**Comparison of Water Vapor Measurements by Airborne Sun Photometer and
Diode Laser Hygrometer on the NASA DC-8**

**\
**

J. M. [Livingston]{.smallcaps}^a^, B. [Schmid]{.smallcaps}^b^, P. B.
[Russell]{.smallcaps}^c^, J. [Redemann]{.smallcaps}^d^, J. R.
[Podolske]{.smallcaps}^c^, G. S. [Diskin]{.smallcaps}^e^

Revision submitted to *J. Atmos. Oceanic Technol.*

4 January 2008

Corresponding author:

John M. Livingston

SRI International

333 Ravenswood Avenue

Menlo Park, CA 94025 U.S.A.

Phone: +1 650 859 4174

Fax: +1 650 322 3218

e-mail: john.livingston@sri.com

*^a^SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 USA\
^b^Pacific Northwest National Laboratory, 902 Battelle Boulevard,
Richland WA 99352 USA\
^c^NASA Ames Research Center, MS 245-5, Moffett Field, CA 94035-1000
USA*

*^d^Bay Area Environmental Research Institute, 560 Third St. West,
Sonoma, CA 95476 USA\
^e^NASA Langley Research Center, Hampton, VA 23681-2199 USA*

ABSTRACT

In January-February 2003 the 14-channel NASA Ames Airborne Tracking
Sunphotometer (AATS) and the NASA Langley/Ames Diode Laser Hygrometer
(DLH) were flown on the NASA DC-8 aircraft. AATS measured column water
vapor on the aircraft-to-sun path, while DLH measured local water vapor
in the free stream between the aircraft fuselage and an outboard engine
cowling. The AATS and DLH measurements have been compared for two DC-8
vertical profiles by differentiating the AATS column measurement and/or
integrating the DLH local measurement over the altitude range of each
profile (7.7-10 km and 1.1-12.5 km). These comparisons extend, for the
first time, tests of AATS water vapor retrievals to altitudes \>\~6 km
and column contents \<0.1 g cm^-2^. To our knowledge this is the first
time suborbital spectroscopic water vapor measurements using the 940-nm
band have been tested in conditions so high and dry. Values of layer
water vapor (LWV) calculated from the AATS and DLH measurements are
highly correlated for each profile. The composite data set yields r^2^
0.998, rms difference 7.7% and bias (AATS minus DLH) 1.0%. For water
vapor densities AATS and DLH had r^2^ 0.968, rms difference 27.6%, and
bias (AATS minus DLH) -4.2%. These results for water vapor density
compare favorably with previous comparisons of AATS water vapor to in
situ results for altitudes \<\~6 km, columns \~0.1 to 5 g cm^-2^ and
densities \~0.1 to 17 g m^-3^.

**1. Introduction**

Water vapor measurements by sunphotometry using the 940-nm water vapor
absorption band have been compared to in situ and other remote (e.g.,
microwave) measurements in several previous publications (e.g., Schmid
et al. 2000, 2001, 2003a,b, 2006; Redemann et al. 2003; Livingston et
al. 2000, 2003, 2007). Those comparisons were all restricted to
sunphotometer altitudes \<\~6 km, with water vapor columns \~0.1 to 5 g
cm^-2^ and water vapor densities \~0.1 to 17 g m^-3^.

In January-February 2003, the 14-channel NASA Ames Airborne Tracking
Sunphotometer (AATS) flew on the DC-8 along with the NASA Langley/Ames
Diode Laser Hygrometer (DLH). The flights were part of the second SAGE
(Stratospheric Aerosol and Gas Experiment) III Ozone Loss and Validation
Experiment (SOLVE II). They provided an opportunity to test AATS water
vapor measurements in higher, drier environments, including altitudes up
to 12 km and water vapor columns \~0.002 to 0.1 g cm^-2^ above 4 km. The
availability of DLH measurements on the same aircraft as AATS provided
an especially good comparison opportunity, since the DLH was designed
and built to perform well in environments this high and dry, and
previous DLH measurements had been compared to other state-of-the-art
water vapor measurements in such regions (e.g., Podolske et al. 2003).
To our knowledge there had not been any previous tests of suborbital
spectroscopic water vapor measurements using the 940-nm band in
conditions so high and dry.

