Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 70)

Forgan, B.W ., 1986: D eterm ination of aerosol optical depth at a sea level station - inves tigations at G ape
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G rim m B A P S. C G BAP S T echnical R eport 5. G ape G rimm BAPS , G ape G rimm , Australia. 55 pages +
figures.
H arrison, L, and J. M ichalsky, 1994: O bjective algorithms for the retrieval of optical depths from ground-
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based m easurem ents. Appl. O ptics, 33(22), 5126 - 5132.
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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 determ ine the quality of the intercept.
Forgan , working in m id-latitude sea-level conditions, suggests the following five m ethods to help 16
determ ine the quality of the intercept value:
(1) The sam pling period m ust be as short as possible but provide at least a 3-airm ass 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 m ore than 1 hPa during the period of observations, V for wavelengths
less than 500 nm should be discarded, or the m olecular scattering norm alized to constant
pressure using a m olecular extinction m odel.
(4) The intercept m ust be less than the intercept calculated for a pure m olecular scattering
atm osphere.
(5) The unbiased estim ate of the standard deviation of the regression should be less than 0.003
The work of Harrison and Michalsky , besides presenting a m eans of determ ining the quality of the 17
Langley regression (see 7.4.2.3), additionally note that:
(6) The m easurem ent interval should be either in the m orning or afternoon and that the data
should not be com bined.
(7) Airm ass below 2 should not be used, even when available, because the rate of change of
the atm osphere is sm all causing an increased probability of contam inating the data with changing
atm ospheric conditions.
Finally:
(8) As the regression is between airm ass and signal, the data should be based on airm ass
observations. Many observation program s are based on tim e and not airm ass. Observations
that are equally spaced in tim e lead to an increased num ber of m easurem ents at sm aller
airm ass, which in turn produce a bias in the intercept toward these values. This can be overcom e
by taking m easurem ents at equal airm ass or weighting the data as a function of airm ass to
offset the increase in the num ber of observations as airm ass decreases.
NOTE: The Langley calibration is based on the actual sun-earth distance at the tim e the calibration
is obtained. The use of these calibrations m ust correct for the change in the sun-earth distance. Com m only,
individual Langley calibrations are norm alized to the m ean 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 tim e were found to correlate with each other. This cross-
correlation for wavelengths affected by m olecules, aerosols and to a lesser extent, ozone (unaffected
by strong absorption bands) is expected because of the extinction functions of each of these com ponents.
The variation in the cross-correlation com es prim arily from the change of aerosol conditions over tim e,