Final Report

for

NOAA Office of Global Programs Grant GC04-396

**"Radiative-Climatic Effects of Aerosols and Water Vapor:**

**Studies in the Context of Intercontinental Transport and Chemical
Transformation (ITCT) with Contributions to Regular Vertical Profiling
(RVP)"**

Philip B. Russell^1,\*^, Beat Schmid^2,\*,#^, Jens Redemann^2,\*^, John
M. Livingston^3^,

Teck Meriam^2,@^, Nicholas Truong^2^, Roy Johnson^1^, and Stephen E.
Dunagan^1^

^1^NASA Ames Research Center, Moffett Field, CA 94035-1000

^2^Bay Area Environmental Research Institute, Sonoma, CA 95476

^3^SRI International, Menlo Park, CA 94025

\*Co-Principal Investigator

^\#^Current affiliation: Pacific Northwest National Laboratory,
Richland, WA 99352

^@^Current affiliation: KLA-Tencor, San Jose, CA 95134

30 March 2007

**1. Introduction**

The overall goal of research on this grant was to advance understanding
of the roles that aerosols and water vapor play in determining regional
and global climates. Approaches included measurements with existing
instrumentation, analyses, and exploration of new instrument concepts
and designs.

Efforts in Year 1 focused on two of the three activities of the FY2004
solicitation of the NOAA Atmospheric Composition and Climate (ACC)
Program, specifically, [ACC Activity 1]{.underline}: The 2004
Intercontinental Transport and Chemical Transformation (ITCT)
experiment, and [ACC Activity 3]{.underline}: Aerosol Indirect Effects.
Results are reported in Section 2 below.

Efforts in Years 2 and 3 focused on [ACC Activity 2]{.underline}:
Regular Vertical Profiling. Results are reported in Section 3 below.

**2. Efforts and Results addressing NOAA ACC Activity 1 (ITCT) and ACC
Activity 3 (Aerosol Indirect Effects).**

Our overall efforts in Year 1 addressed the question:

*What are the distributions, optical properties, and radiative effects
of aerosols and water vapor in the continental outflows studied in
ITCT-2004?*

Our approach was to make measurements of aerosols and water vapor in
ITCT-2004 using the 14-channel Ames Airborne Tracking Sunphotometer
(AATS-14) on a Jetstream-31 (J-31) aircraft. The J-31 also carried the
Solar Spectral Flux Radiometer (SSFR) of Dr. Peter Pilewskie (funded
separately by NOAA and NASA). The combination of AATS-14 and SSFR on the
same aircraft enabled measurements relevant to both the direct and the
indirect effects of aerosols, as elaborated in our proposal.

ITCT 2004 was closely coordinated with NASA's Intercontinental Transport
and Chemical Transformation (INTEX) experiment. INTEX-ITCT coordination
was enhanced by the International Consortium for Atmospheric Research on
Transport and Transformation (ICARTT), which also facilitated
coordination with simultaneous experiments led by European research
agencies. The J-31 flights were coordinated with satellite overpasses
and with other measurements, both in situ and remote, on the NOAA ship
Ronald Brown and on other aircraft.

The ultimate accomplishments resulting from the measurements made in
2004 included three journal papers \[*Redemann et al.,* 2006; *Russell
et al.,* 2006; *Livingston et al.,* 2006\] led by our group, plus
contributions to two more \[*Fehsenfeld et al.,* 2006; *Singh et al.,*
2006\]. As noted in our proposal, the integrated analyses reported in
those papers were funded by our NASA Earth Observing System
InterDisciplinary Science (EOS-IDS) task, whereas the AATS measurements
and initial data reduction and archival were funded by NOAA Grant
GC04-396. (J-31 modification and deployment costs were funded by other
NASA sources.)

*Redemann et al.* \[2006\] present aerosol radiative forcing
efficiencies obtained by combining J-31 AATS and SSFR measurements of
aerosol optical depth (AOD) and solar energy fluxes. In 10 case studies
they found a high variability in the derived aerosol forcing
efficiencies for the visible wavelength range (350-700 nm). They also
estimated aerosol absorption by matching calculations with the measured
gradients of aerosol optical depth and radiant flux. In this way they
found larger aerosol absorption early in the 12 July-8 August 2004
period than later, consistent with independent trajectory analyses and
satellite imagery that showed the stronger influence of smoke aerosols
from Alaska fires during the early period.

*Russell et al.* \[2006\] report J-31 AOD measurements using AATS and
compare them to retrievals by three satellite sensors: MISR on Terra and
MODIS on both Terra and Aqua. Comparisons of MISR and AATS on a day with
strong AOD gradients over the Gulf of Maine (29 July 2004) in 8 MISR
grid cells show that MISR (Versions 15 and 16) captured the
AATS-measured AOD gradient (R^2^=0.9) for all 4 MISR wavelengths (446 to
866 nm), with the MISR gradient somewhat weaker than the AATS gradient.
Comparisons of MODIS and AATS AOD values on 8 overpasses using 61 grid
cells yielded R^2^=0.97 and RMS difference 0.03, with about 87% of the
MODIS AOD retrievals agreeing with AATS values within the MODIS
over-ocean uncertainty. In contrast, the MODIS-AATS differences in
Ångstrom exponent (-dlnAOD/dln) were rather large but improved
substantially when cases with small AOD (\<0.1 at 855 nm) were excluded.

*Livingston et al.* \[2006\] report J-31 water vapor measurements by
AATS and compare them to in situ and satellite measurements. They found
that layer-integrated water vapor (LWV) results from AATS and the J-31
in situ Vaisala sensor had RMS difference 8.8% and bias -7.1% (AATS
minus Vaisala). Similarly good agreement existed between AATS and sondes
released from the RV Ron Brown. AIRS LWV retrievals within 80 km of J-31
profiles had rms difference 13-19% and bias 5.8% (AIRS minus AATS).
MODIS IR retrievals of column water vapor (CWV) in 205 grid cells (5 x
5-km at nadir) were biased wet by 10.4% compared to AATS over-ocean near
surface retrievals. The MODIS-Aqua subset (79 grid cells) exhibited a
wet bias of 5.1%, and the MODIS-Terra subset (126 grid cells) yielded a
wet bias of 13.2%.

Our contributions to the *Fehsenfeld et al.* \[2006\] and *Singh et al.*
\[2006\] overview papers summarize the roles of the J-31 in the ITCT and
INTEX experiments.

Other accomplishments supported by NOAA Grant GC04-396 include:

• Dr. Russell attended the ICARTT Study Planning Team Meeting in
Boulder, CO in January 2004 and the ICARTT Planning Workshop in Orlando,
FL in April 2004. He presented goals and plans for this research
\[*Russell,* 2004\] and the results of the Working Group on Aerosol
Direct Radiative Effects \[*Russell and Bates,* 2004\].

