Asymmetry parameters in the lower troposphere derived from aircraft measurements of aerosol scattering coefficients over tropical India

Abstract

Aerosol scattering coefficients (total βsca and backscatter βbacksca) are measured on board an aircraft using an integrating nephelometer at 450, 550, and 700 nm in the 0 to 3000‐m region over four locations in India in an air campaign held during March–May 2006. βsca is a factor of two higher in the east (Bhubaneshwar, Chennai) than in the west (Trivandrum, Goa). βsca is about 5–10 × 10−5 m−1 over Bhubaneshwar and Chennai. βbacksca is about an order of magnitude lower than βsca. Seven‐day air back trajectory analysis indicate that air masses originating from arid/semiraid regions, continents, and marine regions are found to influence the aerosol characteristics, in addition to local urban sources. No elevated aerosol layers are seen during the campaign. b, the aerosol backscatter fraction, is greater than 0.13 in the lower troposphere. The columnar mean Ångström exponent (α) is >1.75. Asymmetry parameter g profiles are derived for the first time over India in the lower troposphere, using the relation between b and g. 550‐nm g corresponding to 30% RH is in the 0.3–0.6 range over India. Higher b, higher α, and lower g values over these locations suggest the dominance of submicron aerosols during the campaign. Scattering aerosols corrected to 30% RH in the 0 to 3000‐m altitude region contribute about 20–35% to MODIS aerosol optical depths (AODs). The variation in the contribution of scattering aerosols to AODs highlights the spatial and vertical differences in aerosol properties.