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European Geosciences Union, Atmospheric Chemistry and Physics, 1(18), p. 65-79, 2018

DOI: 10.5194/acp-18-65-2018

European Geosciences Union, Atmospheric Chemistry and Physics Discussions, p. 1-30

DOI: 10.5194/acp-2017-426

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Influence of temperature on the molecular composition of ions and charged clusters during pure biogenic nucleation

Journal article published in 2017 by Carla Frege ORCID, Ismael K. Ortega, Matti Petteri Rissanen ORCID, Arnaud P. Praplan ORCID, Gerhard Steiner ORCID, Martin Heinritzi, Lauri Reino Antero Ahonen ORCID, António Amorim, Anne-Kathrin Bernhammer ORCID, Federico Bianchi ORCID, Sophia Brilke ORCID, Martin Breitenlechner, Lubna Dada ORCID, António Dias, Jonathan Duplissy and other authors.
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

Abstract. It was recently shown by the CERN CLOUD experiment that biogenic highly oxygenated molecules (HOMs) form particles under atmospheric conditions in the absence of sulfuric acid, where ions enhance the nucleation rate by 1–2 orders of magnitude. The biogenic HOMs were produced from ozonolysis of α-pinene at 5 ∘C. Here we extend this study to compare the molecular composition of positive and negative HOM clusters measured with atmospheric pressure interface time-of-flight mass spectrometers (APi-TOFs), at three different temperatures (25, 5 and −25 ∘C). Most negative HOM clusters include a nitrate (NO3-) ion, and the spectra are similar to those seen in the nighttime boreal forest. On the other hand, most positive HOM clusters include an ammonium (NH4+) ion, and the spectra are characterized by mass bands that differ in their molecular weight by ∼ 20 C atoms, corresponding to HOM dimers. At lower temperatures the average oxygen to carbon (O : C) ratio of the HOM clusters decreases for both polarities, reflecting an overall reduction of HOM formation with decreasing temperature. This indicates a decrease in the rate of autoxidation with temperature due to a rather high activation energy as has previously been determined by quantum chemical calculations. Furthermore, at the lowest temperature (−25 ∘C), the presence of C30 clusters shows that HOM monomers start to contribute to the nucleation of positive clusters. These experimental findings are supported by quantum chemical calculations of the binding energies of representative neutral and charged clusters.