Soluble Iodine Speciation in Marine Aerosols Across the Indian and Pacific Ocean Basins

Abstract

Iodine affects the radiative budget and the oxidative capacity of the atmosphere and is consequently involved in important climate feedbacks. A fraction of the iodine emitted by oceans ends up in aerosols, where complex halogen chemistry regulates the recycling of iodine to the gas-phase where it effectively destroys ozone. The iodine speciation and major ion composition of aerosol samples collected during four cruises in the East and West Pacific and Indian Oceans was studied to understand the influences on iodine’s gas-aerosol phase recycling. A significant inverse relationship exists between iodide (I–) and iodate (IO3–) proportions in both fine and coarse mode aerosols, with a relatively constant soluble organic iodine (SOI) fraction of 19.8% (median) for fine and coarse mode samples of all cruises combined. Consistent with previous work on the Atlantic Ocean, this work further provides observational support that IO3– reduction is attributed to aerosol acidity, which is associated to smaller aerosol particles and air masses that have been influenced by anthropogenic emissions. Significant correlations are found between SOI and I–, which supports hypotheses that SOI may be a source for I–. This data contributes to a growing observational dataset on aerosol iodine speciation and provides evidence for relatively constant proportions of iodine species in unpolluted marine aerosols. Future development in our understanding of iodine speciation depends on aerosol pH measurements and unravelling the complex composition of SOI in aerosols.

On the Speciation of Iodine in Marine Aerosol

“…The higher fractions of SOI and I − in fine aerosol (respectively ∼50% and ∼30% on average for the complete data set) and higher fraction of iodate in coarse aerosol (∼50%) but with nonnegligible iodide (∼20%) have been previously documented for individual cruise data sets (Baker, 2004 , 2005 ; Baker & Yodle, 2021 ; Droste et al., 2021 ). Figure 3 indicates that SOI, both in coarse and fine aerosol, has an equatorial maximum, minima in the tropical “desert ocean” region, and again enhanced values at middle‐high latitudes.…”

“…Nevertheless, the correlations with MI species in Figure 4 are informative about potential links between acidity and the iodine speciation. The positive correlation between the iodate fraction and alkali cations (coincident with a negative correlation for the SOI and iodide fractions) suggests a role of acidity in iodate reduction, since more alkaline ions would mean less acidic sea‐salt or dust aerosol and accumulation of iodate (Baker & Yodle, 2021 ; Droste et al., 2021 ). Coarse aerosol collected close to the coast shows 1 order of magnitude higher content of crustal ions, which explains the enhanced correlations in Figure 4b .…”

“…Conversely, the SOI and I − fractions are positively correlated + concentrations are generally very low in coarse aerosol (Figure 6). Removing a few points of C20 (three points off the West South American coast, latitude <−15° and longitude <−76°) and of C4 (one point off the East South American coast, latitude = 11° and longitude = −59°, and one point off the West African coast, latitude = 11° and longitude = −17°) enhances these correlations (Figure 4b), which is likely a consequence of those data points being under open ocean rather than coastal conditions (Droste et al, 2021). Classification of data by air mass using back trajectory calculations may give higher correlations between iodide species and MI than with the simple coastal/open ocean classification used here .…”

“…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 in Supporting Information S1 , 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 ion (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 + , NH 4 + , Mg 2+ , Ca 2+ , K + , Cl − , NO 3 − , SO 4 2− , oxalate (C 2 O 4 2− ), Br − , and methanesulfonate (CH 3 SO 3 − ) and derived quantities such as non‐sea‐salt (nss) K + , Ca 2+ , and SO 4 2− as defined in Baker and Yodle ( 2021 ).…”

Review on the physical chemistry of iodine transformations in the oceans

“…The information in the preceding paragraphs along with the thermodynamic data from Martıń et al (2022), Figure 2 (the half reaction for O 3 to O 2 and H 2 O) and Figure 9 predict that IO − 3 should be the dominant species in the atmosphere. Although reduction of IO − 3 is not predicted in an oxidizing atmosphere, analyses of rainwater (Campos et al, 1996b;Truesdale and Jones, 1996;Baker et al, 2001;Hou et al, 2009), aerosols (Gilfedder et al, 2008;Droste et al, 2021) and snow (Gilfedder et al, 2008) in the marine boundary layer indicate that aqueous iodide and iodate coexist. Hou et al (2009) reviewed wet iodine speciation data and reported that IO − 3 predominates over Ifrom marine sources/air masses whereas Ipredominates from continental air masses.…”