Climate change could offset ozone cleanup
Global warming leads to higher smog levels, according to new research.
Cleaner tailpipes and smokestacks are helping to clear the air in smoggy cities, but climate change threatens to undo some of that progress. An emerging body of research shows that warming global temperatures can increase the production of smog-forming ozone, possibly by enough to overtake predicted improvements in air quality from cleaner technology.
This new research is revealing that climate change affects ground-level ozone via complex interactions of warming and increased production of natural smog-forming volatile organic compounds (VOCs), such as isoprene made by trees. Although the basic cause-and-effect mechanisms have been understood for some time, only recently have models been able to incorporate emissions, climate, and chemical transport to make reliable global and regional predictions. Some of these new data were presented at the December 2006 American Geophysical Union (AGU) conference, and other results were published recently in the Journal of Geophysical Research.
Atmospheric chemists have been studying the effects of meteorology on ozone for years, says Peter Adams of Carnegie Mellon University. “Their question wasn’t about climate change,” he says, but they did learn that temperature can shift chemical reactions in a direction that either favors or hinders ozone formation. At cooler temperatures, ozone precursors called nitrogen oxides (NOx) react to form peroxyacetyl nitrates (PANs) instead of catalyzing ozone formation. But as temperatures climb, ozone formation is favored over PANs. “This is a major reason you tend to have high ozone on hot days,” Adams says.
Predicting future climate effects has not been straightforward, however. Global warming tends to increase water vapor, which destroys ozone in the atmosphere through chemical reactions. Yet, warmer temperatures also drive the reactions that form ozone and increase production of ozone precursors by plants. The balance between increased ozone production and destruction, especially at regional scales, has been difficult to quantify until recently.
Adams and graduate students Pavan Racherla and John Dawson of Carnegie Mellon University tied together NASA’s global climate models with global- and regional-scale chemical transport models for ozone to understand the net effect of a changing climate. “Even though background ozone may decrease under future climate change, you can still have more frequent and intense ozone pollution episodes on short-term regional scales because of PAN chemistry and isoprene emissions,” Adams says. Their work was published December 16 in the Journal of Geophysical Research (2006, DOI: 10.1029/2005JD006939).
The group looked at the eastern U.S., which has both high anthropogenic emissions and high natural emissions of VOCs from trees. “In more polluted areas, there are processes that are not captured if you only look at global changes,” Racherla says, noting that over the southeastern U.S. where isoprene emissions are highest, climate change alone can increase summertime ozone concentrations by 10 parts per billion (ppb). The U.S. EPA’s current ozone standard is 80 ppb.
To understand the effects of climate change in the drier western U.S., Allison Steiner of the University of Michigan and collaborators at University of California, Berkeley, are studying central California, where ozone and smog in some areas regularly exceed EPA attainment levels. Steiner modeled future ozone levels in combination with projected reductions in human-caused emissions of smog-forming NOx, VOCs, and carbon monoxide for the year 2050. She published the model results in the October issue of the Journal of Geophysical Research (2006, DOI: 10.1029/2005JD006935), showing increases of as much as 10 ppb in peak ozone events, amounting to a 3–10% change in the daily ozone maximum. “This is enough to push a moderate day into a severe-ozone-quality day,” she says, adding that it’s comparable to the amount by which ozone is expected to be reduced through emissions controls in that time frame
Steiner presented data at the AGU meeting in December on the contributions of different VOC sources to ozone levels under present and future climates. She predicted that under future climate scenarios, emissions of natural VOCs, such as isoprene, and the secondary reactions of VOCs in the atmosphere will become more important. A regulatory focus on curbing primary, or “tailpipe”, emissions may therefore not be adequate to suppress ozone production in central California, she says.
Both research groups were funded through EPA’s Global Change Research Program in the first-ever round of funding to study the potential effects of climate change on air quality. The funding is focused on ozone and particulate matter, both of which are sensitive to climate.
“Climate change is happening now, so this is not solely a problem for 10, 20, or 50 years from now,” says Kris Ebi, an independent consultant who is the lead author on the human-health chapter of the upcoming 2007 climate change assessment report by the Intergovernmental Panel on Climate Change. Ebi notes that predicting ozone levels remains challenging because both future emissions and cloud cover are uncertain, but she says that a warmer climate will impact health issues in ways that include floods, vector-borne diseases, and air pollution. She adds that ozone poses a difficult health problem “because there’s little that people can do to control their own exposure.” Instead, she says, “Ozone has to be controlled through regulations.”
