Study Links Spring Ozone Over North America With Emissions Abroad

Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.buis@jpl.nasa.gov

Feature: 2010-021 January 20, 2010

Study Links Spring Ozone Over North America With Emissions Abroad

The full version of this story with accompanying images is at: http://www.jpl.nasa.gov/news/news.cfm?release=2010-021&cid=release_2010-021

Springtime ozone levels above western North America are rising, primarily due to air
flowing eastward from the Pacific Ocean, a trend that is most significant when the air
originates in Asia. 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, according to a
new international study published online Jan. 20 in the journal Nature.

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/ .

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