dispersion Urban Heat Islands (UHIs) have become a growing concern in recent years due to their impact on the environment and human health. UHIs are defined as areas in urbanised regions where the temperature is significantly higher than in surrounding rural areas. The formation of secondary pollutants is a serious concern in urban environments, and the influence of UHIs on this process has been extensively studied. This blog post discusses how UHIs influence secondary pollutant formation and explores some of the implications of this phenomenon for human health and the environment.
UHIs refer to the phenomenon whereby the city core, or the downtown region, tends to experience higher temperatures compared to the less developed, rural surroundings (Fig 1). Rapid urbanization leads to increased vehicular emission, more highrise buildings and surface modifications in the city areas. These factors then worsen air quality in urban areas. A team of researchers suggested that high temperatures and VOC emissions will be responsible for high PM2.5 concentrations.
Fig 1: Temperature profile of a city and its surroundings (Source: Lawrence Berkeley National Laboratory, 2019)
Secondary pollutants are formed when primary pollutants, such as nitrogen oxides and volatile organic compounds, react with each other in the presence of sunlight. The most common secondary pollutants include ozone, nitrogen dioxide, and particulate matter. These pollutants can have significant negative impacts on human health, including respiratory problems, cardiovascular disease, and cancer.
UHIs contribute to secondary pollutant formation by increasing the concentration of primary pollutants in urban areas. Urban regions experience increased emissions of nitrogen oxides and volatile organic compounds from vehicles and other sources, which can lead to higher concentrations of these pollutants in the atmosphere. UHIs also alter the chemical reactions that cause the formation of secondary pollutants. For example, a study published in Atmospheric Environment noted that daytime formation of ozone was affected by thermally induced air circulation and dilution of nitrous oxides due to a change in boundary-layer height. Hence, ozone concentrations tend to peak during midday when strong sunlight prevails in photochemical formation.
The diagram in Fig 2 illustrates how anthropogenic dust emissions from various sources, including heavy urban construction and traffic, are often accompanied by severe non-dust aerosol emissions in the planetary boundary layer (PBL). Pollutant mixing and dispersion occur, thus forming “urban dome” and creating “urban plume”. Theyremain in the atmosphere for prolonged periods, ultimately aggravating climate and human health impacts.
Fig 2: Air pollution generated in anthropogenic environments (Source: Xia et al., 2022)
The implications of UHIs on secondary pollutant formation have significant implications for public health and the environment. Exposure to high levels of ozone and nitrogen dioxide has been linked to increased mortality rates, particularly among vulnerable populations such as the elderly and those with pre-existing respiratory conditions. This results in rising healthcare costs and lower standards of living for those residing in urban areas.
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