Abstract
Abstract. Nucleation of atmospheric vapours produces more than half of global cloud condensation nuclei and so has an important influence on climate. Recent studies show that monoterpene (C10H16) oxidation yields highly oxygenated products that can nucleate with or without sulfuric acid. Monoterpenes are emitted mainly by trees, frequently together with isoprene (C5H8), which has the highest global emission of all organic vapours. Previous studies have shown that isoprene suppresses new-particle formation from monoterpenes, but the cause of this suppression is under debate. Here, in experiments performed under atmospheric conditions in the CERN CLOUD chamber, we show that isoprene reduces the yield of highly oxygenated dimers with 19 or 20 carbon atoms – which drive particle nucleation and early growth – while increasing the production of dimers with 14 or 15 carbon atoms. The dimers (termed C20 and C15, respectively) are produced by termination reactions between pairs of peroxy radicals (RO2⚫) arising from monoterpenes or isoprene. Compared with pure monoterpene conditions, isoprene reduces nucleation rates at 1.7 nm (depending on the isoprene ∕ monoterpene ratio) and approximately halves particle growth rates between 1.3 and 3.2 nm. However, above 3.2 nm, C15 dimers contribute to secondary organic aerosol, and the growth rates are unaffected by isoprene. We further show that increased hydroxyl radical (OH⚫) reduces particle formation in our chemical system rather than enhances it as previously proposed, since it increases isoprene-derived RO2⚫ radicals that reduce C20 formation. RO2⚫ termination emerges as the critical step that determines the highly oxygenated organic molecule (HOM) distribution and the corresponding nucleation capability. Species that reduce the C20 yield, such as NO, HO2 and as we show isoprene, can thus effectively reduce biogenic nucleation and early growth. Therefore the formation rate of organic aerosol in a particular region of the atmosphere under study will vary according to the precise ambient conditions.
Highlights
Nucleation of aerosol particles is observed in many environments, ranging from boreal forests to urban and coastal areas, from polar to tropical regions, and from the boundary layer to the free troposphere (Kerminen et al, 2018)
Pure biogenic nucleation was first described for α-pinene oxidation in the Cosmics Leaving Outdoor Droplets (CLOUD) chamber (Kirkby et al, 2016)
Global evaluation of this process with the help of atmospheric modelling found an over-prediction of cloud condensation nuclei (CCN) concentrations in the Amazon, leading to speculation about an as yet unaccountedfor chemical suppression mechanism for new-particle formation involving isoprene (Gordon et al, 2016)
Summary
Nucleation of aerosol particles is observed in many environments, ranging from boreal forests to urban and coastal areas, from polar to tropical regions, and from the boundary layer to the free troposphere (Kerminen et al, 2018). Numerous studies report suppression of nucleation in isoprene-rich environments, even if sufficient monoterpenes are present (Lee et al, 2016; Kanawade et al, 2011; Yu et al, 2014; Kiendler-Scharr et al, 2009, 2012; Varanda Rizzo et al, 2018; Wimmer et al, 2018). Further it was shown that ozonolysis is crucial for HOM formation (Ehn et al, 2014; Kirkby et al, 2016) Another proposed possibility for isoprene suppression of nucleation is the deactivation of sulfuric acid cluster growth due to the addition of isoprene oxidation products (Lee et al, 2016). HOMs can nucleate without sulfuric acid (Kirkby et al, 2016), and sup-
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