Submitted 23 February 2005 to:

Proceedings, IEEE Workshop on Remote Sensing of Atmospheric Aerosols

An Honorary Workshop for Prof. John A. Reagan, Tucson, AZ, 5-6 April 2005

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. Holben7

1NASA Ames Research Center, MS 245-5, Moffett Field, CA94035-1000

2SRI International, 333 Ravenswood Avenue, Menlo Park, CA94025

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

4Bay Area Environmental Research Institute, 560 3rd Street West, Sonoma, CA95476

5Brookhaven National Laboratory, Upton, NY, 11973

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

7NASA GoddardSpaceFlightCenter, Laboratory for Atmospheres, Code 912, Greenbelt, MD20771

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Submitted 23 February 2005 to:

Proceedings, IEEE Workshop on Remote Sensing of Atmospheric Aerosols

An Honorary Workshop for Prof. John A. Reagan, Tucson, AZ, 5-6 April 2005

I Background

The NASA Ames airborne Sunphotometry 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

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.,

IIIExample Results

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 <~1°), but they can be as large as 10% of AOD at 354 nm wavelength for Sun photometers with =1.85°.

Acknowledgment

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, PiscatawayNJ, 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,” 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, NASAAmesResearchCenter, 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.

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Submitted 23 February 2005 to:

Proceedings, IEEE Workshop on Remote Sensing of Atmospheric Aerosols

An Honorary Workshop for Prof. John A. Reagan, Tucson, AZ, 5-6 April 2005

IEEERussellDiffuseAbsSubmtd_formatted5Page 1 of 211:18 AM, 11/16/18

Submitted 23 February 2005 to:

Proceedings, IEEE Workshop on Remote Sensing of Atmospheric Aerosols

An Honorary Workshop for Prof. John A. Reagan, Tucson, AZ, 5-6 April 2005

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.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.Ca is the correction factor, AOD/AOD’, where AOD’ is apparent (i.e., uncorrected) AOD. Results are for photometer field-of-view half-angle =1.85°, which applies to the 6- and 14-channel NASA Ames Airborne Tracking Sunphotometers.

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