• Our team integrated AATS-14 onto the J-31 in May and June 2004.

• We calibrated AATS-14 at Mauna Loa Observatory in May 2004.

• We conducted 19 science flights of the J-31 from Portsmouth, NH in
support of ITCT-INTEX-ICARTT in July-August 2004. Table 1 gives a
synopsis of flights. As noted there, flights were coordinated with
satellite overpasses and with other measurements on the NOAA ship Ron
Brown and other aircraft. They sampled both cloud-free and cloudy areas,
so as to acquire data relevant to both direct and indirect effects of
aerosols.

• We made presentations on goals, plans, and early results at an ICARTT
Science Team Meeting in Portsmouth, NH, and the ICARTT Mid-Campaign
Science Meeting, in Durham, NH in July 2004 \[*Russell et al.,*
2004a,b\].

• We submitted J-31 flight reports, which are posted at
<http://cloud1.arc.nasa.gov/intex-na/flight_reps.html> and posted flight
summaries and presentations with data examples at
[http://geo.arc.nasa.gov/sgg/INTEX/]{.underline}.

• We archived the first version of AATS-14 data at
<http://www-air.larc.nasa.gov/cgi-bin/arcstat#1> and the final version
in the INTEX archive,
<http://www-air.larc.nasa.gov/cgi-bin/arcstat>[.]{.underline}

• We presented a paper on our early ITCT results to the AGU 2004 Fall
Meeting \[Livingston et al., 2004\] and additional results at six other
conferences \[*Livingston et al.,* 2005b; *Redemann et al.,* 2005b-d;
*Russell et al.,* 2005a-d\]. We made additional presentations at the
2005 AAAR Annual Conference \[*Redemann et al.,* 2005e, *Russell et
al.,* 2005e\], and an invited presentation at the AGU 2005 Fall Meeting
\[*Redemann et al.,* 2005f\]. In addition to leading those
presentations, we also contributed as coauthors to several other
presentations that used AATS J-31 data \[e.g., *Ferrare et al.,* 2005;
*Kahn et al.,* 2005; *McNaughton et al.,* 2005; *Pilewskie et al.,*
2005\].

• We organized and co-led the J-31 ITCT-INTEX-ICARTT Data Workshop, 9-10
March 2005 at the NOAA Aeronomy Lab in Boulder, CO. We chose the same
week and building as the Ron Brown and Chebogue Point workshops, to
facilitate collaboration between J-31 investigators and those from other
platforms (Ron Brown, its sondes, DC-3, P-3, DC-8). The J-31 workshop
agenda and presentations are accessible via the ICARTT web page
<http://www.al.noaa.gov/ICARTT/Calendar.shtml> and also posted at
<http://geo.arc.nasa.gov/sgg/INTEX/presentations/J31WkspPowerPoints/J31Wksp.html>.

• We served as co-convener for the aerosol sessions at the August 2005
ICARTT data workshop and also as co-convener for an associated session
(A12) at the Fall 2005 AGU meeting.

• We prepared an "ICARTT Fact Sheet" describing the early J-31 results
on aerosol radiative forcing in terms that can be understood by a broad
audience. The fact sheet, based on results in *Redemann et al.* \[2006\]
is posted at <http://esrl.noaa.gov/csd/ICARTT/factsheets/russell.pdf>

**3. Efforts and results addressing NOAA ACC Activity 2 (Regular
Vertical Profiling)**

Our work addressing [NOAA ACC Activity 2]{.underline} (Regular Vertical
Profiling of Aerosols) was conducted in Years 2 and 3. This work was led
by Co-PI Beat Schmid (see, e.g., *Schmid*, 2003; *Schmid and Russell,*
2004a, *Schmid et al.,* 2004b, 2005a,b, 2006). It focused on producing
concepts and designs that address the issues of marrying smaller
aircraft with smaller, lighter, lower-power, automated instruments. Our
concepts include instruments that are both smaller and more capable than
our current instrument, AATS-14. Concepts are described in Section 3.1.
To evaluate and increase the feasibility of the various concepts, we
developed (jointly with NASA Ames funding) a ground-based prototype that
is described in Section 3.2.

**3.1 Concepts and Conceptual Designs**

The existing Ames airborne sunphotometers have solved many experimental
challenges to airborne measurements of the direct solar beam. For
example, motors controlled by sun-sensing detectors maintain solar
pointing accuracies of a few tenths of a degree in spite of aircraft
maneuvers, turbulence, and aerodynamic drag. Feedback-controlled heaters
have prevented window fogging and maintained detector temperatures in a
range of a few degrees or less over wide excursions in ambient
temperature. Pointing the window inward ("parking the head") has
prevented dirt deposition during flight legs in salt spray and clouds.
Weather shields and seals have protected the instruments sufficiently to
permit measurements both before and after flight legs through clouds and
rain.

Our miniaturization efforts under NOAA Grant GC04-396 have explored the
use of optical fibers with direct solar beam optics as a means of
significantly reducing the size of both the instrument and the port
required for its installation. Reducing instrument size and weight will
yield easier, more cost-efficient integration of the instrument onto
smaller aircraft. Our efforts to enhance instrument capabilities have
investigated the ability to perform airborne sky radiance measurements
to derive aerosol size distribution and absorption much like the
retrieval of those parameters from ground-based AERONET
sun/sky-radiometers. Making such sky-scanning measurements with an
airborne instrument presents additional challenges, mostly related to
the great brightness of the Sun relative to the sky. This requires a
large dynamic range of the detection system (up to 6 decades) and very
efficient stray light rejection for skylight measurements close to the
Sun. Conventional methods to exclude that stray light have, in
surface-based instruments, entailed long protrusions in front of the
detector, which may be incompatible with accurate sun-tracking under the
aerodynamic loads of aircraft flight.

All the concepts we have explored in this work provide spectrally
continuous measurements by using a spectrometer (rather than using
optical filters at discrete wavelengths as done currently with AATS-14).
Measuring a continuous spectrum with a spectrometer will yield more
information and better wavelength accuracy. This in turn will lead to
more accurate retrievals of water vapor and ozone, and allow measurement
of gases not attainable with the AATS instruments such as nitrogen
dioxide and potentially others.

Figure 1 shows a conceptual design that incorporates an optical fiber
with direct solar beam optics and a spectrometer in a housing (can) just
below the tracking dome. Figure 2 compares the size of this conceptual
design to the existing AATS-14, showing that both the tracking dome and
the electronics/spectrometer can of the new concept are smaller than for
AATS-14. These smaller dimensions would allow reductions in both the
size of the mounting port in the aircraft skin and the volume of
in-cabin space needed.

