Improvements in the profiles and distributions of nitric acid and nitrogen dioxide with the LIMS version 6 dataset

Abstract

The quality of the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) nitric acid (HNO3) and nitrogen dioxide (NO2) profiles and distributions of 1978/1979 is described after their processing with an updated, Version 6 (V6) algorithm and subsequent archival in 2002. Estimates of the precision and accuracy of both of those species are developed and provided herein. The character of the V6 HNO3 profiles is relatively unchanged from that of the earlier LIMS Version 5 (V5) profiles, except in the upper stratosphere where the interfering effects of CO2 are accounted for better with V6. The accuracy of the retrieved V6 NO2 is also significantly better in the middle and upper stratosphere, due to improvements in its spectral line parameters and in the reduced biases for the accompanying V6 temperature and water vapor profiles. As a result of these important updates, there is better agreement with theoretical calculations for profiles of the HNO3/NO2 ratio, day-to-night NO2 ratio, and with estimates of the production of NO2 in the mesosphere and its descent to the upper stratosphere during polar night. The improved precisions and more frequent retrievals of the profiles along the LIMS orbit tracks provide for better continuity and detail in map analyses of these two species on pressure surfaces. It is judged that the chemical effects of the oxides of nitrogen on ozone can be examined quantitatively throughout the stratosphere with the LIMS V6 data, and that the findings will be more compatible with those obtained from measurements of the same species from subsequent satellite sensors.

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Atmos. Chem. Phys. Discuss., 10, 2769–2808, 2010
www.atmos-chem-phys-discuss.net/10/2769/2010/
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Discussions
This discussion paper is/has been under review for the journal Atmospheric Chemistry
and Physics (ACP). Please refer to the corresponding final paper in ACP if available.
Improvements in the profiles and
distributions of nitric acid and nitrogen
dioxide with the LIMS version 6 dataset
E. Remsberg1, M. Natarajan1, T. Marshall2, L. L. Gordley2, R. E. Thompson2, and
G. Lingenfelser3
1NASA Langley Research Center, 21 Langley Blvd., Mail Stop 401B,
Hampton, VA 23681, USA
2GATS Incorporated, 11864 Canon Blvd., Suite 101, Newport News, VA 23606, USA
3SSAI, 1 Enterprise Parkway, Hampton, VA 23661, USA
Received: 14 December 2009 – Accepted: 22 January 2010 – Published: 3 February 2010
Correspondence to: E. Remsberg (ellis.e.remsberg@nasa.gov)
Published by Copernicus Publications on behalf of the European Geosciences Union.
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Abstract
The quality of the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) nitric
acid (HNO3) and nitrogen dioxide (NO2) profiles and distributions of 1978/1979 is de-
scribed after their processing with an updated, Version 6 (V6) algorithm and subse-
quent archival in 2002. Estimates of the precision and accuracy of both of those
species are developed and provided herein. The character of the V6 HNO3profiles
is relatively unchanged from that of the earlier LIMS Version 5 (V5) profiles, except in
the upper stratosphere where the interfering effects of CO2are accounted for better
with V6. The accuracy of the retrieved V6 NO2is also significantly better in the middle
and upper stratosphere, due to improvements in its spectral line parameters and in the
reduced biases for the accompanying V6 temperature and water vapor profiles. As a
result of these important updates, there is better agreement with theoretical calcula-
tions for profiles of the HNO3/NO2ratio, day-to-night NO2ratio, and with estimates of
the production of NO2in the mesosphere and its descent to the upper stratosphere
during polar night. The improved precisions and more frequent retrievals of the pro-
files along the LIMS orbit tracks provide for better continuity and detail in map analyses
of these two species on pressure surfaces. It is judged that the chemical effects of
the oxides of nitrogen on ozone can be examined quantitatively throughout the strato-
sphere with the LIMS V6 data, and that the findings will be more compatible with those
obtained from measurements of the same species from subsequent satellite sensors.
