Reevaluating the black carbon in the Himalayan and Tibetan Plateau: concentration and deposition

Abstract. Black carbon (BC) is the second most important warming component in the atmosphere after CO2. The BC in the Himalayan and Tibetan Plateau (HTP) has shaped the evolution of the Indian Monsoon and accelerated the retreat of glaciers, thereby resulting in serious consequences for billions of Asian residents. Although a number of related studies of this region have been conducted, the BC concentration and deposition indexes remain poorly understood. Because of the presence of arid environments and the potential influence of carbonates from mineral dust (MD), the reported concentrations of BC from the HTP are overestimated. In addition, large discrepancies in the deposition of BC have been reported from lake cores, ice cores, snowpits and models. Therefore, the actual BC concentration and deposition values in this sensitive region must be determined. A comparison between the BC values of acid (HCl) fumigated and original aerosol samples from the HTP showed that the BC concentrations previously reported for the Namco station (central part of the HTP) and the Everest station (northern slope of the central Himalayas) were overestimated by approximately 47 ± 37 % and 35 ± 26 %, respectively, because of the influence of carbonates from MD. Additionally, the organic carbon (OC) levels were overestimated by roughly 22 ± 10 % and 22 ± 12 % for the same reason. Based on previously reported values from these two areas, we propose that the actual BC concentrations at the Namco and Everest stations are 44 ng m−3 and 164 ng m−3, respectively. Second, a comprehensive comparison of the BC deposition levels obtained via different methods indicated that the BC deposits derived from lake cores of the HTP were mainly caused by river sediments transported from the lake basin as a result of climate change (e.g., increases in temperature and precipitation), and fewer BC deposits were related to atmospheric deposition. Therefore, previously reported BC deposition levels from lake cores overestimated the atmospheric deposition of BC in the HTP. Correspondingly, BC deposition derived from snowpit, ice core and model from the HTP were not only agree very well with each other, but also were close to those of other remote areas (e.g., Arctic), implying that the BC deposits calculated from these three methods reflect the actual values. Therefore, based on reported values of snowpits and ice cores, we propose that the BC deposits of the HTP range from 10 mg m−2 a−1 to 25 mg m−2 a−1, with high and low values appearing along the fringes and central areas of the HTP, respectively. The adjusted BC concentration and deposition values in the HTP observed here are critical for performing accurate evaluations of other indexes of BC such as atmospheric distribution, radiative forcing and chemical transport in the HTP.