They also found that the combined effect of urban form on NO₂ is larger for small cities—an important finding given that cities less than 500,000 people contain a majority of the urban population and are where much of the future urban growth is expected to occur.

More than half of the world population, 3.9 billion people (54%) in 2014, live in urban areas, with an additional 2.5 billion urban dwellers expected by 2050. Cities serve as economic and social centers, concentrating people, activities, ideas, and industries. Cities may also concentrate environmental hazards such as air pollution, and potentially health inequities. As such, cities are a focal point for understanding and addressing environmental health issues.

… Transportation is one of the largest contributors to urban air pollution for pollutants such as carbon monoxide, nitrogen oxides, benzene, ozone, and fine particulate matter (PM_2.5). For example, recent estimates suggest that globally, approximately 25% of ambient urban PM_2.5 is attributable to motor vehicles. Strategies to reduce motor vehicle use may play a role in improving urban air quality. Evidence suggests that changes in urban form can impact travel behavior such as travel distance, trip frequency, and mode choice.

… Here we employ satellite-based measurements of nitrogen dioxide (NO₂ , a proxy for traffic-related air pollution and a major constituent of urban air pollution) and a global data set of 1,274 urban areas to explore the relationship between urban form and air quality. Our work builds on prior research by employing a much larger sample of global cities, allowing for the exploration of factors (i.e., city population, country-level income, environmental performance & policy) that may influence the relationship between urban form and air quality. In this work we are able to explore how urban form-air quality relationships vary for a consistent set of cities in low- and high-income countries, giving insight into the impacts of urban form in developing countries where existing literature is limited. Additionally, more than half of the cities in our data set (n = 675 cities; 53% of all cities) are small cities (100,000 - 500,000 people), allowing us to explore the relationship of urban form and air pollution for a large global sample of small cities.

—Bechle et al.

In the study, the team considered four urban form metrics: circularity, contiguity, percent vegetation, and percent impervious surfaces. Circularity is a measure of urban compactness, quantifying the relative closeness of the built-up area to the geographic center. A city exhibiting this type of compactness may indicate a more efficient use of urban area, and may induce fewer and shorter vehicle trips.

The contiguity index is the ratio of the largest contiguous polygon of built-up area to the total built-up area for a given city. Large amounts of leapfrog or exurban development, and polycentricism (particularly with satellite cities) would result in a low contiguity index.

Among their findings:

• Higher population had the largest effect; i.e., higher populations are associated with worsened air quality.

• Increased urban contiguity, circularity and vegetation metrics are significantly associated with lower urban NO₂ concentrations.

• The impervious surface metric has a non-significant effect on urban NO₂.

• Although the magnitude of the effect size of the three significant urban form metrics are small relative to population, their combined effect could be large.

• Meteorology and country-level income play an important role in describing differences in urban NO₂ concentrations among cities; income, solar insolation, and precipitation together describe 52% of the variation in urban NO₂ concentration.

Overall, our findings demonstrate that urban form has a statistically significant relationship with urban NO₂ concentrations. Our cross-sectional investigation highlights the need for further study of urban design and planning as a potential strategy to address air quality. While meteorology (aside from dilution rate), country-level income, and city population size all had a larger effect on urban NO₂ than the urban form metrics, the combined impact of the three statistically significant metrics (contiguity, circularity, and vegetation) could have large consequences for concentrations. While these findings are generally consistent with our prior work, in this study we find that certain factors may alter the relationship between urban form and air pollution. For example, we find that urban form may have a greater impact on urban NO₂ for small cities than for large cities.

This is an important finding given that more than half of the world’s urban population lives in small urban areas, and because changes to urban form may be easier for small urban areas (owing to less existing infrastructure and the potential for greater relative impact from future growth). We also find that direction of effect for impervious surfaces differs between cities in high- and low-income countries, suggesting that urban form strategies may differ at various stages of growth and development.

—Bechle et al.

Resources

• Matthew J. Bechle, Dylan B. Millet, and Julian D. Marshall (2017) “Does Urban Form Affect Urban NO2? Satellite-Based Evidence for More than 1200 Cities” Environmental Science & Technology doi: 10.1021/acs.est.7b01194