March 14, 2011 — Atmospheric chemist William Keene and his research group from the University of Virginia are in Erie, Colo., working with scientists from the National Oceanographic and Atmospheric Administration and several academic institutions on a month-long study of the wintertime atmosphere. The study is shedding new light on how air pollution forms.
The researchers are trying to understand why and how nitryl chloride – a smog-precursor chemical that until very recently was thought to form only in polluted coastal regions – is produced during the winter night in continental regions far from oceans, such as the foothills of the Rocky Mountains. The study will provide a better understanding of both air quality over land and associated influences on Earth's climate.
Nitryl chloride breaks apart quickly as the sun rises, releasing chlorine atoms, which in turn react with many other compounds, contributing to smog formation. Chlorine atoms also influence chemical cycles that destroy or produce various greenhouse gases, including ozone and methane.
"Measurements during the experiment have confirmed that nitryl chloride is present at significant levels in polluted continental air at night," said Keene, an environmental science researcher professor in U.Va.'s College of Arts & Sciences. "These and related results are fundamentally reshaping our understanding of processes that influence the air we breathe and may lead to improved regulatory strategies for minimizing smog formation."
For more than three decades, Keene has investigated air quality and its climatic implications at numerous locations around the world. The National Science Foundation is funding his group's participation in this current study.
Scientists have learned that during the night, chloride associated with particles in the atmosphere interacts with nitrogen oxides to form nitryl chloride. Most nitrogen oxides are emitted from combustion sources, but exactly where the atmospheric chloride comes from in a region so far from the oceans, and how the nighttime chemistry unfolds to produce nitryl chloride, are not fully understood.
To gain a grasp on the phenomenon, scientists designed an experiment – dubbed "Nitrogen, Aerosol Composition, and Halogens on a Tall Tower" – using NOAA's Boulder Atmospheric Observatory, a 985-foot tall tower that allows air chemistry sampling at various elevations.
Since air particles and chemical interactions vary with altitude, they can't be fully understood by making measurements only at ground level. The researchers built a sophisticated one-ton chemistry laboratory and packed it into a room-sized container that travels up and down the tower, sampling the atmosphere from the ground up. The tower provides a distinct advantage in looking at the lower atmosphere over continents, which is made up of layers that typically don't mix well at night.
Likely sources of atmospheric chlorine over continents include emissions from wood burning, coal-fired power plants and waste incineration, evaporation from chlorinated water supplies, wind-blown soil dust and road de-icing chemicals. Instruments operated by Keene's group and his colleagues, Alex Pszenny of the University of New Hampshire and Ann Middlebrook of NOAA, may help explain parts of the puzzle regarding the sources of particulate chloride and its conversion to nitryl chloride.
"By measuring what's in airborne particles of different sizes, we'll also get a handle on not only the chlorine sources and chemical processing, but also the large but poorly constrained climatic influences of the particles themselves," Keene said.
Some particles in the atmosphere reflect sunlight and enhance the brightness of clouds, which have a net cooling effect on climate, while others, such as soot, absorb heat, which has a net warming effect.
The study is expected to advance the understanding of what goes on in the air in the dead of winter, and at night – two little-studied aspects of the chemistry of the atmosphere. Since both nitrogen oxides and chloride are ubiquitous constituents of the continental atmosphere, it is likely that this chemistry also occurs in other regions and other seasons. Results from this experiment have potentially broad-reaching implications.