PM vertical variation during a fog episode in a rural area of the Yangtze River Delta, China.

A dense radiation fog event occurred at the Shouxian site, Anhui Province, China, from the evening of January 2 to noon on January 3, 2017. During this event, vertical profiles of particulate matter (PM) and meteorological parameters within the lower troposphere (0-1000 m) were collected using a tethered balloon. This study assessed the evolution of the PM profile with the planetary boundary layer (PBL) structure and the effects of fog on the PM concentration. The results showed the following: (1) At the surface, the average diurnal variation in Aitken mode, accumulation mode and coarse mode particles had bimodal patterns before fog formation and was mainly influenced by diurnal variation in the mixing level depth (MLD). The aerosol number concentrations decreased remarkably, and the PM was strongly scavenged from 150 μg/m to 45 μg/m during the fog process. (2) In the vertical direction, the PM distribution was affected by the PBL height and the vertical fog structure. At 05:00 LT (local time) (i.e., early morning before the fog event), the PM concentration was slightly higher in the stable layer (260 μg/m) than in the residual layer (200 μg/m). At 14:00 LT (haze period), PM was well mixed below 500 m, with a concentration of 310 μg/m. After 20:00 LT, when fog formed, PM was scavenged from the surface to the upper layers, and the scavenging height was controlled by the fog top height. (3) The vertical development of fog was promoted by turbulent mixing and radiation cooling at the fog top. Turbulent mixing enhanced the particle scavenging efficiency of fog droplets by the collision-coalescence process. The PM scavenging height was corresponded to the turbulence height. Therefore, turbulence development in the fog was the essential dynamic factor driving PM reduction.