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’.