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

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- 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.