The influence of simplified peroxy radical chemistry on the interpretation of NO2–NO–O3 surface exchange
Abstract
Reactions of NO with peroxy radicals XO2 (=HO2+RO2) are shown to be the most
important chemical reactions causing flux divergence of NO2 and NO close to the
surface, in addition to the "NOx–O3 only" reactions of NO2+O2+hν→NO+O3 and
NO+O3→NO2+O2. A one-dimensional model was constructed to investigate the
impact of omitting these XO2 reactions on calculated surface exchange of NO2, NO
and O3. In simulations of mid-latitude day-time conditions (J(NO2)=5×10−3 s−1,
surface deposition of 38 ng NO2 m−2 s−1 and 320 ng O3 m−2 s−1 and surface emission
of 10 ng NO m−2 s−1), the "NOx–O3 only" reactions reduced apparent NO2 deposition
flux at 1 m height from 38 to 36 ng NO2 m−2 s−1 and apparent NO emission flux from
10 to 8.4 ngNO m−2 s−1. Apparent fluxes decreased further when XO2 was included,
e.g., for XO2=28 pptV, apparent NO2 deposition flux decreased to 33 ng NO2 m−2 s−1
and NO emission flux to 6.3 ng NO m−2 s−1. The model was constrained to fit real
NO2, NO and O3 concentration profiles measured up to a height 2.85 m above
grassland at Halvergate Marshes, UK, using the magnitude and direction of surface
exchange as variable parameters, and with or without inclusion of chemical reaction.
At night the constant flux approach underestimated NO2 deposition and NO emission
fluxes by around 10–20% compared with inclusion of "NOx–O3 only" chemistry, and
by as much more when XO2 of a few tens of pptV was also assumed to be present.
During the day the constant flux approach overestimated NO emission by up to 40%
compared with the NOx–O3 only chemistry but was similar to the emission flux
derived when 50 pptV XO2 was assumed also to be present. Although the impact of
chemistry on derived surface exchange depends sensitively on the relative values of
species concentrations, solar flux, and transport parameters, this work shows that
failure to take into account the presence of XO2 at realistic concentrations can lead to
systematic error of comparable, or greater, magnitude in derivation of surface fluxes
as neglect of chemistry altogether. Measurement, or accurate estimation, of XO2 is
recommended for accurate surface flux derivation in future field campaigns.