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Study finds that cities with highly contiguous built-up areas have on average lower NO2 concentrations

Unique effect of independent variables for the linear regression model (log-linear plots). Shaded blue regions indicate 95% confidence intervals. Hash marks along the x-axes indicate independent variable value for each of the 83 cities. Credit: ACS, Bechle et al. Click to enlarge.

In the first empirical study using satellite measurements to explore the relationship between urban form and air pollution, a team from the University of Minnesota has found that cities with highly contiguous built-up areas have, on average, lower concentrations of nitrogen dioxide (NO2, a key component of urban air pollution).

NO2 is mainly produced in the atmosphere from photochemical oxidation of directly emitted nitric oxide (NO) on a timescale of minutes, so that NO2 concentrations are essentially a marker for combustion-related emissions, Bechle et al. note in their study. Major sources of combustion-related emissions in urban areas are transportation, power generation, and industrial processes. NO2 is linked to numerous adverse health effects including lung cancer, cardiopulmonary mortality, and type 2 diabetes. It is also a precursor to the formation of ground-level ozone and particulate matter, and as a cause of acid rain.

The study, based on a stratified global sample of 83 cities and published in the ACS journal Environmental Science & Technology, shows quantitatively that urban design can influence NO2 concentrations, and demonstrates that urban NO2 levels around the world track the wealth of the city in a nonlinear way and according to an environmental Kuznets curve (EKC).

The team combined satellite measurements of NO2 from the Ozone Monitoring Instrument (OMI) and a global data set of 83 urban areas from the World Bank’s Urban Growth Management Initiative to define urban extent and characteristics. The researchers constructed a linear regression model for the logarithm of arithmetic mean NO2 concentration in each city to determine its dependence on the urban characteristics.

The findings indicated that more-populous cities tend to have worse air quality, but the increase in NO2 associated with a population increase of 10% may be offset by a moderate increase (4%) in urban contiguity. Among the other findings of the study were:

  • Urban circularity (“compactness”) is not a statistically significant predictor of NO2 concentration. (Compactness is a measure of the circularity of the main built-up area of a city, calculated as the ratio of built-up area to total buildable area (areas without bodies of water or extreme slopes) within a circle surrounding the main built-up area of the city.)

  • Anti-leapfrogging policies may improve air quality. (Contiguity is a measure of urban patchiness (degree of “leapfrog” development), calculated as the ratio of the main contiguous built-up area to the total built-up area of the city. )

  • Urban NO2 levels vary nonlinearly with income (Gross Domestic Product), following an EKC. The environmental Kuznets curve (EKC) suggests that rising income increases pollution when per capita gross domestic product (GDP) is low, but decreases pollution when per capita GDP is high.

    The peak occurs at approximately US$30,000 per capita GDP. Starting at this peak and employing mean values for all other independent variables, they found that a 1-SD increase or decrease in per capita GDP yields a 22% reduction in NO2 concentrations.

  • The team estimates that if high-income countries followed urban pollution-per-income trends observed for low-income countries, NO2 concentrations in high-income cities would be 10× larger than observed levels.

Our findings indicate that cities with highly contiguous built-up areas have, on average, lower NO2 concentrations. All else being equal, urban areas with large amounts of development detached from main built-up areas will tend to have higher NO2 concentrations. The urban form metrics employed here do not allow us to distinguish among types of isolated development (e.g., residential-only subdivisions, versus satellite cities, versus business/industrial development) or their differential effect on NO2. It is possible, for example, that self-sufficient isolated satellite cities might reduce travel and/or NO2 concentrations. More work is needed in this regard.

Overall our findings suggest that policies encouraging contiguous rather than isolated development may be an effective part of urban design strategies seeking to minimize NO2 air pollution.

—Bechle et al.


  • Matthew J. Bechle, Dylan B. Millet, Julian D. Marshall (2011) Effects of Income and Urban Form on Urban NO2: Global Evidence from Satellites. Environmental Science & Technology Article ASAP doi: 10.1021/es103866b



To me, this suggests that highly concentrated population indicates less driving. Primary transportation would be walking, bicycles, buses and subways. A more spread out urban setting is the pattern for suburbs where people drive a significant distance to work.

I would be very hesitant to suggest that cities be designed to be more compact as this generally sacrifices park land. Better public transportation (subways) and electric cars could allow a more pleasant spread out city without the pollution drawback.


I always laugh at these studies.. did they track polution around the city and along the main freeways connecting it to everywhere else? While the people may move about less often thier crap sure doesnt.


Pollution averaged over what? Per person? If so, then what they're saying is, "If you're crammed in next to a lot of people, the pollution per person isn't as high...but the absolute pollutions levels certainly could be higher than in a less densely populated your lungs are still hating it."


If you have electric light rail perhaps, but it is my experience that light rail goes from point A to B ONLY. So PHEV/CNG buses are more versatile and with enough of them they could actually get you from points A to Z and everywhere in between without huge waits for transfers.

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