**7. Acknowledgment**

240 Atmospheric Model Applications

of the abundance of this contaminant, our results may have implications in tropospheric chemistry. For larger aldehydes, the rate constant is also consistently smaller than in the gas

Acetaldehyde 0.0068 3.00 0.22 97.00 0.23 1.50, 1.50, 1.44 Propanal 0.30 42.86 0.40 57.14 0.70 1.90, 2.06, 1.99 Butanal 1.37 65.87 0.71 34.13 2.08 2.88, 2.38, 2.38 Pentanal 0.38 35.18 0.70 64.82 1.08 2.48, 2.61, 2.76

Table 2. Calculated total rate constants (in cm3 molecule-1 s-1) and branching ratios at 298 K, for OH H-abstraction reactions of C2-C5 aldehydes on the Si(OH)4 model. Experimental values correspond to the gas phase reaction have been taken from the NIST data base.

Adsorption of other contaminants, such as carboxylic acids and several polyaromatic heterocycles have also been studied on silicate model clusters. In general, these compounds use their most reactive groups to add on the silanol groups, and consequently their OH

In this article, we have reviewed our work on the mechanisms and kinetics of selected VOCs

Quantum chemistry and computational kinetics methods have been used to model mechanisms and kinetics of the reactions of OH radicals with several groups of organic compounds (alkanes, alkenes, dienes, aromatics, aldehydes, carboxylic acids, alcohols, ethers, etc.) under tropospheric conditions. We have calculated reaction profiles, rate constants and branching ratios for numerous volatile organic compounds with OH and other radicals. Insight into reactivity trends, both in the gas phase and in the presence of mineral aerosol

*Aldehydic* 

(x 10-11) *keff* (x 10-11) % *keff* (x 10-11) %

*H-abstraction koveralll*

(x 10-11)

*kexperimental*

*EADS* = 3.05 kcal/mol

VOC

**6. Conclusions** 

towards OH free radicals.

Fig. 9. Formaldehyde adsorbed complex.

phase. Results are summarized in Table 2.

*Alkyl H-abstraction* 

NIST data base (http://kinetics.nist.gov/kinetics/index.jsp).

reactivity is smaller than in the gas phase.

This work is a result of the FONCICYT Mexico-EU 'RMAYS' network, Project Nº 94666. We gratefully acknowledge the Laboratorio de Visualización y Cómputo Paralelo at Universidad Autónoma Metropolitana-Iztapalapa and the Dirección General de Cómputo y de Tecnologías de Información y Comunicación (DGCTIC) at Universidad Nacional Autónoma de México for computer time.
