Modelling iodide – iodate speciation in atmospheric aerosol: Contributions of inorganic and organic iodine chemistry S. Pechtl1,*, G. Schmitz2, and R. von Glasow1,** 1Institute for Environmental Physics, University of Heidelberg, Heidelberg, Germany 2Faculté des Sciences Appliquées, Université Libre de Bruxelles, Bruxelles, Belgium *now at: Deutsches Patent- und Markenamt, Munich, Germany **now at: School of Environmental Sciences, University of East Anglia, Norwich, UK
Abstract. The speciation of iodine in atmospheric aerosol is currently poorly understood.
Models predict negligible iodide concentrations but accumulation of iodate in aerosol,
both of which is not confirmed by recent measurements.
We present an updated aqueous phase iodine chemistry scheme for use in
atmospheric chemistry models and discuss sensitivity studies with the
marine boundary layer model MISTRA.
These studies show that iodate can be reduced in acidic aerosol by inorganic reactions, i.e.,
iodate does not necessarily accumulate in particles.
Furthermore, the transformation of particulate iodide to volatile iodine species
likely has been overestimated in previous model studies due to
negligence of collision-induced upper limits for the reaction rates.
However, inorganic reaction cycles still do not seem to be sufficient to reproduce
the observed range of iodide – iodate speciation in atmospheric aerosol.
Therefore, we also investigate the effects of the recently suggested reaction of
HOI with dissolved organic matter to produce iodide. If this reaction is fast enough
to compete with the inorganic mechanism, it would not only directly
lead to enhanced iodide concentrations but,
indirectly via speed-up of the inorganic iodate reduction cycles,
also to a decrease in iodate concentrations.
Hence, according to our model studies, organic iodine chemistry, combined
with inorganic reaction cycles, is able to reproduce observations.
The presented chemistry cycles are highly dependent on pH and thus offer
an explanation for the large observed variability of the iodide – iodate speciation
in atmospheric aerosol.
Citation: Pechtl, S., Schmitz, G., and von Glasow, R.: Modelling iodide – iodate speciation in atmospheric aerosol: Contributions of inorganic and organic iodine chemistry, Atmos. Chem. Phys., 7, 1381-1393, doi:10.5194/acp-7-1381-2007, 2007.