Modelling iodide – iodate speciation in atmospheric aerosol: Contributions of inorganic and organic iodine chemistry

Abstract

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 MIS-TRA. 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.

Constraints on inorganic gaseous iodine in the tropical upper troposphere and stratosphere inferred from balloon-borne solar occultation observations

“…It is subject of current debate how iodine forms aerosols through polymerization and to what extent inorganic or organic iodine compounds can be taken up by aerosols (e.g. Pechtl et al, 2006Pechtl et al, , 2007Gilfedder et al, 2008). If considerable amounts of iodine are bound by aerosols or ice particles in the upper tropical troposphere and lower stratosphere, release of iodine species to the gas-phase would probably occur through evaporation in the middle to upper stratosphere where our measurements are less sensitive.…”

“…However, reactions in the aqueous phase important for the lower troposphere (e.g. Pechtl et al, 2006Pechtl et al, , 2007 are not included.…”

“…Removal of iodine-bearing compounds from the gas-phase can occur through uptake by aerosols and through nucleation of new particles initiated by polymerization of iodine oxides (e.g. Pechtl et al, 2006Pechtl et al, , 2007Gilfedder et al, 2008).…”

Potential impact of iodinated replacement compounds CF<sub>3</sub>I and CH<sub>3</sub>I on atmospheric ozone: a three-dimensional modeling study

“…Unfortunately, most of the existing measurements of iodine in marine aerosols are based on bulk measurement of total iodine, so the individual chemical iodine species contributing to particulate iodine have rarely been measured (O'Dowd and Hoffmann, 2005). Although recent modeling work tried to include aerosol phase reactions with organic substances (Pechtl et al, 2007), any aerosol chemistry was not included in our current study. Therefore, our derived ODPs of iodine species are the upper limit.…”

Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions – Part 1: General equations, parameters, and terminology

“…For example, they lead to the formation of reactive halogen species and depletion of ozone in the stratosphere and marine boundary layer (Austin et al, 2003;Sander et al, 2003;Pszenny et al, 2004;Coe et al, 2006;von Glasow, 2006;Pechtl et al, 2007), to the decomposition of N 2 O 5 (Meilinger et al, 2002;Stewart et al, 2004;Seisel et al, 2005;Karagulian et al, 2006;McNeill et al, 2006), and have been extensively discussed as a source for HONO (Broske et al, 2004;Stemmler et al, 2007;and references therein).…”