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

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_H has scarcely been studied in the context of frequent urban pollution in recent years. In this study, K_H under urban pollution conditions was directly calculated based on the K-theory. We found an obvious diurnal variation in K_H and its varying vertical distributions for each case and with time. Interestingly, the height of negative K_H rises gradually after sunrise, peaks at noon, and falls near sunset. Negative K_H is unusually significant at sunrise and sunset and approximately 140 m during most of the night. The magnitude and fluctuation in K_H 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_2.5 concentration at the CS because of the negative correlation between PM_2.5 concentration and the absolute value of K_H 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_H and promote the study of turbulent diffusion, air quality forecasting, and weather and climate modeling.