A climatology of aerosol optical and microphysical properties over the Indian subcontinent from 9 years (2000–2008) of Multiangle Imaging Spectroradiometer (MISR) data

Abstract

We present the first detailed analysis of a 9 year (2000–2008) seasonal climatology of size‐ and shape‐segregated aerosol optical depth (AOD) and Ångström exponent (AE) over the Indian subcontinent derived from the Multiangle Imaging Spectroradiometer (MISR). Our analysis is evaluated against in situ observations to better understand the error characteristics of and to corroborate much of the space‐time variability found within the MISR aerosol properties. The space‐time variability is discussed in terms of aerosol sources, meteorology, and topography. We introduce indices based on aerosol size‐ and shape‐segregated optical depth and their effect on AE that describe the relative seasonal change in anthropogenic and natural aerosols from the preceding season. Examples of major new findings include the following: (1) winter to premonsoon changes in aerosol properties are not just dominated by an increase in dust, as previously thought, but also by an increase in anthropogenic components, particularly in regions where biomass combustion is prevalent; (2) ∼15% of the AOD over the high wintertime pollution in the eastern Indo‐Gangetic basin is due to large dust particles, resulting in the lowest AE (<0.8) over India in this season and likely caused by rural activities (e.g., agriculture, etc.) from the densely populated rural area; (3) while AOD decreases from the Indo‐Gangetic basin up to the Tibetan Plateau, a large peak in AE and the fraction of AOD due to particle radii <0.7 μm exists in the foothills of the Himalayas, particularly in the premonsoon season; and (4) the AOD due to nonspherical particles exhibits a strong ocean‐to‐land gradient over all seasons because of topographical and meteorological controls.