In:
Atmospheric Chemistry and Physics, Copernicus GmbH, Vol. 20, No. 20 ( 2020-10-20), p. 11809-11821
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.
Type of Medium:
Online Resource
ISSN:
1680-7324
DOI:
10.5194/acp-20-11809-2020
DOI:
10.5194/acp-20-11809-2020-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2020
detail.hit.zdb_id:
2092549-9
detail.hit.zdb_id:
2069847-1
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