Fog scavenging of particulate matters in air pollution events: Observation and simulation in the Yangtze River Delta, China

Stable atmospheric boundary layer is conducive to the accumulation of atmospheric pollution and the occurrence of fog, and fog has a removal effect on air pollution. In this study, we use the observation and WRF-Chem (Weather Re-search and Forecasting Model with Chemistry) simulation to analyze the factors affecting the removal efficiency in a continuous fog and haze episode from November 26 to 28, 2018 in Jiangsu Province, such as fog thickness and dura-tion. The results show that the WRF-Chem simulation well reproduces the boundary layer characteristics in the stages of fog formation, development and dissipation. The atmospheric boundary layer provides favorable conditions for the maintenance of fog and air pollution. The inversion layer, with the maximum intensity of 3 degrees C per 100 m, creates fa-vorable thermal conditions, and the water vapor advection is also conducive to the fog maintenance. The ground ob-servation verifies the wet scavenging of PM2.5 during dense fog events. The scavenging effect is related to the fog duration, and the correlation is positive when the fog is just formed and negative when the fog is dissipating. The PM2.5 concentration decreases from 159 mu g m-3 to 38 mu g m-3 after the fog lasts for 11 h. The fog has a remarkable scavenging effect on PM2.5 in the vertical direction, due to the deposition effect of fog droplets on the pollutant parti-cles. The PM2.5 concentration on the ground is lower than the vertical average in the fog area, and the scavenging ef-fects during the dense fog periods on November 27 and 28 are 47.7 mu g m-3 and 36.1 mu g m-3, respectively. The fog duration is mostly concentrated in 3-17 h. When the duration of fog is 4-8 h, the scavenging effect on PM2.5 reaches the strongest, with an average PM2.5 concentration decrease of >70 mu g m-3.