Impacts of urban form on air quality in metropolitan areas in the United States

Abstract

Among 221 metropolitan areas (MAs) in the United States (US), this study explored the impact of urban form, either urban compactness or urban sprawl, on two types of air quality in 2014: NOx emissions from road traffic and annual average NO2 concentrations. Urban form was quantified using Smart Growth America (SGA) sprawl indexes with density, land use mixing, centeredness, and street connectivity. NOx emissions from road traffic were derived from the National Emissions Inventory (NEI). Through modeling NO2 concentrations using land use regression (LUR), with satellite-based estimates and kriging, this study measured NO2 concentrations within MAs in the US The study results showed that higher levels of urban form scores (i.e., higher compactness) and land use mixing were associated with lower per-person NOx emissions from road traffic. In addition, higher levels of centeredness were associated with lower NO2 concentrations, but the effect was moderate. On the other hand, regional rainfall and solar insolation had more significant associations with NO2 concentrations than metropolitan urban form. Meanwhile, localized emissions sources had significant associations with local-level NO2 concentrations. This study provides additional evidence on the relationship between urban form and air quality in the US MAs. The study suggests that high compactness-oriented development and the reduction of localized emission sources may be effective in reducing NOx emissions from road traffic and local NO2 concentrations, respectively. However, future studies need to explore the impact of urban form at both the MA and local levels on NO2 concentrations and develop a more accurate national NO2 concentration prediction model.