Capability enhancements of the direct-beam spectrometer shown in Figs. 1
and 2 over the existing AATS-14 would be (1) the ability to measure
trace gases not possible with AATS-14, such as SO~2~ and NO~2~, plus
improved accuracy of our current H~2~O and O~3~ measurements and (2)
improved aerosol optical depth measurements via improved aerosol-gas
separation. Advantage (2) is dependent on instrument calibration
stability comparable to that of AATS-14, i.e., stability to \~1% over
periods between high-altitude calibrations (which are typically several
months apart). The increased trace-gas measurement capability of
continuous spectra (i.e., spectrometry) has been demonstrated by many
NASA space and air instruments, but never in a compact airborne
instrument also capable of the well calibrated transmission measurements
needed for aerosol studies.

Appendix A provides additional drawings showing different aspects of
this direct-beam spectrometer concept.

Figure 3 shows another direct-beam spectrometer concept that houses two
spectrometers in a cabin-mounted rack. The rack-mounted spectrometers
are linked to the tracking dome by fiber optics and electronic cables.
As shown by Figure 3, mounting the spectrometers in a rack instead of
the instrument head allows the head to be even smaller than shown in
Figure 2. Appendix B provides additional drawings of the direct-beam
instrument concept with rack-mounted spectrometers.

A still more capable concept adds the ability to measure diffuse
skylight intensity, including its dependence on angular distance from
the sun. Adding skylight (scattered sunlight) measurements would enable
determination of the size of larger particles (e.g., mineral dust) than
is possible with the direct-beam transmission measurements by AATS-14,
plus the ability to retrieve aerosol type, composition and absorption
information via retrievals of complex refractive index and shape. The
increased information content of scattered sunlight measurements has
been demonstrated by NASA's AERONET program, but never on an airborne
instrument. Figure 4 shows a concept for the airborne sun-sky
spectrometer.

We have named the envisioned instrument 4STAR, for Spectrometer for
Sky-Scanning, Sun-Tracking Atmospheric Research. The primary unique
feature of the envisioned airborne instrument (4STAR-Air) will be its
combination, in a relatively small, rugged, flyable device, of a
spectrometer with sun-tracking and sky-scanning motors and entrance
ports, all linked in a configuration with the calibration stability
required for aerosol transmission measurements. Feasibility of 4STAR
depends on overcoming the following technological hurdles:

1\. Maintaining calibration (output counts per input radiance or
irradiance) to 1% stability over a period of months.

2\. Demonstrating stray light rejection to permit measuring skylight
within a few degrees of the sun.

3\. Devising a fiber optic coupling that maintains 1% calibration
stability with as many as possible of the following desirable
charactertics: detachable during assembly before calibration; detachable
between calibration and scientific measurements; rotatable during
measurements.

As a tool for investigating ways to overcome these hurdles we have
developed a ground-based prototype, 4STAR-Ground, that is described
below.

**3.2 Ground-based Prototype** **(4STAR-Ground)**

4STAR-Ground, shown in Figure 5, was developed with joint funding from
NOAA Grant GC04-396, the NASA Ames Director's Discretionary Fund
\[*Schmid*, 2003; *Schmid et al.,* 2004b, 2005\], and the Ames
Instrument Working Group. The purpose of this ground prototype is to
provide a low-cost functional test bed for addressing the technical
challenges of developing the airborne instrument described in Section
3.1 above. As shown in Figure 5, the prototype has azimuth and elevation
motors and a quad sensor for accurate solar tracking or sky scanning. A
Y-shaped fiber optics cable leads from two separate entrance optics (one
for direct solar beam, one for sky radiance) to a rack-based Zeiss
spectrometer. The rack also houses all non-moving parts such as power
supply, motion controls, data acquisition PC, etc. Figure 6 shows a
cutaway drawing of the dual-barrel entrance port.

The heart of the prototype is a spectrometer shown in Figure 7, the
Zeiss MCS with Tec5 Electronics. Its detector is a cooled Hamamatsu CCD
S7031-1006 with 1044x64 pixels. Spectral range is 220-1000 nm. Spectral
width of each CCD pixel is \~0.8 nm. Resolution is 1.7 - 2.5 nm.

We have used the ground prototype to demonstrate overcoming hurdle #2
above. Examples of data measured by 4STAR-Ground are shown in Figure
8.Tests and modifications to demonstrate #1 and #3 are currently in
progress. Figure 9 demonstrates the ability of the 4STAR-Ground
direct-beam channel to provide output that tracks analogous output from
AATS when both are tracking the sun through a varying atmosphere. Figure
10 shows a typical setup for acquiring such AATS-4STAR comparison data.
Figure 11 shows measurements from the 4STAR-Ground direct-beam channel,
scanning across the sun in elevation (principal plane) and azimuth
(almucantar scan) to measure the channel field of view (FOV) and
demonstrate that, near the FOV-center tracking point, channel output
varies by \<= 2% per degree, which is the performance criterion we
require for AATS. Figure 12 shows analogous data for the diffuse sky
channel. Figure 10 shows 4STAR-Ground output counts for four wavelengths
during an elevation scan that combines data from diffuse (sky) and
direct (sun) channels.

With the completion of NOAA Grant GC04-396, we are continuing to pursue
4STAR development through collaborative efforts at Ames and PNNL (Dr.
Schmid's new employer), using new funding from NASA Headquarters and
PNNL internal funds.

**References**

**\*Presentation or publication supported wholly or partly by funding
for this grant**

**(**excludes J-31 workshop presentations posted at
<http://geo.arc.nasa.gov/sgg/INTEX/presentations/J31WkspPowerPoints/J31Wksp.html>)

\*Fehsenfeld, F.C., G. Ancellet, T.S. Bates, A.H. Goldstein, R.M.
Hardesty, R. Honrath, K.S. Law, A.C. Lewis, R. Leaitch, S. McKeen, J.
Meagher, D.D. Parrish, A.A.P. Pszenny, P.B. Russell, H. Schlager, J.
Seinfeld, M. Trainer, R. Talbot, R. Zbinden, International Consortium
for Atmospheric Research on Transport and Transformation (ICARTT): North
America to Europe: Overview of the 2004 summer field study, *J. Geophys.
Res.,* VOL. 111, D23S01, doi:10.1029/2006JD007829, 2006.

\*Ferrare, R., C. Butler , E. Browell, J. Hair, V. Brackett, M. Fenn, A.
Notari, S. Kooi, S. Ismail, M. Clayton, P. Russell, J. Redemann, J.
Livingston, B. Schmid, G. Chen, A. Clarke, E. Eloranta, DIAL Aerosol
Measurements During INTEX-NA, *ICARTT Data Workshop*., Durham NH, 9-12
August 2005.

\*Kahn, R., J. Livingston, C. McNaughton, P. Quinn, J. Redemann, P.
Russell, and the MISR, AATS, Ron Brown, and SSFR teams, INTEX-NA
Campaign\--Aerosol Retrieval Validation and Regional Aerosol Patterns,
INTEX-A data workshop, Virginia Beach, VA, March 28 - April 1, 2005.

