ESRL Quarterly Newsletter - Spring 2010

Ozone Tied to Emissions Abroad

Springtime ozone above U.S. West is highest when winds sweep in from Asia

Springtime ozone levels above western North America are increasing primarily due to air flowing in from the west, an ESRL-led study concluded in January. The trend, which is strongest when air parcels originate in Asia, could challenge U.S. efforts to reduce ozone pollution, wrote the authors, headed by ESRL's Owen Cooper (Chemical Sciences Division and the Cooperative Institute for Research in Environmental Sciences, CIRES, at the University of Colorado).

The new study, published in Nature, focused on "background" tropospheric ozone, in a slice of the atmosphere from two to five miles above the surface of western North America. That's far below the protective ozone layer, but above the ozone-related, ground-level smog that is harmful to human health and crops.

There in the middle, ozone levels have been rising steadily since 1984, Cooper et al. found. The international team analyzed the nearly 100,000 tropospheric ozone observations gathered since 1984 in separate studies by instruments on aircraft, balloons, and other platforms. The researchers used meteorology records and sophisticated models to trace each measurement back to the broad region where the air originated.

When the dominant airflow came from south and east Asia, the scientists discovered the largest increases in ozone measurements.

"In springtime, pollution from across the hemisphere- not nearby sources- contributes to the ozone increases above western North America," Cooper said. "The big question is if this ozone is impacting the surface. We still don't know, but if it is, there's less margin for pollution in the United States."

The U.S. Environmental Protection Agency is currently considering a proposal to lower allowable ozone levels in the atmosphere. Current regulations in North America and Europe have already reduced the emissions of ozone's precursors- mostly byproducts of fossil fuel consumption- but a 2009 National Research Council study showed that emissions of those pollutants are up across much of Asia.

Kathy Law of the French Laboratoire Atmosphères, Milieux, Observation Spatiales commented on the new research in Nature, calling it "the most conclusive evidence so far of increasing ozone levels in the free troposphere over western North America."

The study's other ESRL authors included Sam Oltmans and Bryan Johnson (Global Monitoring), and David Parrish, Michael Trainer, Ken Aikin, Rushan Gao, and Tom Ryerson (Chemical Sciences).