3.3. Soluble iodine speciation: iodide, iodate and SOI
Table S3 lists descriptive statistics of the bulk iodine speciation ratios measured in the field campaigns included in Table S1 and S2. Except for one station at high latitude (S35), the largest contribution to TI is TSI. It can be seen that the contributions of iodide, iodate and SOI to TSI are highly variable, and that in most cases the iodide fraction is non-zero.
Zonal averages in 10° latitude bins of the different X/TSI ratios (X = SOI, IO3, I-) are shown in Figure S4. There is an increase of SOI towards high latitudes (more pronounced in the southern hemisphere, SH), local SOI minima at tropical latitudes and a local SOI maximum at the equator. Iodide is enhanced towards high latitudes in the northern hemisphere (NH). Iodate is enhanced at tropical and equatorial latitudes in both hemispheres, with a possible local minimum at the equator. The global average contributions of SOI, iodate and iodide to soluble iodine in bulk aerosol are respectively 43%, 29% and 28% (Figure S4, right panel). Figure 3a shows 10° zonal averages as Figure S4 but using a 100% stacked column style, which allows better visualization of these trends. Iodide and SOI show opposite hemispheric gradients in bulk aerosol. Figures 3b and 3c show 100% stacked column plots for soluble iodine species in the fine (PM1) and coarse fraction, respectively. Trends toward high latitudes cannot be seen in these plots because of the lack of size-segregated speciation measurements beyond 60°N and 50°S. The fine fraction (Figure 3b) is dominated by SOI, which shows a local maximum in the equatorial NH and local minima at 15°S-35°S and 25°N -40°N. I- appears to anticorrelate to SOI, while IO3- is more or less constant, representing at most 20% of TSI. The coarse fraction (Figure 3c) is dominated by iodate (35% to 80%), except in the zonal band centered at 55°N, where almost all soluble iodine is SOI. Such high SOI fraction at this latitude is dominated by the MAP 2006 campaign at Mace Head and the Irish Sea (S32). The higher IO3-fraction in coarse aerosol with respect to PM1 is at the expense of SOI (~20%) and to a lesser extent of iodide (~10%). SOI and IO3-appear anticorrelated in the coarse fraction, while I-is approximately constant. The local maximum of SOI in the equatorial NH persists in coarse aerosol, concurrent in this case with an IO3- minimum.