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Paris, January 20, 2010

Pollutants from Asia arrive over the Western United States

With Asia remaining a region with rapid economic, population and industrial growth, the large-scale impact of emissions leading to air pollution is fast becoming an issue. For the first time, a study has shown that in the Western United States rising levels of tropospheric ozone, a pollutant that is harmful to humans, are due to emissions coming from Asia. Because of the specific meteorological conditions, such emissions are carried eastwards across the Pacific. The study, carried out by NOAA (1), compares large quantities of data from various observation tools over the period 1984-2008, coupling transport of air masses with an inventory of emissions. Over half the data from these observations came from the passenger airliners involved in INSU-CNRS's MOZAIC observation program. The study is published in the 21 January 2010 issue of the journal Nature.

The ozone found at an altitude of between 0 and 12 km is known as tropospheric ozone. It is not only one of the main greenhouse gases that play a role in the Earth's climate equilibrium, but also a pollutant that affects air quality. High ozone concentrations can have numerous physiological effects, such as slowing the growth rate of vegetation or causing lung disorders. This pollutant results from emissions due to human activity, and overall it has been increasing in quantity since the Industrial Revolution. Asia is a major producer of emissions that produce ozone in the air (3) and, especially in the spring, meteorological conditions favor the transport of this ozone eastwards across the Pacific. Until now, there was no convincing evidence connecting the increase in ozone resulting from Asian emissions with rising levels of ozone in the Western US.

The study shows that tropospheric ozone in the Western US has steadily increased since the early 1980s (by 30% compared to 1984 measurements). Taking meteorological regimes of long-range transport as a basis, the study also showed that the greatest increases observed over the past fifteen years were associated with air masses affected by emissions from China, India and the countries of Southeast Asia. For the study, a data base was established from measurements carried out over the period 1984-2008 by various tools: the international network of sounding balloons, laser remote sensing stations, research campaigns organized by NOAA and NASA, and the instrument-carrying airliners of the MOZAIC program. A retro-trajectory calculation was then carried out on each measurement of ozone to determine the past movement of the air mass. Three billion nine hundred million retro-trajectories of air masses over fifteen days were calculated using a Lagrangian dispersion model. The ozone observations were then filtered (those marked by the effect of American continental emissions being excluded) and sorted according to the various air mass trajectories.

With regard to air quality, the large-scale increase in ozone concentrations is worrying. In a case like this where there is migration, controlling the increase is far more difficult than controlling regional concentrations, which depend on local sources. If this increase continues, bringing air quality into line with current legislation will become impossible in some regions of the US.

The study shows the decisive role played by routine observation programs such as MOZAIC. The statistical significance of the results is largely due to the MOZAIC measurements, which make up 54% of the data base established on this occasion. The result benefited from INSU-CNRS's strategy of setting up "Observation and experimental systems for environmental research" (SOERE), of which MOZAIC is part.
pollutionasie

© Valérie Thouret

Distribution of ozone, by percentile class, in the mid-troposphere (3-8 km, and PV<1.5 pvu) in spring, for 1984 and 1995-2008, including the number of data for each year.
Mean residence time for 1984-2008 of particles during the 15 days preceding the measurement, in the entire tropospheric column (b, c and d) or as a footprint, i.e. location of air masses with an altitude less than 300 m over the preceding 15 days (e, f, g). The three lines correspond to three different levels of ozone: 0-33% (b and e), 34-66% (c and f) and 67-99% (d and g).


Notes:

(1) US agency for the study of the ocean and atmosphere (National Oceanic and Atmospheric Administration).
(2) The MOZAIC program (Measurements of Ozone, Water Vapour, Carbon Monoxide, and Nitrogen Oxides by Airbus In-service Aircraft) is coordinated by researchers from the Aerology Laboratory in Toulouse (CNRS).
View web site
(3) Ozone is not directly produced by human activity. It is a secondary pollutant. It results from photochemical reactions (triggered by the Sun) involving, among other things, emissions of nitrogen oxides and carbon monoxide.

References:

Increasing springtime ozone mixing ratios in the free troposphere over western North America.
O. R. Cooper, 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.
Nature, 21 January 2010

Contact information:

Researcher l Valérie Thouret l T 05 61 33 27 40 l valerie.thouret@aero.obs-mip.fr

CNRS press office l T 01 44 96 51 51 l presse@cnrs-dir.fr

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