The distribution of positive and negative turbulent heat diffusivity under urban pollution conditions

<p>Turbulent diffusion efficiently transports momentum, heat, and matter and affects their transfers between the surface and the atmosphere. As an important parameter in describing turbulent diffusion, turbulent heat diffusivity K<sub>H</sub> has scarcely been studied in the context of frequent urban pollution in recent years. In this study, K<sub>H</sub> under urban pollution conditions was directly calculated based on the K-theory. We found an obvious diurnal variation in K<sub>H</sub> and its varying vertical distributions for each case and with time. Interestingly, the height of negative K<sub>H</sub> rises gradually after sunrise, peaks at noon, and falls near sunset. Negative K<sub>H</sub> is unusually significant at sunrise and sunset and approximately 140 m during most of the night. The magnitude and fluctuation in K<sub>H</sub> are smaller in the pollutant accumulation stage (CS) at all levels than in the pollutant transport stage (TS) and pollutant removal stage (RS). Turbulent diffusion may greatly affect PM<sub>2.5</sub> concentration at the CS because of the negative correlation between PM<sub>2.5</sub> concentration and the absolute value of K<sub>H</sub> at the CS accompanied by weak wind speed. The applicability of the K-theory is not very good during either day or at night. Note that these problems are inherent in K-theory when characterizing complex systems, such as turbulent diffusion, and require new frameworks or parameterization schemes. These findings may provide valuable insights for improving or establishing a new parameterization scheme for K<sub>H</sub> and promote the study of turbulent diffusion, air quality forecasting, and weather and climate modeling.</p>