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.
Highlights
Iodine (I) plays a significant role in the destruction of ozone (O3) in the atmosphere (Davis et al, 1996; Saiz-Lopez et al, 2012), being responsible for ∼ 30 % of O3 loss in the marine boundary layer (MBL) (Prados-Roman et al, 2015)
Analysis was by ion chromatography (IC) for MIs, inductively coupled plasma mass spectrometry (ICP-MS) for total soluble iodine (TSI) and by IC–ICP-MS for iodide (I−) and iodate (IO−3 )
The major air mass types encountered during AMT21 were similar to those reported for earlier AMT cruises (Baker et al, 2006), with air arrivals from Europe (EUR), North Africa (SAH) and southern Africa (SAF, or SAB if biomass burning tracers were present), as well as air that had passed over the North and South Atlantic (RNA and RSA, respectively) for the preceding 5 d
Summary
Iodine (I) plays a significant role in the destruction of ozone (O3) in the atmosphere (Davis et al, 1996; Saiz-Lopez et al, 2012), being responsible for ∼ 30 % of O3 loss in the marine boundary layer (MBL) (Prados-Roman et al, 2015). Since acoustic cavitation during ultrasonication generates H2O2 and other reactive oxygen species (Kanthale et al, 2008), the result of Yu et al (2019) is further evidence that the use of ultrasonic agitation is likely to alter both inorganic and organic speciation of iodine prior to its determination This raises the possibility that much of the published literature on I speciation in aerosols over the oceans is potentially unreliable. Using results from a one-dimensional MBL model (MISTRA), Pechtl et al (2007) suggested a significant role for acidity in controlling iodine speciation in sulfate (fine) and sea-salt (coarse) aerosols Their chemical mechanism incorporated reactions capable of reducing IO−3 under acidic conditions (Eq 1) , as well as generating I− from the reaction of HOI (which can be produced from the reduction of IO−3 (Eq 1) and other sources) with organic matter (Eqs. 2, 3). The major-ion and soluble-metal chemistry of the samples was used to gain insights into the controls on I speciation, with particular focus on the potential impacts of aerosol acidity
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