Seasonal transition of Black carbon aerosols over Qinghai-Tibet Plateau: Simulations with WRF-Chem

Black carbon (BC) particles over the Qinghai-Tibet Plateau (Q-TP), whether suspended in the air or deposited on snow, can affect snow melt and accelerate glacier retreat. Since BC over the Q-TP may originate from different regions, it is necessary to understand the transport mechanism of BC and then provide concentrations and spatial-temporal distributions of BC over Q-TP, in order to study its radiative and climatic effects. In this study, Weather Research and Forecasting model with Chemistry (WRF-Chem), with MIX emission inventory (a mosaic Asian anthropogenic emission inventory for the MICS-Asia and the HTAP projects), was used to study the BC transport from South Asia, one of the major BC source regions, to Q-TP in April (pre-monsoon season), July (summer monsoon season) and December (winter monsoon season). The model's ability to simulate the BC transport from South Asia to the Q-TP was firstly evaluate using various reanalysis data, including 2 m temperature, precipitation, 10 m wind speed and direction and BC concentration at surface. The simulated meteorological fields and seasonal distribution and variation of BC are generally in good agreement with reanalysis data. The model showed good performance in simulating transport of BC as well. Our study showed that during pre-monsoon season, BC aerosols accumulated in the foothills of the Himalayas and were transported to southern Q-TP and even Southeast Asia via dry and wet deposition and westerly winds below 200hpa. Although the cyclonic airflow, which was favorable to transport, prevailed in South Asia, slight BC aerosols would be transported to Q-TP through wet deposition due to the efficient wet scavenging by abundant precipitation during summer monsoon season. During winter monsoon season, the prevailing westerly winds at 500hpa would transport BC aerosols from South Asia to Q-TP by dry deposition. A sensitivity simulation using PKU emission inventory (Peking University's global BC emission inventory) was then conducted to investigate the impact of different emission inventory on the modeling results. It is shown that the simulations using the different BC emission inventories overall did not have significant impacts on spatial distributions of surface concentration and column burden during three studies seasons. However, the simulated BC concentrations and burden based on PKU were overall higher than that based on MIX, that is, main differences were found in the southern part of Q-TP (i.e. adjacent to South Asia), especially during pre-monsoon and winter monsoon seasons. The differences of mean BC surface concentration ranging from 0.1 to 1 μg/m3, and burden ranging from 0.1 to 1 mg/m2 due to both higher dry and wet deposition (0.1–2 μg/m2/s for dry deposition flux, and 0.1–3 μg/m2/s for wet deposition flux) during pre-monsoon. During winter monsoon seasons, relatively large differences (0–3 μg/m3 for concentration, and 0–2 mg/m2 for burden, and 0.1–3 μg/m2/s for dry deposition flux) were found in southern Q-TP.