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Modification of Ozone Deposition and I2 Emissions at the Air–Aqueous Interface by Dissolved Organic Carbon of Marine Origin
journal contribution
posted on 2016-02-18, 19:13 authored by Marvin D. Shaw, Lucy J. CarpenterThe 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.