Absorbing Refractive Index and Direct Radiative Forcing of Atmospheric Brown Carbon over Gangetic Plain

Atmospheric carbonaceous aerosols consisting of black carbon and organic carbon influence Earth’s radiative balance by interacting with solar radiation. A subset of organic aerosols known as brown carbon is absorbing in nature and poorly characterized in terms of optical properties. Brown carbon can warm the local and regional atmosphere depending upon its absorbing capacity, mixing state, and meteorological conditions. We report a diurnal spectral absorbing refractive index of brown carbon over North India and its influence on regional radiative forcing. Measurements show the presence of highly absorbing brown carbon consisting of soluble and non-soluble fractions having distinct spectral absorption. The brown carbon refractive index at 365 nm shows a 50% reduction during daytime when compared to nighttime as a result of combined effects of reduced primary emissions and photobleaching/volatilization. Brown carbon and the lensing effect as a result of a thin absorbing coating exert a forcing of −0.93 ± 0.27 and 0.13 ± 0.06 W m–2, respectively, at the top of atmosphere. Externally mixed absorbing organic carbon in radiative forcing calculations produces 48% less cooling when compared to the forcing induced by scattering organic carbon. The presence of internally mixed absorbing organic carbon as a shell over black carbon induces 31% more warming compared to a similar shell made of scattering organic carbon. Overall results suggest that brown carbon and the lensing effect need to be included in global climate models while calculating radiative forcing parameters.