Atmospheric Chemistry Program Seminar
Reduction of Iodate in aqueous organic and inorganic thin films
Margarita Reza, ANYL 3rd year,
"Iodine species are known to catalytically destroy ozone, and in the case of Iodic acid (HIO3), efficiently forms particles. Iodic acid is a source of particulate iodate (IO3-); this has been detected in aged stratospheric air alongside gas-phase IO radical, suggesting the existence of recycling mechanisms reducing IO3- to form volatile iodine species. The reduction of iodate is explored through a series of photochemical coated wall flow tube (CWFT) experiments. A quartz glass tube was coated with an aqueous solution containing sodium iodate in a matrix (i.e., either ammonium bisulfate (ABS), 1,2,3,4-butanetetracarboxylic acid (BTCA), citric acid (CA), or Fe(III) citrate (Fe-Cit) and CA) at a concentration ratio iodate:matrix = 1:100. A thin aqueous film forms on the inside walls by passing a stream of humidified air through the CWFT (~80% RH). For each film, the formation of gaseous iodine (I2) was followed in three types of experiments: (1) dark reaction with H2O2 involved flowing H2O2 through the CWFT for several hours in the dark; (2) photochemical experiments involved irradiating the CWFT with visible lights and UVA lights, separately; and (3) dark-aging experiments involved flowing H2O2 through the CWFT for several hours in the dark, followed by irradiation with visible light. The evaporation of gas-phase I2 from the aqueous films was measured by cavity enhanced differential optical absorption spectroscopy (CE-DOAS) coupled to the CWFT. The cleanliness of the materials, cleaning procedures used, and the absence of uncontrolled chromophores reducing iodate, that might be intrinsic to the CWFT setup was confirmed in blank experiments. The I2 released from aged films irradiated with visible light (type 3 experiments) was found to be substantially greater than that from irradiated fresh films (type 2), or fresh films exposed to H2O2 in the dark (type 1). This increase of I2 in H2O2 aged films, independently observed in both inorganic and organic matrices, suggests that a secondary inorganic chromophore is formed from the reaction of iodate with H2O2. A photochemical pathway was discovered in which visible light is sufficient for reducing iodate to I2. This is relevant in the atmosphere because it helps to explain the co-existence of particulate IO3- and gas-phase IO radicals recently observed by aircraft measurements in the stratosphere. Multiphase re-cycling of I2 from particulate IO3- in absence of ultraviolet light further suggests that a catalytic reaction cycle is more active than previously thought, and involves three steps: (1) gas-phase HIO3 formation from I2 photolysis, (2) condensation and dissociation of HIO3 to form IO3-, and (3) IO3- reduction to re-cycle I2. This catalytic reaction cycle destroys O3 by multiphase chemistry, and highlights a possible catalytic role of HIO3 in particle formation. This photochemistry is currently missing in atmospheric models, seems relevant to predictions about the partitioning of iodine between the gas- and particle phases, invigorates particle formation from iodine oxoacids, and seems relevant to better understand iodine’s role in the recovery of the ozone layer."