Report on the Photochemical Induced Halogen Activation of Fecontaining Aerosols

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Iron (Fe) occurs in minerals and highly saline media (e.g., sea salt aerosol, salt brines etc.) and induces photochemical processes. High salinity, low pH, and organic constituents promote the dissolution of iron, forming photosensitive complexes that are responsible for a production of gaseous Cl atoms when irradiated by sunlight. We studied the production of atomic Cl, Br and OH radicals from modeled salt pans and artificial sea-salt aerosols containing Fe(III) ions or pyrogenic Fe2O3 particles (Sicotrans Orange, BASF) at various compositions in a Teflon smog-chamber. The samples were either spread on a Teflon sheet or they were nebulized from dilute brines (most abundant particle diameter: ~0.4 μm, initial surface area: up to 3·10-2cm2cm-3) and exposed to simulated sunlight at 60-90% relative humidity. The photochemical formation of Cl, OH (and Br if possible) in the gas phase was quantified by the radical clock method resulting in time profiles of the radical-production rates and total productions. Simultaneous monitoring of the aerosol surface area enabled us to determine the initial Cl production rate per cm2 aerosol surface. Whereas no significant Cl production was detected employing a molar Cl-/Fe(III) ratio of 955, the rate increased to ~2.8·1017 atoms cm-2 s-1 (at a ratio of 101), ~4.1·1017 atoms cm-2 s-1 (at a ratio of 53) and ~1.9·1018 atoms cm-2 s-1 (at a ratio of 13). The presence of NO2 (~20 ppb) or O3 (630 ppb) in the gas phase additionally increased the Fe(III)-induced chloride activation to ~7·1018 atoms cm-2 s-1 and ~9·1018 atoms cm-2 s-1 (at a Cl-/Fe(III) ratio of 13), respectively. SO2 slightly restrained the Cl formation. Artificial sea salt aerosols doped with Fe2O3 particles (Cl-/Fetot~13) did not result in detectable Cl concentrations. However, decreasing the pH below 2 favored the dissolution of Fe and led to Cl production rates comparable to the Fe(III) experiments (~7·1018 atoms cm-2 s-1) whereas accelerating the erosion of the particles by freezing and thawing did not lead to detectable results. These observations are in accord with the speciation of the photolabile, aqueous Fe(III) complexes obtained from an equilibrium model (PHREEQC). The aerosol experiments result in much higher effective Cl production rates with ~50% of active Fe(III) in zero air, as compared to the previous salt-pan experiments in the same Teflon smog-chamber with a portion of 0.05-0.07% of active Fe(III)). This is caused by the larger active surface area of the homogeneously distributed aerosols.