Passive Cavity Aerosol Spectrometer Probe
PCASP-100 Aerosol Probe
1. Introduction
The Passive Cavity Aerosol Spectrometer
Probe (PCASP) Model 100 is an instrument developed by Particle
Measuring Systems (PMS Inc., Boulder, Co) for the measurement
of aerosol particle size distributions. This sensor is utilized
in studies of tropospheric chemistry and aerosol physics.
2. Operating Principles
The PCASP 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 an aerodynamically focused particle laden air stream. Particles that encounter this beam scatter light in all directions and some of this light is collected by a mangin mirror over angles from about 35o - 135o. This collected light is focussed onto a photodetector and then amplified, conditioned, digitized and classified into one of fifteen size channels. 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 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 PCASP inside a 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: 0.1 mm - 3.0 mm
Accuracy: ±20% (Diameter)
±16% (Concentration)
3b. Primary Output
RAF Parameter Name Plain Language Name Description
APC01-15 Channels 1-15 15 channels of
accumulated counts
3c. Derived Output
RAF Parameter Name Plain Language Name Description
CONCP Concentration # of particles per unit volume - number per cubic centimeter
SFCP Surface Area Total surface area - micrometers squared per cubic centimeter
VOLP Volume Total particle volume - Cubic micrometers per cubic meter
DBARP 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 PCASP was developed as an aerosol particle
measurement instrument. The size that is determined by the PCASP
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. Corrections are applied to
account for these losses but still lead to concentration uncertainties.
The PCASP 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.
