Contrasting aerosol characteristics and radiative forcing over Hyderabad, India due to seasonal mesoscale and synoptic-scale processes

Abstract

Regular measurements of spectral Aerosol Optical Depth (AOD) at ten wavelengths, obtained from multi-wavelength radiometer (MWR) under cloudless conditions in the outskirts of the tropical urban region of Hyderabad, India for the period January 2008 to December 2009, are examined. In general, high AOD with a coarse-mode abundance is seen during the pre-monsoon (March to May) and summer monsoon (June to September) with flat AOD spectra and low Ångström wavelength exponent (α), while in post-monsoon (October–November) and winter (December–February) seasons, fine-mode dominance and steep AOD spectra are the basic features. The aerosol columnar size distribution (CSD) retrieved from the spectral AOD using King's inversion showed bimodal size distributions for all the seasons, except for the monsoon, with an accumulation-mode radius at 0.12–0.25 µm and a coarse-mode one at 0.86–1.30 µm. On the other hand, the CSD during the monsoon follows the power law for fine mode and the unimodal distribution for coarse mode. The fine-mode aerosols during post-monsoon and winter appear to be associated with air masses from continental India, while the coarse-mode particles during pre-monsoon and monsoon with air masses originating from west Asia and western India. The single-scattering albedo (SSA) calculated using the OPAC model varied from 0.83 ± 0.05 in winter to 0.91 ± 0.01 during the monsoon, indicating significant absorption by aerosols due to larger black carbon mixing ratio in winter, whereas a significant contribution of sea-salt in the monsoon season leads to higher SSAs. Aerosol radiative forcing (ARF) calculated using SBDART shows pronounced monthly variability at the surface, top of atmosphere (TOA) and within the atmosphere due to large variations in AOD and SSA. In general, larger negative ARF values at the surface (−65 to −80 W m⁻²) and TOA (∼−17 W m⁻²) are observed during the pre-monsoon and early monsoon, while the atmospheric heating is higher (∼50–70 W m⁻²) during January-April resulting in heating rates of ∼1.6–2.0 K day⁻¹.