Forgan, B.W ., 1986: Determination of aerosol optical depth at a sea level station - investigations at Gape
16
Grimm BAPS. CGBAPS Technical Report 5. Gape Grimm BAPS, Gape Grimm, Australia. 55 pages +
figures.
Harrison, L, and J. Michalsky, 1994: Objective algorithms for the retrieval of optical depths from ground-
17
based measurements. Appl. Optics, 33(22), 5126 - 5132.
58
7.4.2.1 Quality Assurance Procedures for Langley Calibration
The acceptance of high values of the coefficient of determ ination (r ) obtained from a regression analysis
2
0
has been shown to lead to erroneous values of V . Therefore, several quality assurance procedures
can be used to better determine the quality of the intercept.
Forgan , working in mid-latitude sea-level conditions, suggests the following five methods to help
16
determine the quality of the intercept value:
(1) The sampling period must be as short as possible but provide at least a 3-airmass range
between 2 < m < 6.5. Data around solar noon should be avoided.
(2) If a plot of the residuals of the regression shows a trend in the deviations or any of the residuals
0
are greater than ± 0.006, the calculated V is unacceptable.
0
(3) If the pressure changes by more than 1 hPa during the period of observations, V for wavelengths
less than 500 nm should be discarded, or the molecular scattering normalized to constant
pressure using a molecular extinction model.
(4) The intercept must be less than the intercept calculated for a pure molecular scattering
atmosphere.
(5) The unbiased estimate of the standard deviation of the regression should be less than 0.003
The work of Harrison and Michalsky , besides presenting a means of determining the quality of the
17
Langley regression (see 7.4.2.3), additionally note that:
(6) The measurement interval should be either in the morning or afternoon and that the data
should not be combined.
(7) Airmass below 2 should not be used, even when available, because the rate of change of
the atmosphere is small causing an increased probability of contaminating the data with changing
atmospheric conditions.
Finally:
(8) As the regression is between airmass and signal, the data should be based on airmass
observations. Many observation programs are based on time and not airmass. Observations
that are equally spaced in time lead to an increased number of measurements at smaller
airmass, which in turn produce a bias in the intercept toward these values. This can be overcome
by taking measurements at equal airmass or weighting the data as a function of airmass to
offset the increase in the number of observations as airmass decreases.
NOTE: The Langley calibration is based on the actual sun-earth distance at the time the calibration
is obtained. The use of these calibrations must correct for the change in the sun-earth distance. Com monly,
individual Langley calibrations are normalized to the mean sun-earth distance and then converted
to the sun-earth distance of the observation as part of the data analyses procedure.
7.4.2.2 Ratio-Langley Technique
The Ratio-Langley technique was developed following the observation that the responsivities of a group
of wavelengths obtained over a period of time were found to correlate with each other. This cross-
correlation for wavelengths affected by molecules, aerosols and to a lesser extent, ozone (unaffected
by strong absorption bands) is expected because of the extinction functions of each of these components.
The variation in the cross-correlation comes primarily from the change of aerosol conditions over time,
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