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Recent advances in chemical ionization mass spectrometry have allowed the detection of a new group of compounds termed highly oxygenated molecules (HOM). These are atmospheric oxidation products of volatile organic compounds (VOC) retaining most of their carbon backbone, and with O/C ratios around unity. Owing to their surprisingly high yields and low vapor pressures, the importance of HOM for aerosol formation has been easy to verify. However, the opposite can be said concerning the exact formation pathways of HOM from major aerosol precursor VOC. While the role of peroxy radical autoxidation, i.e., consecutive intramolecular H-shifts followed by O2 addition, has been recognized, the detailed formation mechanisms remain highly uncertain. A primary reason is that the autoxidation process occurs on sub-second timescales and is extremely sensitive to environmental conditions like gas composition, temperature, and pressure. This, in turn, poses a great challenge for chemical kinetics studies to be able to mimic the relevant atmospheric reaction pathways, while simultaneously using conditions suitable for studying the short-lived radical intermediates. In this perspective, we define six specific challenges for this community to directly observe the initial steps of atmospherically relevant autoxidation reactions and thereby facilitate vital improvements in the understanding of VOC degradation and organic aerosol formation.

Gas-phase oxidation routes of biogenic emissions, mainly isoprene and monoterpenes, in the atmosphere are still the subject of intensive research with special attention being paid to the formation of aerosol constituents. This laboratory study shows that the most abundant monoterpenes (limonene and α-pinene) form highly oxidized RO2 radicals with up to 12 O atoms, along with related closed-shell products, within a few seconds after the initial attack of ozone or OH radicals. The overall process, an intramolecular ROO→QOOH reaction and subsequent O2 addition generating a next R'OO radical, is similar to the well-known autoxidation processes in the liquid phase (QOOH stands for a hydroperoxyalkyl radical). Field measurements show the relevance of this process to atmospheric chemistry. Thus, the well-known reaction principle of autoxidation is also applicable to the atmospheric gas-phase oxidation of hydrocarbons leading to extremely low-volatility products which contribute to organic aerosol mass and hence influence the aerosol-cloud-climate system.

Auf welchen Wegen biogene Emissionen (vor allem Isopren und Monoterpene) in der Gasphase der Atmosphare oxidiert werden, ist Gegenstand intensiver Forschung. Besonderes Interesse gilt hierbei der Bildung von Aerosolbestandteilen. Diese Laborstudie zeigt, dass die haufigsten Monoterpene (Limonen und α-Pinen) nach dem Angriff von O3 oder OH-Radikalen innerhalb von Sekunden hochoxidierte RO2-Radikale mit bis zu 12 O-Atomen und zugehorige geschlossenschalige Endprodukte bilden. Der Gesamtprozess, intramolekulare ROO→QOOH-Reaktion mit anschliesender O2-Addition und Bildung eines neuen R′OO-Radikals, ahnelt den bekannten Autoxidationsprozessen in flussiger Phase (QOOH steht fur ein Hydroperoxyalkyl-Radikal). Feldmessungen belegen die Bedeutung des Prozesses fur die Atmospharenchemie. Somit ist das Prinzip der Autoxidation auf die Gasphasenoxidation von Kohlenwasserstoffen in der Atmosphare anwendbar; sie liefert extrem schwerfluchtige Produkte, die zur organischen Aerosolmasse beitragen und dadurch das Aerosol-Wolken-Klimasystem beeinflussen.