Lee, Y.S., Y. H. Kim, Y. Yi and J. Kim, A baffle design for an airglow
photometer on board the Korea Sounding Rocket-III*. J. Korean Astron.
Soc., 33*, 165-172, 2000.

\*Livingston, J., J. Redemann, B. Schmid, P. Russell, J. Eilers, P.
Pilewskie, J. Pommier, E. Browell, J. Hair, A. Chu, R. Kahn, M.
Wendisch, M. Hardesty, C. Senff, Airborne Sunphotometer and Related
Measurements Acquired over the Gulf of Maine During INTEX-ITCT 2004,
*Eos Trans. AGU, 85*(47), Fall Meet. Suppl., Abstract A33D-0110, 2004.

\*Livingston, J. M, J. Redemann, B. Schmid, P. Russell, J. Eilers, R.
Kahn, A. Chu, Coincident airborne sunphotometer and satellite aerosol
optical depth measurements during INTEX/ITCT 2004, *European Geosciences
Union*, General Assembly 2005, Vienna, Austria , 24 - 29 April 2005.

\*Livingston, J., B. Schmid, J. Redemann, P. Russell, S. Ramirez, J.
Eilers, W. Gore, S. Howard, J. Pommier, E. Fetzer, S. Seeman, E. Borbas,
D. Wolfe, Comparison of Water Vapor Measurements by Airborne
Sunphotometer and Near-Coincident In Situ and Satellite Sensors during
INTEX-ITCT 2004, *J. Geophys. Res.,* INTEX-A special section, submitted
June 2006.

\*McNaughton, Cameron, Steven Howell, Antony Clarke, Ralph Kahn, Philip
Russell, John Livingston, Beat Schmid, Jens Redemann, Patricia Quinn,
Timothy Bates, Aerosol optical properties and microphysics from the NASA
DC-8, J-31 and R/V Ron Brown compared to satellite retrievals by MISR
during INTEX-A, *ICARTT Data Workshop*., Durham NH, 9-12 August 2005d.

\*Pilewskie, P., O. Hoffman, B. Kindel1, W. Gore, P.Russell, J.
Redemann, J. Livingston, J. Daniel, H. Stark, T. Garrett., The radiative
properties of clouds derived from solar spectral irradiance during
ICARTT, *ICARTT Data Workshop*., Durham NH, 9-12 August 2005.

\*Redemann, J., P. Pilewskie, J. Livingston, S. Howard, P. Russell,
B. Schmid, J. Eilers, M. Wendisch, Airborne measurements of spectral
direct aerosol radiative forcing - a new aerosol gradient method applied
to data collected in INTEX/ITCT, 2004, *INTEX-A data workshop*, Virginia
Beach, VA, March 28 - April 1, 2005a.

\*Redemann J., P. Pilewskie, J. Livingston, S. Howard, P. Russell, B.
Schmid, J. Eilers, M. Wendisch. Airborne measurements of direct aerosol
radiative forcing in INTEX/ITCT, 2004. *European Geosciences Union*,
General Assembly 2005, Vienna , Austria , 24 - 29 April 2005b.

\*Redemann J., P. Pilewskie, J. Livingston, S. Howard, P. Russell, B.
Schmid, J. Eilers, M. Wendisch. Airborne measurements of spectral direct
aerosol radiative forcing in INTEX/ITCT, 2004. *International
Association of Meteorology and Atmospheric Sciences*, Beijing, China,
August 2-11, 2005c.

\*Redemann J., S. Howard, B. Schmid, J. Pommier, P. Pilewskie, P.
Russell, J. Eilers, J. Livingston, M. Wendisch. Airborne measurements of
spectral direct aerosol radiative forcing - a new aerosol gradient
method applied to data collected in INTEX/ITCT/ICARTT, 2004, *ICARTT
Data Workshop*., Durham NH, 9-12 August 2005d.

\*Redemann J., S. Howard, B. Schmid, J. Pommier, P. Pilewskie, P.
Russell, J. Eilers, J. Livingston, M. Wendisch. Airborne measurements of
spectral direct aerosol radiative forcing - a new aerosol gradient
method applied to data collected in INTEX/ITCT/ICARTT, 2004, *Am. Assn.
Aerosol. Research. Annual Conf*., Austin, TX, 17-21 Oct 2005e.

\*Redemann J., S. Howard, B. Schmid, J. Pommier, P. Pilewskie, P.
Russell, J. Eilers, J. Livingston, M. Wendisch. Spectral direct aerosol
radiative forcing determined by airborne measurements in
INTEX/ITCT/ICARTT, 2004, Invited talk, *Fall Meeting, Amer. Geophys.
Union.*., San Francisco, CA, 5-9 December 2005f.

\*Redemann, J., P. Pilewskie, P. B. Russell, J. M. Livingston, S.
Howard, B. Schmid, J. Pommier, W. Gore, J. Eilers, and M. Wendisch
(2006), Airborne measurements of spectral direct aerosol radiative
forcing in the Intercontinental Chemical Transport
Experiment/Intercontinental Transport and Chemical Transformation of
anthropogenic pollution, 2004, J. Geophys. Res., 111, D14210,
doi:10.1029/2005JD006812.

\*Russell, P., Jetstream-31 in INTEX-NENA: Measurements & Analyses of
Aerosol Radiative Properties and Effects, *ICARTT Study Planning Team
Meeting*, Boulder, CO, 21-22 January 2004. Posted at
[http://geo.arc.nasa.gov/sgg/INTEX/.]{.underline}

\*Russell, P., and T. Bates, Working Group Report on Aerosol Direct
Radiative Effects in ICARTT, *ICARTT Planning Workshop*, Orlando, FL,
27-29 April 2004.

