Iodine chemistry in the remote troposphere

Measurements of the flux of I2 and HOI from the heterogeneous reaction of gaseous ozone with aqueous iodide were performed by conversion to iodine oxide particles and detection using a nano-differential mobility analyser. Fluxes were measured as a function of [O3], [I-], salinity, temperature, and in the presence of organic species. A linear response was observed for O3 and salinity; iodide showed some deviation at higher concentrations. The temperature dependence of the I- + O3 reaction was found not to be significant and none of the organic species investigated had a significant effect on the resulting I2 and HOI fluxes. A kinetic model of the interfacial layer was then employed to verify the experimental results and good agreement was found, apart from observations at high iodide where the I2 flux was over-predicted. Parameterised expressions for the flux of I2 and HOI were then produced from the model outputs using multiple linear regression analysis.

The differential optical absorption spectroscopy (DOAS) technique was used to measure IO on the Galapagos Islands in the Eastern Pacific. The long path (LP) DOAS instrument was unable to measure IO above the detection limit (0.9 pptv) and measurements from the multi-axis (MAX) DOAS instrument ranged from below the detection limit to 0.9±0.2 pptv. IOx (IO + I) was inferred from the measured O3 concentrations and correlation analyses were conducted with available ancillary measurements (O3, wind speed, temperature) and satellite data (Chl-a, CDOM, SST, and salinity). A significant positive correlation was observed with both SST and salinityand this was linked to variations in sea surface I-.

The parameterised expressions for I2 and HOI were then input into the 1-D chemistry-transport model THAMO (Tropospheric HAlogen chemistry MOdel) to compare the predicted IO and IOx mixing ratios with measurements performed on the Cape Verde islands in the Atlantic ocean and during the HaloCarbon Air Sea Transect-Pacific (HaloCAST-P) cruise in the Eastern Pacific. The predicted fluxes were in agreement at higher wind speeds, however, at lower wind speeds, IO was over-predicted byaround a factor of three; O3 concentrations were reduced to 2 ppb (20 times lower than calculated) to match the observations.