Radical budget analysis in a suburban European site during the MEGAPOLI summer field campaign
Résumé
Measurements of hydroxyl (OH) and peroxy (RO2) radicals by a Chemical Ionisation Mass Spectrometer (CIMS) were conducted during the MEGAPOLI summer field campaign at the SIRTA observatory near Paris, France, in July 2009. OH and 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. The PSS calculation largely overestimates the OH mixing ratio compared with observations by 50%, especially at NOx mixing ratios lower than 10 ppb. This suggests 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. Various modelling procedures were set up to cope with the unavoidable lack of constraining measurements. They led to significant differences in simulated radical concentrations. OH and RO2 concentrations estimated by two selected model version were compared with measurements. These versions of the model were chosen because they represented the two extremes. The comparisons showed better results than the PSS calculations, with a slight overestimation of 12% and 5%, for OH and RO2 respectively, in average for the reference model, and an overestimation of approximately 20% for OH and an underestimation for RO2 for the 1h dilution model. Thus, we can conclude from our study that OH and 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, O3 photolysis, and aldehydes and ketones photolysis are the main radical initiation pathways. According to the MCM modelling, the reactions of RO2 with NO2, leading mainly to PAN formation, present a significant termination pathway in addition to the main net loss via reaction of OH with NO2.
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