Radical budget analysis in a suburban European site during the MEGAPOLI summer field campaign

Abstract. Chemical Ionisation Mass Spectrometer measurements of hydroxyl radical (OH) and the sum of hydroperoxy and organic peroxy (HO₂+RO₂) radicals were conducted during the MEGAPOLI summer field campaign at the SIRTA observatory near Paris, France, in July 2009. OH and (HO₂+RO₂) showed a typical diurnal variation with averaged daytime maxima values around 5×10⁶ and 1.2×10⁸ molecule cm⁻³, respectively. Simultaneously, a large number of ancillary measurements, such as NO_x, O₃, HONO, HCHO and other VOCs were also conducted. These data provide an opportunity to assess our understanding of the radical chemistry in a suburban environment by comparing the radical observations to calculations. First, OH mixing ratios were estimated by a simple Photo Stationary State (PSS) calculation. PSS calculations overestimate the OH mixing ratio by 50%, especially at NO_x mixing ratios lower than 10 ppb, suggesting that some loss processes were missing in the calculation at low NO_x. Then, a photochemical box model simulation based on the Master Chemical Mechanism (MCM) and constrained by ancillary measurements was run to calculate radical concentrations. Three different modelling procedures were tested, varying the way the unconstrained secondary species were estimated, to cope with the unavoidable lack of their measurements. They led to significant differences in simulated radical concentrations. OH and (HO₂+RO₂) concentrations estimated by two selected model version were compared with measurements. These versions of the model were chosen because they lead, respectively, to the higher and lower simulated radical concentrations and are thus the two extremes versions. The box model showed better results than PSS calculations, with a slight overestimation of 12% and 5%, for OH and (HO₂+RO₂) respectively, in average for the reference model, and an overestimation of approximately 20% for OH and an underestimation for (HO₂+RO₂) for the other selected model version. Thus, we can conclude from our study that OH and (HO₂+RO₂) radical levels agree on average with observations within the uncertainty range. Finally, an analysis of the radical budget, on a daily basis (06:00–18:00 UTC), indicates that HONO photolysis (~35%), O₃ photolysis (~23%), and aldehydes and ketones photolysis (~16% for formaldehyde and 18% for others) are the main radical initiation pathways. According to the MCM modelling, the reactions of RO₂ with NO₂ (~19%), leading mainly to PAN formation, is a significant termination pathway in addition to the main net loss via reaction of OH with NO₂ (~50%).