\*Russell, P., & The J31 Team, Jetstream 31 (J31) in INTEX-ITCT:
Measurements of Aerosol, Radiative Properties and Effects, *ICARTT
Science Meeting*, Portsmouth, NH, 19 Jul 2004a. Posted at
[http://geo.arc.nasa.gov/sgg/INTEX/.]{.underline}

\*Russell, P., P. Pilewskie, J. Redemann, M. Wendisch, J. Livingston, J.
Eilers, B. Schmid, A. Chu, & The Rest Of The J31 Team, Jetstream 31
(J31) at Mid-Campaign in INTEX-ITCT: Measurements of Aerosol, Cloud, and
Surface Radiative Properties and Effects, *ICARTT Mid-Campaign Science
Meeting*, Durham, NH, 24 Jul 2004b. Posted at
[http://geo.arc.nasa.gov/sgg/INTEX/.]{.underline}

\*Russell, P., P. Pilewskie, J. Redemann, M. Wendisch, J. Livingston, J.
Eilers, B. Schmid, A. Chu, & The Rest Of The J31 Team, Jetstream 31
(J31) in INTEX-ITCT: Measurements of Aerosol, Cloud, and Surface
Radiative Properties and Effects, NASA *Ames Earth Science Seminar*,
Moffett Field, CA, 7 October 2004c. Posted at
<http://geo.arc.nasa.gov/sgg/INTEX/>[.]{.underline}

\*Russell, P., P. Pilewskie, J. Livingston, , J. Redemann, B. Schmid, W.
Gore, J. Eilers, G. Feingold, R. Kahn, A. Chu, M. Wendisch, P. Quinn, C.
Senff, O. Cooper, A. Stohl, C. McNaughton, A. Clarke, E. Browell, S.
Ramirez The J31 in INTEX-A / ITCT / ICARTT: Measurements of Aerosol,
Cloud, Water Vapor, and Surface Radiative Properties and Effects,
*INTEX-A data workshop*, Virginia Beach, VA, March 28 - April 1, 2005a.

\*Russell, P. B., P. Pilewskie, J. Redemann, J. Livingston, B. Schmid,
R. Kahn, A. Chu, J. Eilers, J. Pommier, S. Howard, Airborne
sunphotometer and solar spectral flux radiometer measurements during
INTEX/ITCT 2004, *Atmospheric Sciences and Air Quality Conference*,
American Meteorological Society, San Francisco, CA 27-29 April 2005b.

\*Russell, P. B., P. Pilewskie, J. Redemann, J. Livingston, B. Schmid,
R. Kahn, A. Chu, W. Gore, J. Eilers, J. Pommier, S. Howard, Aerosol,
cloud, water vapor, and sea surface radiative properties and effects
measured by airborne sunphotometer and solar spectral flux radiometer
off New England in summer 2004, *Gordon Conference on Radiation and
Climate*, Waterville, Maine, 24-29 July 2005c.

\*Russell, P. B., P. Pilewskie, J. Redemann, J. Livingston, B. Schmid,
R. Kahn, A. Chu, W. Gore, J. Eilers, J. Pommier, S. Howard, Aerosol,
cloud, water vapor, and sea surface radiative properties and effects
measured by airborne sunphotometer and solar spectral flux radiometer
off New England in summer 2004, *ICARTT Data Analysis Workshop*, Durham,
New Hampshire, 9-12 August 2005d.

\*Russell, P. B., J. Livingston, P. Pilewskie, J. Redemann, B. Schmid,
R. Kahn, A. Chu, W. Gore, J. Eilers, J. Pommier, S. Howard, Measurements
of aerosol radiative properties and effects using airborne sunphotometer
and solar spectral flux radiometer in ICARTT 2004, *Am. Assn. Aerosol.
Research. Annual Conf*., Austin, TX, 17-21 Oct 2005e.

\*Russell, P., and B. Schmid, Partnership for Airborne Sun-Sky
Spectrometry, Proposal to Innovative Partnerships Program Office, NASA
Ames, in response to Partnership Seed Fund Call for Proposals---2006,
submitted September 2006.

\*Russell, P. B., J. M. Livingston, J. Redemann, B. Schmid, S. A.
Ramirez, J. Eilers, R. Kahn, A. Chu, L. Remer, P. K. Quinn, M. J. Rood,
W. Wang, Multi-grid-cell validation of satellite aerosol property
retrievals in INTEX/ITCT/ICARTT 2004, *J. Geophys. Res*., INTEX-A
special section, accepted November 2006.

Schmid, B., Towards an Autonomous Airborne Sun/Sky Scanning
Spectrometer, Proposal to NASA Ames Director's Discretionary Fund (DDF),
1 December 2003.

Schmid, B. (BAER PI), and P. Russell (NASA Ames PI), Collaborative
Research: An Autonomous Airborne Direct Solar Beam Spectrometer for
HIAPER, Proposal Numbers 0425910 and 0425890 to National Science
Foundation Announcement NSF 04-517, submitted 17 February 2004a.

Schmid, B., T. Meriam, J. Redemann, and P. Russell, Towards an
Autonomous Airborne Sun/Sky Scanning Spectrometer, Interim Report for
NASA Ames Director's Discretionary Fund (DDF) Project, November 2004b.

Schmid, B., N. Truong, T. Meriam, P. Russell, and J. Redemann, Towards
an Autonomous Airborne Sun/Sky Scanning Spectrometer, poster presented
at NASA Ames Director's Discretionary Fund (DDF) poster show, March
2005a

Schmid, B., Autonomous Airborne Sun/Sky-Scanning Spectrometer, Ames
Instrumentation Working Group, Proposed Instrument Concept Form,
September 2005b.

Schmid, B., N. Truong, R. Johnson, P. Russell, and J. Redemann,
Autonomous Airborne Sun/Sky-Scanning Spectrometer, Ames Instrumentation
Working Group, Request for funds, May 2006.

\*Singh, H. B., W. H. Brune, J. H. Crawford, D. J. Jacob, and P. B.
Russell, Overview of the Summer 2004 Intercontinental Chemical Transport
Experiment-North America (INTEX-A), *J. Geophys. Res.,* VOL. 111,
D24S01, doi:10.1029/2006JD007905, 2006.

