Abstract. New primary and secondary organic aerosol modules have been added to PMCAMx, a three dimensional chemical transport model (CTM), for use with the SAPRC99 chemistry mechanism based on recent smog chamber studies. The new modelling framework is based on the volatility basis-set approach: both primary and secondary organic components are assumed to be semivolatile and photochemically reactive and are distributed in logarithmically spaced volatility bins. This new framework with the use of the new volatility basis parameters for low-NO x and high-NO x conditions tends to predict 4-6 times higher anthropogenic SOA concentrations than those predicted with the older generation of models. The resulting PMCAMx-2008 Aerosol (HOA) and Oxygenated Organic Aerosol (OOA) concentrations and diurnal profiles. The small OA underprediction during the rush-hour periods and overprediction in the afternoon suggest potential improvements to the description of fresh primary organic emissions and the formation of the oxygenated organic aerosols, respectively, although they may also be due to errors in the simulation of dispersion and vertical mixing. However, the AMS OOA data are not specific enough to prove that the model reproduces the organic aerosol observations for the right reasons. Other combinations of contributions of primary and secondary organic aerosol production rates may lead to similar results. The model results strongly suggest that, during the simulated period, transport of OA from outside the city was a significant contributor to the observed OA levels. Future simulations should use a larger domain in order to test whether the regional OA can be predicted with current SOA parameterizations. Sensitivity tests indicate that the predicted OA concentration is especially sensitive to the volatility distribution of the emissions in the lower volatility bins.

Abstract. The contribution of HONO sources to the photochemistry in Mexico City is investigated during the MCMA-2006/MILAGO Campaign using the WRF-CHEM model. Besides the homogeneous reaction of NO with OH, four additional HONO sources are considered in the WRF-CHEM model: secondary HONO formation from NO 2 heterogeneous reaction with semivolatile organics, NO 2 reaction with freshly emitted soot, NO 2 heterogeneous reaction on aerosol and ground surfaces. The WRF-CHEM model with the five HONO sources performs reasonably well in tracking the observed diurnal variation of HONO concentrations. The HONO sources included are found to significantly improve the HO x (OH+HO 2 ) simulations during daytime and the partition of NO/NO 2 in the morning. The HONO sources also accelerate the accumulation of O 3 concentrations in the morning by about 2 h and subsequently result in a noticeable enhancement of O 3 concentrations over the course of the day with a midday average of about 6 ppb. Furthermore, these HONO sources play a very important role in the formation of secondary aerosols in the morning. They substantially enhance the secondary organic aerosol concentrations by a factor of 2 on average in the morning, although they contribute less during the rest of the day. The simulated particle-phase nitrate and ammonium are also substantially enhanced in the morning when the four HONO sources are included, in good agreement with the measurements. The impact of the HONO sources on the sulfate aerosols is negligible because of the inefficient conversion of H 2 SO 4 from SO 2 reacting with OH.

No abstract

Abstract. An episodic simulation is conducted to characterize midday (12:00-17:00 CDT) ozone (O 3 ) photochemical production and to investigate its sensitivity to emission changes of ozone precursors in the Mexico City Metropolitan Area (MCMA) during an "O 3 -South" meteorological episode using the Comprehensive Air Quality Model with extensions (CAMx). High O x (O 3 +NO 2 ) photochemical production rates of 10-80 ppb/h are predicted due to the high reactivity of volatile organic compounds (VOCs) in which alkanes, alkenes, and aromatics exert comparable contributions. The predicted ozone production efficiency is between 4-10 O 3 molecules per NO x molecule oxidized, and increases with VOC-to-NO 2 reactivity ratio. Process apportionment analyses indicate significant outflow of pollutants such as O 3 and peroxyacetyl nitrate (PAN) from the urban area to the surrounding regional environment. PAN is not in chemical-thermal equilibrium during the photochemically active periods. Sensitivity studies of O 3 production suggest that O 3 formation in the MCMA urban region with less chemical aging (NO z /NO y <0.3) is VOC-limited. Both the simulated behavior of O 3 production and its sensitivities to precursors suggest that midday O 3 formation during this episode is VOC-sensitive in the urban region on the basis of the current emissions inventory estimates, and current NO x levels depress the O 3 production.

Ab initio molecular orbital calculations have been performed to investigate the structures and energetics of the peroxy radicals arising from the OH-initiated oxidation of isoprene. Geometry optimizations of the OH-O 2 -isoprene peroxy radicals were performed using density functional theory at the B3LYP/6-31G** level, and individual energies were computed using second-order Møller-Plesset perturbation theory (MP2) and coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). At the CCSD(T)/6-31G* level of theory the zero-point-corrected OH-O 2 -isoprene adduct radical energies are 47-53 kcal mol -1 more stable than the separated OH, O 2 , and isoprene reactants. In addition, we find no evidence for an energetic barrier to O 2 addition and have calculated rate constants for the O 2 addition step using canonical variational transition state theory (CVTST) based on Morse potentials to describe the reaction coordinate. These results provide the isomeric branching between the six isoprene-OH-O 2 adduct radicals.

