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 Research 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 duration. 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 °C per 100 m, creates favorable thermal conditions, and the water vapor advection is also conducive to the fog maintenance. The ground observation 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 µg m-3 to 38 µg 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 particles. The PM2.5 concentration on the ground is lower than the vertical average in the fog area, and the scavenging effects during the dense fog periods on November 27 and 28 are 47.7 µg m-3 and 36.1 µg 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 µg m-3.

[Trends of urban haze in Jiangsu Province China over the past 33 years].

Based on the surface meteorological data of Jiangsu Province during 1980-2012, climatic characteristics and trends of the weather phenomenon of haze were analyzed. The result indicates that during 1980-2012, the haze day increased, and the severe and moderate haze days significantly increased. In the northern and coastal cities, haze days had a significant increase. Haze is often appeared in the fall and winter, and is rare in the summer, it also occurs inland, and is less in the coast. The haze happens more frequently in June since the straw burning during.summer harvest time. The haze day was evenly distributed during the 80's, and increased during the 90's over southern and southwestern Jiangsu Province, middle and northern Jiangsu haze days had a growing after 2000, especially after 2010. The continuous, regional, and regional-continuous haze days were in an increasing trend. As the urban built-up area has been expanded each year, industrial emissions, coal consumption, and car ownership increase every year, resulting in regional temperature increase and relative humidity decrease, resulting in urban heat island and dry island effects. Thence the haze formation and maintenance conditions increased, which led to the increasing of haze days, and the continuous, regional and regional- continuous haze days were significantly increased.