2.2. Description and treatment of datasets
In this work we keep the same campaign labels used in our previous publication on total iodine (Gómez Martín et al., 2021), for consistency. Tables S1 and S2 list respectively the 16 cruises (C#) and 12 coastal ground-based (S#) campaigns where aerosol iodine measurements have been carried out. Iodine speciation measurements are heterogeneous and do not always cover the same species or group of species. The data can be classified in five groups according to the iodine species reported and their size segregation in fine and coarse aerosol, as shown in Tables S1 and S2, and summarized in Table 1. For some of the cruises where the size distribution of soluble iodine species was reported (C4, C6, C10, C14, C17, C19 and C20) there are also measurements of major ions (MI) available (Allan et al., 2009; Baker et al., 2006, 2007; Droste et al., 2021; Martino et al., 2014; Powell et al., 2015), which are used in this work to investigate potential relationships with the iodine observations. MI observations include Na+, NH4+, Mg2+, Ca2+, K+, Cl-, NO3-, SO42-, oxalate (C2O4-2), Br- and methanesulfonate (CH3SO3-), and derived quantities such as non-sea-salt (nss) K+, Ca2+ and SO42- as defined in (Baker & Yodle, 2021). Na+ is the sea-salt tracer. The precursor of C2O4-2, oxalic acid is emitted from fossil fuel combustion, biomass burning and biogenic activity, and it is also an oxidation product of both biogenic (isoprene) and of anthropogenic (e.g. cycloalkanes) emissions (Zhou et al., 2015). Combustion of fossil fuel and biomass are a source of NO3-, while NH4+ is linked to ammonia emissions from agriculture, although it also has background marine sources. The non-sea-salt ions are tracers of biomass burning, mineral dust and sulfur emissions (volcanic, biogenic or anthropogenic), respectively (Martino et al., 2014). CH3SO3- and nss_SO4- are oxidation products of marine biogenic DMS (Andreae et al., 1999), but can also be linked to biomass burning, and agricultural and urban emissions.
Table 1. Summary of available field observations of iodine speciation in aerosol