Simulation of carbonaceous aerosols over the Third Pole and adjacent regions: distribution, transportation, deposition, and climatic effects

Carbonaceous aerosols including black carbon and organic carbon over the Third Pole regions are simulated using a regional climate model (RegCM4.3) coupled with a chemistry–aerosol module. Results show that the model can simulate well the climatology of the Third Pole region in monsoon and non-monsoon seasons, but the model shows a cold bias and an overestimation of precipitation over the Himalayas and the northern Tibetan Plateau. The model also performs reasonably well in terms of aerosol optical depth and near surface aerosol concentration when compared with satellite datasets and in situ observations. BC wet deposition in monsoon seasons is more (less) than that in non-monsoon seasons in the southern (northwestern) parts of the Third Pole region. Westerly winds prevail throughout the year and transport carbonaceous particles from central Asia to the northern Tibetan Plateau. In the monsoon period, aerosols can cross the Himalayas and can be transported to high altitudes by the southwesterly winds over South Asia. Dry deposition shows a topography-controlled distribution, with low fluxes within and high fluxes outside of the Tibetan Plateau. Mixed carbonaceous aerosols produce positive shortwave radiative forcing in the atmosphere and negative forcing at the surface. Shortwave forcing is with less magnitude over the Third Pole region. Longwave radiation forcing is negative over the Pamir Plateau and positive over the Tibetan Plateau during monsoon season. In non-monsoon season, longwave radiative forcing is negative in the Himalayas and southern parts of the Tibetan Plateau. Aerosols increase surface air temperatures by 0.1–0.5 °C over the Tibetan Plateau and decrease temperatures in South Asia during the monsoon season. In the non-monsoon period, temperatures decrease by 0.1–0.5 °C over the southern Tibetan Plateau. Spatial changes in temperature are consistent with the distribution of longwave radiative forcing, which indicates that aerosols’ longwave radiative forcing probably plays an important role in the climatic impact of aerosols over the Third Pole region.