Photochemical aspects of tropospheric iodine behaviour

Abstract

The I-atom sensitised decomposition of ozone in air at 1 atm pressure and ambient temperature has been investigated. Iodine atoms were produced by photolysis of 12 using visible light or of CH31 using ultraviolet light. In both cases, the quantum yield for 03 decomposition was 1.25 (-+0.11) per I atom. An important role is pxoposed for the self-reaction of IO radicals leading to higher oxides of iodine, IO + IO(+ M) ~ I20 2 (+ M) ~ higher oxides, which predominated over the bimolecular reaction leading to regeneration of I atoms, IO + IO ~ 2I + 02 , with k2a]k2b >/4. Simple computer modelling calculations indicate that reaction (2a) may be important in determining the fate of photolabile iodine species in the atmosphere. The consequences for the behaviour of radioiodine releases are also discussed.

On the mechanism of iodine oxide particle formation

“…[7][8][9][10] In fact, it has been shown that emission fluxes of molecular iodine from coastal macroalgae under oxidative stress at low tide are strong enough to sustain substantial secondary aerosol formation in the atmosphere. [11][12][13] This potential link between marine biota and a climatically sensitive variable such as the aerosol loading, besides photosensitized iodine-driven ozone depletion, has motivated numerous laboratory field studies in recent years.…”

Modelling molecular iodine emissions in a coastal marine environment: the link to new particle formation

“…The relevance of iodine in the chemistry of the lower troposphere has been the subject of numerous studies over the past two decades (Chameides and Davis, 1980;Jenkin et al, Correspondence to: J. M. C. Plane (j.plane@uea.ac.uk) 1985; Davis et al, 1996;Vogt et al, 1999;McFiggans et al, 2000;Carpenter, 2003). These investigations have concentrated on the potential of iodine to affect the oxidising capacity of the MBL in a number of ways: catalytic destruction of O 3 by cycles involving the iodine species IO, HOI and OIO; altering the partitioning of NO x (NO 2 /NO) and HO x (HO 2 /OH) (Bloss et al, 2005); activating chlorine (McFiggans et al, 2002) and particularly bromine from sea-salt aerosol; and the reaction between IO and dimethyl sulphide (DMS) (Gravestock et al, 2005).…”

The UV-visible absorption cross-sections of IONO<sub>2</sub>

“…In competition, IO can be sequestered into reservoir species. One of the main gaseous reservoir species for iodine in the atmosphere is believed to be IONO 2 (Chameides and Davis, 1980;Davis et al, 1996;Jenkin, 1992;Jenkin et al, 1985). …”