The influence of simplified peroxy radical chemistry on the interpretation of NO2–NO–O3 surface exchange
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.