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1 Background
The Limb Infrared Monitor of the Stratosphere (LIMS) experiment was launched on
24 October 1978, on the near polar-orbiting Nimbus 7 satellite. LIMS operated from
25 October until 28 May 1979, measuring vertical radiance profiles across the atmo-
spheric limb of the Earth (Gille and Russell, 1984). Its daily orbital data extend from
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64◦S to 84◦N and are available at two local times per latitude (at about 1300h and
2300h 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 (O3), wa-
ter vapor (H2O), nitric acid (HNO3), and nitrogen dioxide (NO2). Thus, LIMS was the
first satellite experiment to provide a simultaneous, near-global view of the key chem-
ical compounds in the ozone/nitrogen oxide photochemical chain. The temperature,
geopotential height, and constituent profiles have been used for many scientific inves-
tigations, including effects of radiation on the net transport, responses of the middle
atmosphere to perturbations, and correlations between the temperature and species
data.
The LIMS Version 5 (or V5) profiles were archived in 1982; its measurements, al-
gorithms, 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 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 undertaken using the high-
resolution, transmission molecular absorption line parameters of HITRAN 91/92 and/or
96. As a result, these V6 data are 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 Observ-
ing System (EOS) Aura satellite and on the Canadian, European, and/or Japanese
satellites SCISAT, ODIN, ENVISAT, and ADEOS.
A single LIMS scan profile began with the center of its FOV array viewing the horizon
at 153km altitude, moving downward to a point 38km below the solid Earth limb and
then returning upward. The angular resolutions (in milliradians) for the detectors of the
FOV array are 1mrad for the NO2and H2O channels and 0.5mrad for the other four
channels, including HNO3. The optical characteristics of all the channels are given in
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Russell and Gille (1978, their Table 1). The V6 dataset was also improved by a better
knowledge of the orbital attitude for the LIMS measurements, by using all the radiance
profile samples, spaced 0.375km in altitude, and by applying a multiple interleave re-
trieval process. 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 two additional papers (Remsberg et al., 2007 and 2009).
This paper is focused on the improvements and scientific implications for the
V6 HNO3and NO2profiles and distributions. 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 0.88km. Note that the UARS
Level 3A profiles have 6 levels of data per decade of pressure and represent a match-
ing 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 O3, HNO3, and NO2to photochemical model output. The effective vertical resolution
for both V6 HNO3and NO2is of order 3.7km, 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 HNO3extend from near the tropopause to just above
the 2-hPa level. The nighttime distributions of NO2extend from about 50hPa to the
lower mesosphere, especially for the polar night. Results for daytime NO2are useful
from about 50hPa to 1hPa.
Section 2 shows several daily, zonally-averaged distributions of the V6 HNO3and
NO2for 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.,
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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 HNO3and NO2profiles. Independent estimates of precision and accuracy for those
V6 data are included, based on calculations of the effects of their known error sources.
The estimates of precision are shown to compare well with the variations in the re-
trieved parameters 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 HNO3and NO2.
Section 5 is a summary of our findings about the changes with V6.
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2 Zonally-averaged distributions of HNO3and NO2
Figure 1a and b are the zonally-averaged distributions of V6 HNO3from the com-
bination of its Level 2 profiles along descending and ascending orbital segments for
15 November 1978, and 16 May 1979, respectively. Maximum mixing ratio values
occur at the high latitudes near 30hPa, and the variation of HNO3with latitude is sim-
ilar to that from the earlier V5 dataset at that pressure level (Gille et al., 1984, 1993).
However, the V6 HNO3profiles no longer exhibit nearly constant values in the upper
stratosphere like those found from V5 by Jackman et al. (1985), due primarily to a better
accounting for the effects of the interfering radiance from a “hot band” of carbon dioxide
(CO2) within the V6 algorithm. The V6 HNO3agrees reasonably well with photochemi-
cal model estimates; however, its day values are notably less than its night values near
the 4-hPa level, most likely due to not having accounted for radiance contributions at
higher altitudes from vibrationally-excited states of CO2and perhaps of O3in the 10 to
12-µm region during daylight (Edwards et al., 1996). V6 HNO3values are somewhat
smaller than those of V5 in the lower tropical stratosphere, due to first-order corrections
for the effects of interfering emissions from aerosols and from the chlorofluorocarbon
(CFC) molecules CFCl3and CF2Cl2. The 1ppbv contour extends to just above the
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50-hPa level at equatorial latitudes; that value is at the upper limit of reactive nitrogen
(NOy) minus odd nitrogen (NOx) from in situ measurements (Jensen and Drdla, 2002,
and references therein).
