Abstract
The influence of organic compounds on iodine (I2) emissions from the O3 + I– reaction at the sea surface was investigated in laboratory and modeling studies using artificial solutions, natural subsurface seawater (SSW), and, for the first time, samples of the surface microlayer (SML). Gas-phase I2 was measured directly above the surface of liquid samples using broadband cavity enhanced absorption spectroscopy. I2 emissions were consistently lower for artificial seawater (AS) than buffered potassium iodide (KI) solutions. Natural seawater samples showed the strongest reduction of I2 emissions compared to artificial solutions with equivalent [I–], and the reduction was more pronounced over SML than SSW. Emissions of volatile organic iodine (VOI) were highest from SML samples but remained a negligible fraction (<1%) of the total iodine flux. Therefore, reduced iodine emissions from natural seawater cannot be explained by chemical losses of I2 or hypoiodous acid (HOI), leading to VOI. An interfacial model explains this reduction by increased solubility of the I2 product in the organic-rich interfacial layer of seawater. Our results highlight the importance of using environmentally representative concentrations in studies of the O3 + I– reaction and demonstrate the influence the SML exerts on emissions of iodine and potentially other volatile species.
Highlights
Biogenic sources contribute to iodine emissions in coastal areas,[17,18] around 80% of atmospheric iodine is believed to arise from abiotic sea-air emissions of inorganic iodine in the form of molecular iodine (I2) and hypoiodous acid (HOI).[2,19,20]
The aqueous iodine reaction scheme used here was the same as in Carpenter et al.[19] except for a modification to reflect that I2(g) emissions observed from artificial seawater (AS) were only around 50% of those from buffered potassium iodide solutions
As discussed in the methods section, a completing oxidation reaction of HOI by HOCl/OCl− was incorporated into the model to account for the ∼50% decrease in I2 emissions observed in AS compared to equivalent KI solutions
Summary
Tropospheric iodine is attracting increasing research interest as insights are gained into its large influence on local and global tropospheric and stratospheric chemistry.[1−8] Reactive iodine species, such as IO radicals, induce cycles of catalytic ozone destruction,[5,9,10] change the oxidative capacity of the troposphere through their perturbation of the HOx and NOx cycles,[1,2,11,12] and are linked to particle nucleation.[13,14] Tropospheric iodine levels have tripled since the mid-20th century in certain regions,[15,16] a robust understanding of iodine sources into the atmosphere is crucial. A suppression of I2 emissions was observed in the presence of a monolayer of octanol,[40] whereas short-chain carboxylic and fulvic acids enhanced I2 emissions.[41] Chemical competition for O3 by phenolate ions at the surface suppresses I2 emissions.[42] The addition of a complex organic matrix, dissolved organic carbon (DOC) extracted from natural seawater, to buffered solutions of iodide has been found to lead to a strong reduction of I2(g) emissions.[43] This reduction could not be explained by the reactivity of DOC toward O3 and I2/HOI, and instead a decrease in the net transfer rate of I2 from the aqueous to gas phase was suggested,[43] as previously observed for octanol.[40] ozonolysis of coastal seawater samples can generate certain halocarbons (CH2I2, CHI3, and CHClI2),[44] implying that reactions of the I2 (or HOI) product in solution can yield organic iodine species. The implications of these first I2 and organic emission measurements using natural SML samples are explored using an adaptation of the aqueous interfacial layer model of Carpenter et al.[19]
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