The contribution of oceanic methyl iodide to stratospheric iodine.

We investigate the contribution of oceanic methyl iodide (CH₃I) to the stratospheric iodine budget. Based on CH₃I measurements during three tropical ship campaigns and the Lagrangian transport model FLEXPART we provide a detailed analysis of CH₃I transport from the ocean surface to the cold point in the upper tropical tropopause layer (TTL). While average oceanic emissions differ by less than 50% from campaign to campaign, the measurements show much stronger variations within each campaign. A positive correlation between the oceanic CH₃I emissions and the efficiency of CH₃I Troposphere--stratosphere transport has been identified for some cruise sections. The mechanism of strong horizontal surface winds triggering large emissions on the one hand and being associated with tropical convective systems, such as developing typhoons, on the other hand, could explain the identified correlations. As a result of the simultaneous occurrence of large CH₃I emissions and strong vertical uplift, localized maximum mixing ratios of 0.6 ppt CH₃I at the cold point have been determined for observed peak emissions during the SHIVA-Sonne campaign in the coastal West Pacific. The other two campaigns give considerable smaller maxima of 0.1 ppt CH₃I for the TransBrom campaign in the open West Pacific and 0.03 ppt for emissions from the coastal East Atlantic during the DRIVE campaign. In order to assess the representativeness of the large local mixing ratios we use climatological emission scenarios to derive global upper air estimates of CH₃I abundances. The model results are compared to available upper air measurements including data from the recent ATTREX and HIPPO2 aircraft campaigns. In the East Pacific region, the location of the available measurement campaigns in the upper TTL, the comparisons give a good agreement indicating that around 0.01 to 0.02 ppt of CH₃I enter the stratosphere. However, other tropical re gions, which are subject to stronger convective activity show larger CH₃I entrainment, e.g., 0.08 ppt in the West Pacific. The strong variations in the geographical distribution of CH₃I entrainment suggest that currently available upper air measurements are not representative of global estimates and further campaigns will be necessary in order to better understand the CH₃I contribution to stratospheric iodine. [ABSTRACT FROM AUTHOR]