One can see from Fig. 1a and b that there is very good seasonal symmetry be-
tween the HNO3distributions for November versus May of the Northern and Southern
Hemispheres, which has already been explained by invoking heterogeneous chemical
mechanisms (Austin et al., 1986). Note that there is an upward extension of moderately
high values of HNO3to the upper stratosphere in November at high northern latitudes
(Fig. 1a). This feature becomes prominent in winter due to a slow accumulation of
HNO3after chemical conversion from NO2and dinitrogen pentoxide (N2O5) under po-
lar night conditions. The distribution of 16 May (Fig. 1b) indicates that there was also
an accumulation of HNO3in the 100 to 200-hPa layer in the Northern Hemisphere by
late spring, due to diabatic descent in the region of the polar vortex.
Figure 2a and b show the zonally-averaged distributions of NO2from the descending
(local nighttime at low and middle latitudes) and from the ascending (daytime at low
and middle latitudes) orbital segments, respectively, for 15 November 1978. Maximum
mixing ratio values for the nighttime LIMS NO2of Fig. 2a are of the order of 14ppbv
centered at about the 4-hPa level. The retrieved nighttime NO2profiles extend into
the lower mesosphere, where they encounter their signal-to-noise (S/N) limit and are
less accurate. The repartitioning of the NOx(nitric oxide (NO)+NO2) at twilight occurs
at about 55 to 60◦S latitude. Those terminator NO2profiles are also less accurate
because the rapidly changing NO2values along the tangent path were not accounted
for (see Solomon et al., 1986).
Maximum mixing ratios for the daytime NO2of Fig. 2b are of the order of 6 to 7ppbv
and occur between 8 and 10hPa. One can clearly see the crossover to polar night
conditions, poleward of about 72◦N. Daytime NO2is small in the upper stratosphere
because most of the NOxis in the form of NO. H2O emission becomes a significant
part of the NO2channel radiance in the upper stratosphere, especially during daytime.
The forward radiance model for the V6 NO2channel assumes that H2O has a constant
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value of 6.5ppmv above the upper limit of 1.3hPa for its V6 profile. Furthermore, there
is a significant amount of extra emission in the mesosphere during daylight from excited
states of the H2O molecule that are difficult to characterize for LIMS, primarily because
its ground-state H2O populations are not known exactly in that region (Lopez-Puertas
and Taylor, 2001; Kerridge and Remsberg, 1989). For this reason no corrections for
non-LTE effects from H2O were developed for the retrieval of the V6 NO2. That source
of positive radiance bias is the primary reason for the spurious, upward extension of
the daytime NO2distributions to the lower mesosphere (see also Sects. 3 and 4).
Figure 3 is the distribution of NO2from the descending (nighttime) orbital segments
of 15 January 1979, and it is similar to that of Fig. 2a in most respects. One exception
is the occurrence of relatively large values of NO2near 50◦S compared with those at
equatorial latitudes. This difference is partly a diurnal effect. The LIMS NO2measure-
ments were obtained at about 2130h (local time) at 50◦S versus 2300h at the equator
for the descending orbital segments, and there is a steady conversion of NO2to N2O5
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 NO2in the mesosphere at the high latitudes of the Northern
Hemisphere are another feature of Fig. 3. Such enhanced values of NO2were 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 NOxin the mesosphere fol-
lowed by a gradual descent of the air to the uppermost stratosphere during polar night.
Findings from more recent satellite sensors support that conclusion (e.g., Randall et
al., 2009). A time series of the V6 polar night NO2will be shown in Sect. 4, indicating
improved estimates for its values and better continuity for its descent during the winter
months of 1978/1979.
