Odin/SMR Limb Observations of Stratospheric Trace Gases: Validation of N2O

The Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched on 20 February 2001, performs regular measurements of the global distribution of stratospheric nitrous oxide (N2O) using spectral observations of the J=20¨19 rotational transition centered at 502.296 GHz. We present a quality assessment for the retrieved N2O profiles (level 2 product) by comparison with independent balloon- and aircraft-borne validation measurements as well as by cross-comparing with preliminary results from other satellite instruments. An agreement with the air-borne validation experiments within 28 ppbv in terms of the root-mean-square (rms) deviation is found for all SMR data versions (v222, v223, v1.2) under investigation. More precisely, the agreement is within 19 ppbv for N2O volume mixing ratios (VMR) lower than 200 ppbv and within 10% for mixing ratios larger than 150 ppbv. Given the uncertainties due to atmospheric variability inherent to such comparisons, these values should be interpreted as upper limits for the systematic error of the Odin/SMR N2O measurements. Odin/SMR N2O mixing ratios are systematically slightly higher than non-validated data obtained from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II). Root-meansquare deviations are generally within 23 ppbv (or 20% for VMR-N2O>100 ppbv) for versions 222 and 223. The comparison with data obtained from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the Envisat satellite yields a good agreement within 9-17 ppbv (or 10% for VMR-N2O> 100 ppbv) for the same data versions. Odin/SMR version 1.2 data show somewhat larger rms deviations and a higher positive bias.