Nitrate (NO3(-)) is an abundant component of aerosols, boundary layer surface films, and surface water. Photolysis of NO3(-) leads to NO2 and HONO, both of which play important roles in tropospheric ozone and OH production. Field and laboratory studies suggest that NO3¯ photochemistry is a more important source of HONO than once thought, although a mechanistic understanding of the variables controlling this process is lacking. We present results of cavity-enhanced absorption spectroscopy measurements of NO2 and HONO emitted during photodegradation of aqueous NO3(-) under acidic conditions. Nitrous acid is formed in higher quantities at pH 2-4 than expected based on consideration of primary photochemical channels alone. Both experimental and modeled results indicate that the additional HONO is not due to enhanced NO3(-) absorption cross sections or effective quantum yields, but rather to secondary reactions of NO2 in solution. We find that NO2 is more efficiently hydrolyzed in solution when it is generated in situ during NO3(-) photolysis than for the heterogeneous system where mass transfer of gaseous NO2 into bulk solution is prohibitively slow. The presence of nonchromophoric OH scavengers that are naturally present in the environment increases HONO production 4-fold, and therefore play an important role in enhancing daytime HONO formation from NO3(-) photochemistry.

Gaseous HCl generated from a variety of sources is ubiquitous in both outdoor and indoor air. Oxides of nitrogen (NO y ) are also globally distributed, because NO formed in combustion processes is oxidized to NO 2 , HNO 3 , N 2 O 5 and a variety of other nitrogen oxides during transport. Deposition of HCl and NO y onto surfaces is commonly regarded as providing permanent removal mechanisms. However, we show here a new surface-mediated coupling of nitrogen oxide and halogen activation cycles in which uptake of gaseous NO 2 or N 2 O 5 on solid substrates generates adsorbed intermediates that react with HCl to generate gaseous nitrosyl chloride (ClNO) and nitryl chloride (ClNO 2 ), respectively. These are potentially harmful gases that photolyze to form highly reactive chlorine atoms. The reactions are shown both experimentally and theoretically to be enhanced by water, a surprising result given the availability of competing hydrolysis reaction pathways. Airshed modeling incorporating HCl generated from sea salt shows that in coastal urban regions, this heterogeneous chemistry increases surface-level ozone, a criteria air pollutant, greenhouse gas and source of atmospheric oxidants. In addition, it may contribute to recently measured high levels of ClNO 2 in the polluted coastal marine boundary layer. This work also suggests the potential for chlorine atom chemistry to occur indoors where significant concentrations of oxides of nitrogen and HCl coexist.

A terthiophene-based quinodimethane, 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (1) was synthesized and crystallized. Compound 1 has a planar quinoid geometry that is stabilized by dicyanomethylene groups at each end of the molecule. In the crystal each molecule is part of a dimerized face-to-face pi-stack, with intermolecular spacings of 3.47 and 3.63 A, respectively. Cyclic voltammetry showed that 1 could be reversibly reduced and oxidized in methylene chloride solution. Thin film transistors (TFTs) were prepared by vacuum evaporation of 1 onto SiO2(300 nm)/Si substrates, followed by evaporation of Ag source and drain contacts. The doped Si substrate served as the gate electrode. X-ray diffraction and atomic force microscopy indicate the films are polycrystalline, with the long axes of the molecules approximately perpendicular to the substrate. The TFT measurements revealed n-channel conduction in films of 1, with room-temperature electron field effect mobilities as high as 0.005 cm2/Vs. The butyl side chains give 1 appreciable solubility in a range of common solvents, and preliminary TFT results on films cast from chlorobenzene show electron mobility as high as 0.002 cm2/Vs. These results indicate that pi-stacked quinoidal thiophene oligomers are a promising new class of soluble n-channel organic semiconductors.

Analysis of a sediment core collected from Siskiwit Lake, located on a remote island in Lake Superior, provides evidence that polybrominated diphenyl ethers (PBDEs) are removed effectively from the atmosphere via deposition processes during long-range transport. A mass balance model based on photochemical rate constants and data from atmospheric samples was created to understand the relative importance of various photochemical and deposition processes in removing PBDEs from the atmosphere. Photolysis rate constants were derived from UV absorption spectra of 25 PBDEs recorded in isooctane over the range of 280-350 nm at 298 K. Photolysis decays measured for BDE-3 and -7 in the gas phase were substantial compared to a well-defined chemical actinometer, indicating that their photolysis quantum yields are significant. Dibenzofuran production was observed when PBDE congeners containing ortho-bromines were photolyzed in helium. From estimates of removal rates of PBDEs from the lower troposphere, we find that wet and dry deposition accountfor >95% of the removal of BDE-209, while photolysis accounts for -90% of the removal of gas-phase congeners such as BDE-47. These results help explain the deposition patterns of PBDEs found in lake and river sediments and have important implications concerning the inclusion of photolysis as a fate process in multimedia models.