A combination of experimental, molecular dynamics, and kinetics modeling studies is applied to a system of concentrated aqueous sodium chloride particles suspended in air at room temperature with ozone, irradiated at 254 nanometers to generate hydroxyl radicals. Measurements of the observed gaseous molecular chlorine product are explainable only if reactions at the air-water interface are dominant. Molecular dynamics simulations show the availability of substantial amounts of chloride ions for reaction at the interface, and quantum chemical calculations predict that in the gas phase chloride ions will strongly attract hydroxl radicals. Model extrapolation to the marine boundary layer yields daytime chlorine atom concentrations that are in good agreement with estimates based on field measurements of the decay of selected organics over the Southern Ocean and the North Atlantic. Thus, ion-enhanced interactions with gases at aqueous interfaces may play a more generalized and important role in the chemistry of concentrated inorganic salt solutions than was previously recognized.

The heterogeneous reaction of NO 2 with water on the surface of laboratory systems has been known for decades to generate HONO, a major source of OH that drives the formation of ozone and other air pollutants in urban areas and possibly in snowpacks. Previous studies have shown that the reaction is first order in NO 2 and in water vapor, and the formation of a complex between NO 2 and water at the air-water interface has been hypothesized as being the key step in the mechanism. We report data from long path FTIR studies in borosilicate glass reaction chambers of the loss of gaseous NO 2 and the formation of the products HONO, NO and N 2 O. Further FTIR studies were carried out to measure species generated on the surface during the reaction, including HNO 3 , N 2 O 4 and NO 2 + . We propose a new reaction mechanism in which we hypothesize that the symmetric form of the NO 2 dimer, N 2 O 4 , is taken up on the surface and isomerizes to the asymmetric form, ONONO 2 . The latter autoionizes to NO + NO 3 À , and it is this intermediate that reacts with water to generate HONO and surface-adsorbed HNO 3 . Nitric oxide is then generated by secondary reactions of HONO on the highly acidic surface. This new mechanism is discussed in the context of our experimental data and those of previous studies, as well as the chemistry of such intermediates as NO + and NO 2 + that is known to occur in solution. Implications for the formation of HONO both outdoors and indoors in real and simulated polluted atmospheres, as well as on airborne particles and in snowpacks, are discussed. A key aspect of this chemistry is that in the atmospheric boundary layer where human exposure occurs and many measurements of HONO and related atmospheric constituents such as ozone are made, a major substrate for this heterogeneous chemistry is the surface of buildings, roads, soils, vegetation and other materials. This area of reactions in thin films on surfaces (SURFACE ¼ Surfaces, Urban and Remote: Films As a Chemical Environment) has received relatively little attention compared to reactions in the gas and liquid phases, but in fact may be quite important in the chemistry of the boundary layer in urban areas.

Tropospheric air pollution has impacts on scales ranging from local to global. Reactive intermediates in the oxidation of mixtures of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) play central roles: the hydroxyl radical (OH), during the day; the nitrate radical (NO3), at night; and ozone (O3), which contributes during the day and night. Halogen atoms can also play a role during the day. Here the implications of the complex VOC-NOx chemistry for O3 control are discussed. In addition, OH, NO3, and O3 are shown to play a central role in the formation and fate of airborne toxic chemicals, mutagenic polycyclic aromatic hydrocarbons, and fine particles.

Nature © Macmillan Publishers Ltd 1998 8 letters to nature NATURE | VOL 394 | 23 JULY 1998 353sites. Clearly, differences in catalyst loading must also be taken into account when assessing the relative activities of the catalytic sites.The technique described here could be extended to monitor multiple reaction products, and thus could also be used to acquire information on catalyst selectivity. This could be accomplished by using different laser frequencies to sequentially generate the REMPI signals of different products. The REMPI signals could then be converted into absolute concentrations, using calibration standards, for the determination of selectivities. In addition, the technique developed should be useful in the study of issues related to the operational lifetimes of catalysts, their resistance to poisoning, their regeneration and their loss during operation. Ⅺ

Contents 1. Introduction 4801 2. Tropospheric Chemistry of Oxides of Nitrogen and Organic Compounds 4802 3. Role of Gas-Phase Halogens in Tropospheric Chemistry 4802 3.1. Gas-Phase Chlorine Reactions in the Troposphere 4803 3.2. Gas-Phase Bromine Reactions in the Troposphere 4803 4. Evidence for Sea Salt Halogen Chemistry in the Troposphere 4804 4.1. Molecular Chlorine 4804 4.2. Molecular Bromine, Bromine Chloride, and Bromine Monoxide 4805 5. Mechanisms of Reactions of Solid NaCl and NaBr and the Role of Adsorbed Water 4807 5.1. Reactions with Oxides of Nitrogen 4807 5.2. Water on NaCl 4809 5.3. Role of Water in NaCl and NaBr Reactions 4810 6. Reactions of Aqueous Solutions of Sea Salt and Its Components 4813 6.1. Aqueous-Phase Chemistry 4813 6.2. Chemistry at the Air−Water Interface 4816 7. Summary 4818 8. Acknowledgments 4818 9. References 4818