Columnar aerosol optical depth, water vapor and ozone over a semi-arid urban location of western India: Potential sources and direct radiative effects

Abstract

Continuous measurements of Aerosol Optical Depth (AOD), columnar water vapor (WV) and Ozone (O3) concentrations were made over a semi-arid urban location Rajkot (22.29°N, 70.74°E, and 142 m AMSL) in the western India. The annual mean values of AOD, WV and O3 were 0.37 ± 0.15, 1.44 ± 0.6 gm cm−2 and 243 ± 18 DU during 2016–2019. AOD showed large seasonal variability mostly dominated by abundant coarse mode aerosols (Angstrom Exponent, AE < 0.65 and AOD ∼ 0.37) during pre-monsoon and monsoon, mainly caused by wind-blown and transported dust from nearby Thar and Kutch desert. Mixture of fine and coarse mode aerosols were abundant (AE = 1.13 ± 0.26) during post-monsoon influenced by both anthropogenic and natural sources. Columnar WV were also higher in pre-monsoon and monsoon seasons. The O3 concentrations is higher during pre-monsoon (247 DU) and lower during monsoon (230 DU). The vertical distribution of aerosol extinction coefficient derived from satellite (CALIPSO) based observations revealed the dominance of dust aerosols (∼74 %) in pre-monsoon; smoke and polluted dust in winter and post monsoon period. The trajectory clusters and concentration weighted trajectory maps also indicated the combined effect of anthropogenic (especially from the northern part of India) and natural aerosol sources during the post-monsoon period. Associated with the higher anthropogenic components during post-monsoon season, aerosol direct radiative forcing depicted surface dimming as high as − 48.7 ± 12.6 Wm−2, resulting in atmospheric forcing of 42.6 ± 12.5 Wm−2.