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 , Haddad Imad El , He Xucheng , Helm Johanna , Hofbauer Victoria , Hoyle Christopher R. , Kangasluoma Juha , Keber Timo , Kim Changhyuk , Kürten Andreas , Lamkaddam Houssni , Lampilahti Janne , Laurila Tiia M. , 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 , 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: February 2020

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.