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New global assessment examines air pollutants falling on the Earth’s surface
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New global assessment examines air pollutants falling on the Earth’s surface

We sat down with Richard Artz, environmental scientist at NOAA’s Air Resources Laboratory, to learn more about the new global assessment he co-edited. The assessment is available online in the journal Atmospheric Environment.

1. Why should we be concerned about chemicals falling onto the Earth’s surface?

Air pollutants can come back down to Earth through precipitation or dry deposition, landing on our soils, vegetation, water bodies, buildings, roads and pavement. This assessment focused on the deposition of major ions, including sulfate, nitrate, ammonium, and phosphorus, which contribute to ecological problems such as the acidification of soils, lakes and streams and an excess of nutrients, called eutrophication, of inland and coastal waters.

2. What are the major findings of the global assessment?

We found that in North America many of the problems associated with nitrogen and sulfur deposition have been substantially reduced over the past decade, and indeed over the past several decades compared to an earlier assessment released in 1996. This is largely due to stringent regulations of air pollutant emissions from electric power plants, industry and automobiles. Canada and the U.S. have reduced the problems associated with chemical deposition in all but the most sensitive ecosystems, such as high elevation forests where soils are thin and easily damaged. Europe has also substantially reduced air pollutants through their emissions regulations. But Asia is where we were in the 1940s, with high concentrations of air pollutants that damage human health and degrade sensitive forests and watersheds.

However, the assessment found that ammonium (a form of nitrogen) deposition is on the rise globally in areas with intensive agriculture. Ammonia emissions are released primarily from animal waste and commercial fertilizer application. While nitrogen is a useful nutrient for crops, too much of it in our waterways can lead to poor water quality.  

3. How can this information be used to assist in reducing chemical pollutants?



A 5th generation Saildrone drifts along the ocean surface.
In North America and parts of Europe, this information helps us remain vigilant to be sure that we continue to maintain the reductions in chemical pollutants that are harmful to human health, our air and water, and sensitive ecosystems. For other parts of the world where there has been little or no regulation, the assessment makes available the best quality data that can be used as the basis for policies to reduce pollution. For example, there is great concern about the cycling of nitrogen and other materials in South America associated with burning of tropical rainforests and industrial processes that accompany rapid growth. Our assessment provides solid information concerning the atmospheric contributions to ecosystems – but admittedly requires corresponding data from terrestrial and aquatic systems. This study contributes to our ability to understand chemical cycling through the full range of ecosystem processes, to better understand the sustainability or consequences of human and natural activities.

4. How does information in this second assessment differ from the first global assessment in 1996?

This second assessment presents a global overview of worldwide deposition using more thorough quality assurance practices than available for the first assessment, and is complemented with the best available global model simulations to fill in gaps. In the first assessment, we looked at deposition from rain and in the second we also begin to factor in chemical substances that are deposited dry from the air to estimate total wet and dry deposition. Total deposition improves the quality of scientific research. While geographical coverage and quality are improved in this assessment, the database remains incomplete in many regions of the world. 

This assessment addresses ongoing issues associated with “acid rain” and also provides information to address environmental issues such as eutrophication of inland and coastal waters, some air pollution issues, and climate change. This research also points out several important gaps. For example, we do not yet have a good handle on organic acid cycling and phosphorus pollution, and we still need better information on the cycling of nitrogen and sulfur, especially in rapidly developing regions.

5. Does this study look at how climate change may be affecting the chemistry of precipitation?

The study doesn’t explicitly examine climate change links. However, it does provide data required for models that examine climate change, most notably for nitrogen and sulfur compounds and organic acids.  A changing climate will alter the amount and type of precipitation that falls and will certainly have effects on energy consumption. We anticipate scientists will use this information to help improve control of pollutants in our water and land resources.

AUV Sentry is secured into its cradle after recovery for the last time during the DEEP SEARCH 2017 expedition. Credit: NOAA OER

6. How did you and the team collect the data to do the analysis for the global assessment?

This assessment was performed under the auspices of the World Meteorological Organization’s Global Atmosphere Watch program. The assessment team involved scientists from Canada, Norway, England, Italy, France, South Africa, Australia, Brazil, Norway, Russia, Australia, Japan, Switzerland, India, and the United States. We combined both actual measurements and model predictions of atmospheric wet and dry deposition. The data were drawn from major deposition monitoring networks of the world, and augmented as much as possible from other high quality measurement programs. Model output was provided by the Task Force on Hemispheric Transport of Air Pollution that operates under the United Nations Economic Commission for Europe Convention on Long Range Transboundary Pollution. Atmospheric deposition knows no political boundaries and affects all aquatic and terrestrial ecosystems. This study would not have been possible without successful collaboration among regional experts from around the world.

For more information, please contact Monica Allen, director of public affairs for NOAA Research, at 301-734-1123 or by email at

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Oceanic and Atmospheric Research (OAR) - or "NOAA Research" - provides the research foundation for understanding the complex systems that support our planet. Working in partnership with other organizational units of the NOAA, a bureau of the Department of Commerce, NOAA Research enables better forecasts, earlier warnings for natural disasters, and a greater understanding of the Earth. Our role is to provide unbiased science to better manage the environment, nationally, and globally.


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