The role of aerosol layer height in quantifying aerosol absorption from ultraviolet satellite observations

Abstract. The purpose of this study is to demonstrate that S-5P/TROPOMI aerosol layer height (ALH) observations can be used to improve the single scattering albedo (SSA) retrieval from ultraviolet satellite observations. We take the Thomas Fire in southern California on 12 December 2017 as a case study. In the first part of this paper, we apply conventional radiative transfer simulations to retrieve the SSA. With forward simulations constrained by TROPOMI ALH, we can determine the uncertainty in SSA due to the assumed spectral dependence of refractive indices between two wavelengths of the near-ultraviolet absorbing aerosol index (UVAI). A significant gap in the retrieved SSA (0.24) between radiative transfer simulations with grey and colored aerosols implies that inappropriate spectral dependences may cause severe misinterpretations of aerosol absorption. In the second part of this paper, we propose a data-driven method to quantify aerosol absorption from long-term measurements of UVAI, the aerosol optical depth (AOD) and ALH using support vector regression (SVR). We present the potential of TROPOMI ALH in this new method. The SVR predicted SSA (0.96 ± 0.01) outperforms that predicted by radiative transfer simulations (0.90 ± 0.05), considering the AERONET SSA measurement is 0.96 and assuming that the aerosol absorption should be homogeneous within the plume (i.e. small SSA standard deviation). We thus believe that the upcoming TROPOMI ALH product can make it feasible to quantify aerosol absorption via data-driven methods, which would play an important role in constructing a long-term global SSA data set.