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3 LIMS V6 retrieval algorithms and error estimates
3.1 The V6 algorithm for HNO3and NO2
New emissivity data tables were prepared for the forward models of the primary gases
in both the HNO3and NO2channels. The HNO3channel contains secondary radiance
contributions from the “hot band” of CO2at 10.4 micrometers and from the primary
CFCs (CFCl3and CF2Cl2). To achieve better accuracy for the emissivity of CO2along
a tangent ray path, emissivity look-up tables were calculated at 55 pressure levels and
33 temperature levels and encompassing their expected atmospheric ranges. In addi-
tion, polynomial fits are no longer applied to account for the temperature dependence
within the emissivity tables; linear interpolation is used instead. These upgrades have
led to a more accurate representation of the effects of the “hot band” of CO2.
The V6 forward model for the primary gas, HNO3, makes use of emissivity tables
generated using its band model data on HITRAN 91 (Rothman et al., 1992a). That
model is based on the laboratory measurements of Giver et al. (1984), and a param-
eterization of them is appropriate for the broadband channel of LIMS. The interfering
effects of CO2and of its “hot band” at 10.4µm were obtained using the line parameters
on 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 summarize, the up-
dated emissivity tables for CO2account for much of the excess of retrieved HNO3that
characterized the V5 HNO3profiles from about 5 to 2hPa, as originally reported in Gille
et al. (1984, 1993) and Jackman et al. (1985) (see also Sect. 4).
Line parameters for the original V5 retrievals of NO2were obtained from the AFGL
trace gas compilation (Rothman et al., 1981). Spin-rotation effects of NO2became
known by the late 1980s, and the fine structure of the lines for the primary ν3cold
band were included in the HITRAN 92 compilation via a perturbation calculation (Perrin
et al., 1992). That change accounts for most of the improvements in the V6 NO2
(Remsberg et al., 1994). Further, minor changes in the retrieved NO2may be found
for those spin-rotation effects from the more explicit calculations of the strengths of the
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resolved lines that were obtained later and are available in HITRAN 96 (Toth, 1992).
The LIMS NO2channel also contains contributions from H2O, CH4, and the oxygen
(O2) continuum. Spectral parameters for the underlying O2were not available, so the
empirical model of Thibault et al. (1997) was used for consistent calculations of the
effects of the continuum-induced absorption from O2in both the H2O and the NO2
channels.
The LIMS V6 profiles were used to correct for the forward radiance contributions
of H2O up to about the 1.5-hPa level, and then they were extended upward using a
constant value of 6.5ppmv. In addition, the entire H2O profile for the forward model
was smoothed by a Gaussian function having a 1.5km half-width at half maximum.
As indicated earlier, no corrections were developed to account for probable non-LTE
radiances from mesospheric H2O during daytime. Even so, there is no indication of
a bias in the retrieved NO2due to that neglect, except at and above about 48km (or
the 1-hPa level). Emissions from excited states of daytime NO2were postulated for
the upper stratosphere by Kerridge and Remsberg (1989), but no correction for those
effects was developed for V6 because the populations of those states are not known
well. The effects of horizontal mixing ratio gradients in the tangent layer have also
been shown to be important for the retrieval of LIMS NO2in 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 considered for the V6 NO2profiles prior to their archival in 2002.
The original V5 profiles of NO2and HNO3are 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 for the radiances is achieved. Estimated
concentrations of the interfering gases were obtained as follows. Updated, seasonal
zonal-mean distributions of CH4were generated from the UARS HALOE dataset for
use with the V6 retrievals of NO2but with an extrapolation of their magnitudes back to
1979, based on the approximate linear emission rate for CH4at the ground. A similar
extrapolation was performed for CFCl3and CF2Cl2for the HNO3channel, based on
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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
HNO3at low latitudes of the lower stratosphere; the LIMS V5 dataset had no such
correction. A first-order broadband emission was generated for V6, based on esti-
mates for the background stratospheric aerosol layer of 1978/1979. Specifically, a
single zonal-mean distribution of aerosol extinction was adopted for the V6 algorithm
based on March/May 1996 results for the HALOE 5.26µm (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
likely not representative of its seasonal cycle variations. However, tropical aerosol dis-
tributions vary most noticeably according to the phase of the QBO cycle, and both the
1996 and 1979 March/May periods occurred at nearly the same QBO phase. Finally,
aerosol extinction at the wavelengths of the LIMS channels versus that at 5.26µm of
HALOE was obtained by employing the sulfuric acid/H2O composition and wavelength-
dependent, refractive index model for its aerosol absorption in the manner of Hervig et
al. (1995).