**Table 1.** J-31 Science Flights out of Portsmouth, NH, July-August
2004, in ITCT/INTEX/ICARTT

+---+----+-------------+-----------------------------------------------+
| * | ** | **Track**   | **Comments**                                  |
| * | Da |             |                                               |
| F | te | (G of M =   |                                               |
| l | ** |             |                                               |
| t |    | Gulf of     |                                               |
| \ |    | Maine)      |                                               |
| # |    |             |                                               |
| * |    |             |                                               |
| * |    |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 7 | 12 | Western G   | Aqua underflight with profiles, Ron Brown     |
|   | J  | of M        | rendezvous                                    |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 8 | 15 | Western G   | Stratus clouds with Ron Brown & Terra,        |
|   | J  | of M        | profiles with 2 RB sondes and Aqua            |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 9 | 16 | Western &   | Profiles and near-surface leg with Aqua &     |
|   | J  | Central G   | near Ron Brown & its sonde                    |
|   | ul | of M        |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 17 | Western G   | Profiles and near-surface leg with Terra &    |
| 0 | J  | of M        | near Ron Brown                                |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 20 | Western G   | Stratus clouds with Ron Brown & Terra (MISR   |
| 1 | J  | of M        | local mode).                                  |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 21 | Western G   | Aerosol profiles and near-surface leg with    |
| 2 | J  | of M        | Aqua. Aerosol gradient at several altitudes.  |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 22 | Western G   | Aerosol profiles and near-surface leg with    |
| 3 | J  | of M        | Ron Brown, DC-8, & Terra (MISR local mode).   |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
|   | 23 | Western G   | Aqua underflight with profiles                |
|   | J  | of M        |                                               |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 26 | Western G   | Water vapor retrievals under cirrus in legs   |
| 5 | J  | of M        | and spirals near Ron Brown & its sonde        |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 29 | Western G   | Terra MISR local mode, Ron Brown with 2 RB    |
| 6 | J  | of M        | sondes, and Aqua. Aerosol gradient 2 ways.    |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 29 | Western G   | Langley calibration at sunset.                |
| 7 | J  | of M        |                                               |
|   | ul |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 31 | Western to  | Terra MISR local mode, Aqua, Ron Brown with   |
| 8 | J  | Eastern G   | sonde, and DC-3. Stratus to east of cirrus.   |
|   | ul | of M &      | Water vapor under cirrus.                     |
|   |    | Atlantic    |                                               |
+---+----+-------------+-----------------------------------------------+
| 1 | 2  | Western G   | Terra overpass. Profiles and aerosol gradient |
| 9 | A  | of M        | 2 ways.                                       |
|   | ug |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 2 | 2  | Western G   | Profile with DC-8, Ron Brown, & its sonde.    |
| 0 | A  | of M        | Langley calibration at sunset.                |
|   | ug |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 2 | 3  | Western G   | Underfly DC-3 lidar at 2 altitudes. Spiral    |
| 1 | A  | of M        | near Ron Brown & its sonde                    |
|   | ug |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 2 | 7  | Central G   | Terra MISR local mode in coordination with    |
| 2 | A  | of M        | DC-8 and Ron Brown, & its sonde               |
|   | ug |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 2 | 7  | Western G   | 2 spirals over Ron Brown, & its sonde.        |
| 3 | A  | of M        | Langley calibration at sunset.                |
|   | ug |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 2 | 8  | Central G   | Aqua, very clean and cloud free conditions.   |
| 4 | A  | of M        | Transect to 68 W.                             |
|   | ug |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 2 | 8  | n/a         | Aborted on runway                             |
| 5 | A  |             |                                               |
|   | ug |             |                                               |
+---+----+-------------+-----------------------------------------------+
| 2 | 8  | Central G   | Langley calibration at sunset                 |
| 6 | A  | of M        |                                               |
|   | ug |             |                                               |
+---+----+-------------+-----------------------------------------------+

![](media/image1.jpeg){width="4.996456692913386in"
height="3.8618110236220473in"}

![](media/image2.jpeg){width="4.090157480314961in"
height="4.095275590551181in"}

Figure 1. A concept for a direct-beam spectrometer applicable to Regular
Vertical Profiling of aerosols \[*Schmid*, 2003\].

![](media/image3.png){width="6.470138888888889in"
height="5.488194444444445in"}

Figure 2. Dimensional comparison of the direct-beam spectrometer concept
and the existing AATS-14. Dimensions are in inches.

![](media/image4.jpeg){width="4.245138888888889in"
height="3.6256944444444446in"}

![](media/image6.png){width="6.521527777777778in"
height="5.100694444444445in"}

Figure 3. A direct-beam instrument concept with spectrometers in a
cabin-mounted rack \[*Schmid and Russell,* 2004\]. Dimensions are in
inches.

![](media/image7.jpeg){width="1.7728346456692914in"
height="0.9362204724409449in"}![](media/image8.jpeg){width="2.038976377952756in"
height="0.9338582677165355in"}![](media/image4.jpeg){width="3.8125984251968505in"
height="2.848031496062992in"}

Figure 4. A concept for a sun-sky spectrometer applicable to Regular
Vertical Profiling of aerosols. To permit both sun and sky measurements,
the entrance port would have dual baffled tubes similar to those used in
the ground prototype, with an attenuator in the direct-sun tube.

![](media/image9.jpeg){width="5.252777777777778in"
height="7.004166666666666in"}

Figure 5. Photo of the ground prototype Spectrometer for Sky-Scanning,
Sun Tracking Atmospheric Research (4STAR-Ground).

![](media/image10.png){width="6.845833333333333in"
height="5.417361111111111in"}

**Figure 6.** Dual-barrel entrance port for 4STAR-Ground. Design adapted
from Lee, Y.S., Y. H. Kim, Y. Yi and J. Kim, A baffle design for an
airglow photometer on board the Korea Sounding Rocket-III*. J. Korean
Astron. Soc., 33*, 165-172, 2000

![](media/image11.png){width="2.895138888888889in"
height="3.1881944444444446in"} ![](media/image12.png){width="3.9125in"
height="3.2069444444444444in"}

Figure 7. (a) Photo of the Zeiss MCS spectrometer and Tec5 electronics
used in 4STAR-Ground.

\(b\) Spectrometer layout diagram. \[*Schmid et al.,* 2005a\]

![](media/image13.wmf){width="3.5846456692913384in"
height="2.6208661417322836in"}![](media/image14.wmf){width="3.3610236220472443in"
height="2.4779527559055117in"}**Figure 8.** Data measured by ground
prototype: (a) Transmitted sunlight spectrum. (b) Skylight angular scan.

![](media/image15.wmf){width="4.55625in" height="3.6597222222222223in"}

**Figure 9**. Time series of concurrent outputs for AATS-14 (thin
traces) and 4STAR-Ground (thick traces), both tracking the sun through a
time-varying atmosphere during a 2.75-h period. 4STAR-Ground direct-beam
spectrometer outputs are shown at 11 wavelengths chosen to match AATS-14
channel filter wavelengths. 4STAR-Ground data are normalized to match
the AATS-14 data in average over the 2.6-h period.

![](media/image16.jpeg){width="5.928472222222222in"
height="4.866666666666666in"}

**Figure 10.** A typical setup for acquiring AATS-4STAR comparison data
like those shown in Figure 9. AATS-14 is on left, 4STAR-Ground on right,
both tracking the sun.

![](media/image17.png){width="3.470138888888889in"
height="2.8513888888888888in"}![](media/image18.png){width="3.520138888888889in"
height="2.84375in"}

**Figure 11.** Measurements using the 4STAR-Ground direct-beam channel,
scanning across the sun in elevation (principal plane, left column) and
azimuth (almucantar scan, right column) to measure the full width at
half maximum (FWHM) of the channel field of view (FOV) and to
demonstrate that, near the FOV-center tracking point, channel output
varies by 2% per degree. Diagonal lines in each frame run from the
autotracking point (near FOV center) to the first point of the scan
(zero counts near left edge of frame), and from the last point of the
scan (zero counts near right edge of frame) to the new autotracking
point (which differs from the first autotracking point because of sun
movement during the scan).

![](media/image19.png){width="3.5in" height="2.8541666666666665in"}
![](media/image20.png){width="3.5in" height="2.8340277777777776in"}
**Figure 12.** As in Figure 11, but for 4STAR-Ground diffuse sky
channel. Note that, by design, the diffuse sky channel has FWHM narrower
than the direct-beam channel.

![](media/image21.wmf){width="6.995138888888889in"
height="5.860416666666667in"}

**Figure 13.** 4STAR-Ground output counts for four wavelengths during an
elevation scan that combines data from diffuse (sky) and direct (sun)
channels. At \~48º elevation, data switch from diffuse channel to direct
channel; at \~53º data switch back from direct to diffuse.

**APPENDIX A**

**Additional Concept Drawings for an Airborne Direct Beam Spectrometer**

![](media/image1.jpeg){width="6.482677165354331in"
height="5.00984251968504in"}

![](media/image22.jpeg){width="4.820078740157481in"
height="3.7236220472440946in"}

![](media/image23.jpeg){width="5.85in" height="4.3527559055118115in"}

![](media/image24.jpeg){width="4.9633858267716535in"
height="4.609055118110236in"}

![](media/image25.jpeg){width="5.85in" height="4.081889763779528in"}

![](media/image26.jpeg){width="4.346456692913386in"
height="2.902755905511811in"}![](media/image27.png){width="5.7in"
height="3.8652777777777776in"}

![](media/image28.jpeg){width="4.604330708661418in"
height="4.179921259842519in"}

![](media/image29.jpeg){width="5.5236220472440944in"
height="3.9354330708661416in"}

![](media/image30.jpeg){width="5.52992125984252in"
height="3.8716535433070867in"}

![](media/image31.jpeg){width="5.970866141732284in"
height="5.903543307086614in"}

**APPENDIX B**

**Concept for an Airborne Direct-Beam Spectrometer Using Cabin-Rack
Mounting of Two Spectrometers**

\[Excerpts from the proposal by *Schmid and Russell,* 2004\]

# Project Summary

A major uncertainty in predicting possible future changes to the Earth
system in general, and its climate in particular, stems from the
necessary inclusion of atmospheric aerosols in climate models. In fact
recent modeling studies claim that tropospheric aerosols are so
important that they may hold the key to combating global warming,
suggesting that reducing the emission of light-absorbing aerosol into
the Earth's atmosphere may be the most feasible way to slow global
warming. However other recent modeling studies seem to, at least in
part, contradict that notion. The current low confidence in the
estimates of aerosol induced perturbations of the Earth's radiation
balance is caused by the highly non-uniform compositional, spatial and
temporal distribution of tropospheric aerosols owing to their
heterogeneous sources and short lifetimes.

To monitor the distribution of aerosols globally requires the
combination of continuous observations from satellites, networks of
ground-based instruments, and dedicated field experiments.

The use of the NASA Ames airborne tracking sunphotometers (AATS) in many
such dedicated campaigns has proven to be a unique link between
space-based retrievals of aerosols (plus also O~3~ and H~2~O) and a
diversity of suborbital measurements. A 6-wavelength AATS made its first
flights in 1985 and was retired in 2003. A 14-channel instrument
(AATS-14) was completed and first flown in 1996. It is now the only
instrument in the world capable of continuous airborne measurements of
aerosol optical depth, H~2~O and O~3~ columns. However, due to its size,
integration on HIAPER would require modification of the airframe, which
is not encouraged under the HIAPER Aircraft Instrumentation
solicitation.

The PI's of this collaborative proposal between Bay Area Environmental
Research Institute and NASA Ames Research Center propose to build a
compact solar spectrometer capable of autonomous transmission
measurements of the direct solar beam from the HIAPER aircraft. Compared
to AATS-14 the proposed instrument will have increased observational
capabilities (continuous spectrum versus discrete wavelengths) yet be
considerably smaller and lighter.

The proposed instrument has been designed specifically for integration
and safe, economical and fully autonomous operation aboard HIAPER. The
design has been iterated with NCAR personnel. As with the existing AATS
instruments the sun tracking head of the instrument is mounted external
to the aircraft skin. However, light is not detected by diodes in the
tracking head but guided by optical fibers into two rack-mounted
spectrometers.

The intellectual merit of this research is demonstrated by the
uniqueness of the proposed instrument and the utility of the proposed
measurements. Science quantities the proposed instrument will deliver
are: Direct solar beam transmittance (at wavelengths 0.35 -- 1.7 m),
direct solar beam irradiance (W/m^2^/m, 0.35 -- 1.7 m), aerosol optical
depth (0.35 -- 1.7 m, outside gaseous absorption bands only), and the
columnar amounts of H~2~O, O~3~, and NO~2~. If the aircraft performs
vertical profiles these quantities can be differentiated with respect to
altitude to obtain profiles of aerosol extinction (0.35 -- 1.7 m,
outside gaseous absorption bands only), H~2~O, O~3~, and potentially
NO~2~ densities. As demonstrated with the existing AATS's, the proposed
instrument will deliver valuable data throughout the vertical range of
the HIAPER aircraft, from the lower troposphere to the upper troposphere
and lower stratosphere.

The broader impact of the proposed activity is that a type of instrument
of which currently only one unit is available worldwide will become
available to the broader science community (scientists, graduate and
undergraduate students). Compared to the current situation where a
single research group operates one instrument, integration of the
proposed instrument into the HIAPER operation will significantly
increase the number of missions where airborne direct beam transmission
measurements will be performed. This will yield a much richer global
data set on aerosols and radiatively important gases providing
significantly more opportunities to link space-based and suborbital
retrievals and to assess closure amongst suborbital measurements. This
in turn will lead to a better understanding of atmospheric aerosols and
their effect on radiation, which is sorely needed to understand and
potentially combat global warming.

\[Segments were skipped here.\]

**3.1 Instrument Design**

The design has been iterated with NCAR personnel ... and takes into
account the considerations/ constraints listed in the HIAPER
solicitation and experimenter handbook, such as instrument volume, power
consumption, safe, economical and autonomous operation, allowed
materials, aircraft speed, vertical and horizontal range. As with the
existing AATS instruments the tracking head of the instrument is mounted
external to the aircraft skin (Figure 9). Potential mounting locations
on HIAPER are shown in Figure 10. Unlike with the AATS instruments light
is not detected by diodes in the head but collected through a quartz
window and entrance optics which is attached to a bundle of optical
fibers (Figure 11). The near-IR enhanced fibers guide the light from the
tracking head into two rack-mounted spectrometers (Figure 12).

![](media/image4.jpeg){width="4.245138888888889in"
height="3.6256944444444446in"}

![](media/image5.png){width="2.292913385826772in"
height="3.8751968503937007in"}Figure 9. Partially sectioned model (left)
and dimensional drawing (right) of instrument head. Dimensions are in
inches.

