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

Abstract. Urban heat island (UHI) and urban air pollution are two major environmental problems faced by many metropolises. The UHI affects changing the local circulation chemical reaction environment, e.g. temperature relative humidity. In this study, WRF-CMAQ (Weather Research Forecasting Community Multiscale Air Quality) model was used to investigate impact of an on vertical distribution aerosol particles, especially secondary inorganic (SIA), taking strong in Hangzhou, China, as example. results show that during day with strongest case 8 d simulation, effect resulted a decrease PM2.5 concentrations boundary layer (BL) about 33 %, accompanied increase lower free troposphere (LFT) 19 %. This is mostly attributed (UHIC) effect, which accounted for 91 % UHI-induced variations PM2.5, rather than or humidity effects, contributed only 5 4 respectively. UHIC plays dominant role, ranging from 72 93 variation all eight cases. not directly transports particles ground level LFT but also redistributes precursors. During case, 80 UHIC-induced due direct transport whereas other 20 formation resulting precursor gases. Of SIA, ammonium nitrate formed ammonia nitric acid. atmosphere, equilibrium acid, depends ambient temperature. BL, higher LFT, BL more toward gas phase LFT; when these gases transported into colder shifts phase. Hence, changes may have potential implications radiation budget, cloud formation, precipitation surrounding areas.