Mineral dust aerosols over the Himalayas from polarization-resolved satellite lidar observations

Abstract

Mineral dust aerosols over the Himalayas are assessed using polarization-resolved observations of Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite over 11 years (2006–2018). The extinction coefficient due to dust aerosols is retrieved using observations of the depolarization ratio which gives the relative contribution of dust aerosols in the scattering volume. Dust extinction coefficients show significant regional and seasonal variation over the Himalayas. High dust loading is observed during the pre-monsoon season (March–May) whereas dust loading is low during the summer monsoon season (June–September). This is due to the reduced dust transport associated with the weak westerlies that prevailed over the Himalayas. Regionally, the mid-Himalayas is characterized by the highest dust extinction coefficient with a 10-fold increase as the season changes from winter (December–February) to pre-monsoon (March–May). Polluted dust (dust combined with anthropogenic aerosols) contributes to 64–74% of total aerosols over the Himalayas. Dry deposition causes a substantial amount of dust aerosols (1–31 mg m−2 day−1) to be deposited over the Himalayas, reducing the albedo by 0.3% on fresh snow and up to 2.7% on aged snow, causing a radiative forcing of 0.38–23.7 Wm-2 at the top of the atmosphere. The Himalayan cryosphere may therefore experience large warming leading to snow melting and enhanced reduction in snow cover.