08 Sep 2023
 | 08 Sep 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

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

Jiangyi Zhang, Jian Zhao, Yuanyuan Luo, Valter Mickwitz, Douglas Worsnop, and Mikael Ehn

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, NOx, and VOC. Understanding the O3 formation sensitivity to changes in VOC and NOx is crucial for making optimal mitigation policies to control O3 concentrations. However, determining the specific O3 formation regimes (either VOC-limited or NOx-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 NOx 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.

Jiangyi Zhang et al.

Status: open (until 28 Oct 2023)

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Jiangyi Zhang et al.

Jiangyi Zhang et al.


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Short summary
Due to the intrinsic connection between the formation pathways of O3 and HOM, the ratio of HOM dimers or non-nitrate monomers to HOM organic nitrates could be used to determine O3 formation regimes. Owing to the fast formation and short lifetimes of HOM, the HOM-based indicating ratios can describe the O3 formation in real time. Despite the success of our approach in this simple laboratory system, the applicability to the much more complex atmosphere remains to be determined.