UPDATE: Diffuse Light Corrections to AATS Data
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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. 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 re as large as 1.75 mm. (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 re = 6 to 177 mm, 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  and Kinne et al.  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
a (l1, l2)=-ln[AOD(l1)/AOD(l2)]/ln(l1/l2)],
and that the correlation improved as wavelengths l1 and l2 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 a were well fitted by exponentials of the form
f = C-1 = A exp(-Ba).
We also found no systematic differences between the f-vs-a 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 a(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 a(380,1020)<0.2. For a(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 a(380,1020)>1, these effects are quite small (<0.025*AOD for 525 nm), and even for a(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 a(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- a relationships.
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