The recession of 2008 and its impact on light-duty vehicle emissions in three western United States cities.

The global economic recession of 2008-2010 severely depressed light-duty vehicle sales in the United States. On-road fleets observed with a remote vehicle exhaust sensor in 2013 at three historical sampling locations in Denver, Los Angeles, and Tulsa showed large reductions in the fleet fractions of 2009 model year vehicles of 40%, 38%, and 35%, respectively, when compared to prerecession 2007 levels with the light-duty truck category suffering the largest percentage declines. The fleet fraction for these ∼ 5 year old vehicles is normally reserved for vehicles more than twice their age. This resulted in a significant increase in the on-road freeway fleet age, which had been relatively stable. The fleet average age increased by two years in Denver and Los Angeles but only by one year in Tulsa, likely due to its faster economic recovery. Using fleet fractions from previous data sets, we estimated age-adjusted mean emissions increases for the 2013 fleet to be 17-29% higher for carbon monoxide, 9-14% higher for hydrocarbons, 27-30% higher for nitric oxide, and 7-16% higher for ammonia emissions than if historical fleet turnover rates had prevailed.