Measurements of hydroxyl (OH) and hydroperoxy (HO 2 ) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric RO x (OH + HO 2 + RO 2 ) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 J -values and the concentrations of 97 chemical species. Several issues related to the RO x chemistry under polluted conditions Correspondence to: S. Dusanter (sdusante@indiana.edu) are highlighted in this study: (i) Measured concentrations of both OH and HO 2 were underpredicted during morning hours on a median campaign basis, suggesting a significant source of radicals is missing from current atmospheric models under polluted conditions, consistent with previous urban field campaigns. (ii) The model-predicted HO 2 /OH ratios underestimate the measurements for NO mixing ratios higher than 5 ppb, also consistent with previous urban field campaigns. This suggests that under high NO x conditions, the HO 2 to OH propagation rate may be overestimated by the model or a process converting OH into HO 2 may be missing from the chemical mechanism. On a daily basis (08:40 a.m.-06:40 p.m.), an analysis of the radical budget indicates that HONO photolysis, HCHO photolysis, O 3 -alkene reactions and dicarbonyls photolysis are the main radical sources. O 3 Published by Copernicus Publications on behalf of the European Geosciences Union. 6656 S. Dusanter et al.: HO x model/measurement comparison for MCMA-2006photolysis contributes to less than 6% of the total radical production.

Measurements of tropospheric hydroxyl (OH) and hydroperoxy (HO 2 ) radicals were made during the MCMA (Mexico City Metropolitan Area) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These radicals were measured using a laser-induced fluorescence instrument developed at Indiana University. This new instrument takes advantage of the Fluorescence Assay by Gas Expansion technique (FAGE) together with direct excitation and detection of OH at 308 nm. HO 2 is indirectly measured as OH by titration with NO inside the fluorescence cell. At this stage of development, IU-FAGE is capable of detecting 3.9×10 5 molecule/cm 3 of both OH and HO 2 , with a signal to noise ratio of 1, an averaged laser power of 10-mW and an averaging time of 5-min. The calibration accuracies (1σ ) are ±17% for OH and ±18% for HO 2 using the water-vapor photolysis/O 2 actinometry calibration technique.OH and HO 2 concentrations were successfully measured at an urban site in Mexico City, with observed concentrations comparable to those measured in other polluted environments. Enhanced levels of OH and HO 2 radicals were observed on several days between 09:30-11:00 a.m. and suggest an intense photochemistry during morning hours that may be due to elevated sources of HO x (OH+HO 2 ) and a fast cycling between the radicals under the high NO x (NO+NO 2 ) conditions of the MCMA.

Abstract. The hydroxyl radical (OH) is one of the most important oxidants in the atmosphere, as it is involved in many reactions that affect regional air quality and global climate change. Because of its high reactivity, measurements of OH radical concentrations in the atmosphere are difficult, and often require careful calibrations that rely on the production of a known concentration of OH at atmospheric pressure. The Indiana University OH instrument, based on the Fluorescence Assay by Gas Expansion technique (FAGE), has been calibrated in the laboratory using two different approaches: the production of OH from the UV-photolysis of water-vapor, and the steady-state production of OH from the reaction of ozone with alkenes. The former technique relies on two different actinometric methods to measure the product of the lamp flux at 184.9-nm and the photolysis time. This quantity derived from N 2 O actinometry was found to be 1.5 times higher than that derived from O 2 actinometry. The water photolysis and ozone-alkene techniques are shown to agree within their experimental uncertainties (respectively 17% and 44%), although the sensitivities derived from the ozone-alkene technique were systematically lower by 40% than those derived from the water-vapor UV-photolysis technique using O 2 actinometry. The agreement between the two different methods improves the confidence of the water-vapor photolysis method as an accurate calibration technique for HO x instruments. Because several aspects of the mechanism of the gas phase ozonolysis of alkenes are still uncertain, this technique should be used with caution to calibrate OH instruments.