A key type of intermediate in atmospheric chemistry reacts faster to form compounds that produce cooling aerosols than previously realized, according to a report in Science (DOI: 10.1126/science.1213229). The experimental results indicate that the role of green plants—emissions from which help form the intermediates—in combating global warming may be larger than currently understood.
Plants emit alkenes, such as isoprene and terpenes, into the air. Alkenes react with ozone to form so-called Criegee intermediates, which are carbonyl oxides with some biradical and some zwitterionic character. The Criegee intermediates go on to react with other airborne chemicals, including NO2 to form NO3• and SO2 to form SO3. Both products help form aerosol particles. Aerosols seed clouds and play a cooling role in the atmosphere by reflecting sunlight.
Despite the importance of Criegee intermediates in atmospheric chemistry, their reactions had never been directly observed. A group led by scientists Craig A. Taatjes of Sandia National Laboratories’Combustion Research Facility and Carl J. Percival of the University of Manchester’s Center for Atmospheric Science, in England, has now closed that gap by reacting CH2I with O2 to produce the Criegee intermediate CH2OO and studying its chemistry using tunable synchrotron photoionization mass spectrometry. The ability to tune the light used for ionization was critical for the experiments, Taatjes says, because that allowed the researchers to distinguish the Criegee intermediate from formic acid and other isomers.
Taatjes, Percival, and colleagues found that although CH2OO reacts with H2O at roughly the same rate as previously believed, it reacts with NO2 and SO2 at rates that are 50 to 10,000 times faster than those estimated in current atmospheric models.
The faster rate constants mean that plants might play a more significant role in cooling the atmosphere than realized. The new research demonstrates that “it’s even more important to preserve the biosphere” than scientists have understood, Percival says.
The work is also likely to inspire further research into Criegee intermediates, says University of Reading chemistry professor George Marston in a commentary accompanying the report. He looks forward to a better understanding of why CH2OO reacts with various reagents at different rates, as well as future experiments with more complex intermediates.