“The idea that reaction of electronically excited NO2 molecules and water can be a source of OH radicals was first proposed by a German group about ten years ago,” team leader Amitabha Sinha told environmentalresearchweb. “However they did not observe any OH radicals being formed and on the basis of their measurements, they concluded the reaction was too slow to be atmospherically important. We have used a slightly more sensitive laser technique and have been able to directly observe the OH radicals formed from this reaction.”
Sinha and colleagues used laser-induced fluorescence to detect the OH. They found that the reaction, which produces OH and nitrous acid (HONO), proceeded 10 times faster than the previous estimate.
“On the basis of this rate measurement, the reaction is expected to be an important source of tropospheric OH radicals and in turn an additional contributor to urban smog production,” said Sinha. “Understanding the underlying factors that control smog production will ultimately allow us set guidelines on how to control it.”
Electronically excited NO2 molecules can be generated in the lower atmosphere by the absorption of visible light.
The team believes the reaction could contribute almost half as much to ozone generation as the traditional source of hydroxl radicals in the lower atmosphere – the reaction of excited oxygen atoms with water vapour.
“The dissociation of ozone at wavelengths shorter than about 320 nm generates the energetic oxygen atoms required for this reaction,” said Sinha. “However, the ultraviolet light reaching the lower atmosphere is quite effectively reduced/filtered by the atmosphere above it. So other sources of OH radicals can become competitive under certain situations.”
The additional source of hydroxyls could explain discrepanices between measured and predicted HOx concentrations reported in Arctic regions, say the researchers in a paper in Science.
Paul Wennberg of California Institute of Technology and Donald Dabdub of the University of California at Irvine used the Sinha team’s findings in a model of air quality in the Los Angeles basin during a summer smog. They found that the new source of hydroxyls boosted ozone concentrations in the simulation by as much as 30–40%, generally giving higher levels than the measurements seen on the ground. This poses the question whether the rate constant found by Sinha and colleagues for the new reaction is too large. “Given the potential importance of this chemistry and the high sensitivity of atmospheric models to the reaction of electronically excited NO2 with H2O, further investigation is clearly needed,” write Wennberg and Dabdub in a perspective in Science.