Modification of Ozone Deposition and I₂ Emissions at the Air–Aqueous Interface by Dissolved Organic Carbon of Marine Origin

Abstract

The reaction between gaseous ozone (O3) and aqueous iodide (I(-)) at the surface microlayer (SML) is believed to be a major chemical contributor to the oceanic dry deposition of O3 over open ocean waters and has also recently been shown to produce environmentally significant quantities of gaseous molecular iodine (I2). Here we investigate how this reaction is affected by the presence of dissolved organic carbon (DOC) of marine origin, using a heterogeneous flow reactor and detection of gaseous I2 by solvent trapping and UV/vis spectroscopy. Ozone deposition measurements over coastal seawater implied an O3 reactivity (λ) toward coastal marine DOC of ∼500 (420-580) s(-1), 2-5 times higher than that toward iodide at typical ocean concentrations (∼0.5-1 × 10(-7) M). We added varying amounts of highly concentrated DOC extracted from coastal seawater to I(-) solutions (1 × 10(-5) M) such that the relative reactivities of DOC and I(-) toward O3 (λDOC/λI) were in the expected range for natural seawater. The evolution of gaseous I2 and the loss of aqueous I(-) both reduced as DOC concentrations increased, with an overall suppression of I2 emissions of about a factor of 2 under conditions of λDOC/λI representative of open ocean waters (0.5-1). A kinetic model of the SML suggested that neither competition of DOC with I(-) for reaction with interfacial O3, nor direct loss of I2 and hypoiodous acid (HOI) through reaction with increasing quantities of DOC, can fully explain these results. We conclude that the suppression of I2 emissions by DOC is largely a physical effect arising from a decrease in the net transfer of I2 from the aqueous to gas phase, as suggested by recent laboratory studies.

A laboratory characterisation of inorganic iodine emissions from the sea surface: dependence on oceanic variables and parameterisation for global modelling

“…Convection is important for mixing when wind is in the low to intermediate speed regime (Rutgersson et al, 2011), thus it is likely that mixing of interfacial iodine to bulk waters is underestimated under low wind speed conditions. Second, it can be hypothesised that reduced wind speeds over the ocean enable the formation of organic films at the surface (Frew et al, 2004), inhibiting the sea-to-air exchange of I 2 and HOI (Reeser and Donaldson, 2011;Shaw and Carpenter, 2013).…”

“…SDS has a carbon chain length of C11 which may explain the lack of an inhibiting effect observed in our experiments. Recently, Shaw and Carpenter (2013) have also reported a suppression of a factor of ∼ 2 of the I 2 emission when dissolved organic carbon (DOC) of coastal seawater origin is added to ozonised iodide solutions, under conditions of O 3 reactivity with iodide and DOC representative of the open ocean, which they have also attributed to a surface physical effect. Magi et al (1997) used the droplet train technique to measure the disappearance of O 3 following uptake in I − solution droplets (0.5-3 M NaI).…”

A laboratory characterisation of inorganic iodine emissions from the sea surface: dependence on oceanic variables and parameterisation for global modelling

“…SDS has a carbon chain length of C11 which may explain the lack of an inhibiting effect observed in our experiments. Recently, Shaw and Carpenter (2013) have also reported a suppression of a factor of ∼ 2 of the I 2 emission when dissolved organic carbon (DOC) of coastal seawater origin is added to ozonised iodide solutions, under conditions of O 3 reactivity with iodide and DOC representative of the open ocean, which they have also attributed to a surface physical effect. Magi et al (1997) used the droplet train technique to measure the disappearance of O 3 following uptake in I − solution droplets (0.5-3 M NaI).…”

“…Sea surface iodide (SSI) data, measured (Tsunogai and Henmi, 1971) or derived from measurements of iodate and total iodine (Truesdale et al, 2000), also show a clear latitudinal variation with an increase in iodide towards the equator. According to Truesdale et al (2000), the polewards increase in IO − 3 (decrease in I − ) corresponds with the overall increase in vertical mixing in the water column, caused by the polewards decrease of SST. Thus, in tropical regions stratification appears to allow IO − 3 to be reduced to I − in the sea surface either by blocking IO − 3 replenishment from below, or by facilitating undetermined reduction processes by prolonged isolation of surface water.…”

“…Note that despite the pre-eminent role attributed to marine biota in the reduction of IO − 3 to I − , IO − Figure 4. The top panel shows measured sea surface iodide from a number of ship campaigns in the Pacific and Atlantic oceans (Tsunogai and Henmi, 1971;Elderfield and Truesdale, 1980;Nakayama et al, 1989;Woittiez et al, 1991;McTaggart et al, 1994;Truesdale et al, 2000;Huang et al, 2005) along with SST measurements in the Pacific Ocean in March-April 2010 . The bottom panel shows IO x volume mixing ratios from campaigns in the open ocean at different latitudes (Allan et al, 2000;Read et al, 2008;Oetjen, 2009;Großmann et al, 2011Großmann et al, , 2013Mahajan et al, 2012;Dix et al, 2013;Gómez Martín et al, 2013).…”