Comment on ‘‘Middle atmospheric O 3 , CO, N 2 O, HNO 3 , and temperature profiles during the warm Arctic winter 2001–2002’’ by Giovanni Muscari et al. Rolf Mu ¨ller 1 and Simone Tilmes 2 Received 13 December 2007; revised 29 June 2008; accepted 29 July 2008; published 20 September 2008. Citation: Mu ¨ller, R., and S. Tilmes (2008), Comment on ‘‘Middle atmospheric O 3 , CO, N 2 O, HNO 3 , and temperature profiles during the warm Arctic winter 2001–2002’’ by Giovanni Muscari et al., J. Geophys. Res., 113, D18303, doi:10.1029/2007JD009709. [1] In a recent paper, Muscari et al. [2007] presented measurements of stratospheric constituents in Arctic winter 2001 – 2002 from mid-January to early March that were obtained using the ground-based millimeter-wave spectrom- eter (GBMS) and a Lidar system at Thule, Greenland (76.5°N, 68.7°W). Among the recent Arctic winters, winter 2001–2002 is one of the warmest winters on record [e.g., Tilmes et al., 2004; Manney et al., 2005; Rex et al., 2006]. Using the GBMS stratospheric O 3 , CO, N 2 O, and HNO 3 measurements together with Lidar temperature observa- tions, Muscari et al. [2007] characterized the polar strato- sphere over Thule in the altitude range between 17 – 45 km focusing on two issues. First, they found low ozone con- centrations in the Aleutian high at 900 K to be well correlated with low solar exposure and, secondly, they quantified ozone loss in the polar vortex in the lower stratosphere. [2] Here, we discuss statements by Muscari et al. [2007] with regard to the lower stratosphere: ‘‘using correlations between GBMS O 3 and N 2 O mixing ratios, in early February a large ozone deficiency owing to local ozone loss is noted inside the vortex. GBMS O 3 -N 2 O correlations suggest that isentropic transport brought a O 3 deficit also to regions near the vortex edge, where transport most likely mimicked local ozone loss’’. [3] We will first discuss possible uncertainties in the way that the chemical ozone loss was derived by Muscari et al. [2007] as a result of the selection criteria used to sort GBMS profiles into different vortex regions. Then, considering the reported ozone loss values, we argue that, for the warm Arctic winter 2001 – 2002, chemical ozone loss to the extent suggested by Muscari et al. [2007] cannot be reconciled with the current understanding of halogen-driven chemical ozone destruction in the Arctic [e.g., Solomon, 1999; World Meteorological Organization, 2007]. [4] To distinguish between data points inside, outside, and at the edge of the polar vortex, Muscari et al. [2007] used GBMS N 2 O measurements instead of considering meteorological fields such as potential vorticity gradients and horizontal wind speed [e.g., Nash et al., 1996; Bodeker et al., 2001; Tilmes et al., 2006b; Manney et al., 2007]. The authors state that they ‘‘trust the GBMS N 2 O observations (O 3 and N 2 O measurements were carried out within a total of 4 to 5 h) more than the temporally and spatially coarser Potential Vorticity data analysis’’. Indeed, Greenblatt et al. [2002] developed a technique to accurately determine the edge of the polar vortex from in situ (aircraft and balloon) measurements of a long-lived trace gas like N 2 O. They found that for high-resolution aircraft data, a potential vorticity analysis may misidentify the inner edge by more than 400 km. However, GBMS measurements have a much coarser spatial and temporal resolution than the in situ data employed by Greenblatt et al. [2002]. Although the width of the instantaneous field of view of the GBMS is 10 km in the lower stratosphere, the integration time of the measurements (1.5 h for O 3 , 3 h for N 2 O) means that the GBMS samples an air mass of a certain horizontal extent. Assuming wind speeds of 20–40 km/h at 10 hPa, Muscari et al. [2007] estimated an effective horizontal resolution of 90 – 180 km. Considering typical wind speeds for the lower polar stratosphere at approximately 480 K [e.g., Chan et al., 1990] between 40 km/h (vortex core) and 180 km/h (toward the vortex edge), we obtain a conserva- tive estimate of the range of horizontal resolution of 180 km to 810 km, a range that includes earlier estimates for GBMS measurements of a single species (about 200–300 km [Muscari et al., 2002]). The vertical resolution of the GBMS is about 7 km in the Arctic lower stratosphere [Muscari et al., 2007]. Current meteorological analyses reach higher spatial resolutions, for example in 2000 the European Centre for Medium-Range Weather Forecasts (ECMWF) introduced a T511/L60 system with a horizontal resolution of about 40  40 km and a vertical resolution of about 1 km in the upper troposphere and lower stratosphere [e.g., Jung and Leutbecher, 2007]. [5] An appropriate criterion to determine whether profiles are measured inside or outside the vortex is essential for the application of ozone-tracer relations to calculate polar ozone loss because the characteristics of air outside the vortex are very different to those of vortex air. A criterion that leads to using a mixture of profiles measured inside and outside of the vortex could cause the chemical ozone loss in the vortex to be underestimated [Tilmes et al., 2004]. [6] We will now discuss the chemical ozone loss in Arctic winter 2001 – 2002. On the basis of correlations between JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D18303, doi:10.1029/2007JD009709, 2008 1 Institute for Stratospheric Chemistry, Forschungszentrum Ju ¨lich, Ju ¨lich, Germany. 2 Atmospheric Chemistry Division, NCAR, Boulder, Colorado, USA. Copyright 2008 by the American Geophysical Union. 0148-0227/08/2007JD009709 D18303 1 of 3