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.

The heterogeneous hydrolysis of NO 2 in thin water films, a major source of HONO and hence OH radicals in polluted urban atmospheres, has been previously reported to be photoenhanced (H. Akimoto, H. Takagi and F. Sakamaki, Int. J. Chem. Kinet., 1987, 19, 539, ref. 1) which has important implications for OH production both in environmental chambers and in the lower atmosphere. We report here studies of the impact of 320-400 nm radiation on HONO formation during the heterogeneous NO 2 hydrolysis at 296 K. The experiments were carried out in a borosilicate glass cell using long path Fourier transform infrared (FTIR) spectroscopy with three initial NO 2 concentrations (20, 46, and 54 ppm) at relative humidities of 33, 39, and 57%, respectively. Nitrous acid was first allowed to accumulate from NO 2 hydrolysis in the dark, and then the mixture of reactants and products was irradiated. The measured concentration-time profiles of the gases were compared to the predictions of a kinetics model developed for this system. The initial loss of HONO upon irradiation was consistent with its photolysis and known secondary gas phase chemistry without any photoenhancement. While the fundamental NO 2 heterogeneous hydrolysis is not itself photoenhanced, there is clear evidence in these experiments for the generation of gas phase HONO by photolysis of adsorbed HNO 3 formed during the heterogeneous hydrolysis. The mechanisms and atmospheric implications of HONO as well as NO 2 formation by the photolysis of surfaceadsorbed HNO 3 are discussed.

Although heterogeneous chemistry on surfaces in the troposphere is known to be important, there are currently only a few techniques available for studying the nature of surface-adsorbed species as well as their chemistry and photochemistry under atmospheric conditions of 1 atm pressure and in the presence of water vapor. We report here a new laboratory approach using a combination of long path Fourier transform infrared spectroscopy (FTIR) and attenuated total reflectance (ATR) FTIR that allows the simultaneous observation and measurement of gases and surface species. Theory is used to identify the surface-adsorbed intermediates and products, and to estimate their relative concentrations. At intermediate relative humidities typical of the tropospheric boundary layer, the nitric acid formed during NO2 heterogeneous hydrolysis is shown to exist both as nitrate ions from the dissociation of nitric acid formed on the surface and as molecular nitric acid. In both cases, the ions and HNO3 are complexed to water molecules. Upon pumping, water is selectively removed, shifting the NO(3-)-HNO3(H2O)y equilibria toward more dehydrated forms of HNO3 and ultimately to nitric acid dimers. Irradiation of the nitric acid-water film using 300-400 nm radiation generates gaseous NO, while irradiation at 254 nm generates both NO and HONO, resulting in conversion of surface-adsorbed nitrogen oxides into photochemically active NO(x). These studies suggest that the assumption that deposition or formation of nitric acid provides a permanent removal mechanism from the atmosphere may not be correct. Furthermore, a potential role of surface-adsorbed nitric acid and other species formed during the heterogeneous hydrolysis of NO2 in the oxidation of organics on surfaces, and in the generation of gas-phase HONO on local to global scales, should be considered.

An experiment suitable for college junior or senior students in the analytical instrumental analysis laboratory that demonstrates the analysis of PAHs (benzo[a]anthracene, benzo[k]fluoranthene, benzo[a]pyrene, chrysene, and phenanthrene) using absorption and fluorescence spectroscopy is described. This experiment is carried out during one seven-hour instrumental analysis laboratory. It could also be used in a physical chemistry laboratory to demonstrate fundamental spectroscopic and photochemical principles. A Beer–Lambert plot for an absorption peak of each PAH was obtained and used to determine the molar absorptivities. The effect of heavy atoms as quenchers of fluorescence was studied by using 1-bromoheptane and 1,7-dibromoheptane, and Stern–Volmer plots were prepared to determine the ratios of the quenching rate constants to the fluorescence rate constant, k Q/k f. The experiment is also useful as an experiment preceding the determination of PAHs by HPLC with absorption and fluorescence detection as described earlier ( J. Chem. Educ. 1998, 75, 1599).

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