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).