It should be noted, however, that several satellite instruments that
view the sun through the Earth's atmospheric limb do use the 940-nm band
for water vapor measurements, and they have been validated. These
instruments include the Stratospheric Aerosol and Gas Experiment (SAGE)
and Polar Ozone and Aerosol Measurement (POAM) families of sensors,
validations of which have been published by, e.g., Nedoluha et al.
(2002), Taha et al. (2004), Thomason et al. (2004), and Lumpe et al.
(2006). The SAGE and POAM measurements benefit from the long viewing
path of their limb-viewing geometry (e.g., \~200 km in a 1-km thick
atmospheric shell, resulting from local solar zenith angle \~90º), which
produces measurable absorption in the 940-nm band even for stratospheric
concentrations of water vapor (typically 3-8 ppmv). The AATS DC-8
measurements reported in this paper had true solar zenith angles (SZA)
ranging from 68.6º to 89.1º, which includes viewing paths (hence airmass
factors) considerably less than those for the SAGE and POAM viewing
geometries.

**2. Instruments and Data Analysis Techniques**

*a. 14-channel Ames Airborne Tracking Sunphotometer (AATS)*

The 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14 or
simply AATS in this paper) has been described previously in the
literature (e.g., Russell et al. 2005, 2007), so we give only a brief
synopsis here. AATS measures the direct beam solar transmission in 14
channels with center wavelengths from 354 to 2138 nm, including a
channel centered at 941 nm. Azimuth and elevation motors rotate a
tracking head to lock on to the solar beam and maintain detectors normal
to it.

The AATS channel wavelengths are chosen to permit separation of aerosol,
water vapor, and ozone transmission along the measured slant path. Our
methods for data reduction, calibration, and error analysis have been
described in detail previously (Russell et al. 1993a, 1993b; Schmid and
Wehrli 1995; Schmid et al. 1996, 2001, 2003). Water vapor analysis
methods are briefly reviewed below. Results for AATS aerosol optical
depth and ozone measurements from the DC-8 in SOLVE II are described by
Russell et al. (2005) and Livingston et al. (2005).

AATS was calibrated by analysis of sunrise measurements acquired at
Mauna Loa Observatory (MLO), Hawaii, for six sunrises in November 2002
prior to SOLVE II and for seven sunrises in March 2003 after SOLVE II.
Exoatmospheric detector voltages, V~0~, were derived using the Langley
plot technique (e.g., Russell et al. 1993a, 1993b; Schmid and Wehrli
1995) for all channels except 941 nm, for which a modified Langley
technique (Reagan et al. 1995; Michalsky et al. 1995; Schmid et al.
1996, 2001) was employed to account for water vapor absorption.

Because absorption by water vapor varies strongly within the 5-nm FWHM
bandpass of the AATS-14 channel centered at 941 nm, the usual
Beer-Lambert-Bouguer expression must be modified to describe correctly
the relationship between the output detector voltage, V(941), and the
atmospheric attenuators on the sun-to-instrument path. In particular,

V(941 nm) = V~0~(941 nm) d^-2^
exp$\left\lbrack - \sum_{i}^{}{m_{i}\tau_{i}(\text{941}\text{nm})} \right\rbrack$
T~w~, (1)

where V~0~(941 nm) is the exoatmospheric calibration voltage, d is the
Earth-Sun distance in astronomical units at the time of observation,
m~i~ is the airmass factor (ratio of slant path optical depth to
vertical optical depth) for attenuating species i (where i represents
gas scattering, non-water vapor gas absorption, or aerosol extinction),
τ~i~ is the optical depth for the ith attenuating species other than
water vapor, and T~w~ is the water vapor transmittance (weighted by
absorption strength, source intensity and filter function). Consistent
with the approach followed by Livingston et al. (2007), we used the
three-parameter expression of Ingold et al. (2000) to parameterize T~w~
as a function of the amount of water vapor, w~s~, along the slant path:

T~w~ = c exp(-aw~s~^b^). (2)

