The remote sensing of radiative forcing by light-absorbing particles (LAPs) in seasonal snow over northeastern China

Light-absorbing particles (LAPs) deposited on snow can decrease snow albedo and affect climate through snow-albedo radiative forcing. In this study, we use MODIS observations combined with a snow-albedo model (SNICAR – Snow, Ice, and Aerosol Radiative) and a radiative transfer model (SBDART – Santa Barbara DISORT Atmospheric Radiative Transfer) to retrieve the instantaneous spectrally integrated radiative forcing at the surface by LAPs in snow (${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$) under clear-sky conditions at the time of MODIS Aqua overpass across northeastern China (NEC) in January–February from 2003 to 2017. ${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$ presents distinct spatial variability, with the minimum (22.3 W m⁻²) in western NEC and the maximum (64.6 W m⁻²) near industrial areas in central NEC. The regional mean ${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$ is $\sim \mathrm{45.1}\pm \mathrm{6.8}$ W m⁻² in NEC. The positive (negative) uncertainties of retrieved ${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$ due to atmospheric correction range from 14 % to 57 % (−14 % to −47 %), and the uncertainty value basically decreases with the increased ${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$. We attribute the variations of radiative forcing based on remote sensing and find that the spatial variance of ${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$ in NEC is 74.6 % due to LAPs and 21.2 % and 4.2 % due to snow grain size and solar zenith angle. Furthermore, based on multiple linear regression, the BC dry and wet deposition and snowfall could explain 84 % of the spatial variance of LAP contents, which confirms the reasonability of the spatial patterns of retrieved ${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$ in NEC. We validate ${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$ using in situ radiative forcing estimates. We find that the biases in ${\mathrm{RF}}_{\mathrm{MODIS}}^{\mathrm{LAPs}}$ are negatively correlated with LAP concentrations and range from ∼5 % to ∼350 % in NEC.