2012 News & Events

An East Side, West Side Story: Ozone Pollution in the United States

15 November 2012

topo map
Topographic map showing the locations of the 53 rural ozone monitoring sites used in the new study comparing ground-level ozone in the eastern and western U.S. Image: O. Cooper, NOAA / CIRES

During the last 20 years, air quality regulations in the United States have reduced the emissions of air pollutants that lead to ground-level ozone pollution. Smaller and smaller amounts of these ozone-forming chemicals are emitted in the United States today. So, ozone pollution is going down everywhere, right?

Not quite. In a new study, academic and CSD scientists examined 20 years of ozone data from rural sites across the United States, to better understand broad, regional trends in the pollutant. Ozone pollution is declining at the rural sites in the East, the team found, but it’s steady or even increasing at most sites in the rural West, with the strongest increases in spring.

"The differences we find are consistent with increased levels of pollution imported into the West from across the Pacific Ocean," said Owen Cooper, Ph.D., lead author of the new paper and researcher working at CSD with the NOAA Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder.

Sunlight can "cook" chemical ingredients—emitted by cars, oil and gas operations, manufacturing and other sources—into ozone pollution. At high levels, ground-level ozone can cause severe respiratory effects in some people and damage crops, trees and other vegetation. The Environmental Protection Agency regulates levels of surface ozone in the United States.

Many model analyses have indicated that decreasing U.S. emissions of those ingredients should reduce ozone from coast to coast. But if the western U.S. is "importing" ozone pollution as previous studies have suggested, that changes the picture. Asian emissions of ozone precursors are known to have increased between 1990 and 2010, and with aircraft and satellites, scientists have observed plumes of ozone-rich air plumes from Asia reaching the U.S. West.

Cooper and his colleagues scrutinized data from rural sites in the United States where ozone was measured from at least 1990 to 2010. Instead of focusing only on averages or exceedances of regulated levels of ozone, the scientists looked at the highest, lowest and median levels of ozone during spring, summer and winter. This allowed them to determine if changes were occurring at the low end of the spectrum, which might indicate changes in background levels of the pollutant, or at the high end, which might be more indicative of effects of regulation.

Overall, the scientists found that the highest levels of ozone pollution dropped at many rural eastern sites and some western ones—probably a consequence of regulations. In the east, summertime's high readings dropped from an average of 80 parts per billion (ppb) in 1990-1994 to an average of 66 ppb. Highest ozone readings out west in summer only dropped from 70 to 69 ppb.

Ozone trends in the West are most surprising in spring, when no site shows decreasing ozone. Instead, half of all sites show increasing median ozone, despite reductions in ozone precursor emissions. These increased ozone levels might be explained by the observed increases in ozone detected in the baseline air that flows into the U.S. West from across the North Pacific Ocean.

The authors ruled out several other factors that might explain east-west differences, such as changing temperatures out west, wildfires and changes in anthropogenic emissions.

The finding may have implications for those attempting to improve air quality, especially in western states. "In the U.S. West, imported pollution from Asia could add to the challenge of reducing ozone pollution," Cooper said.

Coauthors of the new paper are Ru-Shan Gao from NOAA ESRL CSD, David Tarasick from Environment Canada, Thierry Leblanc from CalTech, and Colm Sweeney from CIRES.

Cooper, O. R., R. S. Gao, D. W. Tarasick, T. Leblanc, and C. Sweeney, Long-term ozone trends at rural ozone monitoring sites across the United States, 1990-2010 Journal of Geophysical Research, doi:10.1029/2012JD018261, 2012.


This analysis provides an up-to-date assessment of long term (1990-2010) rural ozone trends using all available data in the western (12 sites) and eastern (41 sites) USA. Rather than focus solely on average ozone values or air quality standard violations, we consider the full range of ozone values, reporting trends for the 5th, 50th and 95th percentiles. Domestic ozone precursor emissions decreased strongly during 1990-2010. Accordingly 83%, 66% and 20% of summertime eastern US sites experienced statistically significant ozone decreases in the 95th, 50th and 5th percentiles, respectively. During spring 43% of the eastern sites have statistically significant ozone decreases for the 95th percentile with no sites showing a significant increase. At the 50th percentile there is little overall change in the eastern US. In contrast, only 17% (2 sites) and 8% (1 site) of summertime western US sites have statistically significant ozone decreases in the 95th and 50th percentiles, respectively. During spring no western site has a significant decrease, while 50% have a significant median increase. This dichotomy in US ozone trends is discussed in terms of changing anthropogenic and biomass burning emissions. Consideration is given to the concept that increasing baseline ozone flowing into the western US is counteracting ozone reductions due to domestic emission reductions. An update to the springtime free tropospheric ozone trend above western North America shows that ozone has increased significantly from 1995-2011 at the rate of 0.41 {plus minus} 0.27 ppbv yr-1. Finally, the ozone changes are examined in relation to regional temperature trends.