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.