Factors influencing the boundary layer height and their relationship with air quality in the Sichuan Basin, China.

We investigated the factors influencing the daily maximum boundary layer height (hmax) and their relationship with air quality in the Sichuan Basin, China. We analyzed the factors influencing hmax on cloudy and sunny days in winter using five years of observational data and a reanalysis dataset and investigated the relationship between hmax and air quality. The inversion layer in the lower troposphere has a critical impact on hmax on cloudy days. By contrast, the sensible heat flux and wind shear are the main influencing factors on sunny days, although the contribution of the sensible heat flux to hmax is less than that of the wind shear. This is because the turbulence is mainly affected by mechanical mixing induced by the topographic effect of the Tibetan Plateau to the west of the Sichuan Basin. The secondary circulation over the Sichuan Basin is weaker on cloudy days than on sunny days. These results are important for understanding the dispersion of air pollutants over the Sichuan Basin.