Quantitative Detection of Iodine in the Stratosphere (Invited Presentation)

Iodine is emitted into the atmosphere mostly from marine sources. In the atmosphere iodine participates in catalytic reaction cycles that destroy ozone, modify oxidative capacity, and can form new particles. The impact on stratospheric ozone is currently not well established, in part due to the lack of quantitative measurements in the daytime lower stratosphere (LS). Previous stratospheric measurements have detected iodine qualitatively in particles, and establish low upper limits of iodine oxide (IO) radicals under twilight conditions; also small amounts of methyl iodide (CH₃I) have been detected. Based on these observations, the WMO2014 report estimated an upper limit of <0.15 pptv I_y (total inorganic gas-phase iodine: sum of I, IO, HOI, I_xO_y, INO_x, CH₃I) are injected into the LS. However, recent IO observations in the daytime tropical tropopause layer (TTL) challenge this view (Volkamer et al., 2015), and suggest that between 0.25 to 0.70 pptv I_y are injected into the LS (Saiz-Lopez et al., 2015). This is 1.6 to 3.5 times the WMO2014 upper limit. At these levels, gas-phase I_y could be responsible for significant ozone destruction in the LS, comparable to the effect of other halogens. A better understanding of iodine is needed.

This presentation discusses recent first attempts to measure IO in the daytime LS. These measurements were obtained using the University of Colorado Airborne MultiAxis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument during the Convection Transport of Active Species in the Tropics (CONTRAST, Jan/Feb 2014) campaign. We also present the first quantification of iodine in LS submicron aerosol by the High Resolution Aerosol Mass Spectrometer (HR-AMS) from the Atmospheric Tomography Mission (Atom1, Jul/Aug 2016, ATOM2, Jan/Feb 2017). The figure shows the locations where iodine has been detected in the upper troposphere/lower stratosphere. We use these new data in conjunction with a global model to re-assess the I_y burden in the LS. The implications for lower stratospheric ozone destruction are also discussed.