Molecular understanding of new-particle formation from alpha-pinene between −50 °C and 25 °C
Authors: Simon Mario , Dada Lubna , Heinritzi Martin , Scholz Wiebke , Stolzenburg Dominik , Fischer Lukas , Wagner Andrea C. , Kürten Andreas , Rörup Birte , He Xu-Cheng , Almeida João , Baalbaki Rima , Baccarini Andrea , Bauer Paulus S. , Beck Lisa , Bergen Anton , Bianchi Federico , Bräkling Steffen , Brilke Sophia , Caudillo Lucia , Chen Dexian , Chu Biwu , Dias António , Draper Danielle C. , Duplissy Jonathan , El Haddad Imad , Finkenzeller Henning , Frege Carla , Gonzalez-Carracedo Loic , Gordon Hamish , Granzin Manuel , Hakala Jani , Hofbauer Victoria , Hoyle Christopher R. , Kim Changhyuk , Kong Weimeng , Lamkaddam Houssni , Lee Chuan P. , Lehtipalo Katrianne , Leiminger Markus , Mai Huajun , Manninen Hanna E. , Marie Guillaume , Marten Ruby , Mentler Bernhard , Molteni Ugo , Nichman Leonid , Nie Wei , Ojdanic Andrea , Onnela Antti , Partoll Eva , Petäjä Tuukka , Pfeifer Joschka , Philippov Maxim , Quéléver Lauriane L. J. , Ranjithkumar Ananth , Rissanen Matti , Schallhart Simon , Schobesberger Siegfried , Schuchmann Simone , Shen Jiali , Sipilä Mikko , Steiner Gerhard , Stozhkov Yuri , Tauber Christian , Tham Yee J. , Tomé António R. , Vazquez-Pufleau Miguel , Vogel Alexander , Wagner Robert , Wang Mingyi , Wang Dongyu S. , Wang Yonghong , Weber Stefan K. , Wu Yusheng , Xiao Mao , Yan Chao , Ye Penglin , Ye Qing , Zauner-Wieczorek Marcel , Zhou Xueqin , Baltensperger Urs , Dommen Josef , Flagan Rick C. , Hansel Armin , Kulmala Markku , Volkamer Rainer , Winkler Paul M. , Worsnop Douglas R. , Donahue Neil M. , Kirkby Jasper , Curtius Joachim 
Published: January 2020

Abstract. Highly-oxygenated organic molecules (HOMs) contribute substantially to the formation and growth of atmospheric aerosol particles, which affect air quality, human health and Earth's climate. HOMs are formed by rapid, gas-phase autoxidation of volatile organic compounds (VOCs) such as α-pinene, the most abundant monoterpene in the atmosphere. Due to their abundance and low volatility, HOMs can play an important role for new-particle formation (NPF) and the early growth of atmospheric aerosols, even without any further assistance of other low-volatility compounds such as sulfuric acid. Both the autoxidation reaction forming HOMs and their new-particle formation rates are expected to be strongly dependent on temperature. However, experimental data on both effects are limited. Dedicated experiments were performed at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN to address this question. In this study, we show that a decrease in temperature (from +25 to −50 °C) results in a reduced HOM yield and reduced oxidation state of the products, whereas the new-particle formation rates (J1.7 nm) increase substantially. Measurements with two different chemical ionization mass spectrometers (using nitrate and protonated water as reagent ion, respectively) provide the molecular composition of the gaseous oxidation products and a 2-dimensional volatility basis set model (2D-VBS) provides their volatility distribution. The HOM yield decreases with temperature from 6.2 % at 25 °C to 0.7 % at −50 °C. However, there is a strong reduction of the saturation vapor pressure of each oxidation state as the temperature is reduced. Overall, the reduction in volatility with temperature leads to an increase in the nucleation rates by up to three orders of magnitude at −50 °C compared with 25 °C. In addition, the enhancement of the nucleation rates by ions decreases with decreasing temperature, since the neutral molecular clusters have increased stability against evaporation. The resulting data quantify how the interplay between the temperature-dependent oxidation pathways and the associated vapor pressures affect biogenic new-particle formation at the molecular level. Our measurements therefore improve our understanding of pure biogenic new-particle formation for a wide range of tropospheric temperatures and precursor concentrations.