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Instrument Description:
1) Instrument Function. 
The FCAS II sizes particles in the approximate diameter
range from 0.07 mm to 1 mm. Particles are sampled from the free
stream with a near isokinetic sampler and are transported to the
instrument. They are then passed through a laser beam and the light
scattered by individual particles is measured (Figure 1). Particle
size is related to the scattered light. The data reduction for the
FCAS II takes into account the water which is evaporated from the particle
in sampling and the effects of anisokinetic sampling (Jonsson et al.,
1995).
2) Scientific Utility of the Data.
The FCAS II and its predecessors have provided accurate
aerosol size distribution measurements throughout the evolution of the
volcanic cloud produced by the eruption of Mt. Pinatubo. (Wilson et al.,
1993). Near co-incidences between FCAS II and SAGE II measurements
show good agreement between optical extinctions calculated from FCAS size
distributions and extinctions measured by SAGE II (Figure 2).
3) Instrument characteristics.
Accuracy: The instrument has been calibrated with monodisperse
aerosol carrying a single charge. The FCAS III and the electrometer
agree to within 10%. Sampling errors may increase the uncertainty
but a variety of comparisons suggests that total uncertainties in aerosol
surface are near 30% (Jonsson, et al., 1995).
Precision: The precision equals 1/ÖN where N is the number of
particles counted. In many instances the precision on concentration
measurements may reach 7% for 0.1 Hz data. If better precision is
desired, it is necessary only to accumulate over longer time intervals.
Response Time: Data are processed at 0.1 Hz. However, the
response time depends upon the precision required to detect the change in
question. Small changes may require longer times to detect.
Plume measurements may be processed with 1 s resolution.
Weight: Approximately 50 lbs.
References:
Jonsson,
H. H., Wilson, J. C., Brock, C. A., Knollenberg, R. G., Newton, R., Dye,
J. E., Baumgardner, D., Borrmann, S., Ferry, G. V., Pueschel, R., Woods,
D. C, Pitts, M. C., Performance of a focused cavity aerosol
spectrometer for measurements in the stratosphere of particle size in the
0.06-2.0 mm diameter range, J TECH:12, 115-129, 1995.
Wilson,
J. C., Jonsson, H. H., Brock, C. A., Toohey, D. W., Avalone, L. M.,
Baumgardner, D., Dye, J., E., Poole, L. R., Woods, D. C., DeCoursey, R.
J., Osborne, M., Pitts, M. C., Kelly, K. K., Chan, K. R., Ferry, G. V.,
Loewenstein, M., Podolske, J. R., Weaver, S., "In situ
observations of aerosol and chlorine monoxide after the 1991 eruption of
Mount Pinatubo: effect of reactions on sulfate aerosol", Science,
271, 1140-1143, 1993.
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