Study reveals critical factors affecting urban pollution
provided by Ohio State University
he most critical factor affecting ozone concentrations in US cities is beyond the control of local regulators: it is the amount of ozone that drifts into a city from outside its boundaries, a new nationwide study confirms. The study also found that the number and placement of road intersections within a city is the largest factor that determines carbon monoxide levels.
Researchers at Ohio State University used a comprehensive statistical analysis to determine what factors most influenced levels of ozone and carbon monoxide both major pollutants in more than 100 US cities. This analysis is the first of its kind to use real-life data to look at how a variety of factors from car emissions to weather to population affected air quality in the nation's cities. The findings also indicated the positive impact that public transportation, forests and farmland in cities can have in lowering levels of ozone and carbon monoxide.
Researchers already know most of the causes of air pollution in our cities, but the advantage of our statistical analysis is that we can measure the relative importance of each of these causes, said Jean-Michel Guldmann, coauthor of the study and professor of city and regional planning at Ohio State.
The study was based on the doctoral dissertation of Hag-Yeol Kim, a former student of Guldmann's at Ohio State and now with the Seoul (Korea) Development Institute. The study was published in a recent issue of the journal Geographical Analysis.
The researchers used US Environmental Protection Agency data for concentrations of ozone and carbon monoxide in US cities from April to September, 1990. Both are major pollutants in most US cities and are among the six principal air pollutants monitored by the federal government. The researchers also examined data on weather conditions, land use, transportation and a variety of socioeconomic factors in the cities in 1990. The final sample included 303 cities in the ozone analysis and 117 cities for the carbon monoxide analysis.
Guldmann said that researchers knew ozone was a national problem because it decays slowly and can travel long distances. But this study confirmed that so-called background ozone ozone that could not be attributed to local sources was the most important factor in determining total ozone levels in cities.
Our results confirm that ozone is transported over long distances with minimal decay, Guldmann said. Local efforts are not going to control ozone in a city. There has to be a comprehensive national policy, which is consistent with what the EPA has been doing.
Another major factor influencing ozone levels was the amount of construction going on in a city. For every 1 percent increase in construction, there was a 0.69 percent increase in ozone levels. Guldmann said construction involves a lot of truck traffic and use of paints and chemicals that can help boost ozone levels.
But the study also revealed how policy-makers can help reduce ozone levels by encouraging public transportation and use of electric-heated homes, and by protecting green space. For example, the study found a 1 percent increase in the use of public transportation decreased ozone levels by 0.51 percent. Similarly, a 1 percent increase in forested land in the city decreased ozone levels by 0.019 percent.
In examining data on carbon monoxide in cities, the researchers found that the placement of pollution-monitoring stations in relation to traffic intersections was critical. A 1 percent increase in the average distance to all intersections within a 5-kilometer distance of the monitoring station led to a carbon monoxide decrease of 0.56 percent. Intersections are important because carbon monoxide emissions are higher when a vehicle is idling. Adjusting traffic flows in congested areas to reduce the amount of time a car must idle can significantly reduce the amount of carbon monoxide released in the air.
The results confirm that intersections are truly important emission sources of carbon monoxide, Guldmann said. Constructing road interchanges instead of intersections, and implementing traffic signal management at intersections could help reduce levels of this pollutant.
Of course, simply reducing the use of cars in the city is also very valuable. Guldmann said that in an urban area with the average number of commuting workers (313,792 persons), if 20 percent of the workers were to reduce their work-trip distance by one mile, carbon monoxide would decrease by 0.8 parts per billion. This could be achieved by higher use of public transportation and by car pooling.
Preserving green space could also help, the study found. A 1 percent increase in the amount of agricultural land in the city led to a decrease of 0.07 percent in carbon monoxide levels.
The statistical model we developed is not yet accurate enough to forecast pollution concentrations, Guldmann said. However, we get a good look at the relative importance of different factors, so the results can help direct pollution control policy.
The study was supported by the Urban Affairs and Urban Assistance Program of the Ohio Board of Regents.