Ubiquity of quasi-aerosol layers in the free troposphere over the Indian region: Results from multiyear satellite observations

Vertical distribution of aerosols and their composition in the lower troposphere is critically important for assessing the Earth’s radiation budget and their impact on monsoon circulation. We combine the extinction coefficient, particulate depolarization ratio obtained from CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) for period of 11 years (2008-2018) over the Indian region to provide an unprecedented climatological overview of the physical and optical characteristics of quasi-aerosol layers and their source and formation mechanism throughout its annual life cycle in the free troposphere. The key findings includes: i)The quasi aerosol layer over the Indian region are found to be persistent between 4-6 km during all seasons and occasionally reach above 6 km and exhibited strong seasonal and regional dependency, ii) Layer thickness varies between 2.0 -3.0 km corresponds to primary peak are more frequent of about 80-90 % of cases over all six regions and while  secondary layer occasionally forms (10-20 %), iii) The aerosol layer thickness increases by about 36.7 and 25% during summer and fall season compared to that of spring, and winter, iv) Layer-AOT showed year-to-year variations of up to a factor of two with a relative variability of about 15-23% (1σ), v) Trend in layer AOT is not very conspicuous and showed oscillatory pattern, vi) Depolarization ratios generally increase with height suggesting that the irregularity of aerosol shape increases with altitude, vii) The polluted dust and smoke are the major aerosol components of the observed quasi aerosol layer  between 4 to 6 km for spring and fall season while these are the polluted dust during winter and summer.