Figure 6. Scatter plots of iodate fraction versus selected MI
species for coastal coarse aerosol. The red squares indicate a group of
measurements off the Peruvian (n=3), the East American (n=1) and West
African coast (n=1) that appear to deviate from the general trend,
possibly because they are not under coastal but under open ocean
conditions.
The strongest and more consistent correlations between the iodine
speciation variables and MI are for coarse aerosol with the ions
classified in the sea-salt, dust and the
NH4+ cluster, and the correlations are
stronger for the coastal data. The SOI and I-fractions are anticorrelated to the ions of the Na+and Ca2+ clusters, while the
IO3- fraction is positively correlated
to the same ions. Conversely, the SOI and I- fractions
are positively correlated to NH4+ and
the IO3- fraction is anticorrelated to
NH4+, with the caveat that
NH4+ concentrations are generally very
low in coarse aerosol (Figure 6). Removing a few points of C20 (3 points
off the West South-American coast, latitude < -15° and
longitude < -76°) and of C4 (1 point off the East South
American coast, latitude = 11° and longitude = -59°, and 1 point off the
West African coast, latitude = 11° and longitude = -17°) enhances these
correlations (Figure 4b), which is likely a consequence of those data
points being under open ocean rather than coastal conditions (Droste et
al., 2021). Classification of data by air mass using back trajectory
calculations may give higher correlations between iodide species and MI
than with the simple coastal/open ocean classification used here (Baker
& Yodle, 2021).
Proxies of pH based on aerosol MI observations are based on the
cation-anion equivalent ratio or the ion balance method (Pye et al.,
2020). In both methods, H+ is assumed to balance the
excess of anions. Two common versions of the cation-anion equivalent
ratio are the degree of sulfate neutralization (DSN), the degree of
neutralization (DON). None of these approaches are universal indicators
of acidity, since they ignore gas-particle partitioning (Hennigan et
al., 2015; Pye et al., 2020). The equivalent ratio in its different
flavors may be able to distinguish alkaline particles from acidic
particles reliably, although it is unable to quantify aerosol acidity
(Hennigan et al., 2015). Figure 4 shows consistent anticorrelation
between DON and iodate. Iodide is positively correlated to DON in coarse
aerosol.