28 Feb 2022
28 Feb 2022
Status: this preprint is currently under review for the journal ACP.

Impact of urban heat island on inorganic aerosol in the lower free troposphere: a case study in Hangzhou, China

Hanqing Kang1,2,3, Bin Zhu1,2, Gerrit de Leeuw1,4,5,6, Bu Yu7, Ronald J. van der A1,4, and Wen Lu1,2 Hanqing Kang et al.
  • 1Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing 210044, China
  • 2Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disaster, Nanjing University of Information Science and Technology, Nanjing 210044, China
  • 3Chongqing Institute of Meteorological Sciences, Chongqing 401147, China
  • 4KNMI (Royal Netherlands Meteorological Institute), R&D Satellite Observations, P.O.Box 201, 3730AE De Bilt, The Netherlands
  • 5Aerospace Information Research Institute, Chinese Academy of Sciences (AirCAS), Beijing 100101, China
  • 6University of Mining and Technology (CUMT), School of Environment Science and Spatial Informatics, Xuzhou 221116, China
  • 7Hangzhou Meteorological Bureau, Hangzhou 310051, China

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.

Hanqing Kang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-93', Anonymous Referee #1, 01 Apr 2022
  • RC2: 'Comment on acp-2022-93', Anonymous Referee #2, 19 May 2022

Hanqing Kang et al.

Hanqing Kang et al.


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Short summary
This study quantified the contribution of each urban-induced meteorological effect (temperature, humidity, and circulation) to aerosol concentration. We found that the urban heat island (UHI) circulation dominates the UHI effects on aerosol. The UHI circulation transports aerosol and its precursor gases from warmer lower boundary layer to colder lower free troposphere and promotes the secondary formation of ammonium nitrate aerosol in the cold atmosphere.