Measurement report: Indirect evidence for the controlling influence of acidity on the speciation of iodine in Atlantic aerosols

Abstract

Abstract. The speciation of soluble iodine and major-ion composition were determined in size-fractionated aerosols collected during the AMT21 cruise between Avonmouth, UK, and Punta Arenas, Chile, in September–November 2011. The proportions of iodine species (iodide, iodate and soluble organic iodine (SOI)) varied markedly between size fractions and with the extent to which the samples were influenced by pollutants. In general, fine mode aerosols (< 1 µm) contained higher proportions of both iodide and SOI, while iodate was the dominant component of coarse (< 1 µm) aerosols. The highest proportions of iodate were observed in aerosols that contained (alkaline) unpolluted sea spray or mineral dust. Fine mode samples with high concentrations of acidic species (e.g. non-sea-salt sulfate) contained very little iodate and elevated proportions of iodide and SOI. These results are in agreement with modelling studies that indicate that iodate can be reduced under acidic conditions and that the resulting hypoiodous acid (HOI) can react with organic matter to produce SOI and iodide. Further work that investigates the link between iodine speciation and aerosol pH directly, as well as studies on the formation and decay of organo-iodine compounds under aerosol conditions, will be necessary before the importance of this chemistry in regulating aerosol iodine speciation can be confirmed.

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

“…Indeed, there appears to be a gradient in the proportions of IO 3 − and I − in samples from continentally/anthropogenically-influenced air masses (e.g., NEA, SCS, NCP, PER, EPP) and those of clean remote air masses (e.g., RSO, RSI, SEP, SWP) (Figure 7). This is consistent with data from the Atlantic Ocean, where IO 3 − proportions were lower and I − and SOI proportions were higher in aerosols in air masses originating over Europe or southern Africa compared to those derived from the remote Atlantic Ocean (Baker and Yodle, 2021a).…”

“…Except for bulk aerosol samples S12 and S13 from NCP and coarse mode aerosols T06 and T09 from NWP, SOI is found in all samples: up to 4.7 pmol m −3 in the fine mode and 10.1 pmol m −3 in the coarse mode. Consistent with previous observations in the Atlantic Ocean (Baker, 2005;Baker and Yodle, 2021a), the SOI fraction in the fine mode samples is generally larger (median 29%) than in the coarse mode samples (median 19%). In many of the TransBrom samples, SOI has higher proportions than both I − and IO 3 − , which has occasionally previously been observed in coastal regions (Gilfedder et al, 2008) and in the southwestern Pacific Ocean (Lai et al, 2008).…”

“…Iodine speciation and proportions in the RSO, RSI, and SEP (and coarse mode SWP) samples are similar to one another (Figure 5) and are comparable to those of aerosols sampled from remote marine air masses in the northern and southern Atlantic Ocean [air masses RNA and RSA in Baker (2005) and Baker and Yodle (2021a)]. Common features of these marine air masses are the dominance of IO 3 − in the coarse mode and high percentages of IO 3 − in the fine mode.…”

“…Coarse mode aerosols (>1 µm) in the datasets presented here are dominated by sea salt aerosols, formed through primary aerosol production mechanisms, such as sea spray and bubble bursting (Smith et al, 1993). The nss-Ca 2+ concentrations in the samples reported here were extremely low, i.e., 1-2 orders of magnitude lower than reported for aerosols containing Saharan dust over the Atlantic Ocean (Baker and Yodle, 2021a). Mineral dust therefore appears to be a minor component of the TransBrom, SHIVA, OASIS, and M138 aerosols.…”

“…More generally, most of the results in Table 6 (with the notable exception of those from TransBrom) indicate higher TSI concentrations in the northern hemisphere than in the south. As noted by Baker and Yodle (2021a), this pattern may be a consequence of higher sea-to-air fluxes of iodine in the northern hemisphere, driven by higher ozone concentrations, and thus dry ozone deposition, in the north.…”