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

Abstract

Atmospheric 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.

Two Typical Patterns of Regional PM2.5 Transport for Heavy Air Pollution Over Central China: Rapid Transit Transport and Stationary Accumulation Transport

“…Strong northerly winds influenced by the East Asian winter monsoons transport high concentrations of air pollutants downstream from the upper reaches of Hebei and Henan, resulting in heavy pollution incidents in the THB (Bai et al, 2021;Hu et al, 2021;Shen et al, 2021). Governed by non-stagnant conditions with strong near-surface winds, unstable atmospheric structures, and non-temperature inversions, severe PM 2.5 pollution episodes in the THB have been aggravated by regional transport (Yu et al, 2020;Zhou et al, 2022). Xiangyang (XY), Jingmen (JM), and Jingzhou (JZ) (Figure 1B), which are in the northwest of THB, are characterized by flat terrain adjoining the eastern slope of the Daba and Wushan Mountains, and the routes of the three cities have been identified as typical regional transport corridors in the THB (Huang et al, 2020).…”

Application of stable index and transport index for regional air pollution over Twain-Hu Basin

“…The average and maximum values of the boundary layer height on the regional pollution day under the influence of P2 type are significantly higher than those under other weather types. For example, the average height of the boundary layer in Xiangy, Jingm and Jingz is about 400 m, while that in the other five stations is about 380 m, and the maximum value basically exceeds 900 m. The boundary layer heights of the other three weather types are similar, with an average height of about 280 m and a maximum value of less than 600 m. The low height of the boundary layer and the weak atmospheric diffusion capacity are conducive to the accumulation of pollutants, which are typical characteristics of stagnant air pollution, while the typical transmission air pollution has a high boundary layer height (Bai et al, 2018;Zhou et al, 2022).…”