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

A Butz,L Kritten,K Pfeilsticker,C Prados-Román,C Camy-Peyret,M Dorf,M P Chipperfield,S Kreycy,H Bösch,J Schwärzle

doi:10.5194/acp-9-7229-2009

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. Thus, iodine-mediated ozone loss plays a minor role for contemporary stratospheric photochemistry but might become significant in the future if source gas emissions or injection efficiency into the upper atmosphere are enhanced. However, photochemical modelling uncertainties are large and iodine might be transported into the stratosphere in particulate form.

Highlights

Inorganic iodine species have been suggested as efficient catalysts for ozone (O3) destruction in the troposphere and in the stratosphere (e.g. Solomon et al, 1994; Davis et al, 1996)
When ratioing solar occultation spectra by the background spectrum as in our Differential Optical Absorption Spectroscopy (DOAS) retrieval, the center-to-limb darkening (CLD) ef- 3 Upper limits of iodine monoxide (IO) and OIO fect is detectable through a change of the optical density of the Fraunhofer lines
Our study reports on the to date lowest upper limits of IO and OIO in the tropical upper troposphere and stratosphere

Summary

Introduction

Inorganic iodine species have been suggested as efficient catalysts for ozone (O3) destruction in the troposphere and in the stratosphere (e.g. Solomon et al, 1994; Davis et al, 1996). An automated heliostat collects direct sunlight and feeds it into the spectrometer optics such that the instruments sample virtually the same air masses along the lines-of-sight from the balloon to the Sun. Here, we use retrievals from solar occultation measurements of the DOAS spectrometer sensitive to the visible spectral range. By ray-tracing the path of the incoming light from the Sun to the balloon-borne detector for each spectrum, we estimate absorber concentrations from the inferred SCDs under the assumption that the absorber volume mixing ratio is constant along individual lines-of-sight This yields an approximate estimate of the absorber concentration at the respective tangent height since, for our solar occultation viewing geometry, the lightpath through the tangent layer dominates the entire lightpath. When ratioing solar occultation spectra by the background spectrum as in our DOAS retrieval, the CLD ef- 3 Upper limits of IO and OIO fect is detectable through a change of the optical density of the Fraunhofer lines.

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Discussion

Conclusions