Molecular understanding of the suppression of new-particle formation by isoprene

Abstract

<jats:p>Abstract. Nucleation of atmospheric vapors 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 vapors. 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 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. </jats:p>

CRI-HOM: A novel chemical mechanism for simulating highly oxygenated organic molecules (HOMs) in global chemistry–aerosol–climate models

“…At low temperatures (blue data), the uncertainty in autoxidation temperature dependence has the greatest effect, while at high temperatures this feature is muted. The increase in yield with temperature is in qualitative agreement with observation (Quéléver et al, 2019;Simon et al, 2020).…”

“…All HOMs were treated as being equally proficient at nucleating new particles, in agreement with the approach and nucleation rates used by Kirkby et al (2016) and Gordon et al (2017). Recent work by Heinritzi et al (2020) suggests that 20-carbon accretion products may be better at nucleating new particles, and therefore the results presented are likely to be an upper bound although nevertheless informative. Representing the different nucleation efficiencies of different HOM species will be investigated in future work.…”

“…New experimental evidence suggests that isoprene may suppress the formation of the most involatile accretion products and thus the smallest aerosol particles (Berndt et al, 2018a;McFiggans et al, 2019;Heinritzi et al, 2020), and reproducing the effect of isoprene has been identified as an important requirement for future mechanisms (Roldin et al, 2019). This inhibition is driven firstly by isoprenescavenging OH radicals (Lee et al, 2016;Kiendler-Scharr et al, 2009;Berndt et al, 2018a), thus reducing the formation of large peroxy radicals by reaction of α-pinene with OH.…”

“…In established schemes such as the Master Chemical Mechanism (MCM; Jenkin et al, 1997;Saunders et al, 2003), the Common Representative Intermediates (CRI; Utembe et al, 2010;Watson et al, 2008;Jenkin et al, 2008Jenkin et al, , 2019b, and the chemistry of the stratosphere and troposphere (StratTrop) used in the climate model UKCA (United Kingdom Chemistry and Aerosols; Archibald et al, 2020), formation of accretion products is not included as it was previously considered negligible or too complex to include. However, experimental work suggests that accretion product formation is a competitive pathway for larger peroxy radicals, such as those formed from α-pinene (Berndt et al, 2018a;Molteni et al, 2019;Simon et al, 2020), and indeed becomes more favourable with increasing function-ality and size; the rate coefficient for the accretion reaction between two α-pinene-derived peroxy radicals was observed to be 16-80 times greater (depending on the extent of oxidation) than for the analogous reaction between two isoprenederived peroxy radicals (Berndt et al, 2018a, b).…”

Contribution of New Particle Formation to Cloud Condensation Nuclei Activity and its Controlling Factors in a Mountain Region of Inland China

“…Recent studies have found that ions and iodine‐containing species also play important roles in nucleation processes (Jokinen et al., 2018; Kirkby et al., 2011, 2016; Kürten et al., 2016; Sipilä et al., 2016). On the other hand, biogenic hydrocarbon such as isoprene was found to suppress NPF, though currently the mechanism is still not fully understood (Heinritzi et al., 2020; Kanawade et al., 2011; Kiendler‐Scharr et al., 2012; McFiggans et al., 2019). Furthermore, NPF can be suppressed under high NO x conditions, particularly in urban atmosphere (Wildt et al., 2014).…”