Abstract
Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood. Globally, around half of cloud condensation nuclei originate from nucleation of atmospheric vapours. It is thought that sulfuric acid is essential to initiate most particle formation in the atmosphere, and that ions have a relatively minor role. Some laboratory studies, however, have reported organic particle formation without the intentional addition of sulfuric acid, although contamination could not be excluded. Here we present evidence for the formation of aerosol particles from highly oxidized biogenic vapours in the absence of sulfuric acid in a large chamber under atmospheric conditions. The highly oxygenated molecules (HOMs) are produced by ozonolysis of α-pinene. We find that ions from Galactic cosmic rays increase the nucleation rate by one to two orders of magnitude compared with neutral nucleation. Our experimental findings are supported by quantum chemical calculations of the cluster binding energies of representative HOMs. Ion-induced nucleation of pure organic particles constitutes a potentially widespread source of aerosol particles in terrestrial environments with low sulfuric acid pollution.
Highlights
Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood[1]
It is well established that oxidation products of volatile organic compounds (VOCs) are important for particle growth[11], but whether their role in the smallest particles is in nucleation or growth alone has remained ambiguous[4,12,13]
It has been shown that oxidized organic compounds do help to stabilize sulfuric acid clusters and probably play a major role in atmospheric particle nucleation[6,14,15]. We refer to these compounds as highly oxygenated molecules (HOMs) rather than ELVOCs16 because the measured compounds span a wide range of low volatilities
Summary
In contrast to negative clusters, the positive clusters nucleate only with dimers, producing distinct mass bands that are detected up to E10 in the APiTOF (Fig. 2c, d) This indicates the importance of dimers for pure biogenic nucleation. Positive and negative clusters nucleate at comparable rates (an example is shown in Extended Data Fig. 5). We further investigated the dependence on ion species by adding small amounts of SO2 to the chamber, up to around 1,000 p.p.t.v. When [H2SO4] exceeds about 1 × 105 cm−3, the major negative ion species shift to HSO−4 , SO−5 and SO−4 (Extended Data Fig. 1c), owing to their lower proton affinity
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