Impacts of urban form on air quality: Emissions on the road and concentrations in the US metropolitan areas.

In the United States (US), one of the effects of urban sprawl is more vehicular travel. The increase in emissions from road traffic renders air quality management unsuccessful, in spite of the efforts of the US Environmental Protection Agency (EPA), operating under the Clean Air Act, which does not regulate land use. As an alternative to urban sprawl, the US-EPA has initiated smart growth strategies based on compact development. In this study, the impacts of metropolitan-level urban form, as a measure of urban compactness/sprawl, was explored with respect to air quality in terms of NOx and PM2.5 emissions on the road and NO2 and PM2.5 concentrations in US metropolitan areas. Ordinary least squares (OLS) and two-level regression models that control for metropolitan-level socioeconomic and local-level geographic covariates were established. The results showed that high land use mixing is one of the compactness/sprawl factors (i.e., low sprawl) that reduces per-person NOx and PM2.5 emissions on the road. While metropolitan-level urban form (i.e. high compactness) had a negative effect on PM2.5 concentrations, local environmental conditions, such as local ambient roads and emissions facilities, had greater impacts on NO2 and PM2.5 concentrations than the urban form. Based on the findings in this study, urban planners and decision makers need to establish policies and practices at two scales when planning urban development: (1) the reduction of emissions from road traffic by managing metropolitan-level urban form and (2) the reduction of emissions concentrations through managing local emissions sources near populated areas.