Fundamental Oxidation Processes in the Remote Marine Atmosphere Investigated Using the NO-NO2-O3 Photostationary State.

The photostationary state (PSS) equilibrium between NO and NO₂ is reached within minutes in the atmosphere and can be described by the PSS parameter, φ. Deviations from expected values of φ have previously been used to infer missing oxidants in diverse locations, from highly polluted regions to the extremely clean conditions observed in the remote marine boundary layer (MBL), and have been interpreted as missing understanding of fundamental photochemistry. Here, contrary to these previous observations, we observe good agreement between PSS-derived NO₂ ([NO₂]PSS ext.) calculated from photochemical model predictions of peroxy radicals (RO₂ and HO₂) and measured NO, O₃, and jNO₂, and observed NO₂ in extremely clean air containing low levels of CO (< 90 ppbV) and VOCs. However, in clean air containing small amounts of aged pollution (CO > 100 ppbV), we observed higher levels of NO₂ than inferred from the PSS, with [NO₂]_Obs./[NO₂]_{PSS ext}. of 1.12–1.68 (25^th–75^th percentile) implying 18.5–104 pptV (25^th–75^th percentile) of missing RO₂ radicals. Potential NO₂ measurement artefacts have to be carefully considered when comparing PSS-derived NO₂ to observed NO₂, but we show that the NO₂ artefact required to explain the deviation would have to be ~ 4 times greater than the maximum calculated from known interferences. If the missing RO₂ radicals have an ozone production efficiency equivalent to that of methyl peroxy radicals (CH₃O₂), then the calculated net ozone production including these additional oxidants is similar to that observed, within estimated uncertainties, once halogen oxide chemistry is accounted for. This implies that peroxy radicals cannot be excluded as the missing oxidant in clean marine air containing aged pollution, and that measured and modelled RO₂ are both significantly underestimated under these conditions. [ABSTRACT FROM AUTHOR]