Radiative forcing of black carbon in seasonal snow of wintertime based on remote sensing over Xinjiang, China

Abstract

Black carbon (BC), which consists of the strongest light-absorbing particles (LAPs) in snow, has been regarded as a potential factor accelerating regional climate change and the melting of snow cover globally. In this study, we used remote sensing (Moderate-resolution Imaging Spectroradiometer, MODIS) observations combined with a snow albedo model (Snow, Ice, and Aerosol Radiation, SNICAR) and a radiative transfer model (Santa Barbara DISORT Atmospheric Radiative Transfer, SBDART) to retrieve the radiative forcing (RF) by BC in snow (RMODIS BC) across Xinjiang, China, for the first time. The observations in January–February show that the concentrations of BC (equivalent BC) in snow ranged from 44.08 to 1949.9 ng g−1, with an average of 536.71 ng g−1. The lowest concentrations of BC were on the border of the Altay region (AR), with a median concentration in snow of 98.5 ng g−1. South of this area in the industrial region (Tianshan Mountain North Slope Economic Development Belt, TMNSEDB), the median concentration of BC in snow was 913.2 ng g−1 RMODIS BC presents distinct spatial variability, with the minimum (3.01W m−2) in the AR and the maximum (40.2W m−2) near industrial areas in TMNSEDB. The regional mean RMODIS BC was 20.43 ± 7.3 W m−2 in Xinjiang, and the average values of the impurity index (ILAPs) and SGS in the region were 0.273 and 241.38 μm, respectively. Moreover, based on the multiple linear regressions, the BC emission intensity values were significantly correlated with ILAPs and RF, and the correlation reached 0.681 and 0.661, respectively; thus, the BC emission could explain above 75% of the spatial variance of BC contents in TMNSEDB, confirming the reasonability of the spatial patterns of retrieved RFMODIS BC in Xinjiang. Additionally, we found that the distribution of RMODIS BC in northern Xinjiang is dominated by ILAPs and BC emissions. We validated RMODIS BC using in situ RF estimates (Rsite estimate), and the error was 24.05 W m−2; furthermore, the biases in RMODIS BC were negatively correlated with the BC concentrations and ranged from 24.3% to 326% in Xinjiang.