In this expression, the coefficients a, b, and c are least squares
fitting parameters. In particular, calculations were performed using the
radiative transfer code LBLRTM V9.2 (Clough et al. 2005) for a variety
of model atmospheres (tropical, mid-latitude summer, mid-latitude
winter, subarctic summer, subarctic winter, 1976 U. S. Standard
Atmosphere) and a range of solar zenith angles to extend the Livingston
et al. (2007) Table 2 results to include all altitudes (maximum of
\~12.5 km) flown by the DC-8 during SOLVE II. These results are shown in
Fig. 1 and the complete set of fitting coefficients is given in Table 1.
We have calculated one set of fitting parameters to LBLRTM results for
the composite set of model atmospheres rather than calculating fitting
parameters separately for each model atmosphere because inclusion of all
the model atmosphere results yields a wide range of water vapor
transmittances and slant path water vapor amounts. These can then be
applied to a wide range of sunphotometer transmittance measurements
obtained at any altitude with no extrapolation to values of water vapor
transmittance and slant amount outside the range of results from LBLRTM
calculations that use a single model (e.g., the subarctic winter
atmosphere).

As noted by Livingston et al. (2007) and illustrated here in Fig. 1, a
retrieval that ignores the instrument altitude can result in an
incorrect determination of slant path (hence, column) water vapor. In
particular, if it is assumed that the instrument is located at sea
level, then column water vapor (CWV) would be underestimated for
instrument altitudes above sea level, and the errors would be greatest
for large solar zenith angles, SZA (high airmass values), and high CWV
(hence, high slant water vapor) amounts.

Column water vapor is calculated from the slant amount by dividing by an
appropriate water vapor airmass factor. These airmass factors were
calculated using the methodology reported in Russell et al. (2005) and
Livingston et al. (2005) by assuming a water vapor vertical distribution
corresponding to the subarctic winter atmospheric model for the 21
January 2003 measurements and the mid-latitude winter atmospheric model
for the 6 February 2003 data. The uncertainty in CWV is computed
following Schmid et al. (1996)**.** The calculated CWV values are
averaged within 50-m vertical bins, and a smoothing spline is then fit
to the resultant CWV profile. Water vapor density (ρ~w~) is obtained as
the derivative of the spline fit. This procedure has been described in
detail in Schmid et al. (2003a) and Livingston et al. (2007).

*b. Diode Laser Hygrometer*

The NASA Langley/Ames Diode Laser Hygrometer (DLH) was designed and
built to measure gas-phase water in the free-stream region of the NASA
DC-8 aircraft, between the fuselage and the cowling of an outboard
engine. The instrument is described in detail elsewhere (Vay et al.
1998; Diskin et al. 2002; Podolske et al. 2003), so only a brief
description is given here.

The DLH instrument is a near-infrared (NIR) spectrometer operating at
wavelengths near 1.4 μm to detect individual rotation-vibration lines of
H~2~O in either the (101) combination band or the (200) overtone band.
Second harmonic detection (Sachse et al. 1977, 1987; Reid and Labrie
1981; Podolske and Loewenstein 1993, and references therein) and long
path length are utilized to achieve high sensitivity, and two or more
lines of different strengths are used to meet the dynamic range
requirements for atmospheric water. The beam of a NIR diode laser is
quasicollimated and transmitted through a quartz window secured in a
DC-8 window plate, toward the cowling of the right (starboard) side
outboard engine. There it strikes a sheet of retroreflector material and
returns to the fuselage window from which it originally emerged. Inside
the window a portion of the return beam is passed through a narrowband
interference filter, collected by a Fresnel lens, and focused onto a
detector. The sample volume of the external path is completely exchanged
every 40--70 ms, depending on aircraft velocity. The laser wavelength is
modulated, and the signal detector output is synchronously demodulated
to produce the second harmonic signal. The second harmonic (2F) and DC
components from the signal detector are recorded to allow power
normalizing of the (2F) signal.

The laser radiation emitted from the rear facet of the diode laser is
sent through a short (50 mm) reference cell containing pure water vapor
and onto a second detector. The reference detector output is
synchronously demodulated at the third harmonic (3F), the central zero
crossing of which is subsequently used to lock the laser wavelength to
the center of the chosen absorption line.

DLH calibration and data retrieval algorithms are described by Podolske
et al. (2003). During SOLVE-II and subsequently, efforts to quantify DLH
accuracy have yielded a typical uncertainty of ±5%, with a precision at
5 ppmv water vapor of approximately 1%. The minimum water vapor
densities (concentrations) measured by DLH for the two profiles
discussed in this paper were 11 ppmv (at 9.7 km) and 4.6 ppmv (at 12.5
km) for the 21 January and 6 February profiles, respectively.

