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 measured NO, O₃, and jNO₂ and photochemical box model predictions of peroxy radicals (RO₂ and Hv), and observed NO₂ ([NO₂]Obs.) in extremely clean air containing low levels of CO (<90 ppbV) and VOCs (volatile organic compounds). 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 underestimation of RO₂ radicals by 18.5–104 pptV. Potential NO₂measurement 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 additional RO2 radicals inferred from the PSS convert NO to NO₂ with a reaction rate equivalent to that of methyl peroxy radicals (CH₃O₂), then the calculated net ozone production rate (NOPR, ppbV h¯¹) including these additional oxidants is similar to the average change in O₃ observed, within estimated uncertainties, once halogen oxide chemistry is accounted for. This implies that such additional peroxy radicals cannot be excluded as a missing oxidant in clean marine air containing aged pollution and that modelled RO₂ concentrations are significantly underestimated under these conditions.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)