title: Temporal variation of aerosol optical depth and associated shortwave radiative forcing over a coastal site along the west coast of India.
abstract: Optical characterization of aerosol was performed by assessing the columnar aerosol optical depth (AOD) and angstrom wavelength exponent (α) using data from the Microtops II Sunphotometer. The data were collected on cloud free days over Goa, a coastal site along the west coast of India, from January to December 2008. Along with the composite aerosol, the black carbon (BC) mass concentration from the Aethalometer was also analyzed. The AOD0.500 μm and angstrom wavelength exponent (α) were in the range of 0.26 to 0.7 and 0.52 to 1.33, respectively, indicative of a significant seasonal shift in aerosol characteristics during the study period. The monthly mean AOD0.500 μm exhibited a bi-modal distribution, with a primary peak in April (0.7) and a secondary peak in October (0.54), whereas the minimum of 0.26 was observed in May. The monthly mean BC mass concentration varied between 0.31 μg/m(3) and 4.5 μg/m(3), and the single scattering albedo (SSA), estimated using the OPAC model, ranged from 0.87 to 0.97. Modeled aerosol optical properties were used to estimate the direct aerosol shortwave radiative forcing (DASRF) in the wavelength range 0.25 μm4.0 μm. The monthly mean forcing at the surface, at the top of the atmosphere (TOA) and in the atmosphere varied between -14.1 Wm(-2) and -35.6 Wm(-2), -6.7 Wm(-2) and -13.4 Wm(-2) and 5.5 Wm(-2) to 22.5 Wm(-2), respectively. These results indicate that the annual SSA cycle in the atmosphere is regulated by BC (absorbing aerosol), resulting in a positive forcing; however, the surface forcing was governed by the natural aerosol scattering, which yielded a negative forcing. These two conditions neutralized, resulting in a negative forcing at the TOA that remains nearly constant throughout the year.
authors.affiliation: Department of Marine Sciences, Goa University, Taleigao Plateau, Panjim, Goa 403602, India. Electronic address: harilalm@gmail.com.
authors.forename: Harilal B
authors.initials: HB
authors.lastname: Menon
authors.forename: Shilpa
authors.initials: S
authors.lastname: Shirodkar
authors.forename: Sumita
authors.initials: S
authors.lastname: Kedia
authors.forename: Ramachandran
authors.initials: R
authors.lastname: S
authors.forename: Suresh
authors.initials: S
authors.lastname: Babu
authors.forename: K Krishna
authors.initials: KK
authors.lastname: Moorthy
mesh_terms: D000336:Aerosols / Q000032:analysis*; D001272:Atmosphere / Q000737:chemistry*; D057231:Climate Change; D007194:India; D008027:Light; D008962:Models, Theoretical; D010783:Photometry / Q000379:methods; D012542:Scattering, Radiation; D014887:Weather
chemical_list: D000336:Aerosols
keywords: Aerosol optical depth (AOD); Black carbon (BC); Direct aerosol shortwave radiative forcing (DASRF); Single scattering albedo (SSA)