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

Ozone (inline-formulaO₃), an important and ubiquitous trace gas, protects lives from harmful solar ultraviolet (UV) radiation in the stratosphere but is toxic to living organisms in the troposphere. Additionally, tropospheric inline-formulaO₃ is a key oxidant and a source of other oxidants (e.g., inline-formulaOH and inline-formulaNO₃ radicals) for various volatile organic compounds (VOCs). Recently, highly oxygenated organic molecules (inline-formulaHOMs) were identified as a new compound group formed from the oxidation of many VOCs, making up a significant source of secondary organic aerosol (SOA). The pathways forming inline-formulaHOMs from VOCs involve autoxidation of peroxy radicals (inline-formulaRO₂), formed ubiquitously in many VOC oxidation reactions. The main sink for inline-formulaRO₂ is bimolecular reactions with other radicals, such as inline-formulaHO₂, inline-formulaNO, or other inline-formulaRO₂, and this largely determines the structure of the end products. Organic nitrates form solely from inline-formulaRO₂ inline-formula+ inline-formulaNO reactions, while accretion products (“dimers”) form solely from inline-formulaRO₂ inline-formula+ inline-formulaRO₂ reactions. The inline-formulaRO₂ inline-formula+ inline-formulaNO reaction also converts inline-formulaNO into inline-formulaNO₂, making it a net source for inline-formulaO₃ through inline-formulaNO₂ photolysis.

There is a highly nonlinear relationship between inline-formulaO₃, inline-formulaNO_x, and inline-formulaVOCs. Understanding the inline-formulaO₃ formation sensitivity to changes in VOCs and inline-formulaNO_x is crucial for making optimal mitigation policies to control inline-formulaO₃ concentrations. However, determining the specific inline-formulaO₃ formation regimes (either VOC-limited or inline-formulaNO_x-limited) remains challenging in diverse environmental conditions. In this work we assessed whether inline-formulaHOM measurements can function as a real-time indicator for the inline-formulaO₃ formation sensitivity based on the hypothesis that inline-formulaHOM compositions can describe the relative importance of inline-formulaNO as a terminator for inline-formulaRO₂. Given the fast formation and short lifetimes of low-volatility inline-formulaHOMs (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 inline-formulaNO_x and VOCs under different inline-formulaNO₂ photolysis rates. We also measured the relative concentrations of inline-formulaHOMs of different types (dimers, nitrate-containing monomers, and non-nitrate monomers) and used ratios between these to estimate the inline-formulaO₃ formation sensitivity. We find that for this simple system, the inline-formulaO₃ sensitivity could be described very well based on the inline-formulaHOM measurements. Future work will focus on determining to what extent this approach can be applied in more complex atmospheric environments. Ambient measurements of inline-formulaHOMs have become increasingly common during the last decade, and therefore we expect that there are already a large number of groups with available data for testing this approach.