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

Abstract

Abstract.A model of iodine chemistry in the marine boundary layer (MBL) has been used to investigate the impact of daytime coastal emissions of molecular iodine (I 2 ). The model contains a full treatment of gas-phase iodine chemistry, combined with a description of the nucleation and growth, by condensation and coagulation, of iodine oxide nano-particles. In-situ measurements of coastal emissions of I 2 made by the broadband cavity ring-down spectroscopy (BBCRDS) and inductively coupled plasmamass spectrometry (ICP/MS) techniques are presented and compared to long path differential optical absorption spectroscopy (DOAS) observations of I 2 at Mace Head, Ireland. Simultaneous measurements of enhanced I 2 emissions and particle bursts show that I 2 is almost certainly the main precursor of new particles at this coastal location. The ratio of IO to I 2 predicted by the model indicates that the iodine species observed by the DOAS are concentrated over a short distance (about 8% of the 4.2 km light path) consistent with the intertidal zone, bringing them into good agreement with the I 2 measurements made by the two in-situ techniques. The model is then used to investigate the effect of iodine emission on ozone depletion, and the production of new particles and their evolution to form stable cloud condensation nuclei (CCN).

Concurrent observations of atomic iodine, molecular iodine and ultrafine particles in a coastal environment

“…The source is most probably exposure of macroalgae, which induces emission of I 2 , ultimately leading to formation of iodine oxide particles as has been reported in the past (O'Dowd et al, 2002;Saiz-Lopez et al, 2006;McFiggans et al, 2010). As mentioned above, a macroalgal belt, 30-50 m wide, was observed within the intertidal zone adjacent to the measurement site along the coast.…”

“…In the past, studies at other coastal locations have reported a daytime maximum of 25-29 pptv through integrated long-path differential optical absorption spectroscopy (LP-DOAS) measurements at Mace Head (Saiz-Lopez and Huang et al, 2010), 115 pptv using in situ measurements at Mace Head (Saiz-Lopez et al, 2006), 87 pptv at Mweenish Bay-I and 302 pptv Mweenish Bay-II, both close to Mace Head using in situ techniques (Huang et al, 2010), 32 pptv (integrated) and 50 pptv (in situ) at Roscoff (Mahajan et al, 2009;McFiggans et al, 2010), and 3 pptv (in situ) at Scripps Pier, La Jolla, California (Finley and Saltzman, 2008). The peak I atom mixing ratio of 10 ± 5 pptv is lower than a maximum of 22 pptv reported by Bale et al (2008) at Mace Head.…”

“…In the past, studies in a similar semi-polluted environment such as Roscoff have indicated much lower I 2 /I ratios, peaking at ∼2 at a height of 4-6 m, where the I atom concentration was modelled from IO observations (Mahajan et al, 2009). A model study by Saiz-Lopez et al (2006) at Mace Head, which is a cleaner environment with respect to NO x , predicted an I 2 /I ratio of ∼5 at a height of 5 m during the daytime; this was later confirmed through I atom measurements made by Bale et al (2008). The ultrafine particle concentrations observed were lower than previous observations from sites such as Mace Head and Roscoff, where >10 5 particles cm −3 have been reported.…”

Observations of I<sub>2</sub> at a remote marine site

“…Iodine radicals can also self-react to form iodine oxides such as I 2 O 4 and other larger iodine oxide clusters (McFiggans et al, 2004;Saiz-Lopez et al, 2006). If iodine levels are sufficiently high, the formation of iodine oxides leads to rapid particle nucleation.…”

Emission of iodine-containing volatiles by selected microalgae species

“…Since molecular iodine and iodocarbons are photochemically unstable (lifetimes between about some tens of seconds for I 2 and a few days for CH 3 I), they are photolysed under UV-visible light to form I· atoms, which are then instantly oxidized by ozone to form the iodine monoxide radical IO (g) (Hoffmann et al, 2001;Saiz-Lopez et al, 2006). Further oxidation reactions of IO in the gas phase then can form low-volatility iodine oxides (I x O y ) which may nucleate under certain conditions and form new particles.…”