## Diffuse Light Corrections to Sun Photometry of Desert Dust and Marine Aerosols

P. B. Russell,^1^ J. M. Livingston,^2^ O. Dubovik,^3^ S. A. Ramirez,^4^
J. Wang,^5^ J. Redemann,^4^ B. Schmid, ^4^ M. Box,^6^ and B. N.
Holben^7^

^1^NASA Ames Research Center, MS 245-5, Moffett Field, CA 94035-1000

^2^SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025

^3^Goddard Earth Sciences & Technology Center, NASA Goddard Space Flight
Center, Code 912, Greenbelt, MD 20771

^4^Bay Area Environmental Research Institute, 560 3^rd^ Street West,
Sonoma, CA 95476

^5^Brookhaven National Laboratory, Upton, NY, 11973

^6^School of Physics, University of New South Wales, Sydney 2052
Australia

^7^NASA Goddard Space Flight Center, Laboratory for Atmospheres, Code
912, Greenbelt, MD 20771

I [Background]{.smallcaps}

The NASA Ames airborne Sun photometry program has benefited in many ways
from the work of John Reagan. Our water vapor retrieval methods \[1, 2\]
are based on methods published by Prof. Reagan \[3, 4\]. Designs for our
existing instruments \[5, 6\] and advanced concepts \[7\] have been
influenced by Prof. Reagan's instruments and by his specialized
consulting \[8\]. And our measurements of the Pinatubo volcanic
stratospheric aerosol \[9, 10\] include corrections for diffuse light
effects derived by Prof. Reagan and colleagues \[11\].

II [Overview]{.smallcaps}

The work described here extends Sun photometric diffuse light correction
methods to include conditions influenced by the large particles present
in airborne desert dust and marine aerosols. Features of the work
include parametric equations that easily determine aerosol optical depth
(AOD) correction factors from Ångström exponents of uncorrected AOD
spectra. The parametric equations are derived from results of an
analytical expression \[12\] that accounts for all orders of scattering.
Aerosol scattering phase functions are derived from a representative
range of dust and marine aerosols with realistic size distributions,
compositions (or complex refractive indices), and shapes. This aerosol
information is obtained from a wide range of retrievals from the AERONET
network of sun-sky radiometers, as well as airborne in situ measurements
especially designed to account for large particles. The formulation also
accounts for realistic mixtures of scattering by the gaseous atmosphere
and extinction by dust and marine aerosols. This, along with the
parametric equations that account for changing particle size, allows for
practical application of the corrections to time series of AOD,
including vertical profiles. The corrections have been applied to our
archived data sets for studies of both African and Asian dust aerosols,
where marine aerosols are also present in the boundary layer. (See,
e.g., http://geo.arc.nasa.gov/sgg/ACE-Asia/data_plots_login_page.html.)

III [Example Results]{.smallcaps}

Fig. 1 shows an example of applying the correction factors to vertical
profiles of multiwavelength AOD acquired in conditions influenced by
Asian desert dust. The derivation of the correction factors, and more
extensive results, are given by \[13\]. In general, we find that the
corrections are negligible (\<\~1% of AOD) for Sun photometers with
narrow FOV (half-angle η\<\~1°), but they can be as large as 10% of AOD
at 354 nm wavelength for Sun photometers with η=1.85°.

[Acknowledgment]{.smallcaps}

This research was supported by the National Aeronautics and Space
Administration and by the National Oceanic and Atmospheric
Administration.

REFERENCES

1.  B. Schmid, K. J. Thome, P. Demoulin, R. Peter, C. Mätzler, and J.
    Sekler. "Comparison of Modeled and Empirical Approaches for
    Retrieving Columnar Water Vapor from Solar Transmittance
    Measurements in the 0.94 Micron Region," *J. Geophys. Res.*, *101*,
    9345-9358, 1996*.*

2.  B. Schmid, J. Michalsky, D. Slater, J. Barnard, R. Halthore, J.
    Liljegren, B. Holben, T. Eck, J. Livingston, P. Russell, T. Ingold,
    and I. Slutsker. "Comparison of columnar water vapor measurements
    during the fall 1997 ARM Intensive Observation Period: solar
    transmittance methods," *Applied Optics,* Vol. 40, No. 12,
    1886-1896, 2001.

