On the potential use of highly oxygenated organic molecules (HOM) as indicators for ozone formation sensitivity

Abstract. Ozone (O₃), an important and ubiquitous trace gas, protects lives from harm of solar ultraviolet (UV) radiation in the stratosphere but is toxic to living organisms in the troposphere. Additionally, tropospheric O₃ is a key oxidant, and source of other oxidants (e.g, OH and NO₃ radicals) for various volatile organic compounds (VOC). Recently, highly oxygenated organic molecules (HOM) were identified as a new compound group formed from oxidation of many VOC, making up a significant source of secondary organic aerosol (SOA). The pathways forming HOM from VOC involve autoxidation of peroxy radicals (RO₂), formed ubiquitously in many VOC oxidation reactions. The main sink for RO₂ is bimolecular reactions with other radicals, HO₂, NO or other RO₂, and this largely determines the structure of the end products. Organic nitrates form solely from RO₂ + NO reactions while accretion products (“dimers”) solely from RO₂ + RO₂ reactions. The RO₂ + NO reaction also converts NO into NO₂, making it a net source for O₃ through NO₂ photolysis.

There is a highly nonlinear relationship between O₃, NO_x, and VOC. Understanding the O₃ formation sensitivity to changes in VOC and NO_x is crucial for making optimal mitigation policies to control O₃ concentrations. However, determining the specific O₃ formation regimes (either VOC-limited or NO_x-limited) remains challenging in diverse environmental conditions. In this work we assessed whether HOM measurements can function as a real-time indicator for the O₃ formation sensitivity based on the hypothesis that HOM compositions can describe the relative importance of NO as a terminator for RO₂. Given the fast formation and short lifetimes of the low-volatile HOM (timescale of minutes), they describe the instantaneous chemical regime of the atmosphere. In this work, we conducted a series of monoterpene oxidation experiments in our chamber while varying the concentrations of NO_x and VOC under different NO₂ photolysis rates. We also measured the relative concentrations of HOM of different types (dimers, nitrate-containing monomers, and non-nitrate monomers) and used ratios between these to estimate the O₃ formation sensitivity. We find that for this simple system, the O₃ sensitivity could be described very well based on the HOM measurements. Future work will focus on determining to what extent this approach can be applied in more complex atmospheric environments. Ambient measurements of HOM have become increasingly common during the last decade, and therefore we expect that there already are a large amount groups with available data for testing this approach.