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

Abstract. 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 (25th–75th percentile) implying 18.5–104 pptV (25th–75th 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.