Nighttime Chemistry Affects Ozone Formation

When it comes to air pollution, what goes on 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 10 April in Geophysical Research Letters.

The scientists found that nighttime chemical processes remove nitrogen oxides (NOx) from the atmosphere in the marine boundary layer off the coast of New England. These gases are one of the two basic ingredients for making ozone pollution. With less nitrogen oxides in the atmosphere, ozone production the next day will almost always be reduced in New England. Ozone is a strong oxidant and can lead to respiratory problems in humans, as well as affect plant life.

Lead author Steven S. Brown and many of his co-authors are at NOAA’s Aeronomy Laboratory and NOAA’s Cooperative Institute for Research in Environmental Sciences (CIRES) in Boulder, Colorado. Scientists at the NOAA Pacific Marine Environmental Laboratory, the University of New Hampshire, and the University of Colorado also participated in 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, both of which are emitted by human activities such as fossil-fuel burning, as well as by 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 nighttime chemistry. After sunset, nitrogen oxide 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 nitrogen from the atmosphere or to store it and re-release it when daylight 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 had been previously either impossible to measure (dinitrogen pentoxide) or measurable only over a large volume of air (nitrate radical). A new capability recently applied by Brown and his colleagues has made it possible to measure each gas in a small volume of sampled air. The scientists got their first look at the nighttime chemistry during the summer of 2002, when the new instrument was deployed off the coast of New England on the NOAA Research Vessel Ronald H. Brown, as part of an air quality study of the region.

They found that the nocturnal gases effectively removed nitrogen oxides from the atmosphere by forming nitric acid, a gas that rapidly deposits to the surface in the marine environment that the scientists investigated. The net result is that the nitrogen oxides that are thus removed can no longer participate in ozone-forming chemistry 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 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 says.

The 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 nitrogen oxide pollution. New nighttime processes are a “must-have” for air quality forecasts and simulations in New England, and perhaps other areas, the researchers say.

“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, a co-author of the study at NOAA’s Aeronomy Laboratory.

The research was funded by the New England Air Quality Study and NOAA.