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PCASP
Dr. Phil Rosenberg
© Crown copyright Met Office
Instrument Status
• PCASP1 (the first one we bought) has been upgraded by
DMT to the SPP200 electronics.
• Originally damaged in transit on return from USA
• Returned for repair
• Faulty upon receipt – turned out to be a bad PCB
• New board fitted and first test flight in Oct
• Measured spectrum intermittently looked odd – very bimodal.
Turned out to be a bad contact one of the PCBs.
• Flying today!
• PCASP2 (the one we’ve been flying for the past two
years) is functioning okay.
• Struggling to get good reference voltage, could be the laser on its
way out, could be contaminated cleaning solvent. New solvent just
arrived and instrument will be recleaned and aligned this week
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Calibration
• Calibration is now performed covering the full
range from ~ 0.1 to 3 microns diameter.
• Using oleic acid or DEHS oil up to ~ 0.5
microns.
• Using PSL beads above this size – Began just
before Volcano.
• Users should be aware of increased uncertainty
where the two methods meet.
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Calibration Using PSL Beads
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Calibration Results
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Calibration Results
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Calibration Accuracy and
Variability
• Accuracy of calibrated bin boundaries generally
better than approx 10 % absolute. Accuracy of
the bin widths is better than 1 %
• Bin boundaries tend to drift by only a few
percent during normal use over a few months.
• Bin boundaries can change by up to 20 % when
the instrument is cleaned and realigned.
© Crown copyright Met Office
How Do We Get Particle
Sizes
• Although PCASP measures scattering cross
section, particle diameter is a much more useful
parameter.
• If we assume the particles are homogeneous
and spherical we can use Mie theory to relate
cross section to diameter.
• The solution is dependant upon the particles
refractive index.
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Theoretical Mie Curves
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Zooming in...
•Derive bin widths by summing
the widths of the sub-bins
•Derive bin centre by averaging
the centres of the sub-bins
weighted by their widths
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Software to do this for you
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How to provide you with cal
data?
Data files used
Particle sizes and times
Calculated fit params
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Resulting bin boundaries
The Impact of Calibration and RI
Correction on Volcanic Ash
Measurements
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Low level mass distribution
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Inlet efficiencies
• Whenever streamlines are bent small particles
tend to follow the airflow, large particles do not.
Subisokinetic
Superisokinetic
Isokinetic
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Image from Aerosol sampling:
science, standards,
instrumentation and
Applications by J. H. Vincent,
Flow Simulation
© Crown copyright Met Office
What Can We Do About the
Sampling Efficiency?
• Modelling may give some insight but requires
large amounts of computing power.
• Wind tunnel calibration can give comparison to
another well calibrated (isokinetic?) inlet, but
only at surface pressures.
• Comparison in flight with (well characterised?)
Low Turbulence Inlet on Fennec.
• Any modelling must be linked with modelling
and measurement of large scale flow round the
aircraft – possible EPSRC bid led by Glasgow
to do this work.
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Summary
• Steady improvement since last year.
• Do you want cal data in the netCDF or do you
want it as a separate text file?
• Do you want counting stats errors in the
netCDF?
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