[1] Collocated measurements on optical and chemical properties made at a coastal urban location Visakhapatnam on the east coast of India were used to assess the relative contribution of different chemical species to composite aerosol radiative forcing. At such a location, the dominant species that decide the atmospheric forcing are the relative mass fractions of Black Carbon (BC) and sulfate. It is observed that the composite forcing at top of the atmosphere follows the BC mass concentration during all the seasons except for some days. In such cases the hypothesis on the role of mixing state of aerosol in deciding the net aerosol radiative forcing is examined to conclude that the BC either independently or in the internal mixture state during winter months would decide the aerosol composite forcing over this coastal urban location. Though the conditions for the formation of such mixtures and their seasonal dependence however remain unclear, drier weather conditions with abundance of sulfate seem to favor the formation of well mixed aerosol.Citation: Niranjan, K., T. Anjana Devi, B. Spandana, V. Sreekanth, and B. L. Madhavan (2012), Evidence for control of black carbon and sulfate relative mass concentrations on composite aerosol radiative forcing: Case of a coastal urban area, J. Geophys.

Abstract. Information on the aerosol intensive properties like Single Scattering Albedo (SSA) and asymmetry parameter are very limited, particularly over the peninsular India, though extensive reports are available on the aerosol bulk properties. In view of the importance of these parameters in evaluating the aerosol radiative forcing, we present for the first time the temporal variation in SSA with measurements on aerosol absorption and scattering coefficients over Visakhapatnam (17.72 • N, 83.32 • E; located on the east coast of India) for the year 2007. The inferred SSA was in the range of 0.65 and 0.9 with an annual mean of 0.76 ± 0.013 and with a probable value of 0.80, indicating a marginal atmospheric warming over the region. The mixed layer contribution to column Aerosol Optical depth is found to be 35 % in summer while it is well above 35 % in winter, indicating the confinement of aerosol within the boundary layer during winter. The asymmetry parameter which represents the angular scattering in radiative forcing estimation is found to be around 0.65 ± 0.1 for the location. The day to day variability in SSA is found to be well correlated with the variations in surface BC mass concentrations and/or the relative dominance of the fine/coarse mode aerosol. The results are discussed in light of the aerosol physical and optical properties and the asymmetry parameter.

Realizing the importance of aerosol physical properties at the adjoining continental and coastal locations in the airmass pathways onto the oceanic region, extensive measurements of aerosol physical properties were made at Visakhapatnam (17.7 • N, 83.3 • E), an eastern coastal location in peninsular India during the ICARB period. The temporal variations of aerosol optical depth, near surface aerosol mass size distributions and BC mass concentrations show significantly higher aerosol optical depth and near surface mass concentrations during the first and last weeks of April 2007. The mean BC mass fraction in the fine mode aerosol was around 11%. The aerosol back scatter profiles derived from Micro Pulse Lidar indicate a clear airmass subsidence on the days with higher aerosol optical depths and near surface mass fraction. A comparison of the temporal variation of the aerosol properties at Visakhapatnam with the MODIS derived aerosol optical depth along the cruise locations indicates a resemblance in the temporal variation suggesting that the aerosol transport from the eastern coastal regions of peninsular India could significantly affect the aerosol optical properties at the near coastal oceanic regions and that the affect significantly reduced at the farther regions.

No abstract