The formation of ozone and particulate matter in the atmosphere is driven by chemical cycles involving hydrocarbons, nitrogen oxides, and highly reactive radical species during the day. A thorough understanding of the chemistry of these compounds in the atmosphere is needed to quantitatively describe ozone formation. The lifecycle of nitrogen oxides begins with their emission, for example from combustion sources and lightning, followed by their chemical cycling between their main forms, nitric oxide (NO) and nitrogen dioxide (NO₂), then their conversion to chemically more stable forms, such as nitric acid, and  finally the loss of these more stable compounds to particles on the ground. Radical species are also responsible for the chemical processing of mercury (Hg) in the atmosphere. Because the various forms of mercury have well known negative effects on human health, knowledge of the chemical processes impacting atmospheric mercury levels is essential.

The Nitrogen, Oxidants, Mercury and Aerosol Distributions, Sources and Sinks (NOMADSS) study was performed in the southeastern U.S. in Summer, 2013 to study the fate of nitrogen oxide, the budget of radicals involved in ozone formation, and the chemistry of mercury (www.eol.ucar.edu/field_projects/nomadss). Observations of a wide range of atmospheric gases and particles were measured on board the National Center of Atmospheric Research C-130 research aircraft. NOMADSS was a collaborative effort between several universities and research institutions. The field experiment and the interpretation of the observations offered unique opportunities for several graduate students and postdoctoral researchers to learn about experimental atmospheric sciences. NOMADSS was also a part of a large multi-agency effort, the Southern Oxidant and Aerosol Study (soas2013.rutgers.edu), and its various outreach activities.

One of the main results of this project was to show that, in a remote marine environment, the loss of nitrogen oxides into particles is not final. The interaction of sunlight with nitric acid in particles releases nitrous acid into the air, which then further dissociates to re-form nitrogen oxides and a hydroxyl radical. This nitrogen oxide recycling pathway, which is currently not included in air quality models, thus reduces the loss of nitrogen oxides, and ultimately leads to an increase in nitrogen oxide levels. Because 70% of the earth is covered with oceans, nitrogen oxide recycling has the potential to impact ozone and particle levels on a global scale.

Mercury is typically emitted in its elemental form and then transformed by chemical processes to oxidized mercury compounds that are more rapidly lost from the atmosphere. The main result from NOMADSS was to provide the first simultaneous observation of oxidized mercury (measured by the Univ. Washington through a NSF sister project) and bromine monoxide in air at approximately 7 km above Texas, in an air mass which originated from the subtropical Pacific. The analysis of this data with an air chemistry model clearly demonstrates that mercury is predominately oxidized by bromine in the free troposphere, i.e. the lower part of the atmosphere not influenced by the ground. Model results also imply that Hg is more quickly converted to oxidized mercury in the free troposphere than thus far assumed, and that other chemical processes need to be considered to correctly quantify the atmospheric mercury budget. Finally, the observations suggest that subtropical anticyclones could be significant global sources of atmospheric bromine and oxidized mercury.

The newly identified chemical processes will allow a more accurate description of atmospheric ozone and mercury in regional and global air quality models. As these models are often used to develop effective air pollution strategies, the results of this study will contribute to the continuing improvement of air quality.

Last Modified: 11/28/2016
Modified by: Jochen P Stutz