AbstractsChemistry

The multifaceted role of halogen chemistry in the troposphere gained increasing attention in the last two decades. In particular, the interaction of Reactive Halogen Species (RHS) with ozone (O3) and nitrogen oxide (NOX) cycles (producing particulate matter and O3), as well as the consumption of methane (CH4) and volatile organic compounds by chlorine atoms (Cl) in competition with OH radicals, or the initiation of sudden O3 depletions by bromine (Br) atoms, are of major interest in atmospheric research. An important issue is the exploration and quantification of RHS sources and sinks, considering homogeneous and heterogeneous processes. The main focus of this work is the heterogeneous, photochemical formation of atomic Cl and Br in the presence of iron-doped saline media at various salt and gas-phase compositions. A Teflon smog chamber was employed to expose the samples to simulated sunlight under controlled conditions (air composition, temperature, relative humidity). The production of the radicals Cl, Br and OH was indirectly quantified by the radical clock method, monitoring the consumption of a test mixture of hydrocarbons with known rate constants for the reactions with these radicals. In a first series of experiments, laboratory-modeled salt pans were spread on a Teflon sheet and irradiated in the Teflon chamber in a zero-air environment (<2 ppb NOX, <1 ppb O3 and <100 ppb CH4) at 55−60 % relative humidity and 20°C. A fraction of 0.5 wt% of iron(III) chloride hexahydrate (FeCl3·6H2O) in sodium chloride (NaCl) revealed a photochemical effect of iron by enhancing the production of atomic Cl from ~4 × 1010 atoms cm-3 h-1 for pure NaCl to ~50 × 1010 atoms cm-3 h-1 for the iron doped samples in the first hour of irradiation. This significant increase can be explained by the photolysis of FeIII-Cl complexes, thus reducing FeIII to FeII and forming Cl radicals (Clº). The recombination of Clº radicals leads to degassing of Cl2 from the quasi-liquid layer of the humidified salt, and gas-phase photolysis forms the detected atoms. The iron-induced activation adds to the known activation processes by NOX and O3 that are responsible for the Cl production in the blank (iron-free) experiment. When 0.5 wt % NaBr were present in the salts, the Br source significantly increased from ~3 × 1010 atoms cm-3 h-1 (iron-free blank) to ~30 × 1010 atoms cm-3 h-1 (2 wt % FeCl3·6H2O). The presence of sulfate, oxalate and catechol in the salt mixtures led to an inhibition of the halogen formation because of competing complexation of FeIII and a scavenging of Clº and OHº by sulfate already in the aqueous phase. A second series of experiments was conducted with aerosol in order to transfer the rather local phenomena of iron-containing salt pans to the potentially more relevant effect of iron on sea-spray aerosol in the troposphere. Artificial seawater solutions, containing typical inorganic anions (Cl−, Br−, SO42−-, Na+, K+, Ca2+, Mg2+), were doped with FeIII, nebulized and irradiated. The aerosol experiments allowed to link the halogen… Advisors/Committee Members: Zetzsch, Cornelius (advisor).