Spectroscopic, kinetic and mechanistic studies of atmospherically relevant I2-O3 photochemistry

Iodine is a key trace element for all vertebrate species as a result of its participation in the thyroid function. The chemical mechanism which facilitates the transfer of iodine from the sea to the continents comprises the photo-oxidation of iodine compounds emitted by the marine biosphere to soluble inorganic compounds and the accumulation of the latter in marine aerosol, which grows further into cloud condensation nuclei. The branching point of this chemistry is iodine monoxide (IO) which can alternatively recycle iodine atoms by depleting ozone in some cases, or generate soluble inorganic compounds by interacting with itself or with key tropospheric oxidants. The IO self reaction appears to offer an alternative route for iodine to the aerosol phase, such that iodine compounds are able to induce new particle formation. A flash photolysis set up combining time-resolved molecular optical and atomic resonance absorption spectroscopy has been employed to investigate the I2/O3 photochemistry at room temperature, and in particular the kinetics of the IO self reaction and the fate of its products, OIO, I2O2. The application of state-of-the-art multivariate mathematical techniques to achieve the separation of overlapping absorptions is shown to significantly improve the analysisof raw spectroscopic and kinetic data. The accurate separation of pure absorbance versus time curves for each absorber facilitated the determination of reliable absolute absorption cross sections, which in turn enabled the scaling of the absorbance curves to concentration. A kinetic simulation and fitting software package based on modern algorithms for integration of stiff systems of differential equations and constrained non linear fitting has been developed and successfully applied to the analysis of the concentration versus time curves. The overall rate coefficient of the IO self reaction at 298 K has been found to be pressure independent in the range of pressures between 10 and 400 hPa of N2 and O2. By contrast, the branching of this reaction has been found to be pressure dependent. The yield ofiodine atoms and of OIO decreases as the pressure increases. Under tropospheric conditions the mayor product of this reaction is the IO dimer. According to the pressure dependence exhibited by the different channels of the IO self reaction, it can be concluded that the most likely geometry of the IO dimer is IOIO. The lifetime of IOIO seems to be significantly large than predicted by theoretical calculations. The rate coefficient of the IO self reaction obtained is about 20 % lower than the IUPAC recommendation. This fact is consistent with a fast reaction of IO and OIO. Its rate coefficient is pressure independent for P> 40 hPa. Evidence for the reaction of OIO and for the attachment of IO and OIO to higher iodine oxides has also been found. These results provide supporting evidence for atmospheric particle formation induced by polymerization of iodine oxides.