Quantum Chemistry and Computational Kinetics of the Reaction between OH Radicals and Formaldehyde Adsorbed on Small Silica Aerosol Models Cristina Iuga, † Annik Vivier-Bunge, † Alfonso Herna ´ ndez-Laguna, ‡ and C. Ignacio Sainz-Dı ´az* ,§ Departamento de Quı ´mica, UniVersidad Auto ´ noma Metropolitana, Iztapalapa, Me ´ xico, D.F., Mexico, Estacio ´ n Experimental del Zaidı ´n, CSIC, C/ Prof. Albareda 1, 18008 Granada, Spain, and Laboratorio de Estudios Cristalogra ´ ficos, IACT, CSIC-UniVersidad de Granada, Spain ReceiVed: September 19, 2007; In Final Form: December 5, 2007 Heterogeneous reactions of atmospheric volatile organic compounds (VOCs) on aerosol particles may play an important role in atmospheric chemistry. Clay particles are present in mineral dust in atmospheric aerosols, and radical reactions are thought to be heterogeneously catalyzed on them. However, the kinetics and mechanisms of adsorption and reaction of atmospheric VOCs on aerosol surfaces are not well understood. In this work, quantum chemical methods are used to study the reaction of OH radicals with formaldehyde adsorbed on small (SiO 4 ) n cluster models, with n ) 1 to 6. We show that surface adsorbed formaldehyde can react in the presence of gas-phase OH radicals to yield surface-bound formyl radicals and water. Significant exothermic adsorption energies are found, supporting the notion that silicate surfaces are good quenchers of VOCs. With the models employed, the reaction appears to be less favored on the silicate surfaces than in the gas phase. The effect of the model surface on the reaction mechanism is analyzed. Introduction A major natural component of atmospheric aerosol is mineral dust, which enters the atmosphere from dust storms in arid and semiarid regions. About 33% of the earth’s land surface is arid, and it represents a potential source region for this atmospheric mineral aerosol. 1 The latter is a general expression for fine particles of crustal origin that are generated by wind erosion, and which consist mostly of silica and silicate minerals. Particles smaller than 10 μm have week-long atmospheric lifetimes, 2 and they may be transported over thousands of kilometers. The transatlantic transport of Saharan dust to North America is a well-studied phenomenon where silicate particles have been found in aerosols (see reviews 2,34 and recent articles 5-7 ). The chemical and mineralogical composition of mineral aerosols is complex. 8 Globally, the most important minerals of the clay fraction (<2 μm) transported in dust storms are phyllosilicates, mainly illite, kaolinite, chlorite, and smectite, 9 whereas coarser particles mainly consist of quartz, feldspars, and carbonates. 10 Dust aerosols in the lower stratosphere consist almost entirely of clay particles. 11 Quartz, feldspars and phyllosilicates are mainly formed by basic units of SiO 4 with substitutions and different arrangements. For instance, phyllosilicates consist of sheets of SiO 4 tetrahe- drons linked to a sheet of Al hydroxide octahedrons. Substitu- tions in any of the sheets give rise to a variety of minerals. They have large specific surfaces and catalytic properties. Therefore, their presence in aerosols can be expected to play an important role in the heterogeneous chemistry of the troposphere. The potentially reactive surface of mineral aerosols may be a significant sink for many volatile organic compounds in the atmosphere, and consequently it could influence the global photooxidant budget. Laboratory studies, together with field observations and modeling calculations, have clearly demon- strated the importance of heterogeneous processes in the atmosphere. The subject has recently been reviewed by Usher et al. 12 Some authors have tried to quantify the effect of dust on tropospheric chemistry. Dentener et al. 13 calculated that ozone concentration would decrease because O 3 production decreased (N 2 O 5 and HO 2 were taken up on dust) and because the O 3 molecules were themselves taken up on dust. In addition, results from this study suggest that a large fraction of gas-phase nitric acid may be neutralized by mineral aerosol. Bian and Zender 14 quantified the effect of dust on tropospheric chemistry due to photolysis and heterogeneous update. They found that, on a global average, O 3 decreases by 0.7%, OH decreases by 11.1%, and HO 2 decreases by 3.5% when dust is added to the atmosphere. As discussed by Ravishankara, 15 the ability for predicting accurately the composition of troposphere will depend on advances in understanding the role of particulate matter in the atmosphere and the extent to which heterogeneous reactions on solids and multiphase reactions in liquid droplets contribute to the chemistry. Thus, the heterogeneous chemistry of trace atmospheric gases on solid-phase particles in the troposphere is a field of great interest. The primordial role of OH radicals in the oxidative trans- formation of volatile organic compounds and other pollutants in the troposphere has stimulated interest in the study of their atmospheric reactions. Both experimental and theoretical studies of atmospheric reactions with OH radicals in the gas phase have been reported for a large number of reactions. 16-21 In most cases, these reactions are very fast, and OH is the main oxidant of volatile organic species in the troposphere. However, the catalytic loss processes of atmospheric pollutants in the presence of OH radicals are not clear. Aerosols may promote the chemical reactions of OH radicals with adsorbed pollutants. 22,23 Formaldehyde is an important component of the polluted troposphere, and it is a precursor of HO x in the troposphere. † Universidad Auto ´noma Metropolitana. ‡ Estacio ´n Experimental del Zaidı ´n, CSIC. § CSIC-Universidad de Granada. 4590 J. Phys. Chem. C 2008, 112, 4590-4600 10.1021/jp077557m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/29/2008