1. Introduction
Iodine has a profound impact on tropospheric chemistry through its role in ozone depletion, particle formation and impact on the oxidative capacity (Saiz-Lopez et al., 2012). In a previous publication (Gómez Martín et al., 2021), we reported the spatial variability of total iodine (TI) in aerosol by compiling and homogenizing a comprehensive dataset of field observations at open ocean, insular and coastal locations, and appending to it previously unpublished measurements, spanning a period of 55 years. The analysis of the latitudinal and longitudinal dependence of TI in aerosol provided for the first time observational evidence from the field showing that the dominant global source of atmospheric iodine to the atmosphere is the reaction between iodide and ozone on the sea-water interface. After uptake on particle surfaces, iodine undergoes a rich aqueous-phase chemistry, which is known to depend on the origin and chemical properties of the aerosol (e.g. acidity (Baker & Yodle, 2021)) but still remains poorly understood. Iodine speciation determines whether recycling to the gas phase can proceed through formation of volatile species, which is thought to occur via iodide (I-), or aerosol becomes essentially an atmospheric iodine sink through accumulation of species assumed to be stable and unreactive, i.e. iodate (IO3-) (Vogt et al., 1999). However, current aerosol chemical schemes cannot explain the concentrations of I-, IO3- and soluble organic iodine (SOI) observed in field campaigns. Models predict negligible concentrations of I- following recycling to the gas phase and high concentrations of IO3-, while they not deal with SOI (Pechtl et al., 2007; Vogt et al., 1999). In contrast, many field observations report highly variable concentrations of I-, IO3- and SOI in aerosol samples (see e.g. (Baker, 2004, 2005; Gäbler & Heumann, 1993; Lai et al., 2008; Wimschneider & Heumann, 1995)).
In this study we adopt the same strategy than in our previous work on total iodine to shed light on the processes that control the speciation of iodine in aerosol. We have compiled the available iodine speciation data from cruises and coastal and insular ground-based stations with the aim of inspecting the spatial variability of the ensemble and comparing them with other global ocean and aerosol variables. For completeness, we also analyze the size distribution of total and soluble iodine, which were not specifically addressed in our previous work (Gómez Martín et al., 2021). The questions that we want to address in this work are: Which species dominates the soluble iodine speciation at different latitudes and longitudes? Where is each species most abundant: in the fine or in the coarse fraction? Can we link the iodine speciation in fine and coarse marine aerosol to other atmospheric and oceanic variables? And is the iodine speciation correlated to the dominant chemical composition of the aerosol substrate?