Ab initio molecular orbital calculations have been employed to investigate the structures and energetics of the adduct isomers arising from the addition reaction of OH to isoprene. Several levels of ab initio theory were evaluated using a set of organic radical species to establish the appropriate level of approximation. The method of gradient corrected density functionals (NLDFT) in conjunction with moderate basis sets was found to yield satisfactory molecular geometries and vibrational frequencies. Single-point energy calculations were performed using various methods, including MP2, MP4, and CCSD(T). The most energetically favorable isomers are those with OH addition to the terminal carbon positions. At the CCSD(T)/6-311G** level of theory corrected with zero-point energy (ZPE), the isomers with OH additions to isoprene at C1 to C4 positions (i.e., isomers I–IV) are 34.8, 24.2, 22.4, and 32.3 kcal mol−1 more stable than the OH and isoprene, respectively. The activation energies against OH migration transforming the higher energy isomers into the lower energy ones (i.e., II to I or III to IV) are significant (25.5–26.5 kcal mol−1), indicating that thermal equilibrium of the OH–isoprene adduct isomers is unlikely to be established. In addition, we have developed and validated a computationally efficient method to calculate the energetics of the OH–isoprene reaction system.

Abstract. We report laboratory kinetic studies of isoprene reactions initiated by the hydroxyl radical OH, using a turbulent flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. The rate constams for the reaction of isoprene with OH have been measured in the pressure range of 70 to 120 torr at 298 +_ 2 K and are found to be independent of pressure with an averaged value of (10.1 + 0.8) x 10 '• cm 3 molecule 4 s 4. The error limit given is within 1 standard deviation; a systematic error is estimated to be +_15%. We also describe direct observation of the OH-isoprene adduct based on ion-molecule reactions by using the CIMS method. The formation of the OH-isoprene adduct was used to extract the rate constam between OH and isoprene; within the uncertainty of the experiments the results were consistent with those obtained from the observed disappearance of OH. By monitoring the formation of the OH-isoprene adduct in the presence of oxygen molecules, an overall rate constam between OH-isoprene adduct and 02 has been first determined, with an averaged value of (2.8 +_ 0.7) x 10 '15 cm 3 molecule 'l s 'l at 76 torr and an estimated systematic error of +_50%. Atmospheric implications of the present results to the photochemical oxidation of isoprene are discussed.

Abstract. Organic aerosol concentrations are simulated using the WRF-CHEM model in Mexico City during the period from 24 to 29 March in association with the MILAGRO-2006 campaign. Two approaches are employed to predict the variation and spatial distribution of the organic aerosol concentrations: (1) a traditional 2-product secondary organic aerosol (SOA) model with non-volatile primary organic aerosols (POA); (2) a non-traditional SOA model including the volatility basis-set modeling method in which primary organic components are assumed to be semi-volatile and photochemically reactive and are distributed in logarithmically spaced volatility bins. The MCMA (Mexico City Metropolitan Area) 2006 official emission inventory is used in simulations and the POA emissions are modified and distributed by volatility based on dilution experiments for the non-traditional SOA model. The model results are compared to the Aerosol Mass Spectrometry (AMS) observations analyzed using the Positive Matrix Factorization (PMF) technique at an urban background site (T0) and a suburban background site (T1) in Mexico City. The traditional SOA model frequently underestimates the observed POA concentrations during rush hours and overestimates the observations in the rest of the time in the city. The model also substantially underestimates the observed SOA concentrations, particularly during daytime, and only produces 21% and 25% of the obCorrespondence to: G. Li (lgh@mce2.org) served SOA mass in the suburban and urban area, respectively. The non-traditional SOA model performs well in simulating the POA variation, but still overestimates during daytime in the urban area. The SOA simulations are significantly improved in the non-traditional SOA model compared to the traditional SOA model and the SOA production is increased by more than 100% in the city. However, the underestimation during daytime is still salient in the urban area and the non-traditional model also fails to reproduce the high level of SOA concentrations in the suburban area. In the nontraditional SOA model, the aging process of primary organic components considerably decreases the OH levels in simulations and further impacts the SOA formation. If the aging process in the non-traditional model does not have feedback on the OH in the gas-phase chemistry, the SOA production is enhanced by more than 10% compared to the simulations with the OH feedback during daytime, and the gap between the simulations and observations in the urban area is around 3 µg m −3 or 20% on average during late morning and early afternoon, within the uncertainty from the AMS measurements and PMF analysis. In addition, glyoxal and methylglyoxal can contribute up to approximately 10% of the observed SOA mass in the urban area and 4% in the suburban area. Including the non-OH feedback and the contribution of glyoxal and methylglyoxal, the non-traditional SOA model can explain up to 83% of the observed SOA in the urban area, and the underestimation during late morning and early afternoon is reduced to 0.9 µ...