Influence of urban heat islands on seasonal aerosol acidity and aerosol liquid water content in humid subtropical Hong Kong, South China

Aerosol acidity governs many important atmospheric processes that modulate the composition and mass concentrations of atmospheric aerosols. Temperature and RH are two key drivers of aerosol acidity. Increasing levels of urbanization in humid subtropical South China have led to the frequent occurrence of urban heat islands (UHIs) in the region. However, the effects that UHIs have on aerosol acidity in cities in South China are currently unknown. In this study, we investigated the effects of UHIs on aerosol acidity and aerosol liquid water concentrations at two urban sites with different land use characteristics in Hong Kong, South China, in winter and summer from 2011 to 2019. Our results showed that the UHI effect resulted in both urban sites having significantly higher temperatures and lower RH than the refence rural site. Higher UHI intensities were observed in winter, but RH differences between the urban and rural sites were the greatest in summer. Temperature and RH changes associated with the UHI effect resulted in urban aerosols having aerosol liquid water concentrations that were up to 6 to 7 times lower than rural aerosols, and urban aerosols being more acidic than rural aerosols by as much as 1.16 pH units. While the pH differences between urban and rural aerosols correlated with temperature and RH differences induced by the UHI effect, these correlations were season-dependent due to differences in the winter and summer aerosol compositions. In addition, although the UHI was most intense in winter, pH differences between urban and rural aerosols were the largest in summer. Changes in temperature, RH, and aerosol liquid water concentrations linked to the UHI effect were found to lead to substantial reductions (by as much as 38%) in the partitioning of semi-volatile basic and acidic species NH₃ and HNO₃ to the aerosol phase. Our results indicate that changes in the aerosol acidity, aerosol liquid water concentrations, and gas-aerosol partitioning associated with UHIs have significant implications for aqueous urban aerosol chemistry in Hong Kong. Since UHIs will intensify in many cities in South China in the future due to their increasing levels of urbanization and a warming climate, changes in urban aerosol acidity and liquid water concentrations induced by intensifying UHIs need to be considered when modeling urban aerosol composition and levels in the region.