Are there interactions of iodine and sulfur species in marine air photochemistry?

Abstract

Portions of the global cycles of sulfur and iodine could be intertwined in reactions resulting from the emissions of dimethyl sulfide (DMS) and methyl iodide. A two‐dimensional photochemical model of a marine tropical tropospheric synoptic system is used to analyze the consequences of these emissions through their atmospheric transformation cycles to aerosols. Following one line of analysis, the reaction of IO+DMS → DMSO+I could play a significant role, ∼30–50%, in destroying DMS, if the reaction proceeds at the published fast rate. Under these assumptions, the concentrations of DMSO (dimethyl sulfoxide), dimethyl sulfone (DMSO2), and sulfate appear to set rather strict upper limits on the extent of the reaction over the global oceans, and hence, on [IO], which is < 2×105 molecule cm−3. DMS plays an important role in the iodine cycle by converting IO into I. The formation of a reservoir species HOI, following H abstraction by IO, may help explain the high concentrations and apparent strong diurnal variation of filter‐sampled inorganic iodine gases and help to explain low [IO] values. In an alternate, preferred, line of analysis, the rate of reaction of IO+DMS may be much slower, < 10−12 cm3 molecule−1s−1 and the I and S cycles remain decoupled: in this case, it appears that the OH+DMS addition reaction could proceed ∼30% to form DMSO. The mean concentration and variations of gaseous and particulate iodine species are also better simulated. A simple aerosol model describing fine and coarse particles is used to close the S budget, and, if only a minor role is played by DMSO, the budget compares well with data. Simulated SO2 concentrations for the upper troposphere are highly dependent on upper tropospheric [OH]. The paper concentrates on remote tropical ocean regions and includes simulations of the spatial and diurnal variation of iodine species I, IO, HI, HOI, IONO2, I2O2 and Itot(particulate) the sulfur species DMS, DMSO, DMSO2, MSA (methane sulfonic acid), SO2, and SO4=(particulate), as well as the species OH, HO2, and H2O2.