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CSD News & Events:

News & Events - 2010


Spring Ozone Increases Above Western North America Linked to Emissions Abroad

2 February 2010

A study led by CSD's Owen Cooper finds that springtime ozone levels above western North America are rising primarily due to air flowing eastward from the Pacific Ocean, and that the trend is largest when the air originates in Asia. These increases in ozone could make it more difficult for the United States to meet its goals of reducing ozone pollution at ground level. The study, published online January 21 in the journal Nature, analyzed large sets of ozone data captured since 1984.

"In springtime, pollution from across the hemisphere contributes to the ozone increases above western North America, not the sources nearby," said Cooper. "We can say that when air is transported from a broad region of south and east Asia, the trend is largest." Cooper is with the NOAA-funded Cooperative Institute for Research in Environmental Sciences (CIRES) at CU in Boulder.

The analysis shows an overall significant increase in ozone of 14 percent from 1995 to 2008. When they included data from 1984, the year with the lowest average ozone level, the scientists saw a similar rate of increase from that time through 2008 and an overall increase in ozone of 29%.

Cooper and his colleagues used historical data of global atmospheric wind records and sophisticated computer modeling to match each ozone measurement with air-flow patterns for several days before it was recorded. This approach essentially let the scientists track ozone-producing emissions back to a broad region of origin.

When the dominant airflow came from south and east Asia, the scientists saw the largest increases in ozone measurements. When airflow patterns were not directly from Asia, ozone still increased but at a lower rate, indicating that it is possible emissions from other places could also be contributing to the ozone increases above North America. The study used springtime ozone measurements because previous studies have shown that air transport from Asia to North America is strongest in spring, making it easier to discern possible effects of distant pollution on the North American ozone trends.

Ozone-measuring research balloons and research aircraft collected a portion of the data. Commercial flights equipped with ozone measuring instruments also collected a large share of the data through the MOZAIC program, initiated by European scientists in 1994. The bulk of the data was collected between 1995 and 2008, but the team also included a large ozone dataset from 1984.

Background: The study focused on springtime ozone in a slice of the atmosphere from two to five miles above the surface of western North America, far below the protective ozone layer but above the ozone-related pollution events near Earth's surface ("smog") that are harmful to human health and crops. Ozone in this intermediate region is indicative of the northern hemisphere "background" or baseline level of ozone in the lower atmosphere. The study was the first to pull together and then analyze the nearly 100,000 ozone observations gathered in separate studies by instruments on aircraft, balloons, and other platforms.

Combustion of fossil fuels releases pollutants like nitrogen oxides and volatile organic compounds (VOCs), which react in the presence of sunlight to form ozone. North American emissions contribute to global ozone levels, but the researchers did not find any evidence that these continental emissions are driving the increasing trend in ozone above western North America.

Significance: These increases in ozone could make it more difficult for the United States to meet Clean Air Act standards for ozone pollution at ground level; however, the influence of ozone from Asia and other sources on ground-level air quality is a question for further study. Scientists will need to routinely measure ozone levels close to the surface at several locations along the West Coast to see whether similar trends are impacting ground-level air quality. The paper has received the attention of both national and international press, including radio interviews in Germany and the UK (BBC radio interview) and print articles in Le Monde in France, the Washington Post, and the New York Times. This research contributes to NOAA's Climate Goal and to the Air Quality Program of NOAA's Weather and Water Goal.


Cooper, O.R., D. D. Parrish, A. Stohl, M. Trainer, P. Nédélec, V. Thouret, J. P. Cammas, S. J. Oltmans, B. J. Johnson, D. Tarasick, T. Leblanc, I. S. McDermid, D. Jaffe, R. Gao, J. Stith, T. Ryerson, K. Aikin, T. Campos, A. Weinheimer, and M. A. Avery, Increasing springtime ozone mixing ratios in the free troposphere over western North America, Nature, doi:10.1038/nature08708, 2010.

Author affiliations:

O.R. Cooper and K. Aikin: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colo., and Chemical Sciences Division of NOAA Earth System Research Laboratory.
D.D. Parrish, M. Trainer, T. Ryerson, R.-S. Gao: Chemical Sciences Division of NOAA Earth System Research Laboratory.
S.J. Oltmans and B.J. Johnson: Global Monitoring Division of NOAA Earth System Research Laboratory.
A. Stohl: Department of Regional and Global Pollution Issues, Norwegian Institute for Air Research.
P. Nédélec: Laboratoire d'Aérologie, Centre National de la Recherche Scientifique, Observatoire Midi-Pyrénées.
V. Thouret: Laboratoire d'Aérologie, Centre National de la Recherche Scientifique, Observatoire Midi-Pyrénées.
J. P. Cammas: Laboratoire d'Aérologie, Centre National de la Recherche Scientifique, Observatoire Midi-Pyrénées.
D. Tarasick: Experimental Studies Research Division, Meteorological Service of Canada, Environment Canada.
T. Leblanc: Table Mountain Facility, Jet Propulsion Laboratory, California Institute of Technology.
I. S. McDermid: Table Mountain Facility, Jet Propulsion Laboratory, California Institute of Technology.
D. Jaffe: Department of Atmospheric and Environmental Chemistry, University of Washington-Bothell.
J. Stith: NCAR Research Aviation Facility, National Center for Atmospheric Research.
T. Campos, A. Weinheimer: Atmospheric Chemistry Division, National Center for Atmospheric Research.
M.A. Avery: NASA Langley Research Center.

More information:

NOAA Press Release: Study Links Springtime Ozone Increases Above Western North America to Emissions Abroad