Non-conventional oxidation of SO2 by iodine oxides: A source of nighttime sulfuric acid in the marine boundary layer

The formation of sulfuric acid (H₂SO₄, SA), a key aerosol precursor in the atmosphere, hinges on the rate-limiting oxidation of SO₂. During the daytime, hydroxyl radical (OH) is the main SO₂ oxidant, but the measured ambient SA concentration suggests the existence of other unaccounted pathways via other oxidants (Berresheim et al., 2014). The nocturnal presence of SA in marine environments is particularly interesting as the formation mechanism is not straightforward due to the lack of photochemical reactions. In marine environments, molecular iodine and iodocarbons are prevalent, and their reactions with the nitrate radical (NO₃) are known sources of nighttime IO and OIO radicals (Saiz-Lopez and Plane, 2004). OIO has low daytime concentrations due to its large photolysis cross-section but can accumulate during nighttime. In the absence of a photolysis sink, OIO predominantly undergoes self-reaction, leading to the generation of the iodine oxide I₂O₄ at nighttime. The reported lifetime of I₂O₄ against the thermal decomposition back to OIO + OIO is about 30 seconds, which means that it is relatively short-lived, but can survive long enough for reactions with other atmospheric trace gases to become relevant (Kaltsoyannis and Plane, 2008).

In this study, laboratory experiments for the reaction of iodine oxides with SO₂ were carried out using a flow reactor coupled with a nitrate-based chemical ionization mass spectrometer (NO₃^--CIMS). The iodine oxides were generated in situ by the reaction of iodine vapors and ozone in the presence of nitrate radical, mimicking the nighttime oxidation of SO₂ to form SO₃ and consequently SA. The experiments were carried out at room temperature and atmospheric pressure conditions. The experiments were complemented by high-level quantum chemical calculations to get detailed insights into the mechanism and feasibility of the oxidation of SO₂ by iodine oxides to produce SA. Among all the formed iodine oxides, I₂O₄ reacts sufficiently fast with SO₂ with a rate coefficient of 2.0×10^-14 molecule^-1 cm³ s^-1 and can thus lead to appreciable concentrations of SO₃. These results suggest that I₂O₄ can be a key SO₂ oxidant in the marine environment and explain a significant fraction of the produced SA in the nighttime.  

References:

Berresheim, H., Adam, M., Monahan, C., O'dowd, C., Plane, J. M., Bohn, B. and Rohrer, F.  Atmos. Chem. Phys. 14, 12209-12223, 2014.

Saiz–Lopez, A. and Plane, J. M. Geophys. Res. Lett. 31, 2004.

Kaltsoyannis, N. and Plane, J.M. Phys. Chem. Chem. Phys., 10, 1723-1733, 2008.