Overseas NOx could be boosting ozone levels in US



A smoggy day in Atlanta. NOx arriving in the U.S. from across the Pacific could be boosting ozone levels, a major component of smog. (Photo courtesy K. Baumann)
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Large amounts of a chemical that boosts ozone production are being transported to North America from across the Pacific Ocean in May, according to a new report by researchers from Georgia Tech. These higher levels of nitrogen oxides (NOx), arriving in late spring, could be contributing to significant increases in ozone levels over North America. The research appeared in volume 33 of the journal Geophysical Research Letters.

"It's well-known that pollutants don't always stay in the region in which they are produced. What's not understood as well is where and when they travel," said Yuhang Wang, associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. "Finding this large amount of NOx traveling from across the Pacific is important because it will allow us to build better models so we can better understand how pollutants created in one region of the world are affecting the other regions."

Wang, along with colleagues from Tech, the University of California, Irvine, and the National Center for Atmospheric Research studied data from the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment when they found much larger amounts of an array of chemicals, including NOx, and ozone than predicted by current models.

Formed when fuel burns at a high temperature, any of the sources of NOx are manmade, with automobile exhaust, electric utilities and industrial activity responsible for the bulk of human-produced NOx. The amount of NOx available largely determines how much ozone, a major component of smog, is produced in most regions of the atmosphere.

"With a very small amount of NOx sitting around, as long as you have all these emissions of carbon monoxide and hydrocarbons, the NOx sits there and continuously produces ozone. So in a way you really don't need a lot of it, but when you have a lot of it, it tends to produce ozone faster," said Wang.

Current models have shown these chemical species coming across the pacific at lower altitudes, but those peak in March and April. Wangs finding that the higher altutide transport occurs in May is significant, not only because they found a large amount of NOx unaccounted for by current models, but the fact that it's occurring in May means the NOx is more efficient at making ozone.

"For the same amount of NOx, ozone production is faster in May than April because there is more ultraviolet light and water vapor available in May," said Wang.

While it's not clear whether this trans-Pacific transport is coming from Asia or as far away as Europe, given Asia's proximity and its burgeoning industrial activity, Wang suggests it is the more likely source. The next step in this research is to study chemical measurements over east Asia to see if there is indeed a link to this seasonal transition and emissions from Asia.

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Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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