DOAS measurements of iodine monoxidefrom satellite

Abstract

Atmospheric columns of the trace gas iodine monoxide, IO, have been investigated by means of spectroscopic measurements in the visible wavelength range. For this purpose, solar radiation scattered and reflected by the Earth's atmosphere and surface is recorded by satellite instrumentation in nadir viewing geometry. These spectra have been analysed for the absorption signal of the IO vibronic absorption lines. Employing the Sciamachy sensor mounted on the ENVISAT satellite, global observations of IO from space become possible for the first time. The importance of iodine in the atmosphere lies in its high potential for destroying ozone as well as in the formation of new particles which is initiated by condensable iodine oxides and may impact on Earth's radiation budget at least locally. A major challenge in this work is the smallness of the observed IO optical depths with respect to the instrument's detection limit. The retrieved IO slant columns are therefore averaged over certain time periods of typically several months. Widespread enhanced IO columns have been detected over the Antarctic region with a detailed spatial and temporal distribution. Further regions with positive IO detection are the Eastern Pacific upwelling region and some Northern Hemispheric coast lines. Additional data such as tropospheric BrO distributions, ice concentrations, phytoplankton amounts and diatom abundances have been considered in specific cases for comparison and discussion purposes, addressing the question of sources of atmospheric iodine, which are most probably biogenic. Successful comparison and validation studies provide confidence in the newly developed satellite IO product, and model calculations have been conducted to investigate the amounts of precursors necessary for the explanation of observed IO abundances. While in some analyses, the limitations of the satellite measurements have been encountered, the presented investigations have advanced the prospects of remote sensing from space for the detection of the minor trace gas IO.