![](media/image32.png){width="6.125694444444444in"
height="8.558333333333334in"}

![](media/image6.png){width="6.521527777777778in"
height="5.100694444444445in"}

![](media/image33.png){width="5.595833333333333in"
height="2.7618055555555556in"}

Figure 12. Schematic of instrument integrated on Hiaper in pad #3. Two
alternate mountings (corresponding to pads 1&2 or 4&5) are shown as
well.

Azimuth and elevation motors point the entrance optics at the sun using
tracking-error signals from a quad-cell photodiode. Position feedback
signals and power are transported through cables to the rack-mounted
power supply, motor amplifiers, motion-control unit and data acquisition
computer (Figure 12).

**3.2 Integration on HIAPER**

The instrument head is designed so it can be mounted in any of the 7 by
10" inlet aperture pads on the upper fuselage numbered 1 through 5 (see
Figure 10). If mounted in pads #1 or #2, #4 or #5 the instrument is
oriented tilted with respect to vertical (see Figure 12) however this
does not impact its operation. The instrument head does not extend into
the cabin (see Figure 12) as virtually all of its volume is absorbed
into the space between the outer aircraft skin and the inner envelope
defined by the aircraft's frames, panels, etc.

If mounted as shown in Figure 9, the instrument head (top of dome)
sticks out by about 10.85" from the aircraft skin which results in a
frontal surface area of 0.77 ft^2^ . Aircraft speed of Mach 0.61 at sea
level, an aerodynamic load factor of 1.25, skin friction drag and
separate drag force coefficients for the dome and the fairing yields a
drag force of 253 lbs and a lift force of 100 lbs. The drag force could
be made smaller by mounting the instrument lower and shortening the
vertical extent of the fairing shown in Figure 9. This lowered mounting
would be possible in pads that are offset form the centerline (#1, #2,
#4 and #5), but likely not in pad #3 without some obstruction in the
cabin. Discussion with the HIAPER Aeronautical Engineer ... indicated
that a drag force of 253 lbs would be acceptable. A strength analysis
for the drag effect (using FEA with the drag force distributed evenly
over the surface) shows that the design is structurally sound. Maximum
stress of 14 kpsi occurs at the mounting-hole locations. Using Aluminum
7075-T7351 (as recommended in the HIAPER Experimenters Handbook) will
provide a safety factor of more than 5.

**3.3 Pressure Seals**

O-ring seals between stationary surfaces and rotary seals between moving
surfaces will hold more than any relevant atmospheric pressure
differential between the inside of the instrument head and the outside
of the aircraft. The rotary seal and motor combination is specified to
operate at the required precision up to a pressure differential of 1013
hPa and over a temperature range of -90°C to +50°C. The inside of the
tracking head is also sealed from the cabin by passing the optical fiber
and the cables through customized sealed connectors. This will prevent
cabin air from leaking into the instrument.

The seals will also prevent moisture from entering the instrument head
where it could condense on the optical entrance window. As an additional
measure the optical window will be heated by a heater foil. Finally, the
head area can also be purged with zero air or nitrogen between flights
to remove residual moisture. Compared to our current AATS-14 instrument
the seals specified for the new design should be more effective and
hence infrequent purging should be sufficient.

**3.4 Automatic Window Cleaning**

Although AATS-14 can be operated autonomously, the above mentioned
"parking the head" to avoid dirt deposition on the optical window
requires an operator. In the proposed design the optical window covering
the apertures of the entrance optics and the quad-sensor comes to rest
on a slanted pedestal when the tracking head is brought into its "park"
position (Figure 13). We envision a design and realization whereby
retractable brushes (using solenoids) clean the optical window in this
"park" position. During flight the head would automatically park itself
briefly at preset intervals for the in-flight cleaning process, thereby
eliminating operator interaction. The cleaning mechanism will be tested
during mountain calibration to be sure dirt removal is sufficient for
the optical accuracies required (i.e. transmission constant to \~0.2%).

**3.5 Spectrometers**

One of the spectrometers will be optimized for the visible and near-IR
optical range (wavelengths 0.35 -- 0.98 µm). We plan to use the Zeiss
MCS instrument. All components in that unit are permanently glued to
each other resulting in a quasi-monolithic module with no moving parts.
The spectrometer body is made of ceramic material and the detector is a
photo-diode array with 1024 pixels resulting in a resolution of 2- 3 nm.
The second spectrometer will cover the near-IR from 0.9 to 1.7 µ m. Here
we are currently evaluating a near-IR version of the Zeiss MCS
(resolution 8-12 nm) or the model NIR 512 from Ocean Optics (resolution
3 nm).

**3.6 Fiber Transmission Tests**

A major difference between the well proven AATS instruments and the
proposed instrument is that in the new design the light will be
transported through optical fibers to rack-mounted spectrometers whereas
in the AATS design the light is detected by individual photodiodes which
are mounted inside the dome. Transporting light from a non-moving head
through optical fibers to rack-mounted spectrometers is a proven concept
used with the SSFR instrument (e.g. *Pilewskie* et al., 2003).

However in the proposed instrument the fibers will endure sustained
bending and twisting: In elevation the tracking head is allowed to move
from its parking position to about 10° beyond zenith. In azimuth the
head is allowed to turn by 400° in each direction before it will
"unwind" quickly and re-acquire the sun. To answer two questions: a)
Does the structure of the fiber endure sustained bending and twisting?
and b) How does the spectral transmission of the fiber change upon
bending and twisting? we performed detailed tests with various types of
optical fibers (fiber bundle vs. mono-fiber, fibers with different core
diameters). The tests were carried out by Dr. Rainer Schmitt (Metcon
Inc., Boulder, CO) in the period of December 2003 to February 2004 on
tests-stand in the laboratory using a highly stable (better than 0.1%)
1000 Watt FEL-lamp. Results indicate negligible transmission change
(\<0.2% over the wavelength range of 0.3 -- 1.7 µm) upon dynamic bending
of the fiber for bending radii used in our design. However we realized
that the fiber design must be chosen carefully for use in a given
wavelength range and geometry.

**3.7 Instrument Calibration**

The instrument will be calibrated by a combination of high-altitude
Langley-plot calibration and laboratory calibration using FEL lamps (see
*Schmid et a*l., 1995 and 1998). As site for the Langley calibration we
propose Mauna Loa Observatory, HI. Based on the long-term experience of
the AATS team this site offers the best chance for cloud-free, stable
and clear atmospheric morning conditions. Due to the high ceiling
altitude of the HIAPER aircraft a calibration using the in-flight
Langley method is particularly attractive. This alternative calibration
method requires the aircraft to fly at high and constant altitude with
no clouds overhead while the sun raises or sets through elevation angles
8-30°. AATS-14 has been successfully calibrated using this in-flight
method during ACE-2 *(Schmid et a*l., 2000).