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locations, and appending to it previously unpublished measurements,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?