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

摘要

We report upper limits of IO and OIO in the tropical uppertroposphere and stratosphere inferred from solar occultation spectra recordedby the LPMA/DOAS (Limb Profile Monitor of the Atmosphere/Differential OpticalAbsorption Spectroscopy) payload during two stratospheric balloon flightsfrom a station in Northern Brazil (5.1° S, 42.9° W). In thetropical upper troposphere and lower stratosphere, upper limits for both,IO and OIO, are below 0.1 ppt. Photochemical modellingis used to estimate the compatible upper limits for the total gaseousinorganic iodine burden (I) amounting to 0.09 to0.16 (+0.10/−0.04) ppt in the tropical lower stratosphere(21.0 km to 16.5 km) and 0.17 to0.35 (+0.20/−0.08) ppt in the tropical upper troposphere(16.5 km to 13.5 km). In the middle stratosphere, upperlimits increase with altitude as sampling sensitivity decreases. Our findingsimply that the amount of gaseous iodine transported into the stratospherethrough the tropical tropopause layer is small. Thus, iodine-mediated ozoneloss plays a minor role for contemporary stratospheric photochemistry butmight become significant in the future if source gas emissions or injectionefficiency into the upper atmosphere are enhanced. However,photochemical modelling uncertainties are large and iodine might betransported into the stratosphere in particulate form.