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Sulfur
dioxide
is
measured
remotely using a correlation spectrometer (COSPEC).
Scattered solar ultraviolet energy of specific
wavelengths is absorbed in proportion to SO2 concentration and plume
thickness. This absorption, with units of pathlength-concentration,
is
measured
by
the COSPEC and calibrated with the absorption of an SO2
gas standard in the instrument. The product of the plume
light-absorption profile and wind speed yields the SO2 emission rate.
The technique is described by Casadevall
and others (1981; 1983; 1987), Stoiber and
others (1983), and Millan and others
(1976). This method is used routinely and worldwide to make volcanic
SO2 measurements. Measurements can be made from the
ground or the air. Wind speed is determined by using a hand-held
anemometer for ground-based measurements. Airborne SO2 and CO2
measurements use wind speeds measured by a variety of methods including
those reported by local airports. Alternatively, wind speed is
determined by comparing true air speed, flying with and against the
wind, and true ground speed. Accurate
determination of wind speed is critical for reliable emission-rate
determinations. Airborne measurements of SO2 tend to be more
reliable than ground-based measurements, because wind speed
is determined at the site of measurement.
Ground-based
COSPEC
measurements
can
be made from a vehicle by traversing beneath
the plume while pointing the instrument up through it. Alternatively,
the COSPEC can be mounted on a tripod near the vent to scan either
vertically or horizontally through the plume.
Airborne
SO2
measurements
are
made by flying below and at right angles to the
plume trajectory. Multiple traverses for either
ground-based or airborne measurements are averaged to calculate SO2
emission rates, which are then scaled up to a daily rate.
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