How nature's glue affects atmospheric chemistry
Organic carbon compounds released by trees are impacting air quality, new research shows. Scientists from Denmark, New Zealand and the US have discovered that the oxidation of isoprene, a hydrocarbon, is generating gas- and aerosol-phase products, and in turn impacting global warming. The results are published in the journal Science. Isoprene (which is formed naturally in plants and animals and is a precursor of ozone) is emitted by many deciduous trees, with oaks playing the biggest role. Global isoprene emissions from plants are estimated at more than 500 teragrams each year. In an accompanying article, Dr Tadeusz Kleindienst, an atmospheric chemist with the US Environmental Protection Agency (EPA), explained that isoprene 'has arguably the most important chemistry of any single nonmethane hydrocarbon'. The reaction time of isoprene with hydroxyl radicals (OH), which researchers consider to be the 'detergent of the atmosphere', is very quick. 'If you mix emissions from the city with emissions from plants, they interact to alter the chemistry of the atmosphere,' explained project leader Paul Wennberg, a Professor of Atmospheric Chemistry and Environmental Science at the California Institute of Technology (Caltech) in the US. 'There is much more isoprene emitted to the atmosphere than all of the gases (gasoline, industrial chemicals) emitted by human activities, with the important exceptions of methane and carbon dioxide.' The scientists conducted a series of laboratory tests showing that when concentrations of nitric oxide (NO) are low (similar to levels found in largely unpopulated areas), the oxidation of isoprene by OH produces substantial amounts of hydroxy hydroxyhydroperoxide. OH further oxidises isoprene to generate chemicals called dihydroxyepoxides (also known as epoxides), which are converted to aerosols and found in small quantities in the air. Lead author Fabien Paulot, a graduate student at Caltech, and his colleagues produced epoxides by placing isoprene and hydrogen peroxide in a Teflon bag filled with 800 litres of unpolluted air. Introducing isoprene and hydrogen peroxide (to act as a source of OH) before illuminating the mix with ultraviolet (UV) light triggered the chemical reactions. According to the researchers, the resulting epoxides are very soluble and easily dissolve into droplets of moisture in the air, effectively forming organic-rich aerosols. 'These epoxides are nature's glue,' Professor Wennberg remarked. 'When these epoxides bump into particles that are acidic, they make glue. The epoxides precipitate out of the atmosphere and stick to the particles, growing them and resulting in lowered visibility in the atmosphere.' The scientists pointed out that converting the epoxides to aerosol is likely to be higher in polluted environments, because the aerosols' acidity is generally higher when human activity is present. So how do the particles affect people? Professor John Seinfeld of Caltech said, 'Particles in the atmosphere have been shown to impact human health, as they are small enough to penetrate deep into the lungs of people. 'Also, aerosols impact Earth's climate through the scattering and absorption of solar radiation and through serving as the nuclei on which clouds form. So it is important to know where particles come from.' This latest finding can help scientists to develop better models of global gas-aerosol chemistry, and researchers will better be able to understand how isoprene chemistry works in some of the most remote parts of the world. 'Perhaps the most important aspect of the Paulot et al. work is its practical value,' noted Dr Kleindienst. '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 parameterised aerosol yields from reacting precursor compounds,' he wrote. '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.' Other participating members of the study included the University of Copenhagen in Denmark and the University of Otago in New Zealand.
Countries
Denmark, New Zealand, United States