**3. Results**

Results from the first comparison profile, which was flown 21 January
2003 on a DC-8 ascent out of Kiruna, Sweden, are presented in Fig. 2.
AATS was able to view the sun at DC-8 altitudes between \~7.7 and 10 km,
thus allowing calculation of CWV at each altitude in that range. The
AATS and DLH CWV profiles are overplotted in Fig. 2a, in addition to the
atmospheric static temperature profile measured by a probe mounted on
the DC-8. Because DLH measures local (not column) water vapor, the DLH
CWV value at profile top is set equal to the AATS value there, and DLH
CWV values below that altitude are obtained by integrating DLH local
values downward. Corresponding AATS and DLH water vapor densities are
overplotted in Fig. 2b. As noted in Section 2a, AATS ρ~w~ is obtained as
the vertical derivative of a spline fit to the AATS CWV profile that
results after averaging the CWV values within 50-m vertical bins.

Scatter plot comparisons of AATS and DLH CWV and ρ~w~ for the 21 January
profile are shown in Fig. 2c and 2d. The statistics shown on each
scatter plot quantify the correlation and agreement between the AATS and
DLH results. For CWV, the data yield r^2^ 0.977, rms difference 0.0004 g
cm^-2^ (13.0%) and bias (AATS minus DLH) 0.0002 g cm^-2^ (8.1%).
Integration of the DLH ρ~w~ values over the layer bounded by the minimum
(7.69 km) and maximum (9.97 km) altitudes for which AATS CWV values were
calculated yields LWV of 0.0038 g cm^-2^, which is 24% drier than the
corresponding AATS LWV value, 0.0050 g cm^-2^. Here, we note that if the
absolute rms difference and bias in CWV (or, equivalently, in LWV) are
expressed as percentages of LWV instead of CWV, the relative values
increase (rms difference: 31.1%, bias: 19.6%) because LWV is less than
CWV. Corresponding values for ρ~w~ are r^2^ 0.887, rms difference 92.1%
and bias (AATS minus DLH) 37.3%. This agreement is noteworthy in light
of the very dry conditions: maximum AATS CWV of 0.007 g cm^-2^ and
maximum ρ~w~ of 0.1 g m^-3^. Table 2 summarizes the AATS-DLH results for
the two SOLVE II profiles and includes ρ~w~ results from three other
campaigns in which coincident AATS and in-situ sensor (not DLH) water
vapor measurements were acquired from the same aircraft.

Examination of Fig. 2a and 2b reveals that the AATS-DLH LWV difference
of 0.0012 g cm^-2^ is dominated by differences between AATS and DLH
water vapor values in the lowest 0.5 km (7.7-8.2 km) of the profile. In
an effort to explain these differences, we have examined the sensitivity
of the AATS results to the water vapor airmass factors -- that is, to
the water vapor vertical distribution used to calculate these airmass
factors. As noted above in Section 2a, the AATS CWV values were derived
using water vapor airmass factors calculated from the water vapor number
density vertical distribution defined by the subarctic winter
atmospheric model. The DC-8 ascent from 7.7 km to 10 km covered a
horizontal distance of \~160 km in 0.25 h. During the ascent the true
SZA decreased from 89.08º to 88.56º, for which water vapor airmass
factors calculated from the subarctic winter atmosphere model decrease
from 35.9 to 25.9, although the airmass factor between 7.7 and 8.2 km
only decreases from 35.9 to 33.5. In fact, DLH measurements are
available between 2.0 km and 10 km and have been combined with the
subarctic winter atmospheric model below and above the DLH profile to
construct a composite water vapor profile, for which a new set of water
vapor airmass factors has been derived. Application of these airmass
factors to the AATS water vapor transmittances decreases the AATS LWV
from 0.0050 to 0.0043 g cm^-2^, in much better agreement with the DLH
LWV value of 0.0038 g cm^-2^, but the new AATS CWV profile (not shown)
still falls within the AATS CWV error band shown in Fig. 2. Use of these
airmasses yields corresponding decreases in AATS-minus-DLH CWV and ρ~w~
bias and rms differences. In particular, the AATS-minus-DLH CWV bias
decreases from 8.1% to 1.5%, and the rms difference decreases from 13.0%
to 7.4%. The AATS-minus-DLH ρ~w~ bias decreases from 37.3% to 19.1%, and
the rms difference decreases from 92.1% to 76.2%. However, the general
shape of the AATS ρ~w~ profile does not change and the divergence
between the AATS and DLH ρ~w~ values in the altitude range between 7.7
and 8.2 km is only marginally reduced. We are left to conclude that
these differences are real and are due to water vapor spatio-temporal
inhomogeneity. Although this exercise is useful for providing some
measure of the sensitivity of the 21 January AATS CWV retrieval to
airmass (i.e., to the water vapor vertical distribution), it is not
practical for calculating real time AATS ρ~w~ profiles because,
typically, no *a priori* information on the vertical distribution of
water vapor is available.

