Multi-Year CALIPSO Observations of Ubiquitous Elevated Aerosol Layer in the Free Troposphere Over South Asia: Sources and Formation Mechanism

The vertical distribution of aerosols in the lower troposphere is critically important for assessing their impact on Earth's radiation budget and modulation of cloud microphysics. This study analyzed cloud-free aerosol extinction coefficient (beta(ext)), aerosol subtypes, and particulate depolarization ratios obtained from CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) over the six regions of India during 2008-2018. We investigated unprecedented climatology of the physical and optical characteristics of elevated aerosol layers (EALs) along with their source and formation mechanism. The key findings include: (a) EALs over the Indian region were persistent between 4 and 6 km during all seasons, (b) geometrical layer thickness of EALs increased up to 36.7% and 25% from the annual mean during summer and fall seasons, respectively, compared to that of spring and winter, (c) dust and polluted dust accounted for up to 50%-80% from near-surface to 6 km and up to 80%-90% of the EALs between 4 and 6 km, respectively for all the seasons and regions, (d) we anticipated that locally confined recirculation coupled with stratified stable layer capped within turbulent layers could be a possible mechanism of formation of stratified EALs between 4 and 6 km during winter-spring-fall, while in summer, vertical transport of pollutants from the PBL to mid-troposphere due to enhanced deep convection served as a key formation mechanism of the EALs, (e) the second Modern-Era Retrospective analysis for Research and Applications Global Modeling Initiative model reasonably simulated the shape and vertical gradient of beta(ext) with significant differences in magnitude below 4 km; however, it fails to reproduce EALs for all seasons and regions during the study period. Plain Language Summary Aerosols distributed in the troposphere can scatter and absorb solar radiation and modify the Earth's radiation budget and cloud properties. The combined effect of scattering and absorption by particles is defined as aerosol extinction coefficient, is a measure of the alteration of radiant energy as it passes through the atmosphere. The aerosol extinction coefficient was derived with the space-borne lidar Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) over the Indian region during 2008-2018. The vertical profiles of aerosol extinction coefficient depicted persistent layers between 4 and 6 km over the Indian region named elevated aerosol layers (EALs). In the summer and fall seasons, the thickness of the EALs increased by 36.7% and 25% from the annual mean, respectively. Throughout the year, dust and polluted dust aerosols accounted for up to 50%-80% of the EALs composition between near-surface and 6 km. Stratified EALs between 4 and 6 km were formed by locally confined wind recirculation coupled with stable atmospheric layers during the winter, spring, and fall seasons. In summer, the vertical transport of pollutants from the planetary boundary layer to the mid-troposphere associated with enhanced deep convection played a crucial role in the formation of EALs.