Three studies find that a family of organic compounds affects the formation and initial growth of atmospheric aerosol particles in clean air — with implications for our knowledge of the climate effects of aerosols. See Letters p.521 & 527
Cloud droplets form when water condenses on microscopic aerosol particles1. A key source of new particles in the atmosphere is nucleation — the formation and growth of molecular clusters, which must then grow about 50 times larger if they are to act as efficient cloud seeds. Sulfuric acid has long been recognized as the key player in particle formation2. But two studies in this issue3,4, and another published in Science5, suggest that molecules called highly oxidized multifunctional organic compounds (HOM compounds) have an under-appreciated role in driving both particle formation and the initial growth of particles, especially in environments largely unaffected by anthropogenic pollution.
Understanding the differences between past and present particle formation and growth rates is crucial in quantifying the aerosol cooling effect6, which has offset warming driven by greenhouse gases over the past century, but remains highly uncertain7. Atmospheric sulfur emissions are higher today than in pre-industrial times because of increased fossil-fuel combustion8, so to understand how particles affected the climate in the past, and how they affect pristine regions of the atmosphere today, it is necessary to characterize particle formation and growth when sulfuric acid concentrations are low. The latest studies together indicate that HOM compounds are key players.
HOM compounds form when hydrocarbons and other volatile organic compounds (VOCs), emitted into the atmosphere from many natural and anthropogenic sources, react with atmospheric oxidants, such as ozone9,10. They are diverse, containing varying numbers of molecules from a wide range of chemical groups, including alcohols and peroxides. Consequently, their vapour pressures — a property that determines their ability to condense — vary by more than 15 orders of magnitude4.
Kirkby et al.3 (page 521) investigated how effective HOM compounds are at producing new particles with diameters larger than 1.7 nanometres at low sulfuric acid concentrations, whereas Tröstl et al.4 (page 527) determined the role of HOM compounds in the particles' subsequent growth (for particles starting at about 2 nm in diameter and increasing to about 20 nm). Both studies were performed in the laboratory, and used a VOC called α-pinene — a molecule emitted by trees and from the ocean — as the source of HOM compounds.
In their study, Kirkby et al. demonstrate that HOM compounds can nucleate to form particles without sulfuric acid, and that the particle-formation rate depends on the presence of Galactic cosmic rays (GCRs). Although previous observations11,12 showed that organic compounds can enhance sulfuric acid-driven particle-formation rates, a direct demonstration of particle formation by organics in the absence of sulfuric acid had been elusive. The dependence of the organic-driven particle-formation rate on GCRs provides a potential connection between the magnetic variability of the Sun (which affects the GCR flux to Earth), particles and climate, an association that remains widely debated.
Newly formed nanoparticles grow through condensation. The growth stage is crucial for particles less than 10 nm in diameter, because they are especially prone to being absorbed by larger particles on collision, thus removing potential cloud seeds from the atmosphere. Nanoparticle-growth rates increase with diameter13, perhaps because of condensation of organic compounds14, but disentangling the controlling factors has been challenging.
Tröstl et al. show that the Kelvin effect — in which the volatility of liquids increases when their interface with the surrounding vapour is curved — rapidly decreases at nanoparticle surfaces as the particles grow. This allows increasingly efficient condensation of HOM compounds that have progressively higher (but always very low) volatilities as the particles grow. Importantly, the accelerating growth rates directly result from the fact that HOM compounds have a distribution of volatilities.
In complement to the two laboratory studies, Bianchi et al.5 used field observations made at the Jungfraujoch research station in Switzerland (Fig. 1) to show that, when sulfuric acid concentrations are low, particle formation and accelerating growth are indeed efficient only when concentrations of HOM compounds are sufficiently large. Although the observed particle-formation rates are in reasonable agreement with Kirkby and colleagues' results, Bianchi et al. were unable to reproduce the observed acceleration in growth rates for particles less than 10 nm in diameter using a mathematical model; by contrast, Tröstl and co-workers were able to model the acceleration in growth rates observed in their study. The authors also found that the HOM compounds at Jungfraujoch were probably anthropogenic in origin, rather than biogenic, suggesting that many VOCs are HOM-compound precursors. Regardless of origin, it seems that the contribution of HOM compounds to nucleation and growth increases as sulfuric acid concentrations fall.
Tröstl and colleagues used their experimental results to constrain simulations made using a global aerosol model. These simulations indicate that the concentration of efficient cloud seeds in the modern atmosphere increases substantially when HOM compounds are included in nanoparticle growth — consistent with the findings of other groups (see ref. 15, for example). Previously reported simulations6 indicated that our understanding of how aerosols affect clouds and climate is limited largely by uncertainties in the natural sulfur cycle, but they considered only particle formation induced by sulfuric acid. The latest results suggest that this view must be reassessed, and that uncertainties stemming from the natural VOC cycle are probably larger than was thought.
One challenge in developing robust predictions from these three studies is that HOM compounds were defined as only those that can be detected using a particular type of mass spectrometer. But the detectabilities of HOM compounds of different compositions and volatilities are not fully established. Although the total concentration of HOM compounds correlated with particle-formation rates and growth rates in both laboratory studies3,4, Tröstl and colleagues' results show that HOM-compound identity cannot be neglected.
Furthermore, different VOCs are not equally efficient at producing HOM compounds10. The empirical, laboratory-derived relationships were determined only for α-pinene, and so it remains to be seen whether they are generally robust; the mismatch between Bianchi and colleagues' field observations and the laboratory-based predictions suggests that more work is needed. Another issue is that the molecular forces that determine the stability of clusters made purely from HOM compounds are unknown. Nonetheless, the three papers provide a solid foundation for understanding the effects of atmospheric organic compounds on particle abundances in the past, present and future.
The current studies focus on the role of HOM-compound vapours in particle formation and the initial stages of growth. But aerosol particles are microreactors in which chemical reactions occur after, or even during, condensation. This transforms particle compositions16 and can therefore influence the overall life cycle and climate impacts of particles by altering their volatilities17,18, interactions with water19 and reactivity20. A better understanding is needed of how the compositions of HOM compounds in vapour (which were measured in these studies) affect the molecular composition of particles, to establish the full life cycle of aerosols and their effects on the atmosphere.
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