The archived V6 profiles were screened for anomalies using several criteria. First,
retrieval variances were calculated and written to an intermediate Level 2 file that was
not archived. Those variances were defined in terms of (NEN/K)2, where NEN is the
noise equivalent radiance for each channel in watts/m2-sr (Gille and Russell, 1984)
and K is the derivative of the measured signal profile as a function of the given species
mixing ratio. During the retrieval process the 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). Note that a restart causes the algorithm to de-
fault to a constant, top-layer, mixing ratio based on the value from the layer of its first,
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successfully-retrieved level; the thickness of that constant, top-layer extends upward
several scale heights. Valid profile segments have positive variances, and only re-
trieved values from those segments were retained in the final Level 2 output file. The
variance parameters effectively set the extreme upper altitude limits for good data for
all the parameters. Negative variances often indicate the low altitude extent of good
data, too. Variance checking occurred only outside of the pressure ranges of 1.9hPa to
46hPa for HNO3and of 0.88hPa to 46hPa for NO2. Independent estimates of random
error indicate that there is adequate tangent layer signal within those ranges. An ad-
ditional algorithm was applied to identify and screen for contaminating radiances from
cloud tops, at least to first order (see Remsberg et al., 2007, for the details).
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3.2Single profile error estimates
The V6 HNO3profile values are not much different from those of V5, except in the
upper stratosphere where the interference from CO2has been accounted for better in
the V6 algorithm. In the lower tropical stratosphere there are differences due to the
corrections for emissions from the CFCs and due to the first-order correction for the
aerosol emission. Random radiometric errors for V6 HNO3are 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 the random uncertainties. The calculated precisions
shown in Fig. 4 vary from 0.15ppbv at 80hPa to 0.05ppbv at 3hPa, based on the av-
erage 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 val-
ues were obtained as minima 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 Febru-
ary 1979. Of course, a part of the empirical SD values in Fig. 4 may be a result of real
variations in the atmospheric HNO3.
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Sources of systematic uncertainty were reported in Gille et al. (1984), based on
simulations of their effects for V5 from a model HNO3profile 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). Uncertainties in the aerosol and CFC corrections are greatest
for the lower tropical stratosphere but do not dominate the total error during 1978/1979.
Biases at the tops of the V6 HNO3profiles are small because the a priori profile values
are also small near the stratopause and extend above the pressure level of the first
retrieved point. 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 HNO3.
The largest elements of potential bias error are the ±20% uncertainties in the inte-
grated 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 Ta-
ble 1. The FOV side lobes from the HNO3channel are not all of the same sign. To judge
their effects, the measured HNO3radiance profiles were analyzed for the vertical dis-
tances of the side lobes from the main HNO3lobe or at effective separations of 17 and
34km at the horizon. For instance, are there any positive or negative radiance correla-
tions 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 34km below? Our investigations indicate no significant correlations
for altitudes below about 40km. Thus, it is concluded that the magnitude of that error
in Table 1 must be an upper limit at all levels, except perhaps at 3hPa. Uncertainties
for the out-of-band spectral response can impart a positive bias in HNO3of order 20%
at 30hPa, as pointed out in Gille et al. (1984). Yet, qualitative comparisons with other
HNO3datasets 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)
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estimates of accuracy in Table 1. In summary, the overall accuracy for V6 HNO3is
believed to be very good from 30hPa to 10hPa – of order 10% at middle latitudes.
Table 2 contains the estimates of precision and accuracy for the profiles of NO2. First,
the reader is reminded that the nighttime values of V6 NO2have peak mixing ratios
near 4hPa 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 was not included in
earlier versions of the AFGL or HITRAN line lists. On the other hand, the peak daytime
V6 NO2values at 8 to 10hPa 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 3hPa, the daytime V6 NO2is larger
than that of V5 due to the improved values from the interfering V6 H2O. Below the 30-
hPa level of the lower stratosphere both the day and night V6 NO2values are somewhat
larger than V5. At this point it is noted that the V5 algorithm contained a merger of its
retrieved NO2with a balloon-based average NO2profile 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 NO2algorithm is not constrained by any such climatological profile.