Fig. 3 shows results from the second comparison profile, which was flown
6 February 2003 as the DC-8 descended into Edwards Air Force Base,
California, on its return from Sweden. In this case, AATS was able to
view the sun at DC-8 altitudes from \~12.5 km to 1.1 km. This profile is
noteworthy for combining very dry conditions above \~5.5 km (similar to
those in the 21 January case, cf. Fig. 2) with significantly increased
CWV and ρ~w~, and strong vertical structure below \~5.5 km (though
values are still small compared to the previous AATS comparisons cited
in Section 1). Another significant feature of this profile is the
availability of water vapor measurements from a Sippican II radiosonde
released from Edwards at 1446 UT (1.75 h before the DC-8 began its
descent). Frames 3a and 3b overplot the results for the entire range of
measurement altitudes; frames 3c and 3d expand the results for altitudes
between 6.0 and 12.5 km; frames 3e and 3f show results between 1.1 and
6.0 km. We note that the spline fitting function was adjusted to smooth
the AATS CWV values above 4.5 km (i.e., for relatively low values of
CWV) more than those below 4.5 km to minimize the effect on the
calculated AATS water vapor density profile due to spatial and/or
temporal water vapor variability (as manifest by increasing CWV with
altitude for selected altitudes above 8.5 km) along the AATS-to-Sun
slant path.

For the entire altitude range traversed by the DC-8 during its descent
on 6 February, the DLH measures LWV (0.291 g cm^-2^) that is 5% greater
than the corresponding AATS LWV value (0.277 g cm^-2^). Corresponding
values of AATS and DLH CWV and ρ~w~ for the 6 February profile are
compared in scatter plots in Fig. 4 and are listed in Table 2. Results
for CWV are r^2^ 0.998, rms difference 0.0041 g cm^-2^ (6.6% in CWV,
7.0% in LWV) and bias (AATS minus DLH) 0.0005 g cm^-2^ (0.8% in CWV,
1.0% in LWV). Corresponding values for ρ~w~ are r^2^ 0.965, rms
difference 0.068 g m^-3^ (25.2%), and bias (AATS minus DLH) -0.013 g
m^-3^ (-4.8%). As can be seen in Table 2, if the analysis (not plotted
separately) of the 6 February data is restricted to altitudes above 5.5
km, the CWV and ρ~w~ r^2^ values are similar to the 21 January results.
The values of absolute rms difference and bias for the
altitude-restricted 6 February analysis are about twice those of the 21
January profile, whereas corresponding values of relative ρ~w~ rms
difference and bias are significantly lower than the 21 January results.
For comparisons between AATS and the radiosonde, the overall agreement
is good: for AATS values interpolated to sonde-reported altitudes, AATS
and sonde CWV have r^2^ 0.998, rms difference 0.010 g cm^-2^ and bias
(AATS minus sonde) 0.006 g cm^-2^. Corresponding values for ρ~w~ are
r^2^ 0.910, rms difference 0.109 g m^-3^ (34.6%) and bias (AATS minus
sonde) 0.026 g m^-3^ (8.3%). The maxima of the scatter plots (CWV\<0.3 g
cm^-2^, ρ~w~ \<1.1 g m^-3^), though larger than those shown in Fig. 2,
are still noteworthy for their small values. For example, in previous
AATS comparisons to in situ and other remote water vapor measurements,
CWV has ranged up to \~5 g cm^-2^ and ρ~w~ up to \~17 g m^-3^.

