Molecular understanding of the suppression of new-particle formation by isoprene
Authors: Heinritzi Martin , Dada Lubna , Simon Mario , Stolzenburg Dominik , Wagner Andrea C. , Fischer Lukas , Ahonen Lauri R. , Amanatidis Stavros , Baalbaki Rima , Baccarini Andrea , Bauer Paulus S. , Baumgartner Bernhard , Bianchi Federico , Brilke Sophia , Chen Dexian , Chiu Randall , Dias Antonio , Dommen Josef , Duplissy Jonathan , Finkenzeller Henning , Frege Carla , Fuchs Claudia , Garmash Olga , Gordon Hamish , Granzin Manuel , El Haddad Imad , He Xucheng , Helm Johanna , Hofbauer Victoria , Hoyle Christopher R. , Kangasluoma Juha , Keber Timo , Kim Changhyuk , Kürten Andreas , Lamkaddam Houssni , Laurila Tiia M. , Lampilahti Janne , Lee Chuan Ping , Lehtipalo Katrianne , Leiminger Markus , Mai Huajun , Makhmutov Vladimir , Manninen Hanna Elina , Marten Ruby , Mathot Serge , Mauldin Roy Lee , Mentler Bernhard , Molteni Ugo , Müller Tatjana , Nie Wei , Nieminen Tuomo , Onnela Antti , Partoll Eva , Passananti Monica , Petäjä Tuukka , Pfeifer Joschka , Pospisilova Veronika , Quéléver Lauriane L. J. , Rissanen Matti P. , Rose Clémence , Schobesberger Siegfried , Scholz Wiebke , Scholze Kay , Sipilä Mikko , Steiner Gerhard , Stozhkov Yuri , Tauber Christian , Tham Yee Jun , Vazquez-Pufleau Miguel , Virtanen Annele , Vogel Alexander L. , Volkamer Rainer , Wagner Robert , Wang Mingyi , Weitz Lena , Wimmer Daniela , Xiao Mao , Yan Chao , Ye Penglin , Zha Qiaozhi , Zhou Xueqin , Amorim Antonio , Baltensperger Urs , Hansel Armin , Kulmala Markku , Tomé António , Winkler Paul M. , Worsnop Douglas R. , Donahue Neil M. , Kirkby Jasper , Curtius Joachim 
Published: October 2020

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.