A chemical reaction long assumed to be unimportant in urban air quality may be a significant source of ozone, the major component of smog.
Hydroxyl (OH) radicals, among the most reactive natural chemicals in the atmosphere, help cleanse the air of some noxious pollutants. In many cases, and especially in urban environments, ozone results from that cleansing, says Amitabha Sinha, a physical chemist at the University of California, San Diego.
Previous studies suggested that the majority of the atmosphere’s hydroxyl radicals are produced when ultraviolet radiation cleaves a molecule of ozone to produce a single oxygen atom, which in turn reacts with water vapor in the air. Now, lab tests by Sinha and colleagues Shuping Li and Jamie Matthews hint that reactions involving nitrogen dioxide and light may in some cases produce substantial quantities of hydroxyl radicals, and therefore more ozone.
In their experiments, the researchers zapped nitrogen dioxide, a common component of vehicle and power-plant emissions, with several wavelengths of light between 450 nanometers (violet) and 650 nm (red). When illuminated at those wavelengths, NO2 molecules can absorb the light’s energy and remain excited for as long as 60 microseconds-long enough for them to collide with another molecule, says Sinha. Most of those collisions involve diatomic molecules of nitrogen and oxygen, gases that make up about 99 percent of the atmosphere. Occasionally, however, the extra-energetic NO2 runs into and reacts with a molecule of water vapor, generating a hydroxyl radical.
Sinha and his colleagues’ models suggest that in environments where NO2 concentrations are 1 part per billion, light-driven chemical reactions create as many as 30,000 hydroxyl radicals each second in a cubic centimeter of air. Such reactions can produce up to 52 percent as many hydroxyl radicals as ozone-cleaving reactions do when the sun is low in the sky and much of its ultraviolet light has been filtered out by the overlying atmosphere. The team reports its findings in the March 21 Science.
“This throws a curve ball at what we thought about urban atmospheric chemistry,” says Paul O. Wennberg, an atmospheric chemist at the California Institute of Technology in Pasadena. However, Wennberg’s analyses suggest that the team’s model may in some cases significantly overestimate the amount of ozone that would result from the light-driven reactions.
Production of hydroxyl radicals by excited nitrogen dioxide molecules “hasn’t been included in models [of atmospheric chemistry] but certainly should be,” says Luisa T. Molina, an atmospheric chemist at the Massachusetts Institute of Technology. However, she notes, in urban areas that have many sources of hydroxyl radicals, such as Mexico City, this reaction’s contribution to pollution may be minimal.