Evidence for control of black carbon and sulfate relative mass concentrations on composite aerosol radiative forcing: Case of a coastal urban area

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.