Dear ACE-Asia AATS collaborator,

In a previous email we announced our plan to apply diffuse light
correction factors to the AATS-6 and AATS-14 AODs measured in ACE-Asia,
and we included a plot of approximate correction factors. We have now
finalized the correction factors and applied them in Versions 4.0 and
3.0 of the AATS-6 and -14 data, respectively. The scientific background
on diffuse light correction factors, and our method of computing them,
are described in the statement below.

[Diffuse light corrections]{.underline}. Diffuse light (skylight)
entering the field of view (FOV) of a sunphotometer causes
overestimation of direct-beam transmission and a corresponding
underestimation of aerosol optical depth (AOD). For most aerosol
conditions and sunphotometer FOVs these effects are negligible. For
example, Eck et al. (1999) report that for the AERONET sun/sky
radiometers, which have FOV half-angle 0.6°, the diffuse-light
correction to apparent AOD is \<0.7% of AOD, even for desert dust with
effective (area-weighted) radius r~e~ as large as 1.75 μm. (To
illustrate, for AOD=0.3 such a correction is \<0.0021.) However, the
effect increases with instrument FOV, particle size, and decreasing
wavelength (e.g., Box and Deepak, 1979). For FOV half-angle 1.2° and
cirrus clouds with r~e~=6 to 177 μm, diffuse light effects can cause
apparent OD to be less than half true OD (Shiobara and Asano,1994; Kinne
et al., 1997) .

The Ames Airborne Tracking Sunphotometers, AATS-6 and -14, are designed
and built with a relatively large FOV (measured half-angle 1.85°) to
help keep the full solar disk in view when sun-tracking during aircraft
maneuvers. This larger FOV makes it necessary to assess quantitatively
the diffuse light effects on AATS-derived AOD when large particles are
dominant. We have previously done this for postvolcanic stratospheric
aerosols (Russell et al., 1993a,b) and for the Saharan dust encountered
in the Puerto Rico Dust Experiment (PRIDE, Livingston et al., 2002).

To quantify the diffuse light effects for the aerosols prevalent during
ACE-Asia we used the analytical formulation of Shiobara and Asano
\[1994\] and Kinne et al. \[1997\] to calculate AOD correction factors,

C = AOD/AOD'

where AOD' is apparent (uncorrected) AOD. Our calculations used the
AATS-6 and -14 FOV (half-angle 1.85°) and aerosol scattering phase
functions derived both from

(1) size distributions and compositions measured on the Twin Otter in
    ACE-Asia (Wang et al., 2002) and

(2) size distributions and complex refractive indices retrieved from Sun
    and sky radiance measurements by AERONET stations in the ACE-Asia
    region during Spring 2001 (Dubovik, personal communication).

We found that the correction factors were well correlated with Angstrom
exponent

α(λ~1~,λ~2~)=-ln\[AOD(λ~1~)/AOD(λ~2~)\]/ln(λ~1~/λ~2~)\],

and that the correlation improved as wavelengths λ~1~ and λ~2~
increased. (Evidently this is because longer wavelengths are more
sensitive to the larger particles in a distribution, and the larger
particles are responsible for the diffuse light effects). Scatter plots
of C-1 vs α were well fitted by exponentials of the form

f = C-1 = A exp(-Bα).

We also found no systematic differences between the f-vs-α scatter plots
for Asian aerosols and analogous plots for Saharan aerosols (the latter
derived from AERONET measurements in the Cape Verde Islands and Puerto
Rico \[Dubovik et al., 2002\]). Therefore, we included both Asian and
Saharan aerosols in the scatter plots for our final fits, to obtain a
robust relationship applicable to both Asian and Saharan aerosols. The
resulting correction factors are shown in the accompanying plot for
selected wavelengths.

Also shown are distributions of the frequency of occurrence of
α(380,1020) for AATS-6 and AATS-14 measurements in ACE-Asia. They show,
for example, that \~11% of AATS-14 measurements at all altitudes had
α(380,1020)\<0.2. For α(380,1020)=0.2, the upper plot shows that f(525
nm) is \~5% for the AATS FOV half-angle of 1.85°. ( As an example, for
AOD'(525)=0.4 this yields an AOD correction of 0.02.)

Note that, for α(380,1020)\>1, these effects are quite small
(\<0.025\*AOD for 525 nm), and even for α(380,1020)=0.5 and AOD\<0.2
they are \<0.01, a typical calibration uncertainty in
sunphotometer-derived AOD. However, for larger optical depths or smaller
α(380,1020) (indicating dominance of larger particles) the corrections
are nonnegligible. They have now been applied to the ACE-Asia AATS data
sets, and AOD uncertainties have been increased to reflect the scatter
in the f-vs-α relationships.

REFERENCES

Box, M. A. and A. Deepak, Atmospheric Corrections to Solar Radiometry,
*Applied Optics*, **12**, 1941-19 , 1979.

Eck, T. F., B. N. Holben, J. S. Reid, O. Dubovik, A. Smirnov, N. T.
O'Neill, I. Slutsker, and S. Kinne, Wavelength dependence of the optical
depth of biomass burning, urban, and desert dust aerosols, *J. Geophys.
Res., 104*, 31,333-31,349, 1999.

Kinne, S., T. P. Ackerman, M. Shiobara, A. Uchiyama, A. J. Heymsfield,
L. Milosevich, J. Wendell, E. W. Eloranta, C. Purgold, and R. W.
Bergstrom, Cirrus cloud radiative and microphysical properties from
ground observations and in situ measurements during FIRE 1991 and their
application to exhibit problems in cirrus solar radiative transfer
modeling, *J.* *Atmos. Sci., 54*, 2320-2344, 1997.

Livingston, J. M., P. B. Russell, J. S. Reid, J. Redemann, B. Schmid, D.
Allen, O. Torres, R. C. Levy, L. A. Remer, B. N. Holben, A. Smirnov, O.
Dubovik, E. J. Welton, J. Campbell, S. A. Christopher, J. Wang, Airborne
sunphotometer measurements of aerosol optical depth and columnar water
vapor during the Puerto Rico Dust Experiment, and comparison with land,
aircraft, and satellite measurements, *J. Geophys. Res.*, submitted,
2002.

Russell, P. B., 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, 1993: \"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.

Russell, P.B., 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, 1993: \"Post-Pinatubo Optical Depth Spectra
vs. Latitude and Vortex Structure: Airborne Tracking Sunphotometer
Measurements in AASE II,\" *Geophys. Res. Lett.*, *20*, 2571-2574, 1993.

Shiobara, M., and S. Asano, Estimation of Cirrus optical thivckness from
Sun photometer measurements, *J. Appl. Meteor, 33*, 672-681, 1994.

Wang, J., R.C.Flagan, J.H.Seinfeld, H.H.Jonsson, D.R.Collins,
P.B.Russell, B.Schmid, J.Redemann, J.M.Livingston, S.Gao, D.A.Hegg,
E.J.Welton, and D.Bates, Clear-column radiative closure during ACE-Asia:
Comparison of multiwavelength extinction derived from particle size and
composition with results from sunphotometry *J. Geophys. Res.,* in
press, 2002

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