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AT UNSW we have been operating a Yankee Environmental
Systems Multifilter Rotating Shadowband Radiometer (MFRSR) for the past
4 years. This instrument measures optical thickness (or attenuation of sunlight)
at 414nm, 502nm, 614nm, 673nm, 862nm and 937nm. Total optical thickness
is made up of contributions due to
Rayleigh or molecular scattering and aerosols. In addition, the
614nm channel has a large ozone contribution, and the 937nm channel has a
major contribution from water vapour. The contribution due to molecular
scattering is easily calculated as a function of wavelength and surface pressure,
but separating the contributions of aerosols from trace gases is more complex.
We have recently published a simple and accurate method, based on
eigenvalue analysis, for performing this
separation.
Our technique makes use of the fact that the aerosol measurements made at
the various wavelengths are not independent of one another because
the kernels of the equations connecting the measurements and the
required properties overlap. Thus, it is possible to predict one measurement as a
linear combination of the others.
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Using our
technique we have generated a set of coefficients which allow the
aerosol contribution to the ozone channel to be calculated from the aerosol channels.
We have applied our technique to data for 1997 and compared the results
with the TOMS (Total Ozone Mapping Spectrometer) satellite
measurements over Sydney for the same period. The results are very impressive:
our measurements agree with the satellite results to within 1 - 2% compared to
10-11% for the old method. The level of
accuracy we achieve compares well with reported comparisons
between ground-based Dobson and Brewer ozone measuring instruments
and TOMS, which agree to within 1 - 1.5%. (It should be noted that
these instruments require manual operation, and are an order of magnitude
more expensive than our instrument, which
operates automatically.) Yankee has expressed an interest in
incorporating this technique into the standard
software package for their instrument, which is fast becoming a standard around
the world. The technique can also be applied to determine the
aerosol contribution to the water vapour channel, and is equally applicable
to other types of radiometers.
Gail Box &
Ghassan Taha
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