Passive Cavity Aerosol Spectrometer Probe (PCASP)

Note this information comes from Darrel Baumgardner's page and has been slightly modified to describe the PCASP-X at SGP.

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 thirty-two 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. In other words 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 2 shows a typical size distribution where the concentration of particles in each size category is shown, normalized by the width of the size channel.

3. Sensor Specifications

3a. General Information

Manufacturer: Particle Measuring Systems Inc., Boulder, Co.

Calibration Method: Monodispersed polystyrene latex beads

Range: 0.1 mm - 10.0 mm

Accuracy: ±20% (Diameter)

±16% (Concentration)

3b. Primary Output

Channels 1-32 32 channels of accumulated counts

3c. Derived Output

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. Note that for the PCASP at SGP the summations would be from 1 to 32 instead of 1 to 15.


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.