5.2. NOx-Determined Layer
 In the NOx-determined layer, low ClO mixing ratios ≪100 pptv were measured both on 27 January and 1 March 2000. Our ClO measurements at these altitudes agree well with ClO measurements obtained by the ClO/BrO instrument on board the HALOZ payload on 27 January, 1 March, and 8 March 2000 [Vömel et al., 2001]. Simulations performed with the SLIMCAT model by A. Robinson et al. (unpublished manuscript, 2002) also overestimate the ClO mixing ratios measured at 550 K by HALOZ on 8 March 2000. Here we performed further sensitivity studies for the NOx-determined layer which are discussed in the following.
 An indication that model simulations overestimate measured ClO mixing ratios at ≈22 km was reported by Klein et al.  for January/February 1999 during the Arctic winter. [Klein et al., 2000] assumed that in this winter uncertainties in UKMO temperatures [Knudsen et al., 2001] could produce the discrepancies between simulated and measured ClO mixing ratios at the altitudes of interest. Uncertainties in UKMO temperatures were also reported for the winter 1999/2000; Knudsen et al.  reported a standard deviation of UKMO temperatures ranging from 1.0–1.3 K. Single events show temperature differences between UKMO temperatures and measurements of up to ±5 K. Therefore we performed model simulations along trajectories with temperatures modified by a constant offset of −2 K, +2 K, +4 K, and +6 K. These simulations show that for 27 January 2000 assuming a temperature enhancement of approximately 4–6 K allows the low measured ClO mixing ratios to be reproduced in the model. Similar simulations for 1 March 2000 reproduce the measured ClO mixing ratios for a temperature enhancement of approximately 2–6 K. For this assumption no PSC occurrence and consequently no chlorine activation was simulated. However, satellite measurements of POAM III [Bevilacqua et al., 2002] demonstrate that PSC occurred during the Arctic winter 1999/2000 at the altitudes and during the time periods considered here. Because PSC occurrence was very inhomogeneous during the Arctic winter 1999/2000, it cannot be demonstrated with certainty that in the air parcels investigated here chlorine was activated. Therefore for 1 March 2000, we checked our model results through a comparison of simulated HCl and NOx mixing ratios with HALOE measurements. The low mixing ratios measured by HALOE demonstrate for the NOx-determined layer that the air was activated during the winter. Thus uncertainties in UKMO temperatures and uncertainties arising from accumulated errors along the calculated trajectories could be the reason for the substantial overestimation of the observed ClO mixing ratios for 27 January 2000; although this is very unlikely for 1 March 2000.
 Further model studies considering the stratospheric NOx source due to galactic cosmic rays [Müller and Crutzen, 1993] were performed, but the simulated ClO mixing ratios are still too large in comparison with measurements. Simulations with an alternative hypothetical NOx source were performed to test the magnitude of such an additional NOx source that would be required to successfully simulate the low observed ClO mixing ratios. Transport of NOx-rich air from the mesosphere and upper stratosphere to the lower stratosphere constitutes a NOx source for the stratosphere; it is, however, unclear whether this NOx source is strong enough to cause a rapid chlorine deactivation which would explain the observed low ClO values [e.g., Callis and Lambeth, 1998; Jackman et al., 2000; Callis et al., 2001]. For 1 March 2000, simulations with an additional permanent NOx emission rate of 300–1000 [molecules cm−3 s−1] can best reproduce the measured ClO mixing ratios as well as the NOx mixing ratios measured by HALOE and, further, the observed ozone loss. Nonetheless, the simulated ClO mixing ratios are slightly too large and the simulated HCl and NOx mixing ratios are somewhat too low compared to the observations. Moreover, solar proton events (SPE) are temporary sources of NOx in the upper atmosphere. In fact, the solar maximum was reached in mid 2000 so that the sun was very active in the winter 1999/2000. For example, on 17 February 2000 two M-class solar flares erupted and were followed by a coronal mass ejection (CME) [Wang et al., 2001]. Further, the NOAA Space Environment Services Center reported a weak solar proton event on 18 February 2000 (http://umbra.nascom.nasa.gov) and a strong SPE on 14–15 July 2000 [Jackman et al., 2001; Randall et al., 2001]. Again, none of these processes is a likely explanation for the noted discrepancy between observed and simulated ClO mixing ratios. Thus the dilemma is that for the winter 1999/2000 we cannot clearly identify a process producing additional NOx in the lower stratosphere at a sufficiently strong rate, neither a permanent source such as galactic cosmic rays or transport processes from the mesosphere and upper stratosphere nor a temporary source such as a solar proton event (SPE).
