1LISA, UMR-CNRS 7583, Université Paris Est Créteil (UPEC), Université Paris Diderot (UPD), Institut Pierre Simon Laplace (IPSL), Créteil, France
2LATMOS, UMR-CNRS 8190, Université de Versailles Saint Quentin, Université Pierre et Marie Curie, Guyancourt, France
3LaMP, UMR-CNRS 6016, Clermont Université, Université Blaise Pascal, Aubière, France
4ISA, UMR-CNRS 5280, Université Lyon 1, ENS-Lyon, Villeurbanne, France
5Université Lille Nord de France, Lille, France
6Department of Chemistry and Environment, Ecole des Mines de Douai, Douai, France
7Centre d'Analyses et de Recherche, Faculty of sciences, Université Saint Joseph, Beirut, Lebanon
8LSCE, Université de Versailles Saint Quentin en Yvelines, CEA, CNRS, Gif sur Yvette, France
9Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland
10LPC2E, UMR-CNRS 6115, Orléans, France
Received: 14 Jun 2012 – Discussion started: 26 Jun 2012
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 NOx, 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 NOx mixing ratios lower than 10 ppb, suggesting that some loss processes were missing in the calculation at low NOx. 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%).
Revised: 08 Oct 2012 – Accepted: 12 Nov 2012 – Published: 17 Dec 2012
Michoud, V., Kukui, A., Camredon, M., Colomb, A., Borbon, A., Miet, K., Aumont, B., Beekmann, M., Durand-Jolibois, R., Perrier, S., Zapf, P., Siour, G., Ait-Helal, W., Locoge, N., Sauvage, S., Afif, C., Gros, V., Furger, M., Ancellet, G., and Doussin, J. F.: Radical budget analysis in a suburban European site during the MEGAPOLI summer field campaign, Atmos. Chem. Phys., 12, 11951-11974, doi:10.5194/acp-12-11951-2012, 2012.