|Springtime ozone distributions for 1984, 1995–2008 in the mid-troposphere (3.0–8.0 km), and air mass source regions. Credit: Nature, Cooper et al. Click to enlarge.|
Springtime ozone levels above western North America are rising primarily due to air flowing eastward from the Pacific Ocean, a trend that is largest 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.
Tropospheric ozone is an important source of the hydroxyl radical, an oxidant that breaks down most pollutants and some greenhouse gases. High concentrations of tropospheric ozone are toxic, however, and have a detrimental effect on human health and ecosystem productivity. (The US EPA recently proposed new tougher ground-level ozone standards. Earlier post.) Moreover, tropospheric ozone itself acts as an effective greenhouse gas.
Much of the present tropospheric ozone burden is a consequence of anthropogenic emissions of ozone precursors resulting in widespread increases in ozone concentrations since the late 1800s. At present, east Asia has the fastest-growing ozone precursor emissions and much of the springtime east Asian pollution is exported eastwards towards western North America.
Despite evidence that the exported Asian pollution produces ozone, the authors note, no previous study has found a significant increase in free tropospheric ozone concentrations above the western USA since measurements began in the late 1970s.
Published in the 21 January issue of the journal Nature, this new study analyzed large sets of ozone data captured since 1984. The study focused on springtime ozone in a slice of the atmosphere from two to five miles above the surface of western North America, 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, or 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 local emissions are driving the increasing trend in ozone above western North America.
In springtime, pollution from across the hemisphere, not nearby sources, contributes to the ozone increases above western North America. When air is transported from a broad region of south and east Asia, the trend is largest.—lead author Owen R. Cooper, Ph.D.
Cooper and colleagues from NOAA’s Earth System Research Laboratory in Boulder, Colo., and eight other research institutes 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 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 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.
The analysis shows an overall significant increase in springtime ozone of 14% 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%.
This study did not quantify how much of the ozone increase is solely due to Asia. But we can say that the background ozone entering North America increased over the past 14 years and probably over the past 25 years.—Owen Cooper
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.
Collaborating institutions include the Norwegian Institute for Air Research, the National Center of Scientific Research Midi-Pyrenees Observatory in Toulouse, France; the Meteorological Service of Canada; NASA Jet Propulsion Laboratory and the California Institute of Technology; the University of Washington; the National Center for Atmospheric Research, Boulder, Colo.; and NASA’s Langley Research Center. The MOZAIC program is supported by the European Communities, EADS, Airbus and the airlines (Lufthansa, Austrian, Air France) who have carried MOZAIC equipment free of charge since 1994.
O. R. Cooper, et al. (2010) Increasing springtime ozone mixing ratios in the free troposphere over western North America. Nature 463, 344-348 doi: 10.1038/nature08708