PM2.5 Exchange Between Atmospheric Boundary Layer and Free Troposphere in North China Plain and Its Long‐Range Transport Effects

Abstract

AbstractThis study estimates long‐term PM2.5 exchange flux between the atmospheric boundary layer (ABL) and free troposphere (FT) in the North China Plain. The effect of PM2.5 long‐range transport (LRT) on air quality in downwind regions is also assessed. The PM2.5 exchange fluxes are calculated based on a mass budget method with data from WRF‐Chem simulations, during wintertime months (November, December, and January) in 2014, 2016, and 2017 (represent moderate, heavier, and less polluted years). The overall PM2.5 exchange exhibits clear topographical correlation and diurnal variation. The plain area is characterized by a considerable upward flux of −0.13 μg m−2 s−1 (in a 3‐year average), acting as an effective source for LRT. ABL height variation contributes significantly to the diurnal cycle of PM2.5 vertical exchange. Over the mountainside and ridge areas, the PM2.5 exchange flux is downward and its diurnal variation is weak, due to the dominant cross‐mountain descent flows. Four LRT patterns in the FT are identified, that is, eastward, southward, recirculation, and local stagnation. They can affect air quality in Northeast China, Yangtze River Delta, South Korea, and Japan. Indicating with averages and standard deviations, the PM2.5 concentration increases (relative contribution) for these four target regions over the 3‐year winter period are respectively 5.5 ± 4.8 μg m−3 (12.6 ± 10.9%), 4.6 ± 4.5 μg m−3 (8.4 ± 6.4%), 4.4 ± 3.9 μg m−3 (5.2 ± 4.4%), and 1.6 ± 0.9 μg m−3 (2.6 ± 1.6%). While the maximum impacts can be 36.7 μg m−3 (59%), 31.1 μg m−3 (41%), 25.7 μg m−3 (30%), and 7.2 μg m−3 (13%), respectively.