Scattering and absorption characteristics of aerosols at an urban megacity over IGB: Implications to radiative forcing

Abstract

Aerosol scattering and absorption characteristics were investigated at an urban megacity Delhi in the western Indo-Gangetic Basin (IGB) during the period from October 2011 to September 2012 using different in-situ measurements. The scattering coefficient (σsp at 550 nm) varied between 71 and 3014 Mm−1 (mean ~710 ± 615 Mm−1) during the entire study period, which was about ten times higher than the absorption coefficient (σabs at 550 nm ~67 ± 40 Mm−1). Seasonally, σsp and σabs were substantially higher during the winter/post-monsoon periods, which also gave rise to single scattering albedo (SSA) by ~5%. The magnitude of SSA (at 550 nm) varied between 0.81 and 0.94 (mean: 0.89 ± 0.05). Further, the magnitude of scattering Ångström exponent (SAE) and back-scattering Ångström exponent (BAE) showed a wide range from −1.20 to 1.57 and −1.13 to 0.87, respectively which suggests large variability in aerosol sizes and emission sources. Relatively higher aerosol backscatter fraction (b at 550 nm) during the monsoon (0.25 ± 0.10) suggests more inhomogeneous scattering, associated with the coarser dust particles. However, lower value of b during winter (0.13 ± 0.02) is associated with more isotropic scattering due to dominance of smaller size particles. This is further confirmed with the estimated asymmetry parameter (AP at 550 nm), which exhibits opposite trend with b. The aerosol optical parameters were used in a radiative transfer model to estimate aerosol radiative forcing. A mean radiative forcing of −61 ± 22 W m−2 (ranging from −111 to −40 W m−2) was observed at the surface and 42 ± 24 W m−2 (ranging from 18 to 87 W m−2) into the atmosphere, which can give rise to the mean atmospheric heating rate of 1.18 K day−1.