 Another possibility of producing sufficient levels of NOx would be if hydrolysis of N2O5 were completely suppressed. Again no such process is known. However, enhanced N2O5 mixing ratios were measured by the MkIV instrument on 3 December 1999 (2.9 ppbv N2O5 at 700–800 K, see Tables 1 and 2) and on 15 March 2000 (up to 0.9 ppbv at 500–600 K). This could also be an indication that such a process exists.
 These results could also indicate that the chemistry controlling the NOy partitioning is not yet completely understood. For example, Stowasser et al.  reported discrepancies in both NOy observations and observed NOy partitioning in comparison with SLIMCAT and KASIMA simulations for the Arctic winter 1998/1999 at similar altitudes as those discussed here. As in our model studies, these model simulations underestimate the observed NOy mixing ratios so that the absence of an additional unknown NOx source in the models could cause these discrepancies.
 For 1 March we assumed in our simulations that during the winter denitrification occurred in the NOx-determined layer. The simulations show that the simulated ClO mixing ratios are very sensitive to the rate of the assumed denitrification and the magnitude of the assumed NOx source. At these altitudes NOy observations are only available from the MkIV instrument on 3 December 1999 and 15 March 2000 [Popp et al., 2001]. Popp et al.  derived a small denitrification for this time period and altitude range (≈2.0–4.5 ppbv NOy at ≈525–550 K). Unfortunately, for the exact time period considered in our studies no NOy measurements are available, therefore we cannot corroborate our assumptions on denitrification by measurements.
 Further, a comparison between model simulations for the NOx-determined layer with and without denitrification shows that in simulations with denitrification no PSC formation occurred during February 1999 at the altitudes considered (see Figure 9). This is consistent with measurements by POAM III [Bevilacqua et al., 2002]: POAM III observed PSCs in the altitude range between 21 and 25 km from December 1999 until the beginning of February 2000 with short interruptions at the beginning and the end of January. In the time period from mid-February to mid-March PSCs only occurred up to an altitude of 20 km. The absence of PSCs at the end of January at altitudes of the NOx-determined layer could also be an indication that denitrification had occurred before end of January in the air masses considered, because there were PSCs before end of January and chlorine was activated, therefore the low measured ClO mixing ratios must be caused by deactivation. This indicates that uncertainties in UKMO temperatures and uncertainties arising from accumulated errors along the calculated trajectories do not cause the measured ClO mixing ratios to be low.
 Moreover, on 27 January 2000 TRIPLE observed chlorine activation from ≈400 K (≈16 km) to ≈575 K (≈24 km). At similar altitudes POAM III observed PSCs early in the winter (generally between 17 and 25 km). On 1 March 2000, TRIPLE observed chlorine activation from ≈400 K (≈17 km) to ≈500 K (≈21 km) and a weak chlorine activation between ≈350 and 375 K (≈13–16 km) probably caused by activation on the background aerosol. Again, POAM III observed PSCs at similar altitudes. PSCs reappear in late February and into March 2000 forming at lower altitudes between ≈13 and 20 km and centered roughly at 16 km. Summarizing the results for 1 March 2000, ClO, HCl, and NOx mixing ratios measured by TRIPLE and by HALOE in the NOx-determined layer can only be simulated simultaneously for a specific parameter setup assuming denitrification and an additional unidentified NOx source.