Abstract
Abstract. The urban heat island intensity (UHII) is the temperature difference between urban areas and their rural surroundings. It is commonly attributed to changes in the underlying surface structure caused by urbanization. Air pollution caused by aerosol particles can affect the UHII through changing (1) the surface energy balance by the aerosol radiative effect (ARE) and (2) planetary-boundary-layer (PBL) stability and airflow intensity by modifying thermodynamic structure, which is referred to as the aerosol dynamic effect (ADE). By analyzing satellite data and ground-based observations collected from 2001 to 2010 at 35 cities in China and using the WRF-Chem model, we find that the impact of aerosols on UHII differs considerably: reducing the UHII in summer but increasing the UHII in winter. This seasonal contrast is proposed to be caused by the different strengths of the ARE and ADE between summer and winter. In summer, the ARE on UHII is dominant over the ADE, cooling down surface temperature more strongly in urban areas than in rural areas because of much higher aerosol loading, and offsets the urban heating, therefore weakening UHII. In winter, however, the ADE is more dominant, because aerosols stabilize the PBL more in the polluted condition, weakening the near-surface heat transport over urban areas in both vertical and horizontal directions. This means that the heat accumulated in urban areas is dispersed less effectively, and thus the UHII is enhanced. These findings shed new light on the impact of the interaction between urbanization-induced surface changes and air pollution on urban climate.
Highlights
The global population has been increasingly concentrated in cities (Heilig, 2012)
The soil-adjusted vegetation index (SAVI), is a modification of the normalized difference vegetation index that corrects for the influence of soil brightness when the vegetative cover is low (Huete, 1988; Qi et al, 1994; Rondeaux et al, 1996)
Where Tu is the average temperature of an urban area, and Tr is the average temperature of the neighboring rural area
Summary
The global population has been increasingly concentrated in cities (Heilig, 2012). Urbanization in China has dramatically increased from 26 % in 1990 to 60 % in 2018, resulting in a marked change in the landscape. Urbanization leads to a dramatic change in the underlying surface structure, properties, and spatial distribution of a city, such as a reduction in green areas and a corresponding increase in urban impervious areas These changes increase the temperature difference between urban and rural areas, known as the urban heat island (UHI) intensity (UHII) (e.g., Kalnay and Cai, 2003; Zhao et al, 2014, 2016; Zhou et al, 2016; Yang et al, 2017). Some numerical modeling studies have demonstrated that landscape changes reduce near-surface concentrations of particulate matter (PM2.5) and that the UHI effect can influence the dispersion of air pollutants (Liu et al, 2009; Liao et al, 2015; Tao et al, 2015; Zhong et al, 2017, 2018). This study aims to fill this gap by analyzing the annual and seasonal effects of aerosols on UHII and proposing mechanisms that may explain the seasonal differences
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