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When it comes to air pollution, what occurs at night can be just as important as what happens during the day, say National Oceanic and Atmospheric Administration (NOAA) scientists and their colleagues in a study published April 10 in Geophysical Research Letters. NOAA is an agency of the U.S. Department of Commerce.
The study found that nighttime processes in the marine boundary layer off the coast of New England remove nitrogen oxides (NOx) from the atmosphere, gases that are one of the two basic ingredients for making ozone pollution. Ozone is a strong oxidant and can lead to respiratory problems in humans as well as affect plant life. With less NOx in the atmosphere, ozone production in the New England region the next day will almost always be reduced.
Lead author Steve Brown and many of the coauthors are at NOAA’s Aeronomy Laboratory and NOAA’s Cooperative Institute for Research in Environmental Sciences (CIRES) in Boulder, Colo. Scientists at the NOAA Pacific Marine Environmental Laboratory, the University of New Hampshire, and the University of Colorado also contributed to the study.
Ozone forms in the presence of sunlight from chemical reactions between hydrocarbons (also known as volatile organic compounds, or VOCs) and nitrogen oxides (NOx), both of which are emitted from human activities such as fossil-fuel burning and from natural sources. Most studies have focused on the daytime processes associated with ozone pollution.
But, Brown notes, “Atmospheric chemistry never sleeps” and more information is needed about the nighttime chemistry. After sunset, the NOx compounds undergo reactions that make two new nitrogen-containing gases that exist mainly at night. These nocturnal nitrogen oxides have the potential to either remove the nitrogen from the atmosphere, or to store it and re-release it when daytime returns—two possibilities that have vastly different consequences for subsequent ozone formation.
The authors studied the two nocturnal gases, known chemically as nitrate radical (NO3) and dinitrogen pentoxide (N2O5). The gases previously have been either impossible to measure (N2O5) or measurable only over a large volume of air (NO3). A new capability recently applied by Brown and colleagues at NOAA has made it possible to measure each gas in a small volume of sampled air. The scientists obtained a first look at the nighttime chemistry during the summer of 2002, when the new instrument was deployed on the NOAA Research Vessel Ronald H. Brown off the coast of New England as part of a regional air quality study.
The authors found in their investigation that the nocturnal gases effectively removed NOx from the atmosphere by forming nitric acid, a gas that rapidly deposits on the surface in the marine environment. The net result is that the NOx that is removed can no longer contribute to ozone-formation the next day. Scientists at the University of New Hampshire provided key measurements of the nitric acid during the study.
“This nighttime process takes out about as much NOx as daytime processes. Under nearly all polluted conditions, this will short-circuit some of the ozone production that would have occurred the next day in New England,” Brown said.
This result is important to include in air quality models of the region, because it affects the amount of ozone that is expected to form per unit of NOx pollution. New nighttime processes are a “must-have” for air quality forecasts and simulations in New England, and perhaps other areas.
“The nighttime chemistry is a new piece of the air quality puzzle. We need to find out more about when and where it is important, so that we will be able to provide more accurate predictions of ozone pollution for the public,” said A.R. Ravishankara, co-author, from NOAA’s Aeronomy Laboratory.
The Commerce Department's National Oceanic and Atmospheric Administration (NOAA) Office of Oceanic and Atmospheric Research is dedicated to enhancing economic security and the nation’s welfare through research to better understand atmospheric and climate variability and to manage wisely our nation's coastal and marine resources.
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(only) may obtain a copy of Brown's paper by contacting Kara LeBeau: