Interactive comment on “ Improvements in the profiles and distributions of nitric acid and nitrogen dioxide with the LIMS version 6 dataset ”

General comments: The LIMS HNO3 and NO2 data sets have been re-processed with an updated retrieval algorithm (version 6). This paper describes the algorithm improvements and evaluates the results. The main improvement to HNO3 is better treatment of interference by CO2 in the upper stratosphere, reducing the high bias in HNO3 that had been seen in the version 5 retrievals. The main improvements to NO2 are in the middle and upper stratosphere. These primarily result from improved spectroscopic parameters, particularly inclusion of spin-rotation effects. Minor improvements also resulted from better retrievals of temperature and water vapor, biases in which previously led to errors in the NO2 retrievals. There still appears to be a high bias in LIMS NO2 above 1 hPa, due to not accounting for non-LTE emissions from H2O that contaminate the NO2


Background
The Limb Infrared Monitor of the Stratosphere (LIMS) experiment was launched on 24 October 1978 on the near polarorbiting Nimbus 7 satellite.LIMS operated from 25 October until 28 May 1979, measuring vertical radiance profiles across the atmospheric limb of the Earth (Gille and Russell, 1984).Single limb radiance profiles were measured about every 12 seconds along an orbital track of the LIMS tangent point locations.Its daily orbital data extend from 64 • S to 84 • N and are available at two local times per latitude (at about 1300 h and 2300 h at the Equator).The radiances were processed to infer middle atmospheric temperature profiles and the concentrations of several chemical compounds important for the chemistry of stratospheric ozone.LIMS provided profiles of ozone (O 3 ), water vapor (H 2 O), nitric acid (HNO 3 ), and nitrogen dioxide (NO 2 ).Thus, LIMS was the first satellite experiment to provide a simultaneous, near-global view of the key chemical compounds in the ozone/nitrogen oxide photochemical chain.The temperature, geopotential height, and constituent profiles have been used for many scientific investigations, including effects of radiation on the net transport (Mlynczak et al., 1999), responses of the middle atmosphere to perturbations (Leovy et al., 1985), and correlations between the temperature and species data (Froidevaux et al., 1989).
The LIMS Version 5 (or V5) profile dataset was archived in 1982; its measurements, algorithms, and data products are described in Gille and Russell (1984) and references therein.Since that time, significant improvements have been realized in the original spectroscopic parameters (Drayson et al., 1984) that were used for the V5 retrieval of temperature/pressure or T(p) and for the gaseous constituent profiles Published by Copernicus Publications on behalf of the European Geosciences Union.that contribute to the radiances measured within the six channels of LIMS.For this reason a reprocessing of the LIMS Level 2 (or profile) data to Version 6 (V6) was considered for several of its primary and interfering gases using the line parameters of HITRAN 92 (Rothman et al., 1992a).As a result, these V6 data are considered more appropriate for comparisons with the temperature and species distributions obtained from the several middle atmosphere instruments onboard the NASA Upper Atmosphere Research Satellite (UARS), the Earth Observing System (EOS) Aura satellite and on the Canadian, European, and/or Japanese satellites SCISAT, ODIN, ENVISAT, and ADEOS.
The V6 dataset contains improvements due to a better knowledge of the orbital attitude for the LIMS measurements, from the application of a multiple interleave retrieval process, and from using all the radiance profile samples, spaced 0.375 km in altitude.Descriptions of the V6 algorithms for the conditioned radiances of all the channels and for the retrieval of the temperature and geopotential height profiles are in Remsberg et al. (2004).Characterizations of the improvements for V6 ozone and water vapor are provided in Remsberg et al. (2007Remsberg et al. ( , 2009)).This paper is focused on the improvements and scientific implications for the V6 HNO 3 and NO 2 profiles and distributions.
The LIMS instrument is essentially a limb-viewing radiometer, and the optical characteristics of all the channels are given in Russell and Gille (1978, their Table 1).A single LIMS scan profile begins with the center of its field-of-view (FOV) array viewing the horizon at 153 km altitude, moving steadily downward to a point 38 km below the solid Earth limb and then returning upward in the same manner.The angular resolutions (in milliradians or mrad) for the detectors of the FOV array are 1 mrad for the NO 2 and H 2 O channels and 0.5 mrad for the other four LIMS channels, including HNO 3 .Their instantaneous FOV at the horizon is 3.6 km for NO 2 and H 2 O and 1.8 km for HNO 3 , O 3 , and for temperature from its two CO 2 channels.Radiance samples are obtained every 0.375 km in altitude for HNO 3 and 0.75 km for NO 2 .The filter for the LIMS HNO 3 channel extends from 842 to 915 cm −1 , its 5% transmission points.That channel measured the radiance emitted by the ν 5 and 2ν 9 bands or in the region of 11.3 µm.The V6 HNO 3 retrievals were obtained using the same laboratory-measured, band parameters used to process the LIMS V5 profiles (Drayson et al., 1984;Goldman et al., 1981).Thus, the primary improvements for V6 HNO 3 occur in the upper stratosphere due to accounting for the CO 2 laser band emission centered near 10.4 µm and in the lower stratosphere from the underlying emissions of the CFCs and aerosols.The filter for the LIMS NO 2 channel has its 5% transmission points at 1561 and 1631 cm −1 .The effects of the spin splitting for its lines are included in the line parameters of HITRAN 92 that we used (Rothman et al., 1992a).The band intensity for the primary NO 2 ν 3 cold band of HITRAN 92 was also unchanged for HITRAN 96 (Rothman et al., 1998), although the strengths of its assigned lines are altered somewhat in HITRAN 96 based on the analyses of Toth (1992).
The LIMS retrievals make use of an onion-peeling approach along with the emissivity growth approximation (EGA) of Gordley and Russell (1981).The V6 Level 2 data files contain both the de-convolved radiances and the retrieved parameters, and they are tabulated at 18 levels per decade of pressure or at a spacing of about 0.88 km.Note that the UARS Level 3A profiles have 6 levels of data per decade of pressure and represent a matching subset to the LIMS V6 data for easy comparison.In addition, the time, location, and solar zenith angle for the tangent point of a LIMS measurement are included in the header lines of each profile.This information makes it easier to relate the LIMS profiles of O 3 , HNO 3 , and NO 2 to photochemical model output.The effective vertical resolution for both V6 HNO 3 and NO 2 is of order 3.7 km, and retrievals were performed for each of the down/up scan pairs spaced about 1.6 degrees of latitude along the LIMS orbital tangent tracks.Distributions of HNO 3 extend from near the tropopause to just above the 2-hPa level.The nighttime distributions of NO 2 extend from about 50 hPa to the lower mesosphere, especially for the polar night.Results for daytime NO 2 are useful from about 50 hPa to 1 hPa.
Section 2 shows several daily, zonally-averaged distributions of the V6 HNO 3 and NO 2 for comparisons with model output and with distributions from more recent satellite experiments that also were observing during periods when the stratospheric aerosol loading was near background levels.An extensive validation of the V6 products was not conducted, although their daily zonal mean cross sections have been assessed against those of the V5 data that had been compared with the few correlative measurements available during the LIMS timeframe (e.g., Gille et al., 1984;Gille, 1987;Russell et al., 1984a;Remsberg and Russell, 1987).Qualitative improvements have been found for all the V6 data products.
Section 3 describes the significant changes in the V6 algorithm that affect the V6 HNO 3 and NO 2 profiles.Precision and accuracy for those V6 data are included, based on calculations of the effects of their known error sources.These calculated precisions are shown to compare well with the variations in the retrieved HNO 3 and NO 2 from among adjacent scans along an orbit, specifically for latitudes and times when zonal variations due to wave activity were minimal.Section 4 presents some scientific findings demonstrating the improved quality of the V6 HNO 3 and NO 2 .Section 5 is a summary of findings about the changes with V6.

Zonally-averaged distributions of HNO 3 and NO 2
There were 13 to 14 orbits of data per day from LIMS, following the Sun-synchronous geometry for the Nimbus 7 satellite (Gille and Russell, 1984).Retrievals were performed on the radiance profiles, located along the ascending (south-to-north) and the descending (north-to-south) node, tangent tracks of each orbit.The local time of a measurement at the low and middle latitudes is near 1 p.m. for the ascending node and 11 p.m. for the descending node data.As an introduction, we show several V6 HNO 3 and NO 2 distributions obtained from the LIMS Level 2 profiles.
Figure 1 provides the daily, zonally-averaged distributions of V6 HNO 3 from the combination of its species profiles from the descending and ascending orbital segments for 15 November 1978, (top panel) and 16 May 1979 (bottom panel).Maximum mixing ratio values occur at the high latitudes near 30 hPa, and the variation of HNO 3 with latitude is similar to that from the earlier V5 dataset at that pressure level (Gille et al., 1984(Gille et al., , 1993)).However, the V6 HNO 3 profiles no longer show nearly constant values in the upper stratosphere, as was the case for V5 in Jackman et al. (1985).This improvement is due primarily to a better accounting for the effects of the interfering radiance from the laser band of carbon dioxide (CO 2 ) at 10.4 µm.The V6 HNO 3 agrees reasonably well with the photochemical model estimates of Jackman et al. (1985).The LIMS V6 daytime values are slightly less than its nighttime values near the 4-hPa level (not shown), possibly due to not having accounted accurately for the effects of the temperature tides on the LIMS HNO 3 radiances.V6 HNO 3 values are somewhat smaller than those of V5 in the tropical lower stratosphere, due to first-order corrections in the V6 algorithm for the effects of interfering emissions from aerosols and from the chlorofluorocarbon (CFC) molecules CFCl 3 and CF 2 Cl 2 .The 1 ppbv contour extends to just above the 50-hPa level at equatorial latitudes; that value is at the upper limit of reactive nitrogen (NO y ) minus odd nitrogen (NO x ) from in situ measurements (e.g., Jensen and Drdla, 2002, and references therein).
One can see from Fig. 1 that there is very good seasonal symmetry between the HNO 3 distributions of the middle and www.atmos-chem-phys.net/10/4741/2010/Atmos.Chem.Phys., 10, 4741-4756, 2010 lower stratosphere for November versus May of the northern and southern hemispheres.Austin et al. (1986) explained that the increases in fall and winter were likely a result of heterogeneous, chemical production mechanisms.Note that the values of HNO 3 are also larger in November at high northern latitudes of the upper stratosphere (top panel of Fig. 1).This feature becomes even more prominent in midwinter due to an accumulation of HNO 3 from a chemical conversion of NO 2 and di-nitrogen pentoxide (N 2 O 5 ) under polar night conditions (e.g., Kawa et al., 1995;Bekki et al., 1997;Stiller et al., 2005).The distribution of 16 May (bottom panel of Fig. 1) also indicates that there was an accumulation of HNO 3 in the lower part of the northern, polar stratosphere from the winter and early spring.
Because NO 2 undergoes large diurnal variations in the upper stratosphere, Fig. 2 shows its zonal-averages from the descending (or local nighttime at low and middle latitudestop panel) and from the ascending (or local daytime at low and middle latitudes -bottom panel) orbital segments for 15 November 1978.Maximum mixing ratio values for the nighttime LIMS NO 2 of Fig. 2 (top panel) are of the order of 14 ppbv centered at about the 4-hPa level.The retrieved nighttime NO 2 profiles extend into the lower mesosphere, where they encounter their signal-to-noise (S/N) limit and are less accurate.The repartitioning of the NO x (nitric oxide (NO)+NO 2 ) at twilight occurs at about 55 to 60 • S latitude.Those terminator NO 2 profiles are also less accurate because their rapidly changing NO 2 values along the tangent path were not accounted for in the same manner as in Solomon et al. (1986).
Maximum mixing ratios for the ascending NO 2 (daytime at most latitudes) of Fig. 2 (bottom panel) are of the order of 6 to 7 ppbv and occur between 8 and 10 hPa.Note that there is a crossover to polar night conditions at about 75 • N. The daytime NO 2 at the lower latitudes is small in the upper stratosphere because most of the NO x is in the form of NO.H 2 O emission becomes a significant part of the NO 2 channel radiance in the upper stratosphere, especially during daytime.The forward radiance model for the V6 NO 2 channel assumes that H 2 O has a constant value of 6.5 ppmv above the upper limit of 1.3 hPa for its V6 profile.Furthermore, there is a significant amount of extra emission in the mesosphere during daylight from excited states of the H 2 O molecule that are difficult to characterize for LIMS, primarily because its ground-state H 2 O populations are not known exactly for those altitudes (Lopez-Puertas and Taylor, 2001;Kerridge and Remsberg, 1989).For this reason no corrections for non-LTE effects from H 2 O were developed for the retrieval of the V6 NO 2 .That source of positive radiance bias is the most likely reason for the spurious, upward extension of the daytime NO 2 distributions above the 1-hPa level from 64 • S to ∼70 • N (see also Sects. 3 and 4).
Figure 3 is the distribution of NO 2 from the descending (nighttime) orbital segments of 15 January 1979, and it is similar to that of Fig. 2 (top panel) in most respects.One exception is the occurrence of larger values of NO 2 in the upper stratosphere near 50 • S in Fig. 3 than in Fig. 2.This difference is partly a diurnal effect.The LIMS NO 2 measurements for the descending orbital segments were obtained at about 2130 h (local time) at 50 • S versus 2300 h at the equator, and there is a steady conversion of NO 2 to N 2 O 5 from just after sunset and until sunrise.Furthermore, sunset occurs later in the day during summer at the high southern latitudes.
The elevated values of NO 2 in the mesosphere at the high latitudes of the northern hemisphere are another feature of Fig. 3.Such enhanced values of NO 2 were first reported by Russell et al. (1984b) based on a special, radiance-averaged version of the LIMS data, and they were attributed to the formation of NO x in the mesosphere followed by a gradual descent of the air to the uppermost stratosphere during polar night.Findings from more recent satellite sensors support that interpretation (e.g., Randall et al., 2009).A time series of the V6 polar night NO 2 will be shown in Sect.4, indicating the improved estimates for its values and a better continuity for its descent during the winter months of 1978/1979.and error estimates   3.1 The V6 algorithms for HNO 3 and NO 2 Emissivity data tables for the forward limb-radiance algorithms were developed as a function of temperature and pressure for each of the gases that make contributions in the HNO 3 and NO 2 channels.The V6 forward model for HNO 3 makes use of the cross section parameters of Goldman et al. (1981), based on the laboratory measurements of Giver et al. (1984).That parameterization is appropriate for the broadband HNO 3 channel of LIMS, and it is essentially unchanged from that used for V5.Although there were uncertainties associated with the line parameters on the HI-TRAN HNO 3 databases during the 1990s, it is noted that the HNO 3 line intensities in the 11-µm region for HITRAN 96 also include normalizations to the band intensities of Giver et al. (1984).

LIMS V6 retrieval algorithms
The HNO 3 channel contains secondary radiance contributions from the laser band of CO 2 at 10.4 micrometers and from the primary CFCs (CFCl 3 and CF 2 Cl 2 ).The line parameters for CO 2 were taken from HITRAN 92 (Rothman et al., 1992a and b;Dana et al., 1992), which are improved over the parameters used with the original LIMS V5 algorithm.To achieve better accuracy for the emissivity of CO 2 along a tangent ray path, its emissivity table values were generated at 55 pressure levels and 33 temperature levels encompassing their expected atmospheric ranges.In addition, polynomial fits were not used to account for the temperature dependence within the emissivity tables; linear interpolation was employed instead.These upgrades have led to a more accurate representation of the effects of the laser band of CO 2 .To summarize, the V6 emissivity tables for CO 2 account for much of the excess of retrieved HNO 3 that characterized the V5 HNO 3 profiles from about 5 to 2 hPa, as originally reported in Gille et al. (1984), Jackman et al. (1985), and Gille et al. (1993) (see also Sect.4).
Line parameters for the original V5 retrievals of NO 2 were obtained from the AFGL trace gas compilation (Rothman et al., 1981).Later, the spin-rotation effects of NO 2 were identified, and the fine structure of the lines for the primary ν 3 cold band was included in the HITRAN 92 compilation via a perturbation calculation (Perrin et al., 1992).This update accounts for most of the improvements in the V6 NO 2 profiles (Remsberg et al., 1994).Only minor changes in the retrieved NO 2 are expected from the more explicit calculations of the strengths of those resolved lines reported by Toth (1992)  and available later in HITRAN 96.The LIMS NO 2 channel also contains contributions from H 2 O, CH 4 , and the oxygen (O 2 ) continuum, and tables were developed for each of them.Since the LIMS NO 2 radiances are interfered with substantially by H 2 O in the stratosphere, an additional emissivity table was employed to account for the effects of their overlapping lines (Remsberg et al., 2004).Spectral parameters for the underlying O 2 were not available, so the empirical model of Thibault et al. (1997) was used for consistent calculations of the effects of the continuum-induced O 2 absorption in both the LIMS H 2 O and NO 2 channels.
The LIMS V6 profiles were used to correct for the forward radiance contributions of H 2 O in the NO 2 channel up to about the 1.5-hPa level, and then they were extended upward using a constant value of 6.5 ppmv.In addition, the entire H 2 O profile for the forward model was smoothed by a Gaussian function having a half-width of 1.5 km at half maximum.As indicated earlier, no corrections were developed to account for non-LTE radiances from mesospheric H 2 O during daytime.Even so, there is no indication of a bias in the retrieved NO 2 due to that neglect, except at and above about 48 km (near 1 hPa).Emissions from excited states of daytime NO 2 were postulated for the upper stratosphere by Kerridge and Remsberg (1989), but no corrections for those effects were developed for V6 because the populations of those states were not known well.More importantly, Funke et al. (2005a) reported finding no evidence for such non-LTE effects in upper stratospheric NO 2 from the higher spectralresolution data of MIPAS on the ENVISAT satellite.The effects of horizontal mixing ratio gradients in the tangent layer were shown to be important for the retrieval of LIMS NO 2 in the region of the day/night terminator (Solomon et al., 1986).
However, because such a gradient correction is needed only for solar zenith angles near 90 degrees, a second-pass processing was not applied to the V6 NO 2 profiles prior to their archival in 2002.
Estimated concentrations of the interfering gases were obtained as follows.Updated, seasonal zonal-mean distributions of CH 4 were generated from the UARS HALOE dataset for use with the V6 retrievals of NO 2 but with an extrapolation of their magnitudes back to 1979, based on the approximate, linear emission rate for CH 4 at the ground.A similar extrapolation was performed for CFCl 3 and CF 2 Cl 2 for the HNO 3 channel, based on their measured distributions from the UARS CLAES Version 8 dataset plus knowledge of the growth rates for their emissions since 1975.Interfering emission due to aerosols has its largest effect on the LIMS-retrieved HNO 3 at low latitudes of the lower stratosphere.Zonal mean distributions of the background aerosol extinction were not available for the seven months of LIMS, and its V5 dataset had no correction for it.A first-order aerosol extinction cross section was generated for the V6 