National Oceanic and Atmospheric Administration WP-3D Orion aircraft were also used to collect ozone data for this study.

The study led by the National Oceanic and Atmospheric Administration (NOAA) analyzed large quantities of ozone data captured since 1984. Among the data sources for the study were profiles of ozone in Earth's troposphere (lowermost atmosphere) measured since 1999 by the differential absorption lidar (laser detection and ranging) system located at the NASA Jet Propulsion Laboratory Table Mountain Facility near Wrightwood, Calif. That remote, high-altitude facility enables research in atmospheric science, optical communication and astronomy. Measurements from atmospheric balloons launched from Table Mountain also contributed to the findings.

"In springtime, pollution from across the hemisphere, not nearby sources, contributes to the ozone increases above western North America," said lead author Owen R. Cooper of the NOAA-funded Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder. "When air is transported from a broad region of south and east Asia, the trend is largest."

The study focused on springtime ozone in a slice of the atmosphere from 3 to 8 kilometers (2 to 5 miles) above the surface of western North America, far below the protective ozone layer but above ozone-related, ground-level smog that is harmful to human health and crops. Ozone in this intermediate region constitutes 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, 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 local emissions are driving the increasing trend in ozone above western North America.

Cooper and colleagues from NOAA's Earth System Research Laboratory in Boulder, Colo., and eight other research institutes, including JPL, 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.

This method is like imagining a box full of 40,000 tiny, weightless balls at the exact location of each ozone measurement, explained Cooper. Considering winds in the days prior to the measurement, the computer model estimates which winds brought the balls to that spot and where they originated.

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 the possibility that emissions from other places could be contributing to the ozone increases above North America. The study used springtime ozone measurements because previous studies had 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 were collected between 1995 and 2008, but the team also included a large ozone dataset from 1984.

The analysis shows an overall significant increase in springtime 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 springtime ozone of 29 percent.

"This study did not quantify how much of the ozone increase is solely due to Asia," Cooper said. "But we can say that the background ozone entering North America increased over the past 14 years and probably over the past 25 years."

The influence of ozone from Asia and other sources on ground-level air quality is a question for further study, Cooper said. 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. More information on the study is online at: http://www.noaanews.noaa.gov/stories2010/20100120_ozone.html .

More information on JPL's Table Mountain Facility is at: http://tmf-web.jpl.nasa.gov/ . For more information on JPL's Table Mountain Facility Atmospheric Lidar Group, see: http://tmf-lidar.jpl.nasa.gov/ .

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