How tiny aerosol particles form and grow from vapours produced by vegetation has been a mystery. The finding that highly oxygenated products form directly from volatile organic compounds may offer the solution. See Letter p.476
On page 476 of this issue, Ehn et al.1 report their identification of large yields of highly oxygenated compounds when volatile organic compounds emitted from biological sources are exposed to atmospherically relevant conditions. This observation may help to close the gap between the measured mass of organic aerosol particles in the atmosphere and that predicted by models. It might also forge a mechanistic link between biogenic volatile organic compounds and the formation of aerosol particles, and provide insight into one of the main climate feedback cycles.
Organic aerosol particles are widespread in the global atmosphere2. They consist of primary particles, which are emitted directly into the atmosphere, and secondary particles, which form from the oxidation products of anthropogenic and biogenic volatile organic compounds. In the overall aerosol budget, the contribution of secondary particles that form from the oxidation of natural plant emissions is complex and highly uncertain. Secondary organic aerosol (SOA) probably dominates primary aerosol and, in some conditions, biogenic sources have been estimated3 to contribute up to 90% of SOA.
Understanding the roles of naturally occurring atmospheric particles in the climate system has proved difficult. Because of mechanistic uncertainties, 'bottom-up' models that predict the concentrations of atmospheric aerosol particles by explicitly describing the emission and oxidation of their precursors have long been unable to predict the observed mass of SOA. Geographically widespread, long-term observations of high numbers of ultrafine particles in the atmosphere, particularly over forested regions (Fig. 1), has stimulated much research into how such aerosol particles form and grow4,5. So far, compounds of sufficiently low volatility to explain the growth of ultrafine particles have not been identified or quantified.
To address these problems, Ehn and colleagues made concurrent, direct measurements of the mass spectra of gaseous organic compounds in a chamber, and of the masses of 'seed' particles injected into the chamber, in experiments conducted under atmospherically reasonable conditions. They studied several biogenic volatile organic compounds (BVOCs), such as monoterpenes (members of the terpene family of naturally occurring hydrocarbons), and found that the oxidation products of these compounds had a high ratio of oxygen to carbon atoms. The products were irreversibly taken up by the seed particles, even at very low particle loadings. This irreversible condensation is expected for compounds that exhibit extremely low volatility, as highly oxygenated compounds generally do. Possibly the most surprising of the authors' findings is that such compounds are formed at high yields at an early stage of oxidation — in oxidation reactions of the primary compounds, rather than in subsequent oxidation reactions.
Observed atmospheric particle-growth rates are generally higher than previously proposed oxidation mechanisms can support, which has led to speculation that other mechanisms might play a part6. Ehn and co-workers' observation of a hitherto unmeasured class of gas-phase oxidation products obviates the need to invoke such mechanisms. The authors convincingly argue that this direct pathway to low-volatility compounds can contribute to observed particle formation and explain particle growth in boreal forest regions, consistent with recent predictions7.
Ehn et al. propose that the products are formed efficiently in the reaction of endocyclic alkenes (chemical structures found in many abundant terpenes) with ozone. They suggest a mechanism consisting of a chain reaction in which a hydrogen atom is rapidly removed from a terpene molecule, forming a free radical to which an oxygen molecule attaches; this cycle repeats several times before terminating (see Extended Data Fig. 9a of the paper1). Their hypothesis may explain why models better approximate biogenic SOA if they allow the products of the first oxidation reactions of terpenes to be involatile8. In any case, inclusion of such products in atmospheric oxidation mechanisms can only improve our predictive capability and potentially close the gap between bottom-up models and atmospheric observations. The hypothesized mechanism also provides a means by which the particle-forming potential of BVOCs can be affected by anthropogenic emissions, because the extent of the chain reaction will be influenced by atmospheric levels of compounds produced by human activities.
The current study should provoke valuable developments in several areas. Attempts should be made to directly establish the oxidation mechanisms that so efficiently generate highly oxidized compounds, to identify the products and the kinetics of the participating reactions. This information will enable detailed analyses of the sensitivity of the SOA-formation process to different conditions, and of the sensitivities of components of previously proposed oxidation mechanisms to the newly described process.
Although Ehn and colleagues' logic is convincing, they did not directly determine the volatility of the observed products. It would therefore be beneficial if their attribution of complete involatility (or extremely low volatility) to the products is substantiated by measurement. That said, measurement will be difficult: the compounds are likely to have several hydroperoxide groups (HOO) and/or peroxy acids (HOOC=O) on a monoterpene backbone, and so will probably be difficult to make. Estimating the compounds' volatilities will also be difficult, because hydroperoxides and peroxy acids are poorly represented in techniques used for such estimations.
The current work highlights the importance of accurately measuring atmospheric oxidant levels and BVOC emissions. Anthropogenic changes in the ratio of ozone to hydroxyl radicals (both of which are key oxidizers in the atmosphere), and in concentrations of nitrogen oxides (which affect atmospheric ozone levels) will influence climate through SOA and its effects on concentrations of cloud condensation nuclei, the particles that act as seeds for cloud-droplet formation.
Biogenic SOA particles probably have a substantial role in natural climate feedback cycles, whereby emissions of BVOCs are affected by the direct or indirect effects of the particles on the intensity of solar radiation that reaches the ground9,10. The influences of climate and of perturbations to terrestrial vegetation caused by human activities will also affect BVOC emissions. Mechanistic11 and sensitivity12 elements of the feedbacks mentioned above have received considerable recent attention. The mechanistic insights into the production of low-volatility compounds provided by Ehn et al. should lead to a better description of how aerosol particles and cloud condensation nuclei form, and of the associated climate feedbacks following BVOC emission changes, thereby improving the predictive capabilities of climate and Earth system models.
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