Gas phase formation of extremely oxidized pinene reaction products in chamber and ambient air

M Ehn,G Lönn,M Dal Maso,H Junninen,T Petäjä,A Trimborn,J Wildt,A Wahner,E Kleist,S Schobesberger,Th F Mentel,M Kulmala,D R Worsnop

doi:10.5194/acp-12-5113-2012

Abstract

Abstract. High molecular weight (300–650 Da) naturally charged negative ions have previously been observed at a boreal forest site in Hyytiälä, Finland. The long-term measurements conducted in this work showed that these ions are observed practically every night between spring and autumn in Hyytiälä. The ambient mass spectral patterns could be reproduced in striking detail during additional measurements of α-pinene (C10H16) oxidation at low-OH conditions in the Jülich Plant Atmosphere Chamber (JPAC). The ions were identified as clusters of the nitrate ion (NO3−) and α-pinene oxidation products reaching oxygen to carbon ratios of 0.7–1.3, while retaining most of the initial ten carbon atoms. Attributing the ions to clusters instead of single molecules was based on additional observations of the same extremely oxidized organics in clusters with HSO4− (Hyytiälä) and C3F5O2− (JPAC). The most abundant products in the ion spectra were identified as C10H14O7, C10H14O9, C10H16O9, and C10H14O11. The mechanism responsible for forming these molecules is still not clear, but the initial reaction is most likely ozone attack at the double bond, as the ions are mainly observed under dark conditions. β-pinene also formed highly oxidized products under the same conditions, but less efficiently, and mainly C9 compounds which were not observed in Hyytiälä, where β-pinene on average is 4–5 times less abundant than α-pinene. Further, to explain the high O/C together with the relatively high H/C, we propose that geminal diols and/or hydroperoxide groups may be important. We estimate that the night-time concentration of the sum of the neutral extremely oxidized products is on the order of 0.1–1 ppt (~106–107 molec cm−3). This is in a similar range as the amount of gaseous H2SO4 in Hyytiälä during day-time. As these highly oxidized organics are roughly 3 times heavier, likely with extremely low vapor pressures, their role in the initial steps of new aerosol particle formation and growth may be important and needs to be explored in more detail in the future.

Highlights

Earth’s atmosphere contains a large variety of different trace gases, produced both from biogenic and anthropogenic sources
The mechanism responsible for forming these molecules is still not clear, but the initial reaction is most likely ozone attack at the double bond, as the ions are mainly observed under dark conditions. β-pinene formed highly oxidized products under the same conditions, but less efficiently, and mainly C9 compounds which were not observed in Hyytiala, where β-pinene on average is 4–5 times less abundant than α-pinene
High molecular weight ions have previously been reported from Hyytiala (Ehn et al, 2010), and additional longterm measurements during May–November 2010 showed that these ions are abundant almost every night

Summary

Introduction

Earth’s atmosphere contains a large variety of different trace gases, produced both from biogenic and anthropogenic sources. Many of these trace gases impact everyday life, either through their influence on air quality (e.g. SO2, tropospheric ozone, NOx) or climate. The climate-influence occurs both through the greenhouse effect from e.g. CO2 and CH4, as well as through oxidation products of trace gases that can condense onto, or even form new, aerosol particles. These aerosol particles can either directly scatter incoming solar radiation, or form cloud droplets that affect radiative transfer even more efficiently. Organic aerosol (OA) can be primary, i.e. emitted directly as aerosol particles, or secondary, formed from

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