Chemical composition of nanoparticles from α-pinene nucleation and the influence of isoprene and relative humidity at low temperature
Authors: Caudillo Lucía , Rörup Birte , Heinritzi Martin , Marie Guillaume , Simon Mario , Wagner Andrea C. , Müller Tatjana , Granzin Manuel , Amorim Antonio , Ataei Farnoush , Baalbaki Rima , Bertozzi Barbara , Brasseur Zoé , Chiu Randall , Chu Biwu , Dada Lubna , Duplissy Jonathan , Finkenzeller Henning , Gonzalez Carracedo Loïc , He Xu-Cheng , Hofbauer Victoria , Kong Weimeng , Lamkaddam Houssni , Lee Chuan P. , Lopez Brandon , Mahfouz Naser G. A. , Makhmutov Vladimir , Manninen Hanna E. , Marten Ruby , Massabò Dario , Mauldin Roy L. , Mentler Bernhard , Molteni Ugo , Onnela Antti , Pfeifer Joschka , Philippov Maxim , Piedehierro Ana A. , Schervish Meredith , Scholz Wiebke , Schulze Benjamin , Shen Jiali , Stolzenburg Dominik , Stozhkov Yuri , Surdu Mihnea , Tauber Christian , Tham Yee Jun , Tian Ping , Tomé António , Vogt Steffen , Wang Mingyi , Wang Dongyu S. , Weber Stefan K. , Welti André , Yonghong Wang , Yusheng Wu , Zauner-Wieczorek Marcel , Baltensperger Urs , El Haddad Imad , Flagan Richard C. , Hansel Armin , Höhler Kristina , Kirkby Jasper , Kulmala Markku , Lehtipalo Katrianne , Möhler Ottmar , Saathoff Harald , Volkamer Rainer , Winkler Paul M. , Donahue Neil M. , Kürten Andreas , Curtius Joachim 
Published: July 2021

Abstract. New Particle Formation (NPF) from biogenic organic precursors is an important atmospheric process. One of the major species is α-pinene, which upon oxidation, can form a suite of products covering a wide range of volatilities. A fraction of the oxidation products is termed Highly Oxygenated Organic Molecules (HOM). These play a crucial role for nucleation and the formation of Secondary Organic Aerosol (SOA). However, measuring the composition of newly formed particles is challenging due to their very small mass. Here, we present results on the gas and particle phase chemical composition for a system where α-pinene was oxidized by ozone, and for a mixed system of α-pinene and isoprene, respectively. The measurements took place at the CERN Cosmics Leaving Outdoor Droplets (CLOUD) chamber at temperatures between −50 °C and −30 °C and at low and high relative humidity (20 % and 60 to 100 % RH). These conditions were chosen to simulate pure biogenic new particle formation in the upper free troposphere. The particle chemical composition was analyzed by the Thermal Desorption-Differential Mobility Analyzer (TD-DMA) coupled to a nitrate chemical ionization time-of-flight mass spectrometer. This instrument can be used for particle and gas phase measurements using the same ionization and detection scheme. Our measurements revealed the presence of C8-10 monomers and C18-20 dimers as the major compounds in the particles (diameter up to ~ 100 nm). Particularly, for the system with isoprene added, C5 (C5H10O5-7) and C15 compounds (C15H24O5-10) are detected. This observation is consistent with the previously observed formation of such compounds in the gas phase. However, although the C5 and C15 compounds do not easily nucleate, our measurements indicate that they can still contribute to the particle growth at free tropospheric conditions. For the experiments reported here, most likely isoprene might enhance growth at particle sizes larger than 15 nm. Besides the chemical information regarding the HOM formation for the α-pinene (plus isoprene) system, we report on the nucleation rates measured at 1.7 nm and found that the lower J1.7nm values compared with previous studies are very likely due to the higher α-pinene and ozone mixing ratios used in the present study