Radiative transfer in the absorption bands of oxygen : Studies of their significance in ozone chemistry and potential for aerosol remote sensing

Sammanfattning: Among the scientific topics of greatest public interest are the ozone question and climatic change. The discovery of the Antarctic ozone hole in 1985 and recognition of large scale ozone depletions in the northern hemisphere focused the world's attention on human induced changes in the atmosphere. Considerable progress has been made in modelling ozone and other atmospheric constituents and it is now possible to simulate individual situations in complex three dimensional models. However there remains, at least in some models, a long standing discrepancy, between measurements and models results.The modelled ozone concentrations tend to be about 30% too low in the upper stratosphere and mesosphere, the so called "the Ozone deficit". This may be the result of too little production or too fast ozone loss. Two aspects of the former are addressed here. One is the production of odd oxygen in the Schumann-Runge bands, the dominant spectral region for the altitude range in question. The high wavelength resolution required to accurately compute ozone production rates within such absorption bands is not available in chemical models. A new parameterisation of this process is presented here and compared with previous ones. The complex relationship between the transmission of the atmosphere in this wavelength region and the photolysis of compounds other than oxygen greatly complicates the effects of the changes. The second aspect concerns an autocatalytic production mechanism for ozone, where the vibrationally excited oxygen resulting from the photolysis of ozone is itself photolysed in the hot bands of the Schumann-Runge system.The effect of aerosols was classed highly uncertain in a recent Intergovernmental Panel on Climate Change assessment of the radiative forcing of our climate. The global mean ra- diative aerosol forcing could mask approximately one third of the anthropogenic increased radiative forcing by CO2. Due to the variability of the aerosol sources satellite measurements are required to obtain global coverage. Most of today's space based instruments rely on one or two-colour based algorithms to retrieve global aerosol concentrations. The Global Ozone Mapping Experiment (GOME) onboard the ERS-2 satellite is an instrument that makes a high resolution-multispectral analysis possible. The spectral region of the molecular oxygen A-Band is measured by GOME at sufficient resolution to obtain coarse information on the altitude distribution of the aerosol. We have applied the optimal estimation method in an altitude dependent retrieval algorithm. The results are still to some degree preliminary since the altitude profiling is still in the evaluation stage. Once fully verified the results can be incorporated in large scale climate models to improve the assessment of the aerosol effect. Future satellites with increased spectral resolution will give enhanced opportunities to employ this method.