The Dark Side of Ozone Chemistry: New Paper Published in Science MagazineJanuary 6, 2006
In a paper published in the 6 January 2006 issue of Science, NOAA and CIRES scientists in the ESRL Chemical Sciences Division and their colleagues at the Georgia Institute of Technology have discovered a new aspect of nighttime chemistry that influences the daytime production of ozone pollution and hence air quality. The work used state-of-the-art technology developed at NOAA to focus on a little-studied aspect of chemistry involving nocturnal trace gases that contain nitrogen. The nighttime chemistry affects compounds that are the "starting ingredients" (precursors) for daytime ozone formation. The authors confirm that a key nocturnal reaction leads to the removal of ozone precursors (thereby reducing the potential for ozone formation the next day), and they show that the process is particularly rapid in the presence of atmospheric particles ("aerosols") that contain sulfur. This work by CSD scientist Steve Brown and colleagues is a first demonstration of the sensitivity of the chemistry to aerosol composition (a nitrogen chemistry/sulfur chemistry linkage).
The production of ozone pollution during the day requires three ingredients: nitrogen oxides (NOx), hydrocarbons (or volatile organic compounds, VOCs), and sunlight. Though nightfall brings an end to this production process, a different set of "dark reactions" then occurs that transforms the NOx-VOC-ozone mixture in ways that affect the next day's ozone production. The key nighttime compounds are nitrate radical (NO3) and dinitrogen pentoxide (N2O5), which are not well characterized because they are very difficult to measure.
CSD scientists applied a novel technique that they developed in the last few years to make the measurements during the 2004 New England Air Quality Study. Observations were made during flights of the NOAA WP-3D aircraft, enabling NO3 and N2O5 abundances to be characterized aloft for the first time. NOAA's ability to directly measure N2O5 in the atmosphere, a key new scientific capability, allowed for these discoveries. Accompanying measurements of other nitrogen oxides, VOCs, and particularly the aerosols enabled the authors to examine the interrelationships in the chemistry.
A particular focus of the paper was the reaction of N2O5 with H2O, which occurs on the surface of atmospheric aerosol particles, to produce nitric acid. Nitric acid is a stable and highly soluble form of nitrogen that is readily washed out of the atmosphere, thereby removing nitrogen precursor compounds and potentially reducing the next-day ozone formation. The authors found that the composition of the aerosol particles makes a difference in how fast N2O5 is converted to nitric acid, and that sulfate-containing aerosols, which in the Northeast U.S. arise primarily from anthropogenic sulfur emissions from coal-fired power plants, greatly enhance the efficiency of the process.
The authors' discovery of the dependence of the nighttime N2O5 chemistry on aerosol sulfate composition shows that there is a link between the emissions of SO2, nitrogen oxide abundances, and regional ozone formation. Changes in sulfur emissions, for example because of emission control strategies, could therefore influence a key class of ozone precursors (nitrogen oxides) and hence ozone formation. The work thus has implications for air quality improvement strategies related to ozone, nitrogen oxide emissions, and sulfur emissions. The previously unrecognized dependence of the N2O5 reaction on aerosol composition has further implications for predictions of the global levels of ozone and other oxidants. Additional work is needed to quantify the importance of this effect and its regional dependence. The research contributes to objectives of the Air Quality Program of NOAA's Weather and Water Mission Goal.