Classification of atmospheric aerosols over Urmia Lake based on lidar observations

Abstract. This study provides new observational evidence on the contribution of salt-dust plumes originating from the desiccated bed of Urmia Lake. The near-surface atmosphere over the lake bed was investigated using a scanning polarization lidar. Nighttime measurements at 532 nm were conducted in September 2022, with the instrument operating in azimuthal scan mode. Investigations show that the aerosol plumes above the lake contain both dust and salt particles. A modified two-step polarization-lidar photometer networking retrieval scheme was applied to lidar azimuthal scans to obtain backscatter ratios and mass concentrations of dust, salt-dust, and wet-salt aerosols. Plume regions were detected and isolated from their surroundings using a multi-scale layer detection algorithm. Averages of particle depolarization ratios, backscattering coefficients, and mass concentrations for each detected plume are retrieved to quantify the contributions of different particle types to the plume composition. The retrievals indicate that salty particles exhibit characteristically lower depolarization ratios but substantially higher backscattering than pure dust particles. The results demonstrate that even relatively low mass fractions of saline aerosols markedly enhance particle backscattering over the dried lake bed. Based on plume-averaged backscattering values, the detected aerosol plumes were classified as dust-dominant, salt-dominant, or mixed mode. Analysis of 64 individual plumes revealed that 47 % of them were salt-dominant, 25 % dust-dominant, and 28 % in mixed mode.