Department of Chemical, Biochemical and Environmental Engineering, University of Maryland , Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, United States.
Environ Sci Technol. 2017 Nov 21;51(22):13095-13103. doi: 10.1021/acs.est.7b02786. Epub 2017 Nov 3.
The urban heat island (UHI) is a widely observed phenomenon whereby urban environments have higher temperatures and different relative humidities than surrounding suburban and rural areas. Temperature (T) and relative humidity (RH) strongly affect the partitioning of semivolatile species found in the atmosphere, such as nitric acid, ammonia, and water. These species are inherently tied to aerosol pH, which is a key parameter driving some atmospheric chemical processes and environmental effects of aerosols. In this study, we characterized the effect of the UHI on aerosol pH in Baltimore, MD, and Chicago, IL. The T and RH differences that define the UHI lead to substantial differences in aerosol liquid water (ALW) content. The ALW differences produce urban aerosol pH that is systematically lower (more acidic) than rural aerosol pH for identical atmospheric composition. The UHI in Baltimore and Chicago are most intense during the summer and at night, with urban-rural aerosol pH differences in excess of 0.8 and 0.65 pH units, respectively. The UHI has been observed in cities of all sizes: the similarity of our results for cities with different climatologies and aerosol compositions suggests that these results have broad implications for chemistry occurring in and around urban atmospheres globally.
城市热岛（UHI）是一种广泛存在的现象，城市环境的温度和相对湿度通常高于周围的郊区和农村地区。温度（T）和相对湿度（RH）强烈影响大气中半挥发性物质的分配，例如硝酸、氨和水。这些物质与气溶胶 pH 值密切相关，而气溶胶 pH 值是驱动一些大气化学过程和气溶胶环境效应的关键参数。在这项研究中，我们描述了巴尔的摩和芝加哥城市热岛效应对气溶胶 pH 值的影响。定义城市热岛的 T 和 RH 差异导致气溶胶液水（ALW）含量的显著差异。ALW 的差异导致相同大气成分下的城市气溶胶 pH 值比农村气溶胶 pH 值更低（更具酸性）。巴尔的摩和芝加哥的城市热岛在夏季和夜间最为强烈，城市与农村气溶胶 pH 值差异分别超过 0.8 和 0.65 个 pH 单位。在各种规模的城市中都观察到了城市热岛效应：我们对于具有不同气候和气溶胶组成的城市的结果具有相似性，这表明这些结果对于全球城市大气中发生的化学过程具有广泛的影响。
