Constraints on inorganic gaseous iodine in the tropical upper troposphere and stratosphere inferred from balloon-borne solar occultation observations

Abstract

Abstract. We report upper limits of IO and OIO in the tropical upper troposphere and stratosphere inferred from solar occultation spectra recorded by the LPMA/DOAS (Limb Profile Monitor of the Atmosphere/Differential Optical Absorption Spectroscopy) payload during two stratospheric balloon flights from a station in Northern Brazil (5.1° S, 42.9° W). In the tropical upper troposphere and lower stratosphere, upper limits for both, IO and OIO, are below 0.1 ppt. Photochemical modelling is used to estimate the compatible upper limits for the total gaseous inorganic iodine burden (Iy) amounting to 0.09 to 0.16 (+0.10/−0.04) ppt in the tropical lower stratosphere (21.0 km to 16.5 km) and 0.17 to 0.35 (+0.20/−0.08) ppt in the tropical upper troposphere (16.5 km to 13.5 km). In the middle stratosphere, upper limits increase with altitude as sampling sensitivity decreases. Our findings imply that the amount of gaseous iodine transported into the stratosphere through the tropical tropopause layer is small and that iodine-mediated ozone loss plays only a minor role for stratospheric photochemistry. However, photochemical modelling uncertainties are large and iodine might be transported into the stratosphere in particulate form.

Injection of iodine to the stratosphere

“…Ground-based observations in Arizona (United States) determined an upper limit for total inorganic iodine (I y ) of 0.2 pptv in the stratosphere [Wennberg et al, 1997]. Balloon-borne UV-Visible spectra derived lower stratospheric upper limits of around 0.1 pptv for both IO and OIO in the northern high and middle latitudes [Bösch et al, 2003], as well as over the tropics [Butz et al, 2009], consistent with previous reports. There is, however, one exception of a study reporting stratospheric IO mixing ratios of 0.65 and 0.8 pptv above Spitzbergen [Wittrock et al, 2000].…”

“…Measurements made at twilight capture the end of the typical top-hat-shaped diurnal profile of IO (supplementary information), when most of the IO is rapidly converted into nighttime reservoir species (IONO 2 and HOI) (Figure 3 and supplementary information). We have performed a model run with assimilated meteorology simulating the location and atmospheric conditions of previous balloon-borne solar occultation measurements [Butz et al, 2009]. Our modeled IO throughout the upper troposphere and lower stratosphere at twilight is in very good accord with the solar occultation method IO upper limits, <0.1 pptv.…”

“…There have been several attempts to detect iodine oxides (IO and OIO) in the upper troposphere-lower stratosphere [Berthet et al, 2003;Bösch et al, 2003;Butz et al, 2009;Pundt et al, 1998;Wennberg et al, 1997;Wittrock et al, 2000]. Ground-based observations in Arizona (United States) determined an upper limit for total inorganic iodine (I y ) of 0.2 pptv in the stratosphere [Wennberg et al, 1997].…”

“…Recent studies have reported values of 0.2-0.4 pptv of iodine monoxide (IO) in the free troposphere over the subtropical station of Izaña in the Atlantic Ocean (Canary Islands) [Puentedura et al, 2012] and 0.1-0.2 pptv range throughout the tropical free troposphere of the Pacific Ocean , demonstrating the ubiquitous presence of the radical in the marine free troposphere. However, due to the low levels of iodine monoxide (IO) previously detected in the lowermost stratosphere at twilight (solar zenith angle (SZA)~90°) [Butz et al, 2009;Wennberg et al, 1997], iodine chemistry is not considered in the photochemistry of stratospheric ozone [WMO, 2014].…”

“…Since then, the subsequent World Meteorological Organization (WMO) and United Nations Environment Programme Ozone Assessment Reports [WMO, 2014], based on available remotesensing measurements at twilight [Berthet et al, 2003;Bösch et al, 2003;Butz et al, 2009;Pundt et al, 1998;Wennberg et al, 1997], suggested upper limits of 0.1 pptv of IO and a total I y of <0.15 pptv, leading to an estimated negligible role of iodinated gases on stratospheric ozone photochemistry [WMO, 2014]. Consequently, global assessments on climate change have thus far only considered the stratospheric ozone depletion potential of long-and short-lived bromocarbons and chlorocarbons [WMO, 2014].…”