Figure 3. Latitudinal dependence of the TSI speciation. Panel a: in bulk aerosol; Panel b: in PM1. Panel c: in coarse aerosol. Red, blue and yellow stacked columns correspond to SOI, IO3-, I-, respectively. Columns indicate data averages. The number of samples are indicated at the bottom of the columns. The latitudinal averages of PM1 pH measured in the ATtom 1 and 2 campaigns is also plotted in panel b (averages: circles, meridional zones: horizontal lines, spread of measurements: error bars). Panel d: Sea surface salinity (SSS, circles) from the Aquarius sattellite mission, and Chlorophyll-a concentration (Chl-a, squares), CDOM and detritus absorption at 443 nm (CDOM, blue columns) and Phytoplankton absorption at 443 nm (Phyt, red columns) from the MODIS-A satellite mission. The satellite averages exclude data for land-locked seas where no measurements of iodine speciation exists (Baltic, Mediterranean, Caspian, Black, Red Seas and Persian Gulf).
The longitudinal variations of the X/TSI ratios (zonal band 55°S-55°N) are plotted in Figure S5. The iodide fraction appears to be enhanced in the Pacific, while the iodate fraction is higher at the Atlantic and Indian oceans. SOI shows a minimum in the eastern Atlantic and western Indian Ocean, which corresponds with regions of lower ocean productivity. For the sake of simplicity, we have not included data for PM2 in Figures 3 and S5 (campaigns C8, C9 and part of S32). Analogous latitudinal and longitudinal plots for PM1 and PM2.5 are shown in Figure S6. Campaigns C8 and S32 (North Atlantic) and C9 (Southern Atlantic) show an extremely high SOI fraction.
3.4 Correlation between iodine speciation and major ions .
Since SOI/TSI = 1 – TII/TSI = 1 - [I-]/TSI - [IO3-]/TSI, the SOI fraction is anticorrelated, by definition, with the TII fraction, and is expected to be anticorrelated with at least one of the two components of TII. Figure 4 shows that the SOI fraction is anticorrelated to both the iodide and iodate fractions in PM1, but only anticorrelated to iodate in the coarse fraction. The iodate and iodide fractions are anticorrelated both in fine and coarse aerosol, but the anticorrelation is weaker in fine aerosol.
Correlations between MI concentrations in aerosol and iodine speciation ratios have been investigated for the seven cruises (C4, C6, C10, C14, C17, C19 and C20) reporting both types of measurements. This includes a total number of 132 iodine speciation measurements, although in practice each correlation pair may have less data as a result of non-detectable levels of a particular MI species. In addition to investigating correlations for the complete dataset (labelled ”All” in Figure 4), the data has been divided in two groups: coastal (labelled ”Coast”) and open ocean (labelled ”Ocean”), based on the distance between the sampling point and the closest continental coast. The idea behind this is highlighting the effect of crustal elements on the iodine speciation. A more rigorous approach would require classification of air masses by origin using back trajectories, which has been done previously for specific cruises (Baker & Yodle, 2021; Yodle & Baker, 2019). The Spearman rank correlation coefficient is used instead of the Pearson correlation coefficient, because the relationship between X/TSI (X = iodide, iodate, SOI) and MI is nonlinear because of the nature of the X/TSI ratio (always <1), as exemplified in Figure 6 for coastal coarse aerosol. Although the Pearson coefficient captures most of the existing correlations, the Spearman rank coefficient is a more robust diagnostic for non-linear dependences and non-normally distributed data. Figure 4 shows Spearman correlation coefficients significant at p = 0.01 level.