As can be seen in Fig. 3d and 3f, the radiosonde measured higher values
of ρ~w~ than did AATS or DLH between 5.5 and 7 km, and also in the lower
stratosphere (LS) between 11 and 12 km, but lower ρ~w~ between 7 and 9
km and also between \~3.2 and 4.5 km. Previous studies by Wang et al.
(2003), Ferrare et al. (2004), and Miloshevich et al. (2006) have found
that the Sippican carbon hygristor sensor fails to respond to humidity
changes at colder temperatures and thus cannot provide reliable
measurements of relative humidity in the upper troposphere (UT). This
may explain the higher Edwards sonde ρ~w~ values measured in the LS and
certainly raises doubts about the validity of the data not only in the
LS but throughout the UT (i.e., above \~6 km). Also, since no radiation
correction is applied to the Sippican temperature data, heating by solar
radiation could lead to an incorrect temperature measurement and thus
contribute to an incorrect estimate of water vapor density. However,
there is no direct evidence of this effect, as examination of Fig. 3a
indicates that measurements of static temperature by the DC-8 sensor and
by the radiosonde are virtually indistinguishable throughout most of the
troposphere, except within the temperature inversion at 3.2 km, where
the sonde is warmer by \~2°C, and in the LS, where the DC-8 sensor is
actually 1-2°C warmer than the sonde.

Fig. 5 shows scatter plots that combine AATS and DLH results for the
profiles on 21 January and 6 February. AATS and DLH CWV (5a) have r^2^
0.998, rms difference 7.2% (7.7% LWV) and bias (AATS minus DLH) 0.9%
(1.0% LWV). Corresponding values for ρ~w~ (5b) are r^2^ 0.968, rms
difference 27.6% and bias (AATS minus DLH) -4.2%. Results for altitudes
\>5.5 km only are shown for CWV and ρ~w~ in 5c and 5d, respectively. For
CWV, values are r^2^ 0.994, rms difference 9.5% (18.0% LWV) and bias
(AATS minus DLH) 5.0% (9.5% LWV). For ρ~w~, values are r^2^ 0.825, rms
difference 49.6% and bias (AATS minus DLH) 9.7%.

**4. Summary and Conclusions**

This paper has compared values of CWV and ρ~w~ calculated from
simultaneous measurements acquired by the AATS and DLH sensors on the
DC-8 for two vertical profiles flown during SOLVE II. This study is
unique because it includes data acquired at altitudes (above 6 km) where
we had previously never taken measurements and where the amount of water
vapor in the atmosphere was the lowest we have ever measured. To our
knowledge this is the first time suborbital spectroscopic water vapor
measurements using the 940-nm water vapor absorption band have been
tested in conditions so high and dry.

Measurements were acquired at altitudes between 7.7 km and 10 km during
the 21 January aircraft ascent, and between 1.1 km and 12.5 km during
the 6 February descent. AATS and DLH CWV values (where the DLH value was
set equal to the AATS CWV value at the top of the profile) yielded r^2^
of 0.997 for 21 January and 0.998 for 6 February. Comparison of AATS and
radiosonde CWV values for sonde measurements (normalized to AATS CWV at
the top of the profile) acquired 1.75-2.15 h before the AATS
measurements on 6 February yielded an r^2^ of 0.997. For measurements
taken at altitudes \>5.5 km, the composite AATS and DLH data set gave an
r^2^ of 0.994, an rms difference of 0.0006 g cm^-2^ (9.5% CWV, 18.0%
LWV), and a bias (AATS minus DLH) of 0.0003 g cm^-2^ (5.0% CWV, 9.6%
LWV). Corresponding values for ρ~w~ were r^2^ of 0.825, an rms
difference of 0.012 g m^-3^ (49.6%), and a bias (AATS minus DLH) of
0.002 g m^-3^ (9.7%). The large relative rms and bias differences
reflect the dry atmosphere with ρ~w~ \<0.1 g m^-3^ at those altitudes.

The AATS-DLH CWV (LWV) results reported for the two profiles in the
current study are not directly comparable to AATS-in situ sensor results
obtained in previous AATS field campaigns (Schmid et al. 2003b, Schmid
et al. 2006, and Livingston et al. 2007), because each of the previous
studies included 26-35 separate aircraft profiles and reported r^2^, rms
difference and/or bias LWV statistics calculated for the ensemble of
profiles. The relative ρ~w~ statistics may be comparable, but caution
must be exercised in drawing any conclusions from such comparisons
because of the much larger values of ρ~w~ measured in the prior studies.
Nevertheless, to put the current results in the context of the previous
measurements, we include in Table 2 the comparative AATS-minus-in situ
sensor ρ~w~ statistics reported in those studies.

