The influence of simplified peroxy radical chemistry on the interpretationof NO2-NO-O-3 surface exchange

Reactions of NO with peroxy radicals XO2 ( = HO2 + RO,) are shown to be the most important chemical reactions causing flux divergence of NO2 and NO cl ose to the surface, in addition to the "NOx-O-3 only" reactions of NO2 + O- 2 + hv --> NO + O-3 and NO + O-3 --> NO2 + O-2. A one-dimensional model was constructed to investigate the impact of omitting these XO2 reactions on c alculated surface exchange of NO2, NO and O-3. In simulations of mid-latitu de day-time conditions (J(NO2)= 5 x 10(-3)s(-1), surface deposition of 38 n g NO2 m(-2)s(-1) and 320 ng O-3 m(-2)s(-1) and surface emission of 10 ng NO m(-2)s(-1)), the "NOx-O-3 only" reactions reduced apparent NO2 deposition flux at Im height from 38 to 36 ng NO2 m(-2)s(-1) and apparent NO emission flux from 10 to 8.4 ng NO m(-2)s(-1). Apparent fluxes decreased further whe n XO2 was included, e.g., for XO2 = 28 pptV, apparent NO2 deposition flux d ecreased to 33 ng NO2 m(-2)s(-l) and NO emission flux to 6.3 ng NO m(-2) s( -l). The model was constrained to fit real NO2, NO and O-3 concentration pr ofiles measured up to a height 2.85 m above grassland at Halvergate Marshes , UK, using the magnitude and direction of surface exchange as variable par ameters, and with or without inclusion of chemical reaction. At night the c onstant flux approach underestimated NO2 deposition and NO emission fluxes by around 10-20% compared with inclusion of "NOx-O-3 only" chemistry, and b y as much more when XO2 of a few tens of pptV was also assumed to be presen t. During the day the constant flux approach overestimated NO emission by u p to 40% compared with the NOx-O-3 only chemistry but was similar to the em ission flux derived when similar to 50pptV XO2 was assumed also to be prese nt. Although the impact of chemistry on derived surface exchange depends se nsitively on the relative values of species concentrations, solar flux, and transport parameters, this work shows that failure to take into account th e presence of XO2 at realistic concentrations can lead to systematic error of comparable, or greater, magnitude in derivation of surface fluxes as neg lect of chemistry altogether. Measurement, or accurate estimation, of XO2 i s recommended for accurate surface flux derivation in future field campaign s. (C) 2001 Elsevier Science Ltd. All rights reserved.