Forward Scattering Spectrometer Probe
FSSP-300 Aerosol Probe
1. Introduction
The Forward Scattering Spectrometer Probe
(FSSP) Model 300 is an instrument developed by Particle Measuring
Systems (PMS Inc., Boulder, Co) for the measurement of aerosol
particle size distributions. The sensor was originally developed
for the study of stratospheric aerosol distributions and polar
stratospheric clouds but is now widely utilized in studies of
tropospheric chemistry and aerosol physics.
2. Operating Principles
The FSSP-300 is of that general class of
instruments called optical particle counters (OPCs) that detect
single particles and size them by measuring the intensity of light
that the particle scatters when passing through a light beam.
The schematic diagram shown in Fig. 1 illustrates the optical
path of this instrument. A Helium Neon laser beam is focused to
a small diameter at the center of an inlet that faces into the
oncoming airstream. This laser beam is blocked on the opposite
side of the inlet with an optical stop, a "dump spot"
to prevent the beam from entering the collection optics. Particles
that encounter this beam scatter light in all directions and some
of that scattered in the forward direction is directed by a right
angle prism though a condensing lens and onto a beam splitter.
The "dump spot" on the prism and aperture of the condensing
lens define a collection angle from about 4o - 12o.
The beam splitter divides the scattered
light into two components, each of which impinge on a photodetector.
One of these detectors, however, is optically masked to receive
only scattered light when the particles pass through the laser
beam within a region 0.5 mm either side of the center of focus.
Particles that fall outside that region are rejected when the
signal from the unmasked detector exceeds that from the masked
detector. This defines the sample volume that is needed in order
to calculate particle concentrations.
The size of the particle is determined by
measuring the light scattering intensity and using Mie scattering
theory to relate this intensity to the particle size. Figure 2
illustrates how the scattered light varies with particle diameter
given that the particle is spherical and that the refractive index
is known. The size is categorized into one of 31 channels and
this information is sent to the data system where the number of
particles in each channel is accumulated over a preselected time
period. Figure 3 shows a typical size distribution where the concentration
of particles in each size category is shown, normalized by the
width of the size channel. Figure 4 is a photograph of the FSSP-300
in the canister that is normally mounted on an aircraft pylon.
3. Sensor Specifications
3a. General Information
Manufacturer: Particle Measuring Systems
Inc., Boulder, Co.
RAF Resident Expert: Darrel Baumgardner
(303) 497-1054
darrel@ncar.ucar.edu
Typical Mounting
Location: Pylons on fuselage or wings
Calibration Method: Monodispersed polystyrene
latex beads
Range: 0.3 mm - 20.0 mm
Accuracy: ±20% (Diameter)
±16% (Concentration)
3b. Primary Output
RAF Parameter Name Plain Language Name Description
AF301-15 Channels 1-31 31 channels of accumulated counts
FRST3 Total Resets Total Particles passing
through the beam
3c. Derived Output
RAF Parameter Name Plain Language Name Description
CONC3 Concentration # of particles per unit volume - number per cubic centimeter
SFC3 Surface Area Total surface area - micrometers squared per cubic centimeter
VOL3 Volume Total particle volume -
Cubic micrometers per cubic meter
DBAR3 Average Diameter Arithmetic average
of particle size - micrometers
where ni is the number of particles
detected in size channel i, di is the diameter represented
by channel i, and V is the sample volume measured in a given sample
period.
4. Data Interpretation
The FSSP-300 was developed as an aerosol
particle measurement instrument. The size that is determined by
the FSSP assumes that the scattered light detected is from a spherical
particle of refractive index 1.58. The size distributions produced
from these measurements must be viewed with great caution when
in mixed composition aerosols. Particles will not be correctly
sized due to their different refractive index and non-spherical
shapes.
The probability of more than a single particle
coinciding in the beam or being missed during the electronic reset
time increases with concentration from about 5% losses at 300
cm-3 to greater than 30% at 1000 cm-3. Corrections
are applied to account for these losses but still lead to concentration
uncertainties.
The FSSP is a particle sizing instrument, not a particle surface area or volume probe. Since the surface areas and volumes are derived by integrating the size distribution, uncertainties in the size measurement lead to root sum squared accuracies in surface area and volume a factor of two and three higher, respectively.