Most recently, Livingston et al. (2007) compared LWV and ρ~w~ measured
by AATS and by a Vaisala HMP243 humidity sensor mounted on the same
aircraft during 35 vertical profiles acquired over the Gulf of Maine,
with maximum LWV \~3.7 g cm^-2^ and maximum ρ~w~ \~16 g m^-3^. For the
ensemble of 35 profiles, they found for ρ~w~ an r^2^ of 0.98, an rms
difference of 20.3% (0.904 g m^-3^), and bias (AATS minus Vaisala) -7.1%
(-0.313 g m^-3^). For coincident measurements acquired with AATS and
with an EdgeTech 137-C3 chilled mirror during 35 vertical profiles over
the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP)
site in 2003, Schmid et al. (2006) reported for ρ~w~ an r^2^ of 0.958,
an rms difference of 19.8% (0.628 g m^-3^), and an AATS-minus-mirror
bias of -5%. In each of these studies, AATS was equipped with the same
941-nm interference filter used during SOLVE II. Schmid et al. (2003b)
compared coincident measurements acquired with AATS (using a different
941-nm interference filter from that used in the 2003 SGP and 2004 SOLVE
II campaigns) and with an EdgeTech Chilled Mirror 137-C3 and a Vaisala
HMP243 mounted on the same aircraft during 36 vertical profiles during
the 2003 ACE-Asia field campaign. For the AATS-mirror ρ~w~ , they
calculated an r^2^ of 0.961 and an rms difference of 24.9% (0.633 g
m^-3^). The AATS-Vaisala results were almost identical. When the data
obtained at all altitudes for 6 February 2003 are included in the
analyses, the AATS-DLH r^2^ values for SOLVE II equal those reported in
the previous AATS studies. For data obtained on 21 January and for the
composite data set restricted to those data acquired above 5.5 km on 6
February, the AATS-DLH r^2^ values (0.82-0.89) are less than the earlier
results. The AATS-DLH ρ~w~ relative rms difference and bias for 21
January are significantly higher than the previous results, as are
relative rms differences for the high altitude 6 February and high
altitude composite data sets. Again, about all that can be concluded is
that the large relative rms and bias differences reflect the dry
atmosphere at those altitudes.

We are encouraged that useful water vapor retrievals were derived from
the AATS measurements on 21 January when data were acquired at high
altitude for relatively high SZAs (89.08º to 88.56º). However, these
results are consistent with successful retrievals of aerosol optical
depth and columnar ozone from AATS measurements taken at high altitude
and high SZAs during SOLVE II (Russell et al. 2005; Livingston et al.
2005). Although horizontal inohomogeneities are more likely to occur
along long slant paths, these same long slant paths can improve
signal-to-noise ratio and reduce sunphotometer calibration uncertainties
to allow extension into an area of low ambient water vapor density.

The comparisons presented here included a very limited set of
measurements at altitudes in the 6-12 km range, and additional
comparisons between sunphotometer and in situ sensors are needed at
these altitudes where the amount of water vapor in the atmosphere is so
low to permit quantification of the uncertainty in the
sunphotometer-calculated ρ~w~ values. Nevertheless, the agreement we
have found between the AATS and DLH retrievals of CWV (essentially, LWV)
gives us hope that airborne sunphotometer measurements can provide
useful data for validation of satellite water vapor retrievals not only
for the full atmospheric column and for ρ~w~ in the lowest few km of the
troposphere, as has been shown in previous studies, but also at
altitudes in the upper troposphere where water vapor is limited.

*Acknowledgments*. We thank James Eilers and Richard Kolyer for
supporting AATS measurements, and Stephanie Ramirez for help with
illustrations and formatting. The SOLVE II measurements were supported
by NASA's Upper Atmosphere Research Program. AATS analyses were
supported by NASA's Solar Occultation Satellite Science Team.

