Seasonal Transport and Dry Deposition of Black Carbon Aerosol in the Southeastern Tibetan Plateau

Abstract

To investigate the regional transport and dry deposition of black carbon (BC) aerosol in the southeastern Tibetan Plateau, continuous equivalent BC (eBC) mass concentrations were measured at a high-altitude remote site of Lulang from July 2008 to July 2009. The bivariate polar plots for eBC mass concentrations showed that large eBC values were often associated with low winds (< 2 m s⁻¹) during the pre-monsoon, post-monsoon, and winter seasons. Moreover, strong winds (> 4 m s⁻¹) from southeast or northeast also contribute to the large eBC loadings during the pre-monsoon, monsoon, and post-monsoon seasons. The concentration-weighted trajectory analysis showed that emissions from the eastern Kingdom of Bhutan, Assam of India, and southern Shannan Prefecture of Tibet had the most important contributions to the eBC pollution at Lulang during the pre-monsoon and monsoon seasons. In contrast, the eBC potential source region shifted to the east and southeast of Lulang during the post-monsoon and to the north India and northwest Nepal during the winter. The estimated eBC deposition rate was the highest for the pre-monsoon (6.3–62.6 μg eBC m⁻² day⁻¹), followed by the post-monsoon (4.6–45.9 μg eBC m⁻² day⁻¹), winter (4.3–43.1 μg eBC m⁻² day⁻¹), and monsoon (2.4–24.5 μg eBC m⁻² day⁻¹). Further calculations of eBC concentrations in the snow surface were 33.3–333.2, 61.5–614.7, 27.0–269.9, and 58.8–587.6 μg kg⁻¹ during the pre-monsoon, monsoon, post-monsoon, and winter seasons, respectively, which resulted in the snow albedos being reduced by 2.6–25.3, 4.7–46.6, 2.1–20.5, and 4.5–44.5% accordingly.