Role of sulphate and carbonaceous aerosols on the radiative effects of aerosols over a remote high-altitude site Lachung in the Eastern Himalayas

Abstract

The rapid changes in the pattern of atmospheric warming as well as the degradation of glaciers in the Himalayas point to the inevitability of accurate source characterization and quantification of the impact of aerosols. In this regard, optical and chemical properties of aerosols, and their role in radiative effects are examined over a remote high-altitude site Lachung (27.4°N, 88.4°E, 2700 m a.s.l.) in the eastern Himalayas during August-2018 to February-2020. It is found that the sulphate (SO42−) and carbonaceous aerosols (both organic carbon - OC and elemental carbon - EC) significantly contribute to the total aerosol mass loading in winter (DJF) and spring (MAM), resulting in high values of scattering and absorption coefficients. Aerosol single scattering albedo (SSA) is relatively higher in winter (> 0.85) due to a significantly higher amount of OC (OC/EC > 8). However, SSA ~ 0.8 in spring despite of higher SO42− concentrations (SO42−/EC > 4.0 and SO42−/OC ~ 1.0) than winter. A reverse pattern is seen in summer-monsoon (JJAS) having lower SO42−/EC < 2 and SO42−/OC < 0.5, resulting in SSA as low as ~0.64. The seasonal values of aerosol direct radiative forcing in the top of the atmosphere (DRFTOA) are as high as −2.9 ± 1.2 Wm−2 during the period of abundant OC in winter and −2.8 ± 0.5 Wm−2 during the period of abundant SO42− in spring. The combined effect of carbonaceous and SO42− aerosols on the surface cooling is highest in spring (−16.7 ± 4.9 Wm−2). DRF in the atmosphere is also ~ 34% higher in spring (13.8 ± 4.5 Wm−2, which translates to an atmospheric heating rate of ~ 0.39 K day-1), than in winter. The seasonal pattern of forcing influenced by the heterogeneous sources and chemical composition of aerosols over the eastern Himalayan site is significantly influenced by the transport of aerosols from the Indo-Gangetic Plains of India.