5 March 2012
Springtime air pollution from Asia, swept across the Pacific Ocean on winds, can contribute to episodes of high surface ozone pollution in the western United States, according to a new study, "Transport of Asian ozone pollution into surface air over the western United States in spring" published in the Journal of Geophysical Research - Atmospheres by NOAA scientists and academic colleagues.
Several recent studies have shown that powerful spring winds can carry Asian pollution into the atmosphere above North America. The new analysis goes further, using high-resolution models and observations to show how some of the imported pollution can descend to the surface, where it affects ground-level ozone, a regulated pollutant. At high concentrations, ground-level ozone can cause severe respiratory effects in some people, and it damages crops, trees, and other vegetation.
"We showed that Asian pollution directly contributes to surface ozone pollution episodes in parts of the western United States," said Meiyun Lin, Ph.D., lead author of the new study from NOAA GFDL and the Cooperative Institute for Climate Science at Princeton University in New Jersey. In several areas, about half of the springtime pollution episodes that exceeded federal limits would probably not have occurred without the contribution of Asian pollution, Lin said. Still, Asian pollution contributed to no more than 20 percent of the ground-level ozone, according to the new study: Other sources of the pollutant include local fossil fuel use, wildfires, and imported pollution from other regions of the globe.
For the analysis, Lin and colleagues from NOAA ESRL CSD and GMD, CIRES, NASA, and NCAR combined detailed observations with high-resolution modeling. The team drew upon data collected by balloon-borne instruments, aircraft, ground instruments, and satellites during an intensive study of air quality and climate in California in 2010.
The researchers found that NOAA GFDL's high-resolution chemistry-climate model, AM3, could accurately reproduce the real-world pattern of ozone levels observed in California. And the model could differentiate the effects of local emissions – from vehicles, power plants and other factors – from Asian emissions.
During episodes of high surface ozone in parts of California and the Southwest, Asian emissions added 8 to 15 parts per billion of ozone to air, comprising up to 20 percent of the total. The Environmental Protection Agency's health-based standard limits ozone to 75 parts per billion (averaged over 8 hours). Roughly half of the pollution episodes that exceeded that health-based standard would not have occurred – the study reported – without the addition of Asian pollution.
Finally, the scientists reported that they could use satellite data to predict when incoming plumes of polluted air might affect western air quality, one to three days ahead of time.
"Advance knowledge about incoming pollution could be helpful to local decision makers, who inform the public about episodes of poor air quality," said Owen Cooper, Ph.D., coauthor of the paper and a researcher at CSD and CIRES.
Lin, M., A.M. Fiore, L.W. Horowitz, O.R. Cooper, V. Naik, J. Holloway, B.J. Johnson, A.M. Middlebrook, S.J. Oltmans, I.B. Pollack, T.B. Ryerson, J.X. Warner, C. Wiedinmyer, J. Wilson, B. Wyman, Transport of Asian ozone pollution into surface air over the western United States in spring, Journal of Geophysical Research, doi:10.1029/2011JD016961, 2012.
Many prior studies clearly document episodic Asian pollution in the western U.S. free troposphere. Here, we examine the mechanisms involved in the transport of Asian pollution plumes into western U.S. surface air through an integrated analysis of in situ and satellite measurements in May–June 2010 with a new global high-resolution (∼50 × 50 km2) chemistry-climate model (GFDL AM3). We find that AM3 with full stratosphere-troposphere chemistry nudged to reanalysis winds successfully reproduces observed sharp ozone gradients above California, including the interleaving and mixing of Asian pollution and stratospheric air associated with complex interactions of midlatitude cyclone air streams. Asian pollution descends isentropically behind cold fronts; at ∼800 hPa a maximum enhancement to ozone occurs over the southwestern U.S., including the densely populated Los Angeles Basin. During strong episodes, Asian emissions can contribute 8–15 ppbv ozone in the model on days when observed daily maximum 8-h average ozone (MDA8 O3) exceeds 60 ppbv. We find that in the absence of Asian anthropogenic emissions, 20% of MDA8 O3exceedances of 60 ppbv in the model would not have occurred in the southwestern USA. For a 75 ppbv threshold, that statistic increases to 53%. Our analysis indicates the potential for Asian emissions to contribute to high-O3episodes over the high-elevation western USA, with implications for attaining more stringent ozone standards in this region. We further demonstrate a proof-of-concept approach using satellite CO column measurements as a qualitative early warning indicator to forecast Asian ozone pollution events in the western U.S. with lead times of 1–3 days.