4. Discussion.
On average, 40% of aerosol iodine (total and soluble) exists in PM1 and about 60% in PM2.5. These percentages are consistent along both geographical coordinates. There is some evidence of higher concentration of iodine in fine aerosol at high latitudes, but only from campaign S28 (Barrie et al., 1994). The fraction of TSI in TI is also consistent across latitude and longitude (~65-80%). There is only one campaign at low latitudes (C12, (Gómez Martín et al., 2021)), reporting almost all TI being soluble, while another campaign (S35, (Zhang et al., 2016)) reports an extremely high non-soluble fraction (82%), with complete absence of iodate and SOI. Insoluble aerosol iodine does not seem to be linked to coastal new particle formation as previously speculated (Baker, 2004), since it appears to exist ubiquitously, and iodine driven new particle formations leads to iodic acid particles (Gómez Martín et al., 2020) and ultimately to iodate. The campaign reporting dominant non-soluble iodine took place in a coastal location, but not in open-ocean waters, and with some influence of continental air (Risø, Denmark, S35). Recent work has demonstrated the presence of abundant non-soluble iodine compounds in a continental location related to anthropogenic activities (Shi et al., 2021).
Iodine in enriched in aerosol compared to seawater, as a result of the uptake of gas-phase iodine compounds (Duce et al., 1983). As schematically depicted in Figure 7, the uptake of HI, HOI, IONO2 leads to the formation of I-, while uptake of iodine oxides is expected to form IO3-. Part of SOI present in seawater may be incorporated into aerosol from bubble bursting (primary SOI), but the fine mode dominance of SOI suggests that a larger fraction forms after sorption of gas phase iodine into particles (secondary SOI). It has been previously suggested that dissolved organic matter (DOM) in aerosol reacts with HOI to form SOI (Baker, 2005). Photolysis of SOI can potentially form I-, as has been observed for alkyl halides (Jones & Carpenter, 2005; Martino et al., 2005). Organic compounds and iodide could also form adducts (i.e. SOI) as reported by (Yu et al., 2019), leading to SOI-iodide interconversion, although the use of long ultrasonication times and cellulose filters makes the organic speciation reported in that work somewhat uncertain (Yodle & Baker, 2019).
The higher fractions of SOI and I- in fine aerosol (respectively ~50% and ~30% on average for the complete dataset), and higher fraction of iodate in coarse aerosol (~50%) but with non-negligible iodide (~20%) has been previously documented for individual cruise datasets (Baker, 2004, 2005; Baker & Yodle, 2021; Droste et al., 2021). Figure 3 indicates that SOI, both in coarse and fine aerosol, has an equatorial maximum, minima in the tropical ‘desert ocean’ region, and again enhanced values at middle-high latitudes. This latitudinal distribution is reminiscent of the average latitudinal profiles of chl-a, phytoplankton absorption at 443 nm and CDOM and detritus absorption at 443 nm measured by MODIS-A (see Figure 3d), suggesting that organic compounds derived from oceanic emissions or incorporated in bubble bursting may exert some control on the SOI and I- fractions. Some MI tracers of biogenic emissions are correlated to the iodine speciation in PM1, suggesting that SOI forms from reactions between organics that have condensed on sulfate aerosol, forming DOM, and an iodine-containing species (HOI or I-). The SOI and IO3- fractions in PM1are respectively correlated and anticorrelated to oxalate, C2O4-2. Oxalate grows towards the NH, as can be expected from its partly anthropogenic sources, but the latitudinal profile shows some evidence of a superposed biogenic oceanic source (local equatorial maximum and tropical minima, Figure S9c). However, SOI is also anticorrelated to the sea-salt tracer Na+ (which itself tracks SSS), which may indicate that there is less organic matter incorporated into aerosol by bubble bursting in the high SSS ocean ‘deserts’.