Evaluation of WRF-Chem simulations on vertical profiles of PM2.5 with UAV observations during a haze pollution event

Abstract

Accurate simulation of PM2.5 vertical distribution is crucial to understand the formation, transport, and dispersion mechanisms of aerosols, and protect the public from exposure to high levels of air pollutants. While most of the current model evaluations are limited to near-surface variables, vertical profiles of simulated PM2.5 are rarely evaluated. In this study, the measurements with unmanned aerial vehicle (UAV) are used to evaluate the WRF-Chem simulated vertical profiles of PM2.5, potential temperature, water vapor, and winds while surface meteorological fields and air pollutants are evaluated with various observational data during a heavy aerosol pollution episode that occurred on December 22–25, 2017 in Nanjing, China. Results show that the simulated surface meteorology and air quality agree well with the observations. Vertical profile shapes and magnitude of PM2.5 are reasonably captured by the model under the convective conditions during the daytime, but not in the nighttime. It is noticed that a well-mixed vertical structure of PM2.5 concentration, a typical feature of the daytime convective boundary layer is observed in the nocturnal boundary layer (NBL) but not captured by the WRF-Chem model and PM2.5 concentrations are over-predicted significantly during the nighttime. The over-predictions of surface PM2.5 and vertical profiles are improved substantially by increasing the minimum value of eddy diffusivity. This study indicates the importance of refinement of parameterization schemes in reproducing the ABL vertical structures and PM2.5 concentrations, especially for the NBL with heavy pollution conditions.