algorithm, based on estimates for the background stratospheric aerosol layer of 1978/1979.Specifically, a single zonalmean distribution of aerosol extinction was adopted, based on March/May 1996 results for the HALOE 5.26 µm (nitric oxide or NO) channel (Hervig et al., 1995).The magnitude of that distribution was then scaled back to 1978/1979 using a factor obtained by comparing the SAGE II 1-µm channel aerosol extinction values of 1996 versus the SAGE I aerosol extinction for the same months of 1979.That single modified, zonal mean aerosol distribution was used for each of the seven months of the LIMS dataset and must be considered as somewhat qualitative with latitude and not truly representative of its seasonal cycle variations.Since tropical aerosol extinctions vary most noticeably according to the phase of the QBO cycle (Hitchman et al., 1994), a scaling of their values between the 1996 and the 1979 March/May periods was used because they occurred at nearly the same QBO phase.Finally, aerosol extinctions at the wavelengths of the LIMS channels versus that at 5.26 µm of HALOE were obtained by employing the sulfuric acid/H 2 O composition and the wavelength-dependent, refractive index model for its aerosol absorption in the manner of Hervig et al. (1995).
The original V5 profiles of NO 2 and HNO 3 were used as a priori estimates for the V6 forward models, although there is no dependence upon them for the onion-peeling retrievals of LIMS once a reasonable S/N level for the radiances was met.The archived V6 profiles were screened for anomalies using several criteria.First, the retrieval process for a tangent layer value was iterative.The variances between successive iterations were calculated, and they were checked for convergence to near the measurement noise.Those variances are defined in terms of (NEN/K) 2 , where NEN is the noise equivalent radiance for each channel in watts/m 2 -sr (Gille and Russell, 1984) and K is the derivative of the measured signal profile as a function of the given species mixing ra-tio.Variances were set to the negative of their actual values whenever convergence was not achieved or the retrieval needed to be restarted (at tops of profiles).Valid profile segments have positive variances, and only the retrieved values from those segments were retained in the final Level 2 output file.Variance limits effectively set the extreme upper altitude of the good data for each of the parameters.Negative variances often indicate the low altitude extent of good data, too.Variance checking occurred primarily outside of the pressure ranges of 1.9 hPa to 46 hPa for HNO 3 and of 0.88 hPa to 46 hPa for NO 2 .Independent estimates of the random error indicate that there is adequate tangent layer signal within those ranges.An additional algorithm was applied to identify and screen for contaminating radiances from cloud tops.The profiles of HNO 3 , H 2 O, and NO 2 were also screened for polar stratospheric cloud features prior to their archival, based on the occurrence of those features in the retrieved ozone profiles (Remsberg et al., 2007).
Species profile segments were also screened within given pressure ranges, when their retrieved mixing ratios exceeded certain threshold values.The pressure range for HNO 3 was 2.2 to 46 hPa and its threshold was 30 ppbv; only a very few points met that criterion, and they were judged to be spurious.H 2 O is a major interference for the retrieval of NO 2 , particularly in the polar lower stratosphere during winter; its screening threshold was set at 12 ppmv over the pressure range of 1.47 to 46 hPa.Unrealistically, large values of H 2 O can occur due to not resolving the temperature structure properly.NO 2 thresholds were set for two separate pressure regions.Those thresholds were 25 ppbv within 1 to 31.6 hPa or 10 ppbv within 36 to 68 hPa, neither of which would be explained by an excess in the retrieved H 2 O. Almost all the profiles where NO 2 exceeded 25 ppbv in the upper stratosphere occurred in early December 1978 or in January 1979, when their temperature structures were associated with stratospheric warming events.The retrieved LIMS H 2 O and NO 2 values are not very accurate in that case.Quantitative studies of the descent of NO 2 from the mesosphere to the stratosphere can be affected somewhat by this 25 ppbv limit, although time series of the retrieved NO 2 for the upper stratosphere do not indicate any bias (see also Sect.4.4).Further, it is noted that no screening threshold was applied to the retrieved NO 2 of the polar mesosphere.Information about specific profile segments that were screened can be found on the LIMS Website (http://lims.gats-inc.com)for each day of the LIMS V6 dataset.

