Figure 7. Chemical scheme of soluble iodine in aerosol adapted from Pechtl et al., 2007. The scheme includes SOI explicitly and highlights plausible routes between SOI, I- and IO3- that may explain the variability observed in the global dataset of iodine speciation. Halogen atoms are noted in general as X (X = Cl, Br, I). InOm (n = 2, m = 2,3,4) denotes iodine oxides. DOM refers to dissolved organic matter. Photolysis is indicated by hv.
The larger fraction of IO3- in coarse aerosol, with a non-negligible fraction of I- (Figure 3), and the anticorrelation between the I- and IO3- fractions both in fine and coarse aerosol (Figure 4), indicate that iodate is not a permanent iodine sink. The Dushman reaction and the Bray-Liebhafsky mechanism have been previously invoked to explain qualitatively the reduction of iodate to iodide under different acidity conditions (Koenig et al., 2020; Pechtl et al., 2007). I2 formed from iodate reduction would be in equilibrium with HOI, which would react with DOM leading to the formation of SOI. Also, under acidic conditions, I-could possibly be oxidized faster to HOI (Figure 7), explaining anticorrelation between SOI and I- in PM1, but not in coarse aerosol (Figure 4). Sulfate aerosol is acidic (Pye et al., 2020), with pH generally between 1 and 3, that can also take negative values, as shown by the boundary layer pH data for PM1 in Figure 3b (ATom campaigns (Nault et al., 2021)). Freshly emitted coarse sea-salt aerosol maintains its high pH for a very short period and then acidifies to values around 4-5 (Angle et al., 2021), and dust aerosol is basic in nature (Pye et al., 2020). The campaigns reporting iodine speciation and MI do not include concurrent measurements or estimates of pH. Observations of aerosol pH are only available for a limited number of field campaigns. Nevertheless, the correlations with MI species in Figure 4 are informative about potential links between acidity and the iodine speciation. The positive correlation between the iodate fraction and alkaline cations (coincident with a negative correlation for the SOI and iodide fractions) suggests a role of acidity in iodate reduction, since more alkaline ions would mean less acidic sea-salt or dust aerosol and accumulation of iodate (Baker & Yodle, 2021; Droste et al., 2021). Coarse aerosol collected close to the coast shows one order of magnitude higher content of crustal ions, which explains the enhanced correlations in Figure 4b. By contrast, Figure 4a shows a positive correlation in acidic PM1 between SOI and nss_SO42-. The pH proxies DSN and DON are anticorrelated to iodate and positively correlated to SOI, which supports the role of acidity in controlling the IO3- fraction. Hence, we suggest that the latitudinal variation of iodate and SOI in coarse aerosol (Figure S10) is mainly controlled by acidity, in contrast to fine aerosol, where the latitudinal variation of iodide and SOI appears to be controlled by organics (Figure S9).