Aug 10, 2009
Aerosol formation by plants: the missing link
Deciduous plants such as trees give off around 500 teragrammes of carbon each year in the form of isoprene (C5H8). In remote areas such as the Amazon, where there is little nitric oxide (NO) produced by biomass and fossil fuel burning, the chemistry of what happens to the compound as it forms aerosol particles has to date been unclear.
Now a team from the California Institute of Technology, US, University of Otago, New Zealand, and University of Copenhagen, Denmark, has found that the hydroxyhydroperoxides formed by oxidation of the isoprene by hydroxyl radicals (OH) are further oxidized by OH to dihydroxyepoxides.
"We identify a new compound, an epoxide, which is formed in large amounts in the atmosphere," Fabien Paulot of the California Institue of Technology told environmentalresearchweb. "Nothing is known about its fate in the atmosphere, which is very exciting. Given the properties of this epoxide, it is likely to make a good candidate for the elusive aerosol precursor from isoprene photooxidation under pristine conditions."
The dihydroxyepoxide formation step also regenerated hydroxyl radicals, which are key in oxidizing many atmospheric pollutants. Previously there was no good explanation for the high levels of OH observed following formation of hydroperoxides.
"Given the importance of isoprene, it constitutes a very large uncertainty in our understanding of atmospheric chemistry," said Paulot. "Furthermore without a good understanding of the oxidation of isoprene, it is very difficult to understand what are the isoprene photooxidation products that can be aerosol precursors."
This formation of epoxides was unexpected; its discovery was enabled by the use of a chemical ionization mass spectrometry system developed by John Crounse at the California Institute of Technology. The system can ionize gases while preserving information about the size or mass of the original molecule. Other methods, in contrast, tend to destroy fragile molecules during the ionization process.
"Isoprene oxidation products under these [pristine] conditions are very unstable and thus very difficult to detect, identify and quantify with conventional techniques – GC (gas chromatography) for instance," said Paulot. "As a result, the oxidation mechanism of isoprene under these conditions was largely unknown."
Paulot and colleagues also used oxygen isotopes to distinguish the epoxide – the major second generation oxidation product – from hydroperoxide, the major first generation product. "These compounds are isobaric which likely explains why the epoxide had not been identified earlier," he explained.
The team reckons that plants produce around 95 Teragrammes of carbon each year in the form of dihydroxyepoxide, with the largest concentrations over the southern tropics and substantial levels in summer over Canada and the southeast US. The dihydroxyepoxide is a likely candidate for forming secondary organic aerosol particles as it tends to stick to acidic particles.
Now Paulot plans to investigate the "implication on the HOx level of the mechanism we put together". Data collected by co-author Paul Wennberg's group during the ARCTAS (Arctic Research of the Composition of the Trophosphere from Aircraft and Satellites) field campaign over Canada can be used to assess the importance of these processes in the atmosphere, he said. "This can then be translated into a more global study using the Geos-Chem model, for instance. In a recent campaign in California (BEARPEX near lake Tahoe), we have also attempted to measure this newly identified epoxide." Researcher John Seinfeld's group, meanwhile, is investigating epoxide uptake by aerosols.
"Perhaps the most important aspect of the Paulot et al. work is its practical value," said Tadeusz Kleindienst of the US Environmental Protection Agency writing in a "Perspective" article in Science. "Air quality models for secondary organic aerosol formation used by regulatory agencies, such as the US EPA, are generally limited in their predictive power by relying on experiments that give parameterized aerosol yields from reacting precursor compounds. Incorporation of the chemical mechanisms derived experimentally by Paulot et al. into deterministic models of gas-aerosol chemistry should help to improve their predictive capabilities."
The researchers reported their work in Science.
About the author
Liz Kalaugher is editor of environmentalresearchweb.