Single profile error estimates
The V6 HNO 3 profile values are not much different from those of V5, except in the upper stratosphere where the interference from CO 2 has been accounted for better in the V6 algorithm.In the tropical lower stratosphere there are important differences due to the corrections for emissions from the CFCs and due to the first-order correction for the aerosol Atmos.Chem.Phys., 10, 4741-4756, 2010 www.atmos-chem-phys.net/10/4741/2010/emission.Random radiometric errors for V6 HNO 3 are reduced from those of V5 by a factor of 2.2 because of the larger number of samples used with the interleave procedure for the retrieval of a V6 profile (Remsberg et al., 2004).Offset errors due to pointing jitter are included in those random uncertainties.The calculated precisions vary from 0.15 ppbv at 80 hPa to 0.05 ppbv at 3 hPa, based on an average profile at 30 • S. Those calculated values are provided in percent in the top row of Table 1, and they compare well with empirical estimates of the precision from the Level 2 data themselves (the standard deviation (SD) profile in Fig. 4).The SD values were obtained as the minimum of the variances for each pressure level from among the sets of scans along each of the orbits that crossed 25 • S to 35 • S latitude on 1 February 1979.
Of course, a part of the empirical SD values in Fig. 4 may be a result of real variations in the atmospheric HNO 3 .Sources of systematic uncertainty were reported in Gille et al. (1984), based on simulations of their effects for V5 from a model HNO 3 profile for 32 • N.Many of those error estimates are retained for V6, as shown in the middle rows of Table 1.However, the effects of temperature bias are based on the revised estimates of T(p) error in Remsberg et al. (2004).Uncertainty for the spectral band model is 5%, according to Goldman et al. (1981).Uncertainties in the aerosol and CFC corrections are greatest for the tropical lower stratosphere but do not dominate the total error during 1978/1979.Biases at the tops of the V6 HNO 3 profiles are small because its a priori mixing ratio values are small near the stratopause, and they are applied above the pressure level of the first retrieved point, too.Uncertainties for the effects of the horizontal temperature gradients are not included, since sensitivity to temperature biases are relatively small for an optically thin species like HNO 3 .
The largest elements of potential bias error are the ±20% uncertainties in the integrated areas of the field-of-view (FOV) side lobes and from a possible 0.05% bias in the total measured signal due to regions of the channel filter that are outside the main spectral band, as discussed in Gille et al. (1984) and shown in the bottom rows of Table 1.The FOV side lobes from the HNO 3 channel are not all the same sign.To judge their effects, the measured HNO 3 radiance profiles were analyzed at the vertical distances of the side lobes from the main HNO 3 lobe or at effective separations of 17 and 34 km at the horizon.For instance, are there any positive or negative radiance correlations at those separations when the main lobe is viewing the low radiances of the mid to upper stratosphere and when the side lobes are viewing the much larger radiances at altitudes 17 or 34 km below?Our investigations indicated no significant correlations for HNO 3 below 40 km.Thus, it is concluded that the magnitude of that error in Table 1 must be an upper limit at all levels, except perhaps at 3 hPa.Uncertainties for the out-of-band spectral response can impart a positive bias in HNO 3 of order 20% at 30 hPa, as pointed out in Gille et al. (1984).Yet, qualitative comparisons with other HNO 3 datasets indicate agreement that is better than 20%, at least for those datasets that were obtained at times of relatively low stratospheric aerosol loading.Therefore, neither of those sources of error is included in the combined root-sum-squares (RSS) calculations of accuracy in Table 1.In summary, the overall accuracy for V6 HNO 3 is believed to be very good from 30 hPa to 10 hPa or of order 10% at middle latitudes.
Table 2 contains the calculations of precision and accuracy for the profiles of NO 2 .First, one is reminded that the nighttime values of V6 NO 2 have peak mixing ratios near 4 hPa that are about 15% to 20% lower than those of V5.Most of that change is due to the effects of the spectral spin splitting of the strong lines that were not considered for earlier versions of the AFGL or HITRAN line lists.On the other hand, the peak daytime V6 NO 2 values at 8 to 10 hPa are reduced from V5 by only about 5% because the effect of saturation for the strongest lines is not nearly so pronounced for the lower mixing ratios of daytime.In the upper stratosphere near 3 hPa, the daytime V6 NO 2 is larger than that of V5 due to the improved determinations of the interfering V6 H 2 O. Below the 30-hPa level of the lower stratosphere both the day and night V6 NO 2 values are somewhat larger than V5.At this point it is noted that the V5 algorithm contained a merger of its retrieved NO 2 with a balloon-based, average NO 2 profile from 30 • N, constraining its results below the 30-hPa level at least when the S/N for the tangent-layer radiance was low.The V6 NO 2 algorithm is not constrained by any such climatological profile.The calculated precision values for the V6 NO 2 in Table 2 were adopted from those of V5 in Russell et al. (1984a), after accounting for the improvements due to the use of all the samples within the measured radiance profiles.Random uncertainties from single profiles of the retrieved H 2 O were adopted from Remsberg et al. (2009) and are included because H 2 O is a major source of interfering emission, especially for daytime NO 2 in the upper and the lowermost stratosphere.The effects of those random H 2 O errors are scaled further, according to the fraction of the signal in the NO 2 channel that is due to H 2 O. Estimates of percentage NO 2 precision profiles from the data are shown in Fig. 5 and are based on average daytime and average nighttime SD profiles at 30 • S for 1 February 1979.Those values range from about 5% in the middle stratosphere, to 8% for nighttime or 30% for daytime at 1 hPa, and then to 15-20% at 40 hPa.The estimated SD values in Fig. 5 agree with the RSS precisions in Table 2 for the middle and upper stratosphere but are larger at 30 and 50 hPa, presumably because of the effects of the natural variability of the atmospheric NO 2 , H 2 O, and temperature.
A linear scaling was applied to the entries of the V5 NO 2 error table in Russell et al. (1984a) for its V6 bias errors, based on a model-generated, daytime profile at 32 • N. Its primary sources of systematic uncertainty are from temperature bias and from the interfering effects of H 2 O, based on the V6 uncertainties in Remsberg et al. (2004) and Remsberg et al. (2009), respectively.There are uncertainties in the lower stratosphere due to the O 2 interference, but they should be considered as an upper limit because corrections for the O 2 emission were applied to the retrievals of both the LIMS H 2 O and NO 2 and therefore carry the same sign.There is also a possible bias due to a 20% uncertainty for the area of the measured FOV side lobes; its effect on the retrieved NO 2 is believed to be small but remains unverified.
Table 2 indicates RSS V6 NO 2 uncertainties of 17% at 3 hPa and about 18% from 5 to 10 hPa.RSS values increase sharply in the lower stratosphere and are dominated by the estimates of error for the interfering O 2 continuum and the H 2 O.At stratopause levels (1 hPa) the ascending (daytime) NO 2 may have a bias of order 33% across most latitudes.However, the larger, descending (nighttime) NO 2 values at that level are not affected much by the interfering H 2 O, so they are more accurate (∼17%).Also, the increasing values of NO 2 in the lower mesosphere at the high northern latitudes of Fig. 3 are judged to be reasonably accurate (see also Sect. 4.4).One source of bias error that has been neglected is the likelihood that the retrieved V6 polar night NO 2 values are too low in the mesosphere due to not accounting for the effects of non-LTE emissions of the ground-state of NO 2 itself (e.g., see Funke et al., 2005a).

