One might reasonably assume that increasingly stringent air quality regulations enacted in the U.S. in recent decades would lead to a decrease in surface-level ozone pollution nationally, but a recent study, authored in part by Boulder scientists, indicates that's only half right.
After studying 20 years' worth of ozone data from rural areas across the U.S., a team of academic and National Oceanic and Atmospheric Administration scientists have concluded levels of the pollutant are decreasing over rural areas in the eastern U.S. but may actually be increasing in the west.
The study, published recently in the Journal of Geophysical Research, examined data collected between 1990 and 2010 at rural monitoring sites across the U.S., including 41 sites east of Texas and 12 west of Texas. The study found that ozone pollution is declining in the east, while levels are holding steady or even increasing in many parts of the rural west, particularly in springtime.
“There have been emissions reductions in the west as well as in the east, but ozone isn't responding in the rural areas,” said Owen Cooper, the study's Boulder-based lead author. “At half the sites we looked at (in the west), ozone is increasing. It's not going down.”
Ground level — or “bad” — ozone is created when chemical reactions between oxides of nitrogen (a nitrogen atom with one or more oxygen atoms bonded to it) and volatile organic compounds take place in the presence of sunlight, according to the Environmental Protection Agency. Major sources of both nitrogen oxides and volatile organic compounds include emissions from industrial facilities, chemical solvents and motor vehicle exhaust, according to the EPA, which regulates levels of surface ozone in the U.S.
Cooper, a researcher with NOAA's Cooperative Institute for Research in Environmental Sciences, or CIRES, on the University of Colorado's Boulder campus, says at high levels, surface ozone can be damaging to the human respiratory system and may negatively impact plants and reduce crop productivity.
Cooper calls U.S. efforts to limit emissions a “success story,” noting that urban areas across the country, including western areas such as in and around Los Angeles, have seen big reductions in ozone pollution, but data in rural areas points to something undermining those efforts.
The study looked beyond simple averages of ozone at monitoring sites, with scientists analyzing highest, lowest and median levels during the summer, spring and winter months, according to NOAA. The study saw decreases in the high end of ozone in the summer months, with average high readings dropping from 80 parts per billion in 1990 to 1994 to 66 at eastern sites by 2010, and from 70 parts per billion to 69 at western sites over the same time frame. The study notes the drop is likely the product of air quality regulations.
The most surprising ozone trends noted by the study were those at western sites in the spring. None of the sites saw decreases in median ozone during the spring from 1990 to 2010 and half of the sites saw increases in their median ozone levels, according to NOAA.
When looking for an explanation of how ozone levels could be increasing in the western U.S. in spite of air quality regulations, Cooper points to previous studies and scientific observations that indicate ozone-rich air is blowing into the U.S. across the North Pacific Ocean from Asia. Emissions of ozone precursors are known to have increased in Asia since 1990, according to the study, and Cooper said “imported ozone” is one explanation that must be considered for the results observed in the western U.S.
“We don't have a smoking gun, but basically we have to use models to simply (theorize) that the excess ozone is coming from Asia,” Cooper said. “We're now working with a lot of atmospheric modeling groups to measure how much ozone from Asia gets down to the surface level here. It's very much an area of ongoing research.”
Cooper noted that increases in Asian ozone emissions do have the potential to offset recent decreases seen in Europe and North America.
“That's really an area of uncertainty – how emissions will continue to change over the next 10 to 20 years,” he said.