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.

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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.