Scientific implications of the V6 data
The previously, archived LIMS V5 HNO 3 and NO 2 , along with the temperature, O 3 , and H 2 O data have been used in scientific studies and reported in the literature (e.g., Callis et al., 1983;Jackman et al., 1985;Natarajan et al., 1986;Solomon et al., 1986;and Considine et al., 1992).In the subsections that follow several of the issues that they raised with respect to the V5 HNO 3 and NO 2 profiles and distributions are re-visited and re-evaluated using the V6 data.

HNO 3 /NO 2 ratio profiles
Simultaneous measurements of HNO 3 and NO 2 by LIMS provide an opportunity to check the partitioning of NO y species in comparison with that predicted with theoretical models.Notably, Jackman et al. (1985) reported that between 5 and 1 hPa the ratio of the LIMS V5 daytime HNO 3 to NO 2 was inconsistent with model values and that HNO 3 derived from photochemical relations should be used instead of those from LIMS.They based their conclusion on a comparison of daytime hydroxyl (OH) values derived from the V5 data using two different procedures.In the first approach they invoked an instantaneous photochemical equilibrium assumption for HNO 3 and, thereby, expressed OH concentrations as a function of the ratios of HNO 3 /NO 2 .Their second approach used an equilibrium assumption for odd hydrogen (HO x ) along with the LIMS O 3 and H 2 O data to derive the daytime OH values.In the upper stratosphere those two approaches yielded different results.For instance, the V5 HNO 3 /NO 2 ratios overestimated OH in the upper stratosphere by a large margin, at least when compared to other available observations and model results.It is noted though that the lifetime of HNO 3 against photolysis in the upper stratosphere is an hour or more, such that their instantaneous assumption is not strictly valid at the local measurement times of LIMS across all latitudes.Even so, their second approach yielded much better agreement with model OH values.Jackman et al. (1985) concluded that the problem with the first approach was due to errors in the LIMS HNO 3 in the 5 to 1 hPa region.Natarajan et al. (1986) reached a similar conclusion regarding the HNO 3 /NO 2 ratio, but on the basis of diurnal photochemical model calculations that were constrained by the LIMS V5 nighttime data.The V5 HNO 3 /NO 2 ratios also showed a positive bias in the upper stratosphere in comparison with their modeled daytime estimates.
Figure 6 is a comparison of the daytime ratio of V6 HNO 3 /NO 2 for 15 February 1979 versus that from the updated, diurnal photochemical model of Natarajan et al. (2002), which incorporates the recommended chemical kinetics data of Sander et al. (2006).Distributions of longlived species, such as nitrous oxide (N 2 O) that are used to initialize the diurnal calculations are taken from a simulation with the NASA Langley two-dimensional model of Callis et al. (2001) using surface source gas mixing ratios correspond- ing to the 1978-1979 time period.The bottom panel of Fig. 6 shows LIMS HNO 3 /NO 2 ratios that are less than 0.2 in the upper stratosphere to near 3 hPa, except at the winter high latitudes where the photochemical partitioning of the atmospheric NO y to HNO 3 and NO 2 is incomplete.In addition, at the top boundary of the data domain (near 2 hPa) the V6 ratios are lower than the ones from the V5 data (not shown).The better determinations of the V6 temperature profile and its effects for the laser band of CO 2 have led to a large decrease in the retrieved, upper stratospheric HNO 3 and a corresponding decrease in the HNO 3 /NO 2 ratio.The model results are shown in the top panel of Fig. 6, and they agree well with the ratios from the V6 data.
The HNO 3 /NO 2 ratios in Fig. 6 from both the model and LIMS are declining from 10 hPa to about 3 hPa, but then the LIMS ratios become larger than the model values at the topmost boundary.The reason for the remaining disagreement near 2 hPa is most likely a high bias in the daytime HNO 3 due to the influence of non-LTE emission in the spectrallybroad, LIMS channel that has not been corrected.Edwards et al. (1996) also observed differences between the day and night radiances from the 10.83-µm blocker channel of the Cryogenic Limb Array Etalon Spectrometer (CLAES) instrument, and they ascribed them to non-LTE effects from ozone and CO 2 near the stratopause during daytime.Estimates of the effects of those emissions on the retrieved LIMS HNO 3 have not been evaluated for this paper.Nevertheless, because of the improved results from LIMS V6 there is no longer any need to substitute model HNO 3 for the retrieved LIMS daytime values throughout most of the upper stratosphere.

Lower stratospheric HNO 3 and NO 2
The LIMS dataset provides global scale information on the latitudinal distribution of stratospheric HNO 3 and NO 2 .Earlier analyses of the V5 data in conjunction with model studies highlighted a deficiency with respect to the HNO 3 mixing ratios of the winter high latitudes as calculated by models using only gas-phase chemistry.For instance, Austin et al. (1986) compared LIMS V5 HNO 3 with other measurements and model results.They reported that an additional source of HNO 3 was needed at high latitudes in winter and suggested that the hydrolysis of N 2 O 5 on aerosol surfaces could be important.Considine et al. (1992) also described results from their two-dimensional model compared with the V5 data and noted the importance of heterogeneous chemistry for their calculations of HNO 3 .They also pointed out that while the inclusion of heterogeneous chemistry improved the model/data agreement for HNO 3 , its addition led to larger discrepancies between the calculated and observed NO 2 .
At this point it is noted that the latitudinal variations from the V6 algorithm are similar to those from V5 for middle and lower stratospheric HNO 3 and for NO 2 (at least above the 30-hPa level).Their profile variations in the middle to lower stratosphere are also similar to the results (not shown) from more recent satellite experiments (e.g., Santee et al., 2007;Wetzel et al., 2007;Kyröla et al., 2010).These results are not surprising because the updated spectral parameters for the primary gases of V6 have little effect on their retrieved mixing ratios at those altitudes.However, changes from V5 to V6 were found in the lower stratosphere at tropical and polar winter latitudes, due to improvements in their associated profiles of T (p) and interfering species.Figure 7 shows a comparison of zonally averaged HNO 3 at 30 hPa as a function of latitude for December 1978.The dashed line represents LIMS V6 data, and the solid line corresponds to the HNO 3 from the two-dimensional model of Callis et al. (2001).The model includes heterogeneous processing of N 2 O 5 and ClONO 2 on stratospheric sulfate aerosol surfaces, leading to the production of HNO 3 .Reaction probabilities were adopted for the model from Sander et al. (2006).The surface area densities for the background aerosol distribution were taken from the climatology based on SAGE II measurements (Thomason et al., 1997).The model does not include reactions on polar stratospheric cloud particles.A heterogeneous reaction path explains the higher mixing ratios at the winter high latitudes and the good agreement with the LIMS data.
The vertical line in Fig. 7 represents the estimated LIMS uncertainty of 8% based on a mid-latitude summer profile (Table 1).It is clear that the LIMS HNO 3 is higher than that of the model across the middle latitudes and has a different slope, particularly for the winter hemisphere.Figure 8 shows the V6 HNO 3 distribution on the 31.6 hPa surface of the northern hemisphere for early January 1979, and it exhibits a zonal wave-3 variation at middle and high latitudes.Remsberg and Bhatt (1996) indicated that the effects of such zonal wave activity could lead to the pattern of the middle latitude surf zone plus a subtropical barrier region by early January, as shown in Fig. 8.It is difficult to simulate such wave forcing and subsequent changes of the HNO 3 distribution with the parameterized transport of a two-dimensional model.Thus, while the model results indicate the proper seasonal variations at the high latitudes, they do not agree well in the middle latitudes with the early January data at this pressure level.In the tropics the observed HNO 3 in Fig. 8 displays almost no zonal variations; Fig. 7 shows that the LIMS data and model results agree well in that region.
Figure 9 is a polar plot of the northern hemisphere distribution of LIMS V6 nighttime NO 2 at 31.6 hPa for 5 January 1979.Note that the distribution shows a decrease along the 270 • E meridian from about 2.4 ppbv at 30 • N to less than 1.0 ppbv by about 50 • N.This change is a clear indication of the so-called "cliff phenomenon" in column NO 2 over North America (Noxon, 1979).Such a pronounced variation with latitude was not as apparent and coherent within the V5 dataset.One can now readily follow the daily evolution of this feature and compare it with the changes in HNO 3 using the V6 data.

