SGP) site in April 1994. These lidar data are used to derive aerosol profiles for altitudes between 0 .015 and 5 km. Since this lidar detects Raman scattering from nitrogen and oxygen molecules as well as the elastic scattering from molecules and aerosols, it measures both aerosol backscattering and extinction simultaneously, The aerosol extinction/backscattering ratio varied bet ween approximately 30 sr and 75 sr at 351 nm. Aerosol optical thicknes ses derived by integrating the lidar profiles of aerosol extinction me asured at night between 0.1 and 5 km are found to be about 10-40% lowe r than those measured by a Sun photometer during the day. This differe nce is attributed to the contribution by stratospheric aerosols not in cluded in the lidar estimates as well as to diurnal differences in aer osol properties and concentrations. Aerosol profiles close to the surf ace were acquired by pointing the lidar nearly horizontally. Measureme nts of aerosol scattering from a tower-mounted nephelometer are found to be 40% lower than lidar measurements of aerosol extinction over a w ide range of relative humidities even after accounting for the differe nce in wavelengths. The reasons for this difference are not clear but may be due to the inability of the nephelometer to accurately measure scattering by large particles.