Seasonal variability in aerosol optical depth over India: a spatio-temporal analysis using the MODIS aerosol product

Abstract

Aerosols are one of the key components of climate systems. They absorb and scatter both solar and terrestrial radiation and produce strong surface as well as atmospheric radiative forcing effects. Aerosol climatology includes the measurement of light extinction by aerosol scattering and absorption, by procedures such as aerosol optical depth (AOD), angstrom exponent (α), single-scattering albedo (ω), and size distribution. This article analyses the dynamics of seasonal AOD over the Indian subcontinent from 2001 to 2009 using the MODIS level 2 data set. The analysis carried out for winter, pre-monsoon, monsoon, and post-monsoon seasons is based on 8 days’ composite AOD data for selected months representative of each season. The spatial variability of AOD has been shown to be 0.47 μm and 0.66 μm for fine- and coarse-mode aerosols, respectively, which illustrates the principle of relative difference. The dynamics of seasonally averaged AOD over the period under study represent an increasing tendency from 0.20 to 0.37 at 0.47 μm and from 0.16 to 0.26 at 0.66 μm during winter (2003–2009), whereas AOD in the pre-monsoon season ranged from 0.24 to 0.16 at 0.47 μm and from 0.24 to 0.16 at 0.66 μm (2005–2009). The monsoon season yielded an AOD of less than 0.15 throughout the study period, and the post-monsoon season recorded an increasing tendency from 0.18 to 0.29 at 0.47 μm and from 0.16 to 0.19 at 0.66 μm (2005–2009), reflecting a similar trend to that of the winter AOD curve. The spatial distribution of AOD shows that the northern part of India – especially the Indo-Gangetic plain – remains most affected by high AOD throughout the year. Such high AOD can be attributed to increasing anthropogenic emission due to an ever-increasing population, and urban, industrial, and other economic activities causing high concentrations of fine-mode organic and inorganic aerosol particles, along with coarse soil and mineral dust over the Indo-Gangetic plain.