Odin is a 250 kg class satellite built in co-operation between Sweden, Canada, France, and Finland and launched in February 2001. It carries two instruments: a 4-band sub-millimetre radiometer used for both astronomy and atmospheric science and an optical spectrometer and infrared imaging system for purely atmospheric observations. As part of the joint mission Odin will observe the atmospheric limb for 50% of the observation time producing profiles of many species of interest in the middle atmosphere with a vertical resolution of 1–2 km. These species include, among others, ozone, nitrogen dioxide, chlorine monoxide, nitric acid, water vapour, and nitrous oxide. An overview of the mission and the planned measurements is given. PACS Nos.: 42.68Mj, 94.10Dy, 95.55Fw

Stratospheric ozone density profiles between 15 and 40 km altitude are derived from scattered sunlight limb radiance spectra measured with the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. The method is based on the analysis of limb radiance profiles in the centre and the wings of the Chappuis‐Wulf absorption bands of ozone. It employs a non‐linear Newtonian iteration version of Optimal Estimation (OE) coupled with the radiative transfer model LIMBTRAN. The derived zonally averaged ozone field for August 2001 is in excellent agreement with the main characteristics of the global morphology of stratospheric ozone, indicating that the limb scatter technique is capable of providing ozone profiles with high accuracy and high vertical resolution on a global scale and a daily basis.

(SAOZ and DOAS). An exception is the in situ Triple profile, in which the balloon and satellite data mostly does not agree within the given errors. In general, the satellite measurements show systematically higher values below 25 km than the balloon data and a change in profile shape above about 25 km.

[1] Vertical profiles of nitrogen dioxide in the 19-40 km altitude range are successfully retrieved over the globe from Optical Spectrograph and Infrared Imager System (OSIRIS) limb scatter observations in late 2001 and early 2002. The inclusion of multiple scattering in the radiative transfer model used in the inversion algorithm allows for the retrieval of NO 2 down to 19 km. The slant column densities, which represent the observations in the inversion, are obtained by fitting the fine structure in normalized radiance spectra over the 435-449 nm range, where NO 2 electronic absorption is readily observable because of long light paths through stratospheric layers rich in this constituent. Details of the spectral fitting and inversion algorithm are discussed, including the discovery of a pseudo-absorber associated with pixelated detectors and a new method to verify altitude registration. Comparisons are made with spatially and temporally coincident profile measurements of this photochemically active trace gas. Better than 20% agreement is obtained with all correlative measurements over the common retrieval altitude range, confirming the validity of OSIRIS NO 2 profiles. Systematic biases in the number densities are not observed at any altitude. A ''snapshot'' meridional cross section between 40°N and 70°S is shown from observations during a fraction of an orbit.