Abstract. We present observations of the 2010-2011 Arctic winter stratosphere from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard ENVISAT. Limb sounding infrared measurements were taken by MIPAS during the Northern polar winter and into the subsequent spring, giving a continuous vertically resolved view of the Arctic dynamics, chemistry and polar stratospheric clouds (PSCs). We adopted a 2-D tomographic retrieval approach to account for the strong horizontal inhomogeneity of the atmosphere present under vortex conditions, self-consistently comparing 2011 to the 2-D analysis of [2003][2004][2005][2006][2007][2008][2009][2010]. Unlike most Arctic winters, 2011 was characterized by a strong stratospheric vortex lasting until early April. Lower stratospheric temperatures persistently remained below the threshold for PSC formation, extending the PSC season up to midMarch, resulting in significant chlorine activation leading to ozone destruction. On 3 January 2011, PSCs were detected up to 30.5 ± 0.9 km altitude, representing the highest PSCs ever reported in the Arctic. Through inspection of MIPAS spectra, 83 % of PSCs were identified as supercooled ternary solution (STS) or STS mixed with nitric acid trihydrate (NAT), 17 % formed mostly by NAT particles, and only two cases by ice. In the lower stratosphere at potential temperature 450 K, vortex average ozone showed a daily depletion rate reaching 100 ppbv day −1 . In early April at 18 km altitude, 10 % of vortex measurements displayed total depletion of ozone, and vortex average values dropped to 0.6 ppmv. This corresponds to a chemical loss from early winter greater than 80 %. Ozone loss was accompanied by activation of ClO, associated depletion of its reservoir ClONO 2 , and significant denitrification, which further delayed the recovery of ozone in spring. Once the PSC season halted, ClO was reconverted primarily into ClONO 2 . Compared to MIPAS observed [2003][2004][2005][2006][2007][2008][2009][2010] Arctic average values, the 2010-2011 vortex in late winter had 15 K lower temperatures, 40 % lower HNO 3 and 50 % lower ozone, reaching the largest ozone depletion ever observed in the Arctic. The overall picture of this Arctic winter was remarkably closer to conditions typically found in the Antarctic vortex than ever observed before.

We present a new retrieval model designed to analyze the observations of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which is on board the ENVironmental SATellite (ENVISAT). The new geo-fit multitarget retrieval model (GMTR) implements the geo-fit two-dimensional inversion for the simultaneous retrieval of several targets including a set of atmospheric constituents that are not considered by the ground processor of the MIPAS experiment. We describe the innovative solutions adopted in the inversion algorithm and the main functionalities of the corresponding computer code. The performance of GMTR is compared with that of the MIPAS ground processor in terms of accuracy of the retrieval products. Furthermore, we show the capability of GMTR to resolve the horizontal structures of the atmosphere. The new retrieval model is implemented in an optimized computer code that is distributed by the European Space Agency as "open source" in a package that includes a full set of auxiliary data for the retrieval of 28 atmospheric targets.

[1] The local chemical impact of sprites is investigated for the first time. This study is motivated by the current understanding of the streamer nature of sprites: streamers are known to produce NO x , thus sprites are expected to lead to a local enhancement of the background abundances, and possibly impact ozone. We adopted the following strategy: middle atmosphere MIPAS/GMTR NO 2 satellite measurements were correlated with ground-based WWLLN detections of large tropospheric thunderstorms as a proxy for sprite activity. We found no evidence of any significant impact at a global scale, but an indication of a possible sprite induced NO 2 enhancement of about 10% at 52 km height in correspondence with active thunderstorms. This local enhancement appears to increase with height from a few percent at 47 km to tens of percent at 60 km.

Abstract. We present a multi-year database of atmospheric fields of the upper troposphere, stratosphere and lower mesosphere retrieved from satellite measurements adopting a 2-dimensional tomographic approach. The full mission of the Michelson

We examine volume mixing ratios (vmr) retrieved from limb emission spectra recorded with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board Envisat. In level 2 (L2) data products of three different retrieval processors, which perform one dimensional (1-D) retrievals, we find significant differences between species' profiles from ascending and descending orbit parts. The relative differences vary systematically with time of the year, latitude, and altitude. In the lower stratosphere their monthly means can reach maxima of 20% for CFC-11, CFC-12, HNO₃, H₂O, 10% for CH₄ and N₂O. Relative differences between monthly means of 1-D retrieval results and of the true atmospheric state can be expected to reach half of these percentage values, while relative differences in single vmr profiles might well exceed those numbers. Often there are no physical or chemical reasons for these differences, so they are an indicator for a problem in the data processing. The differences are generally largest at locations where the meridional temperature gradient of the atmosphere is strong. On the contrary, when performing the retrieval with a tomographic two dimensional (2-D) retrieval, L2 products generally do not show these differences. This suggests that inhomogeneities in the temperature field, and possibly in the species' fields, which are accounted for in the 2-D algorithm and not in standard 1-D processors, may cause significant deviations in the results. Inclusion of an externally given adequate temperature gradient in the forward model of a 1-D processor helps to reduce the observed differences. However, only the full tomographic 2-D approach is suitable to resolve the horizontal inhomogeneities. Implications for the use of the 1-D data, e.g. for validation, are discussed. The dependence of the ascending/descending differences on the observation strategy suggests that this problem may affect 1-D retrievals of infrared limb sounders, if the line of sight of the instrument has a significant component in the direction of the horizontal temperature variation