Abstract. Data from a recent field campaign in Mexico City are used to evaluate the performance of the EPA Federal Reference Method for monitoring the ambient concentrations of NO 2 . Measurements of NO 2 from standard chemiluminescence monitors equipped with molybdenum oxide converters are compared with those from Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS) and Differential Optical Absorption Spectroscopy (DOAS) instruments. A significant interference in the chemiluminescence measurement is shown to account for up to 50% of ambient NO 2 concentration during afternoon hours. As expected, this interference correlates well with non-NO x reactive nitrogen species (NO z ) as well as with ambient O 3 concentrations, indicating a photochemical source for the interfering species. A combination of ambient gas phase nitric acid and alkyl and multifunctional alkyl nitrates is deduced to be the primary cause of the interference. Observations at four locations at varying proximities to emission sources indicate that the percentage contribution of HNO 3 to the interference decreases with time as the air parcel ages. Alkyl and multifunctional alkyl nitrate concentrations are calculated to reach concenCorrespondence to: E. J. Dunlea (edward.dunlea@colorado.edu) trations as high as several ppb inside the city, on par with the highest values previously observed in other urban locations. Averaged over the MCMA-2003 field campaign, the chemiluminescence monitor interference resulted in an average measured NO 2 concentration up to 22% greater than that from co-located spectroscopic measurements. Thus, this interference has the potential to initiate regulatory action in areas that are close to non-attainment and may mislead atmospheric photochemical models used to assess control strategies for photochemical oxidants.

Abstract.A detailed analysis of OH, HO 2 and RO 2 radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify timeresolved RO x (sum of OH, HO 2 , RO 2 ) radical production rates from day-and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O 3 ), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH-and NO 3 radicals, O 3 ; and (4) NO, NO 2 , meteorological and other parameters. The RO x production rate was calculated directly from these observations; the MCM was used to estimate further RO x production from unconstrained sources, and express overall RO x production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO 2 and HO 2 radicals into OH radicals).Daytime radical production is found to be about 10-25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: Oxygenated VOC other than HCHO about 33%;Correspondence to: R. Volkamer (rainer.volkamer@colorado.edu) HCHO and O 3 photolysis each about 20%; O 3 /alkene reactions and HONO photolysis each about 12%, other sources <3%. Nitryl chloride photolysis could potentially contribute ∼15% additional radicals, while NO 2 * + water makes -if any -a very small contribution (∼2%). The peak radical production of ∼7.5 10 7 molec cm −3 s −1 is found already at 10:00 a.m., i.e., more than 2.5 h before solar noon. O 3 /alkene reactions are indirectly responsible for ∼33% of these radicals. Our measurements and analysis comprise a database that enables testing of the representation of radical sources and radical chain reactions in photochemical models.Since the photochemical processing of pollutants in the MCMA is radical limited, our analysis identifies the drivers for ozone and SOA formation. We conclude that reductions in VOC emissions provide an efficient opportunity to reduce peak concentrations of these secondary pollutants, because (1) about 70% of radical production is linked to VOC precursors; (2) lowering the VOC/NO x ratio has the further benefit of reducing the radical re-cycling efficiency from radical chain reactions (chemical amplification of radical sources); (3) a positive feedback is identified: lowering the rate of radical production from organic precursors also reduces that from inorganic precursors, like ozone, as pollution export from the MCMA caps the amount of ozone that accumulates at a lower rate inside the MCMA. Continued VOC reductions will in the future result in decreasing peak concentrations of ozone and SOA in...

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.