The 2008 recession hammered the U.S. auto industry, driving down sales of 2009 models to levels 35% lower than those before the economic slump. A new study has found that because sales of new vehicles slowed, the average age of the U.S. fleet climbed more than expected, increasing the rate of air pollutants released by the fleet (Environ. Sci. Technol. 2014, DOI:10.1021/es5043518).

Passenger cars and trucks in the U.S. are the second-largest emitters of nitrogen oxides and volatile organic compounds, precursors to harmful smog. The age distribution of the vehicle fleet is an important variable that scientists use in their models to determine if emissions regulations will deliver on promises of cleaner air, says Gary A. Bishop, an atmospheric scientist at the University of Denver and lead author of the study. If drivers buy newer, cleaner cars, then older, dirtier ones get taken off the road, and air quality improves.

Since 1995, Bishop and his colleagues have collected real-time data on vehicle exhaust at sites throughout the western U.S. In 2013, he and U of Denver emeritus professor Donald H. Stedman hit the road with a remote exhaust detector that shoots infrared and ultraviolet light across the road to sensors on the other side. As cars pass through the beam, the sensors measure levels of pollutants in the exhaust plumes. By measuring carbon dioxide in the plumes, the scientists can also calculate the amount of fuel being burned. A video camera snaps a photo of the license plate, matching the make, model, and age of each vehicle with its pollutant emissions. The scientists captured data on more than 68,000 vehicles at sites in three cities—Los Angeles, Denver, and Tulsa.

Bishop and Stedman then plotted the number of cars and trucks on the road for each model year. Los Angeles had 30% fewer 2009 models than 2007 cars and trucks, whereas Denver and Tulsa had 40% and 35% fewer, respectively. On the basis of these observations, the scientists determined that the mean age of the 2013 fleet was nine years old. For nearly two decades before the recession, the average age had remained stable at seven years old.

Next, the researchers calculated the average amount of pollution released per kilogram of fuel burned for the 2013 fleet and compared the rates to those that would have occurred if the 2013 fleet had the same age distribution as the prerecession fleet. For the three cities, carbon monoxide emissions were greater by 17 to 29%, hydrocarbons by 9 to 14%, nitrogen oxide emissions by 27 to 30%, and ammonia by 7 to 16%.

Bishop cautions that the study looks only at emissions per kilogram of fuel burned and not the total amount of pollution emitted by the fleet. “Because people were driving less after the recession, the aging of the fleet may not have worsened air quality,” he says.

“This paper shows that a small change in consumer behavior can have an unexpectedly large impact on average fleet age,” says Russell R. Dickerson, an atmospheric chemist at the University of Maryland, College Park. The findings suggest that air quality modelers should examine their assumptions about the stability of the age distribution of the vehicle fleet, he says.

This article is reproduced with permission from Chemical & Engineering News (© American Chemical Society). The article was first published on December 11, 2014.