Day-to-night NO 2 ratios
The availability of daytime and nighttime measurements of a diurnally varying species like NO 2 allows one to examine the photochemistry of this species using theoretical models in conjunction with data.For example, Solomon et al. (1986) used LIMS nighttime NO 2 data at high northern latitudes in May to show that the decay of NO 2 to form N 2 O 5 was consistent with theory.On the other hand, Kerridge and Remsberg (1989) examined profiles of the day-to-night ratios of NO 2 and found a region of discrepancy with theory in the upper stratosphere.In particular, the LIMS V5 data showed much larger ratios than expected from photochemical models above about 42 km (or the 2-hPa level).
An updated comparison of the day-to-night NO 2 ratio profiles from the V6 dataset is shown in Fig. 10 along with those from a contemporary, diurnal photochemical model.The model is an updated version of the zero-dimensional model described in Natarajan et al. (2002) and Natarajan and Callis (1991).The numerical procedure uses a stiff equation solver to integrate the system of species continuity equations.Chemical kinetics and photochemical data are adopted from Sander et al. (2006).For each latitude and altitude location considered, the model is constrained by the nighttime observations of LIMS.Initialization for species that are not measured by LIMS, for example chlorine species, is based on the results of the two-dimensional model of Callis et al. (2001).During initialization, the calculations are repeated for a few diurnal cycles, and in each cycle at the local time corresponding to the LIMS nighttime measurement the mixing ratios of the observed species are reset to the measured values.Once the unmeasured reactive species reach a steady diurnal variation, the time integration is continued for another diurnal cycle and the required parameters are evaluated, i.e., the day/night ratios for NO 2 .The monthly-averaged LIMS results for May 1979 are in the bottom panel of Fig. 10, and they show steadily declining ratios above the 5-hPa level, reaching a minimum of less than 0.05 at 1.0 hPa but then increasing to about 0.15 at 0.5 hPa.The model day-to-night NO 2 ratios are shown in the top panel and correspond to the local times of the LIMS day and night measurements.The model ratios are in good agreement with the data in the 5 to 1 hPa region but continue to show decreasing values in the lower mesosphere.Interfering radiance from H 2 O is the primary source of uncertainty for the LIMS forward model of NO 2 in the mesosphere, especially for daytime.H 2 O was accounted for by considering the retrieved H 2 O up to 1.5 hPa plus a fixed H 2 O value above that level, even though non-LTE emissions from H 2 O are known to be important (Remsberg et al., 2009).As with V5, the LIMS V6 algorithm does not correct for such non-LTE effects in H 2 O, leading to a positive bias in the retrieved daytime NO 2 .Funke et al. (2005a) found no evidence for non-LTE emissions from the higher-vibrational states of NO 2 itself in the MIPAS data, at least for altitudes below 50 km.Whether there may be such effects from NO 2 in the lower mesosphere is unclear.It is tentatively concluded that any non-LTE effects from NO 2 in the mesosphere are secondary to the influence of the non-LTE emissions from H 2 O, and it is estimated that the upper boundary region of accurate V6 daytime NO 2 values is 1.5 to 1.0 hPa.

Mesospheric NO 2 during polar night
The production of NO x in the mesosphere and lower thermosphere by energetic particle precipitation has been described in connection with its downward transport into the high latitude winter stratosphere (e.g., Callis et al., 2001;Natarajan et al., 2004;Funke et al., 2005b;Randall et al., 2006).Russell et al. (1984b) reported the first satellite-based observation of wintertime increases in upper stratospheric NO 2 .Upon averaging 5 days of LIMS radiances within 60 • longitude sectors, they were able to extend their retrievals of LIMS NO 2 to higher altitudes.By focusing on polar night conditions in January 1979, they obtained NO 2 mixing ratios exceeding 100 ppbv in one 60 • sector of the lower mesosphere.Odd nitrogen can be produced directly by solar proton events in the high latitude upper stratosphere.However, the study by Jackman et al. (1990) for the period 1963-1984 does not indicate the occurrence of any such event during January 1979.The enhanced NO 2 in the LIMS data must be due to high latitude descent within the mesosphere following the production of NO x by energetic particle precipitation, the so-called "indirect effect" (Randall et al., 2006).
The V6 algorithm does not employ a radiance-averaging procedure, so the single profiles of V6 NO 2 extend only up to about 0.15 hPa at the winter high latitudes.Although elevated, zonally-averaged mixing ratios of NO 2 are still found in the mesosphere (Fig. 3), they are lower than those reported in Russell et al. (1984).Even so, the V6 values are considerably higher than what is expected if there are only stratospheric sources of NO x .The occurrence of the smaller NO 2 mixing ratios from V6 is primarily because the V6 radiance profiles are registered better and the V6 temperatures are warmer by as much as 6 K at 0.1 hPa (Remsberg et al., 2004).Although the single profile error estimates for NO 2 in Table 2 apply strictly to its climatological profile and do not extend to altitudes above 1 hPa, the effects of a 2 K temperature bias and of an H 2 O bias at 1 hPa are 10% and 28%, respectively.Total error in Table 2 for NO 2 is estimated to be 33% at 1 hPa.However, the EOS Aura MLS experiment confirms that H 2 O has relatively small values in the wintertime, polar vortex regions of the mesosphere (http://mls.jpl.nasa.gov/).Thus, the uncertainties for the LIMS V6 NO 2 from Table 2 due to the interfering effects of H 2 O should be reduced considerably for the polar night.
Figure 11 is a time series of the zonally-averaged V6 descending node (or nighttime) NO 2 mixing ratio as a function of pressure for latitudes greater than 74 • N. Data from 1 November 1978 to 31 March 1979 are shown.For the several days when the LIMS instrument did not take data, their NO 2 values are averages from the adjacent days at all pressure levels.NO 2 mixing ratios exceeding 10 ppbv occurred in the 0.2 to 0.5 hPa region from late November 1978 to early February 1979 along with a clear indication of atmospheric descent with time of that elevated NO 2 .Comparisons of the fields of geopotential height and nighttime NO 2 during January at 0.46 hPa indicate that the highest NO 2 values correspond to observations within the polar vortex region (not shown).The reaction of NO x with O 3 in the upper stratosphere leads to a gradual conversion of NO 2 to nitrogen trioxide (NO 3 ) and then to N 2 O 5 in the polar night region.During November and December and between 2 and 10 hPa, the NO 2 variations in Fig. 11 are mostly a result of transport of air from lower latitudes followed by the photochemical conversion of its NO 2 .In particular, early December 1978 was the time of a wave-1, stratospheric (Canadian) warming event.The LIMS observations of Fig. 11 provide support for the hypothesis that under such dynamic conditions there can be increases of high-latitude, upper stratospheric NO x as a result of accelerated descent within the polar vortex.More dramatic perturbations, such as solar proton events, are not a necessary condition for moderate enhancements of upper stratospheric NO x .

