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 measured NO, O3,
and jNO2 and photochemical box model predictions of peroxy radicals
(RO2 and HO2), 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 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 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.
