The Spatio-Temporal Variability in the Radiative Forcing of Light-Absorbing Particles in Snow of 2003-2018 over the Northern Hemisphere from MODIS

Light-absorbing particles (LAPs) deposited on snow can significantly reduce surface albedo and contribute to positive radiative forcing. This study firstly estimated and attributed the spatio-temporal variability in the radiative forcing (RF) of LAPs in snow over the northern hemisphere during the snow-covered period 2003-2018 by employing Moderate Resolution Imaging Spectroradiometer (MODIS) data, coupled with snow and atmospheric radiative transfer modelling. In general, the RF for the northern hemisphere shows a large spatial variability over the whole snow-covered areas and periods, with the highest value (12.7 W m(-2)) in northeastern China (NEC) and the lowest (1.9 W m(-2)) in Greenland (GRL). The concentration of LAPs in snow is the dominant contributor to spatial variability in RF in spring (similar to 73%) while the joint spatial contributions of snow water equivalent (SWE) and solar irradiance (SI) are the most important (>50%) in winter. The average northern hemisphere RF gradually increases from 2.1 W m(-2) in December to 4.1 W m(-2) in May and the high-value area shifts gradually northwards from mid-altitude to high-latitude over the same period, which is primarily due to the seasonal variability of SI (similar to 58%). More interestingly, our data reveal a significant decrease in RF over high-latitude Eurasia (HEUA) of -0.04 W m(-2) a(-1) and northeastern China (NEC) of -0.14 W m(-2) a(-1) from 2003 to 2018. By employing a sensitivity test, we find the concurrent decline in the concentration of LAPs in snow accounted for the primary responsibility for the decrease in RF over these two areas, which is further confirmed by in situ observations.