The precision estimates for the V6 NO2in 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 from the measured radiance profiles. Random uncertainties from single
profiles of the retrieved H2O were adopted from Remsberg et al. (2009) and included
because H2O is a major source of interfering emission, especially for daytime NO2in
the upper and the lowermost stratosphere. The effects of those random H2O errors
are scaled further, according to the fraction of the signal in the NO2channel that is
due to H2O. Estimates of percentage NO2precision profiles from the data are shown
in Fig. 5 and are based on average day 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 1hPa, and then to 15–20% at 40hPa. 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 50hPa, presumably because of the
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effects of the natural variability of the atmospheric NO2.
For the V6 NO2bias errors a linear scaling was applied to the entries in the V5 error
table of Russell et al. (1984a), which were appropriate for a model-generated, daytime
profile at 32◦N. Its primary sources of systematic uncertainty are from temperature bias
and from the interfering effects of H2O, based on the V6 uncertainties in Remsberg
et al. (2004, 2009), respectively. There are uncertainties in the lower stratosphere
due to the O2interference, but they should be considered as an upper limit because
corrections for its emission were applied to the retrievals of both the LIMS H2O and
NO2and 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, but its effect on the retrieved
NO2is small and remains unverified.
Table 2 indicates RSS V6 NO2uncertainties of 17% at 3hPa and about 18% from 5
to 10hPa. RSS values increase sharply in the lower stratosphere and are dominated
by the estimates of error for the interfering O2continuum and the H2O. At stratopause
levels (1hPa) the ascending (daytime) NO2may have a bias of order 33% across most
latitudes. However, the larger, descending (nighttime) NO2values at that level are
not affected much by the interfering H2O, so they are more accurate (17%). Also, the
increasing values of NO2in 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 for that situation is the likelihood that the retrieved V6
polar night NO2values are too low due to not accounting for the effects of non-LTE
emissions of the ground-state of NO2itself (e.g., see Funke et al., 2005).
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4 Scientific implications of the V6 data
The previously, archived LIMS V5 HNO3and NO2, along with the temperature, O3,
and H2O data have been used in scientific studies and reported in the literature. In
the subsections that follow several of the issues that they raised with respect to the
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V5 HNO3and NO2profiles and distributions are re-visited and re-evaluated using the
V6 data.
4.1HNO3/NO2ratio profiles
Simultaneous measurements of HNO3and NO2by LIMS provide an opportunity to
check the partitioning of NOyspecies in comparison with that predicted with theoretical
models. In particular, Jackman et al. (1985) reported that between 5 and 1hPa the
ratio of LIMS daytime HNO3to NO2was inconsistent with model values and that the
HNO3derived from photochemical relations should be used instead of those from the
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 in-
voked an instantaneous photochemical equilibrium assumption for HNO3and, thereby,
expressed OH concentrations as a function of the ratios of HNO3/NO2. Their second
approach used an equilibrium assumption for odd hydrogen (HOx) along with the LIMS
O3and H2O data to derive the daytime OH values. In the upper stratosphere those
two approaches yielded different results. The V5 HNO3/NO2ratios overestimated OH
in the upper stratosphere by a large margin, at least when compared to other available
observations and model results. The second approach yielded much better agree-
ment. Jackman et al. (1985) concluded that this discrepancy was due to errors in the
LIMS HNO3in the 5 to 1hPa region. Natarajan et al. (1986) reached a similar conclu-
sion regarding the HNO3/NO2ratio, but on the basis of diurnal photochemical model
calculations that were constrained by LIMS V5 nighttime data. The V5 HNO3/NO2
ratios also showed a positive bias in the upper stratosphere in comparison with their
modeled estimates.
Figure 6 is a comparison of the daytime ratio of V6 HNO3/NO2for 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 long-lived species, such as nitrous oxide (N2O), that are used to ini-
tialize the diurnal calculations are taken from a simulation with the NASA Langley
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