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Table 1. Coefficients of Ingold et al. (2000) 3-parameter functional fit
(2-parameter at altitudes below 4 km) to LBLRTM_v9.2 calculations of
water vapor transmittance as a function of slant path water vapor w~s~
(where w~s~ is in units of cm or g cm^-2^) for the AATS-14 channel
centered at 940.6 nm. Values for altitudes 0-8 km are the same as those
shown in Table 2 of Livingston et al. (2007).

  --------------------- ----------------- ----------------- -----------------
  [Altitude             [a]{.underline}   [b]{.underline}   [c]{.underline}
  \[km\]]{.underline}                                       

  0                     0.51623           0.6439            1.00000

  1                     0.49669           0.6331            1.00000

  2                     0.47492           0.6238            1.00000

  3                     0.45191           0.6186            1.00000

  4                     0.43700           0.6053            1.00540

  5                     0.42217           0.5929            1.00924

  6                     0.40499           0.5859            1.01006

  7                     0.38888           0.5892            1.00855

  8                     0.38005           0.6059            1.00614

  9                     0.38692           0.6354            1.00394

  10                    0.43234           0.6897            1.00195

  11                    0.53860           0.7600            1.00077

  12                    0.60497           0.7952            1.00040

  13                    0.33551           0.6835            1.00086
  --------------------- ----------------- ----------------- -----------------

Table 2. SOLVE II AATS, DLH and radiosonde water vapor retrieval
statistics; AATS-in situ results from previous field campaigns.

Date Alt Range LWV r^2^~CWV~ CWV bias CWV rms diff. r^2^ρ~w~ ρ~w~ bias
ρ~w~ rms diff.

\[km\] \[g cm^-2^\] \[g cm^-2^\] \[g cm^-2^\] \[g m^-3^\] \[g m^-3^\]

\-\-\-\-\-\-\-\-\-\-\-\-\-- \-\-\-\-\-\-\-\-\-\-\-\--
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-- \-\-\-\-\-\-\--
\-\-\-\-\-\-\-\-\-\-\-\-\-\-- \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--
\-\-\-\-\-\-- \-\-\-\-\-\-\-\-\-- \-\-\-\-\-\-\-\-\-\-\-\-\-\--

[AATS]{.underline} [DLH]{.underline}

21 Jan 2003 7.69-9.69 0.0050 0.0038 0.977 0.0002 0.0004 0.887 0.006
0.015

(8.1^a^, 19.6^b^) (13.0^a^, 31.1^b^) (37.3%) (92.1%)

06 Feb 2003 5.50-12.45 0.0182 0.0190 0.993 0.0004 0.0007 0.874 0.001
0.010

(alt \> 5.5 km) (4.7^a^, 8.7^b^) (8.7^a^, 16.2^b^) (3.2%) (38.4%)

06 Feb 2003 1.10-12.45 0.277 0.291 0.998 0.0005 0.0041 0.965 -0.013
0.068

(all altitudes) (0.8^a^, 0.9^b^) (6.6^a^, 7.0^b^) (-4.8%) (25.2%)

[sonde]{.underline}

06 Feb 2003 1.10-12.45 0.277 0.252 0.998 0.0057 0.0101 0.910 0.026 0.109

(all altitudes: radiosonde) (7.2^a^, 7.5^b^) (12.8^a^, 13.4^b^) (8.3%)
(34.6%)

21 Jan, 6 Feb 0.998 0.0005 0.0037 0.968 -0.009 0.062

(all altitudes) (0.9^a^, 1.0^b^) (7.2^a^, 7.7^b^) (-4.2%) (27.6%)

21 Jan, 6 Feb 0.994 0.0003 0.0006 0.825 0.002 0.012

(alt \> 5.5 km) (5.0^a^, 9.6^b^) (9.5^a^, 18.0^b^) (9.7%) (49.6%)

ACE-Asia, 2003 \~0.0-3.8 0.961 - 0.633

(Schmid et al. 2003b: AATS vs. EdgeTech Chilled Mirror 137-C3) (24.9%)

SGP, 2003 \~0.2-5.7 0.958 - 0.628

(Schmid et al. 2006: AATS vs. EdgeTech Chilled Mirror 137-C3) (-5.0%)
(19.8%)

ICARTT, 2004 0.05-6.0 0.977 -0.313 0.904

(Livingston et al. 2004: AATS vs. Vaisala HMP243) (-7.1%) (20.3%)

^a^ value in %CWV ^b^ value in %LWV