Reply on RC2

Abstract

Urban heat island (UHI) and urban air pollution are two major environmental problems faced by many metropolises. The UHI affects air pollution by changing the local circulation and the chemical reaction environment, e.g., air temperature and relative humidity. In this study, the WRF-CMAQ model was used to investigate the impact of an UHI on the vertical distribution of aerosol particles, especially secondary inorganic aerosol (SIA), taking the strong UHI in Hangzhou, China, as an example. Results show that due to the UHI effect, PM2.5 concentrations over Hangzhou decreased by about 26 % in the boundary layer (BL) but increased by about 21 % in the lower free troposphere (LFT). This is mostly attributed to the UHI circulation (~90 %) rather than changes in the air temperature (~5 %) and humidity (~4 %). The UHI circulation not only directly transports aerosol particles from ground level to the LFT, but also redistributes aerosol precursors. About 80 % of the increase of the aerosol particles in the LFT due to the UHI circulation effect is contributed by direct transport of aerosol particles, whereas the other 20 % is due to secondary aerosol formation resulting from the transport of aerosol precursor gases. Of this 20 %, secondary inorganic aerosol (SIA), especially nitrate and ammonium aerosol formed from ammonia and nitric acid, contributes 91 %. In the atmosphere, ammonium nitrate is in equilibrium with ammonia and nitric acid and the equilibrium depends on the ambient temperature. In the lower urban BL, the temperature is higher than in the LFT and the ammonium nitrate equilibrium in the lower BL is more toward the gas phase than in the LFT; when these gases are transported by the UHI circulation into the colder LFT, the equilibrium shifts to the aerosol phase. Hence, the UHI circulation changes the vertical distribution of SIA, which may have potential implications on the radiation budget, cloud formation, and precipitation in the urban and surrounding areas.