Summary findings
The radiances of the Nimbus 7 LIMS experiment were reconditioned and new retrievals of them conducted with an updated (V6) algorithm to generate HNO 3 and NO 2 profiles that are more compatible with those of follow-on satellite experiments.The V6 HNO 3 profiles are nearly unchanged in the middle and lower stratosphere from those of the earlier V5 algorithm.However, in the upper stratosphere V6 HNO 3 is smaller due to improvements in the accounting for interfering emissions from CO 2 .That change yields daytime HNO 3 /NO 2 ratios that agree better with results from theoretical model calculations than before.The V6 NO 2 profiles have smaller values than those of V5, most noticeably for nighttime in the upper stratosphere due to the accounting of spin-rotation effects within the updated, spectroscopic line parameters of NO 2 .More accurate retrievals of the V6 temperature and H 2 O from the lower mesosphere to near the stratopause are responsible for the more realistic V6 NO 2 profiles of that region of the atmosphere, most notably in the polar night regions.Day-to-night ratios of V6 NO 2 also agree well with photochemical model calculations in the middle and upper stratosphere.
Individual V6 profiles of HNO 3 and NO 2 have points every 0.375 km and an effective vertical resolution of 3.7 km.The V6 retrievals were applied to adjacent pairs of profiles along orbits, yielding a spacing of 1.6 degrees of latitude between retrieved profiles, on average.Single HNO 3 profiles have calculated precisions of 3-4% from 10 to 50 hPa but of order 10% at 3 hPa and 21% at 80 hPa.Single NO 2 profiles have precisions of about 3% from 3 to 10 hPa, 7% at 30 to 50 hPa, and 14% at 1 hPa.The improved precisions and more frequent retrievals of profiles along the orbit tracks provide better continuity and detail for map analyses of these two species on pressure surfaces.
Calculated accuracies for single profiles of V6 HNO 3 are better than 10% from 10 to 50 hPa, and about 12% at 3 hPa and 16% at 80 hPa.Accuracy for the V6 NO 2 profiles is of order 18% from 3 to 10 hPa, but degrades to about 30% at 30 hPa and 1 hPa.Even so, good quality retrievals of V6 NO 2 extend downward to at least the 50-hPa level at polar latitudes in northern winter, which was not the case for NO 2 from the earlier V5 dataset.A time series plot of V6 NO 2 at northern high latitudes illustrates NO x enhancement, most likely due to wintertime descent from the lower mesosphere to the upper stratosphere.This finding is significant, especially when one considers that there were no large solar disturbances or enhanced geomagnetic storm activity in late 1978 or early 1979.
The V6 HNO 3 distributions still support the finding that one must include heterogeneous chemical mechanisms in stratospheric models for the partitioning of NO y , even during the 1978-1979 period of low volcanic aerosol loading.However, it is also apparent from the V6 data that there are deficiencies for the seasonal, meridional transport of the N 2 O source molecule and/or of its end product NO y , as simulated with a two-dimensional model of the lower stratosphere.
The V6 Level 2 (profile) data can be obtained by ftp download from the Goddard Earth Sciences and Data Information Services Center or GES DISC (http://daac.gsfc.nasa.gov/).A LIMS V6 Level 3 or zonal Fourier coefficient product is also available at GES DISC (Remsberg and Lingenfelser, 2010).

Fig. 1 .
Fig. 1.V6 HNO 3 distributions in ppbv for (top) 15 November 1978 and (bottom) 16 May 1979, based on a combination of the profile data from their ascending and descending orbital segments.Orangeto-red colors represent high values and magenta colors are low values; blackened regions indicate where there are no data.

Fig. 2 .
Fig. 2. LIMS V6 distributions for 15 November 1978 of (top) descending (or nighttime) NO 2 and of (bottom) ascending (or daytime) NO 2 both in ppbv.Orange colors are high values, while magenta-to-purple colors are low values; blackened regions indicate where there are no data.There is a crossover to polar night conditions at about 75 • N for the ascending NO 2 .

Fig. 3 .
Fig. 3.As in the top panel of Fig. 2, but for descending (or nighttime) NO 2 in ppbv for 15 January 1979.Orange-to-red colors are high values and magenta-to-purple colors are low values; blackened regions are where there are no data.

Fig. 4 .
Fig. 4. Estimates of precision based on the minimum standard deviations (SD) of the retrieved LIMS V6 HNO 3 profiles from the sets of ascending (solid) or descending (dashed) orbital crossings of the latitude band of 25 • S and 35 • S on 1 February 1979.

Fig. 7 .
Fig. 7. LIMS V6 (dashed) and model (solid) comparisons for HNO 3 in ppbv from the LIMS descending (Des or nighttime) orbital segments of December 1978.The vertical line on the LIMS data at 32 • S denotes its estimated uncertainty.

Fig. 8 .
Fig. 8. Ascending plus descending V6 HNO 3 in ppbv for 5 January 1979.Bluish regions are low values and orange-to-pink regions are high values; contour interval is 0.8 ppbv.

Fig. 9 .
Fig. 9. V6 descending NO 2 in ppbv for 5 January 1979.Bluish regions are low values and greenish regions are high values; contour interval is 0.6 ppbv.

Fig. 10 .
Fig. 10.Ascending/descending (or day/night) NO 2 ratio (top) from the diurnal model and (bottom) from the LIMS V6 data.Orangeto-red colors are high ratios, while magenta colors are low ratios.

Fig. 11 .
Fig. 11.Time series of LIMS V6 descending (or nighttime) NO 2 in ppbv for latitudes >74 • N. The time span is from 1 November 1978 (1101) to 31 March 1979 (0331).For those few days when no data were taken, the NO 2 values are averages from the adjacent days.Yellow colors are high values, while blue-to-magenta colors are low values.