Impacts of urbanization on atmospheric circulation and aerosol transport in a coastal environment simulated by the WRF-Chem coupled with urban canopy model

Abstract

This study examines the impacts of urbanization on the local atmospheric circulation and the dispersion of air pollutants over the populated city of Ulsan, South Korea, located in the coastal region. Two experiments are conducted using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and the urban canopy model (UCM) at 1-km horizontal resolution. The model experiments are conducted in August for three consecutive years of 2016–2018, with an updated land use category and diurnally-varying surface emission and anthropogenic heat flux. The impacts of urbanization are isolated by comparing the control run (CTRL) with realistic land use conditions and the sensitivity run (NOURB) that replaces the urban surfaces with non-urban grasslands. CTRL reproduces the urban heat island (UHI) and associated wind patterns realistically with reliable local land-sea breeze circulations. The positive temperature anomalies develop over the urban area due to enhanced surface heat flux at the urban surfaces, driving low-level convergence and secondary circulation. Enhanced heating by UHI changes the ground-level aerosol concentration differently. While the concentration does not change significantly by UHI in the daytime, it is reduced considerably at night in the urban areas due to enhanced vertical mixing. The dominant process that the urbanization modifies the aerosol concentration is the thermodynamical effects, which are also supported by the observation.