Abstract. An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, during the Mexico City Metropolitan Area field study (MCMA-2003) from 31 March-4 May 2003 to investigate particle concentrations, sources, and processes. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 µm (NR-PM1) with high time and size-resolution. In order to account for the refractory material in the aerosol, we also present estimates of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from Proton-Induced X-ray Emission Spectrometry (PIXE) analysis of impactor substrates. Comparisons of AMS + BC + soil mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a PM2.5 Tapered Element Oscillating Microbalance (TEOM), and a PM2.5 DustTrak Aerosol Monitor) show that the AMS + BC + soil mass concentration is consistent with the total PM2.5 mass concentration during MCMA-2003 within the combined uncertainties. In Mexico City, the organic fraction of the estimated PM2.5 at CENICA represents, on average, 54.6% (standard deviation σ=10%) of the mass, with the rest consisting of inorganic compounds (mainly ammonium nitrate and sulfate/ammonium salts), BC, and soil. Inorganic compounds represent 27.5% of PM2.5 (σ=10%); BC mass concentration is about 11% (σ=4%); while soil represents about 6.9% (σ=4%). Size distributions are presented for the AMS species; they show an accumulation mode that contains mainly oxygenated organic and secondary inorganic compounds. The organic size distributions also contain a small organic particle mode that is likely indicative of fresh traffic emissions; small particle modes exist for the inorganic species as well. Evidence suggests that the organic and inorganic species are not always internally mixed, especially in the small modes. The aerosol seems to be neutralized most of the time; however, there were some periods when there was not enough ammonium to completely neutralize the nitrate, chloride and sulfate present. The diurnal cycle and size distributions of nitrate suggest local photochemical production. On the other hand, sulfate appears to be produced on a regional scale. There are indications of new particle formation and growth events when concentrations of SO2 were high. Although the sources of chloride are not clear, this species seems to condense as ammonium chloride early in the morning and to evaporate as the temperature increases and RH decreases. The total and speciated mass concentrations and diurnal cycles measured during MCMA-2003 are similar to measurements during a previous field campaign at a nearby location.

[1] We present the first direct measurements of glyoxal (CHOCHO) in the atmosphere, and demonstrate that glyoxal measurements are possible by differential optical absorption spectroscopy (DOAS). Glyoxal was routinely detected during the daytime in Mexico City, where mixing ratios ranged from <0.15 ppbv (detection limit) to 1.82 ppbv. These time-resolved measurements resolve the rapid diurnal variation of glyoxal, and indicate the onset of volatile organic compound (VOC) oxidation about 1hr after sunrise. The atmospheric lifetime of glyoxal is determined to be 1.3 hr for overhead sun conditions. Then elevated glyoxal levels indicate a persistently active VOC chemistry during most of the day. Glyoxal forms from the oxidation of numerous VOCs, which foster the formation of 'photochemical smog' including ozone and aerosol particles; atmospheric levels are essentially unaffected by direct vehicle emissions in Mexico City. Satellite measurements of glyoxal seem feasible, making possible the better identification of photochemical hot spots in the Earth's atmosphere. Citation: Volkamer, R., L. T. Molina, M. J. Molina, T. Shirley, and W. H. Brune (2005), DOAS measurement of glyoxal as an indicator for fast VOC chemistry in urban air, Geophys. Res. Lett., 32, L08806,

Abstract. The Mexico City Metropolitan Area (MCMA) study in April 2003 had measurements of many atmospheric constituents, including OH and HO 2 . It provided the first opportunity to examine atmospheric oxidation in a megacity in a developing country that has more pollution than typical U.S. and European cities. At midday, OH typically reached 0.35 pptv (∼7×10 6 cm −3 ), comparable to amounts observed in U.S. cities, but HO 2 reached 40 pptv, more than observed in most U.S. cities. The OH reactivity was also measured, even during the highly polluted morning rush hour. MCMA's OH reactivity was 25 s −1 during most of the day and 120 s −1 at morning rush hour, which was several times greater than has been measured in any U.S. city. Median measured and modeled OH and HO 2 agreed to within combined uncertainties, although for OH, the model exceeded the measurement by more than 30% during midday. OH production and loss, which were calculated from measurements, were in balance to within uncertainties, although production exceeded loss during morning rush hour. This imbalance has been observed in other cities. The HO 2 /OH ratio from measurements and steady-state analyses have essentially the same dependence on NO, except when NO was near 100 ppbv. This agreement is unlike other urban studies, in which HO 2 /OH ratio decreased much less than expected as NO increased. As a result of the active photochemistry in MCMA 2003, the median calculated ozone production from measured HO 2 reached 50 ppb h −1 , a much higher rate than observed in U.S. cities.

[1] Hydroxyl (OH), hydroperoxy (HO 2 ) radicals, collectively known as HO x , and OH reactivity, were measured during the PMTACS-NY (PM2.5 Technology Assessment and Characteristics Study-New York) summer 2002 intensive at Whiteface Mountain, Wilmington, New York. The measurement results of OH and HO 2 for 4 weeks are presented. Diurnal cycles show that the average noontime maximum mixing ratios were about 0.11 pptv (2.6 Â 10 6 cm À3 ) for OH and 20 pptv for HO 2 . Measured HO 2 to OH ratios were typically between 40 and 400, which are greater than those obtained in polluted and semipolluted rural environments. Low but significant mixing ratios of OH and HO 2 persisted into early evening and were frequently observed during nighttime, consistent with previous studies in different environments. Steady state OH and HO 2 were calculated with a zero-dimensional chemical model using a complete Regional Atmospheric Chemical Mechanism (RACM) and a parameterized RACM which was constrained to the measured OH reactivity. Good agreement was obtained between the complete RACM and the parameterized RACM models. On average, the complete RACM model reproduced the observed OH with a median measured-to-modeled OH ratio of 0.82 and daytime HO 2 with a median measured-to-modeled HO 2 ratio of 1.21. The reasonably good agreement in this study is inconsistent with the significant underestimation of OH in the Program for Research on Oxidants: Photochemistry, Emissions, and Transport in 1998 (PROPHET98) study at a similar forested site. HO x budget analysis indicates that OH was primarily from the photolysis of HONO and O 3 during the day and from O 3 + alkenes reactions at night. The main HO x loss was the self reaction of HO 2 . The good agreement between the measured and calculated OH reactivity in this environment contrasts with findings in the PROPHET2000 study, in which significant OH reactivity was missing and the missing OH reactivity was temperature-dependent.

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