Investigation of the uncertainties of simulated optical properties of brown carbon in two Asian sites using a modified bulk aerosol optical scheme of the Community Atmospheric Model version 5.3 (CAM5.3)

Recent studies have suggested that brown carbon (BrC), an absorbing component in organic aerosol (OA), has strong absorption in the near-ultraviolet wavelengths, and contributes to regional and global radiative forcing. However, the inclusion of BrC in global climate models leads to significant uncertainties in estimated radiative forcing, mainly attributed to uncertain BrC properties and relevant BrC parameters assigned in the model. In this study, we modified the bulk aerosol optical scheme (BAOS) in the Community Atmospheric Model version 5.3 (CAM5.3) by including BrC absorption and evaluated the performance of the modified BAOS by comparing the simulated aerosol absorption with two-year surface observational data in two Asian cities, Kanpur, India and Nanjing, China. The mean relative errors in the simulated total aerosol absorption (B_abs) and absorption Angstrom exponent (AAE) in modified BAOS are around 30% in Kanpur and even below 20% in Nanjing. Our results show that the inclusion of BrC remedies the underestimated total aerosol absorption by 20% and 14% on average at Kanpur and Nanjing, respectively, exhibiting a better agreement with ground-based observations of aerosol absorption at both sites. We also conducted a series of sensitivity experiments to quantify the uncertainties caused by varying parameters related to BrC. The model simulations suggest that the imaginary refractive index of BrC is the most significant factor contributing to the uncertainties in aerosol optical properties calculated in BAOS at the Kanpur site. While in the Nanjing site, both the particle size distribution and mixing state have dominant impacts on the calculated aerosol optical properties.