Comment on “Origin of the January–April 2004 increase in stratospheric NO2 observed in northern polar latitudes” by Jean‐Baptiste Renard et al.

[1] In a recent letter Renard et al. [2006] (hereinafter R06) have analyzed stratospheric NO2 enhancements in the polar Northern hemisphere (NH) during January – April 2004 using data from the GOMOS and MIPAS instruments onboard Envisat, and by ground-based column measurements of the zenith-sky UV-visible spectrometer (SAOZ) [Pommereau and Goutail, 1988] at Sodankyla, Finland. R06 claim that the large concentrations of NO2 were produced locally at altitudes of 60 km due to energetic (>100 keV) electron precipitation (EEP) from the outer trapping region of the magnetosphere associated with a strong geomagnetic perturbance on 22 January 2004. R06 do not find such NO2 enhancements in the Southern hemisphere (SH) and propose a particular nighttime ion chemistry to explain the observed asymmetry. The authors base their explanation of the origin of the NH upper stratospheric NO2 enhancements in early 2004 on SAOZ ground-based column measurements which show a pronounced increase after 22 January 2004, peaking on 25 January with values of about 100% higher than the background level. [2] The suggestion of R06 on the origin of the NH NO2 enhancements, however, is in disagreement with the conclusions drawn in several other studies related to this topic. Randall et al. [2005] discussed the NOx enhancements in the upper stratosphere from March to July 2004 by using several satellite measurements. These authors suggested that the NOx enhancement during that period was caused by energetic particle precipitation that led to substantial NOx production in the upper atmosphere beginning with the solar storms in late October 2003 and possibly persisting through January, followed by downward transport favored by the strong upper stratospheric vortex from mid January until March 2004. López-Puertas et al. [2005] analyzed operational MIPAS NO2 data from October 2003 to April 2004 and concluded, in agreement with Randall et al. [2005], that the upper stratospheric NO2 enhancements that appeared in mid January 2004 were most probably caused by the unusually strong vortex and downward transport together with an uncommonly large auroral activity over the whole winter. A gap in the MIPAS version 4.61 NO2 data used in that study from the end of December until mid February, however, inhibited a detailed analysis of the NH stratospheric NO2 distributions around 22 January. The missing period was examined in a subsequent paper using data version 4.62 [López-Puertas et al., 2006] (hereinafter L06). The temporal evolution of MIPAS nighttime NO2 at 65–90 N, presented in L06, shows a pronounced increase in NO2 with volume mixing ratios (VMR) of more than 100 ppb around 0.1 hPa on 15 January 2004, i.e., already one week before the EEP event reported by R06. This NO2 increase was associated with low values of CH4, typical of the upper atmosphere, which strongly supports the hypothesis that the observed NO2 enhancements were produced by downward transport from the mesosphere and lower thermosphere. The low (high) stratospheric NO2 (CH4) abundances measured by MIPAS before 15 January could be related to a major stratospheric warming [Manney et al., 2005] which led to transport of NOx-poor and CH4-rich air from mid-latitudes to the polar region, and induced excursions of the upper stratospheric vortex toward lower latitudes where NOx was lost due to photo-dissociation of NO. In order to corroborate these results we have carried out an independent analysis of MIPAS NO2 and CH4 distributions retrieved with the scientific processor developed at Institut für Meteorologie und Klimaforschung (IMK) and Instituto de Astrofı́sica de Andalucı́a (IAA) [von Clarmann et al., 2003; Funke et al., 2005b] which confirm the results obtained from the operational data, and further show an excellent spatial anticorrelation of NO2 and CH4 (not shown). [3] The experimental evidence described above clearly support a dynamical origin of the early 2004 NH NO2 enhancements in the stratosphere by subsiding NOx-rich mesospheric air masses, thus calling in question the explanation by R06 that these enhancements could be related to in situ production associated with the EEP event on 22 January 2004. R06 discard descent of NOx from the upper atmosphere because no NO2 increase above 70 km was detected by GOMOS inside the Arctic circle in December 2003. However, increased NOx at these altitudes would be in the form of NO even under dark conditions [Callis and Lambeth, 1998], and hence, not detectable by the GOMOS instrument. Since MIPAS measures NO only GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L07813, doi:10.1029/2006GL027518, 2007