Figure 4 . Spearman rank correlation coefficients between major ions in aerosol (MI), pH proxies (DSN and DON) and the soluble iodine speciation ratios X/TSI for campaigns C4, C6, C10, C14, C17, C19 and C20. Only correlations with p ≤ 0.010 are shown. Panel a: correlation coefficients for PM1 aerosol. Panel b: correlation coefficients for coarse aerosol. Full, empty and crossed symbols correspond respectively to the complete dataset, open ocean data and coastal data. Panel b also contains a correlation re-analysis of the coarse coastal dataset excluding 5 datapoints of possible oceanic influence (small crossed symbols).
Iodine speciation variables and some MI may be causally linked, but they may also be correlated simply because existing correlations between the different MI resulting from their common sources. Correlation-based hierarchical cluster analysis of the MI data (Supplementary Text S3) indicates five groups or clusters of variables in fine aerosol (Figure 5) that appear both in coastal (Figure S7) and open ocean data (Figure S8): the Na+ group (sea-salt), the Cl- group (Cl- not linked to sea-salt in fine aerosol), the nss_SO42- group (marine biogenic emissions), the nss_Ca2+ group (mineral dust) and the nss_K+ group (biomass burning). NO3, NH4+, CH3SO3- and C2O42- appear associated to different groups in the coastal and open ocean subsets. In fine aerosol most of K+, Ca2+ and SO42- are non-sea salt ions, i.e. nss_X ≈ X, with X = K+, Ca2+ or SO42-. Five groups of variables can also be identified for coarse aerosol MI (Figures 5, S7 and S8): the sea-salt group (which includes tightly correlated Na+, Cl-, Mg2+ and K+, as well as Br- and SO4), the mineral dust group (nss_Ca2+), the NH4+ group, the NO3-and C2O42‑ group (possibly fossil fuel combustion), and the biomass burning group (nss_K+). For coarse aerosol Ca2+it is also a non-sea salt ion (nss_ Ca2+ ≈ Ca2+). The concentration of sea-salt- and dust-related ions is much higher in coarse aerosol (1-2 orders of magnitude).