Understanding spatio-temporal variability of aerosol composition based on MISR aerosol product over the Indian Subcontinent

Understanding the spatio-temporal heterogeneity of aerosol composition is critical in improving climate projection and health impact assessment of air pollution. Systematic data on aerosol composition is lacking over the Indian subcontinent. In this work, we processed 22 years (2001 to 2022) of Level 2 version 23 Multi-angle Imaging Spectro-Radiometer (MISR) aerosol products to derive information about aerosol composition over the Indian Subcontinent.

We broadly categorized aerosols into four types: Secondary Inorganic Aerosol (SIA), Absorbing (BC, OC), Sea- Salt and Dust. MISR aerosol retrieval algorithm assumes 74 aerosol mixtures considering 8 aerosol models based on size, shape and absorbing properties. We calculated the % retrieval frequency of all 74 aerosol mixtures over 22 years, month-wise, in a grid of 0.05 x 0.05. Utilizing this, we then mapped the monthly climatology of aerosol retrieval frequency of four broad types calculated summing over the frequency of a particular mixture multiplied by the fraction of aerosol models assumed in that mixture. We also investigated aerosol optical properties such as size and shape-segregated AODs, Angstrom exponent and SSA. The result shows a very high retrieval frequency for SIA (>50%), while a very low value (<10%) for absorbing aerosol particles almost throughout the year. The second most frequently retrieved aerosol type is sea salt, ranging between 25% to 40 %, but increasing to >50% during monsoon months. Dust aerosol’s retrieval frequency is very high (>50%) during the pre-monsoon and monsoon months over the oceans surrounding Southern India, while the value is much lower over the land. SIA retrieval frequency is >60% over the land and nearby oceanic regions in the winter season, which decreases to 40%-50% in the pre-monsoon season, increasing dust fractions from <10% to 15-25% over the land. Overall, particles with high AE and high small mode AODs dominate over India region, which supports the high fraction of small anthropogenic particles. The aerosol species AODs derived utilizing this information will help in understanding the differential impacts of aerosol species on the Radiation Budget.