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).