3.  J.A. Reagan, K. Thome, B. Herman, and R. Gall, "Water vapor
    measurements in the 0.94 micron absorption band: Calibration,
    measurements and data applications," in *Proceedings of IGARSS \'87
    Symposium*, pp. 63-67, IEEE Press, Piscataway NJ, 1987.

4.  J. A. Reagan, K. Thome, and B. Herman. "A Simple Instrument and
    Technique for Measuring Columnar Water Vapor Via Near-IR
    Differential Solar Transmission Measurements," *IEEE Transactions
    Geosci. Remote Sensing*, *30*, 825--831, 1992a.

5.  T. Matsumoto, P. Russell, C. Mina, W. Van Ark, and V. Banta,
    "Airborne Tracking Sunphotometer," *J. Atmos. Ocean. Tech.*, *4*,
    336-339, 1987.

6.  P. Russell, J. Livingston, B. Schmid, J. Redemann, and J. Eilers,
    "Ames Airborne Tracking Sunphotometer, AATS-14,"
    <http://geo.arc.nasa.gov/sgg/AATS-website/AATS6_AATS14/AATS6_AATS14.html>,
    2002.

7.  E. A. Hildum, and W. Vallotton, "Conceptual design of a sun tracker
    and control/data system for the ER-2 sun photometer," Document
    9ER2-9002-XR, NASA Ames Research Center, January 10, 1991.

8.  J. A. Reagan, and Z. Huang, "Conceptual design and simulation of a
    spectral sunphotometer for the ER-2 aircraft," Final Report to NASA
    Ames, University of Arizona, 1990.

9.  P. B. Russell, J. M. Livingston, E. G. Dutton, R. F. Pueschel, J. A.
    Reagan, T. E. DeFoor, M. A. Box, D. Allen, P. Pilewskie, B. M.
    Herman, S. A. Kinne, and D. J. Hofmann, "Pinatubo and pre-Pinatubo
    optical-depth spectra: Mauna Loa measurements, comparisons, inferred
    particle size distributions, radiative effects, and relationship to
    lidar data," *J. Geophys. Res*., *98*, 22,969-22,985, 1993a.

10. P. B. Russell, J. M. Livingston, R. F. Pueschel, J. A. Reagan, E. V.
    Browell, G. C. Toon, P. A. Newman, M. R. Schoeberl, L. R. Lait, L.
    Pfister, Q. Gao, and B. M. Herman, "Post-Pinatubo optical depth
    spectra vs. latitude and vortex structure: Airborne tracking
    sunphotometer measurements in AASE II," *Geophys. Res. Lett*., *20*,
    2571-2574, 1993b.

11. J.A. Reagan, Q. Gao, B.M. Herman, T. Caudill, and D. Flittner,
    "Aureole corrections to optical depths determined by solar
    photometry under turbid volcanic conditions," American Geophysical
    Union Chapman Conference on Climate, Volcanism, and Global Change,
    Hilo, Hawaii, Conference Abstracts, p. 28, March 23-27, 1992b.

12. M. Shiobara, and S. Asano, "Estimation of cirrus optical thickness
    from Sun photometer measurements," *J. Appl. Meteor, 33*, 672-681,
    1994.

\[13\] P. B. Russell, J. M. Livingston, O. Dubovik, S. A. Ramirez, J.
Wang, J. Redemann, B. Schmid, M. Box, and B. N. Holben (2004), "Sunlight
transmission through desert dust and marine aerosols: Diffuse light
corrections to Sun photometry and pyrheliometry," *J. Geophys. Res*.,
109, D08207, doi:10.1029/2003JD004292, 2004.

![](media/image1.png){width="7.0472222222222225in"
height="3.4006944444444445in"}

**Fig. 1.** An example of applying the diffuse-light correction factors
to vertical profiles of aerosol optical depth (AOD) measured in
conditions affected by Asian desert dust. *α*~AOD~(1020,1558) is the
Ångström exponent of the overlying AOD, derived as a fit to uncorrected
AOD at wavelengths from 1020 to 1558 nm. *C*~a~ is the correction
factor, AOD/AOD', where AOD' is apparent (i.e., uncorrected) AOD.
Results are for photometer field-of-view half-angle η=1.85°, which
applies to the 6- and 14-channel NASA Ames Airborne Tracking
Sunphotometers.