Atmospheric Boundary Layer—Free Troposphere Air Exchange in the North China Plain and its Impact on PM2.5 Pollution

Abstract

AbstractThis study presents the first climatology of the atmospheric boundary layer (ABL)—free troposphere (FT) exchange during wintertime in a region covering the North China Plain (NCP) and further discusses its impact on PM2.5 pollution. The exchange flux is calculated based on a mass budget equation using the simulated meteorological field from 2013 to 2019. The mean ABL‐FT exchange is well related to topography and sea‐land distribution, manifesting as strong downward flux over the western/northwestern mountains, moderate exchange along the eastern coastline, and small upward flux over the NCP. Eight weather types are classified to analyze the synoptic influence on ABL‐FT exchange. Strong exchanges exist under the types of “low pressure trough,” “cold front,” and “high pressure” (about 63% of total days), characterized by the downward spatial mean flux of 0.019–0.026 kg m−2 s−1 over the whole research domain. But over the plain area, upward transport is dominant with the air flux from 0.004 to 0.013 kg m−2 s−1 under the first two weather types (about 32% of total days), corresponding to PM2.5 fluxes estimated in the range 0.30–0.54 μg m−2 s−1. This indicates the NCP being an effective pollution source for long‐range transport. We also find that weather types with large downward flux and straightforward winds can refresh the underneath ABL air in about 6 h, which explains the dispersion of air pollution. A typical pollution case illustrates convergence and uplift to the FT of the pollutants in the NCP and the possible influence on downstream air quality.