Roles of Atmospheric Turbulence and Stratification in a Regional Pollution Transport Event in the Middle Reaches of the Yangtze River

Abstract

AbstractAtmospheric turbulence and the planetary boundary layer (PBL) can significantly modulate the fine particulate matter (PM2.5) concentration near the ground surface. However, the mechanism by which they affect the variation in PM2.5 is poorly understood. Using surface‐based meteorological and PM2.5 observations, turbulence measurements, and high‐resolution soundings, this study analyzed the properties of turbulence and PBL stratification during a severe regional pollution transport event in Xiangyang, a city in the middle reaches of the Yangtze River, from January 3 to January 9, 2019. During the growth stage, PM2.5 was observed to increase continuously under a northerly wind of 2–4 m s−1 that brought a polluted air mass to central China. During the entire pollution event, the turbulent kinetic energy and sensible heat flux tended to be lower at high PM2.5 concentrations. This correlation varied greatly depending on the stage of the event. A negative correlation existed in the initial and maintenance stages, whereas the relationship was positive during the growth stage, when regional transport dominated the variation in PM2.5. There was no obvious inversion layer below 500 m during the entire pollution episode, and a weak inversion layer appeared at higher altitudes in the initial and maintenance stages. Vertically homogeneous northerly winds favored the transboundary transport of pollutants, causing a significant increase in ground‐level PM2.5 concentration. By contrast, the presence of wind shear in the prevailing northerly winds suppressed pollutant transport to the ground, resulting in a low‐value gap in PM2.5 concentration between high altitudes and the ground surface.