Forward Scattering Spectrometer Probe

FSSP-100 Cloud Probe

1. Introduction

The Forward Scattering Spectrometer Probe (FSSP), model 100, is an instrument developed by Particle Measuring Systems (PMS Inc., Boulder, Co) for the measurement of cloud droplet size distributions. The sensor is used primarily for the study of cloud microphysical processes, particularly the nucleation and growth of cloud droplets through condensation and coalescence.

2. Operating Principles

The FSSP 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 diameter of 0.2 mm 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 4^o - 12^o.

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 displaced greater than approximately 1.5 mm either side of the center of focus. Particles that fall in that region are rejected when the signal from the masked detector exceeds that from the unmasked detector. This defines the sample volume needed 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 15 channels and this information 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 droplets in each size category is shown, normalized by the width of the size channel. Figure 4 is a photograph of the FSSP 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 glass beads

Range: 2.0 mm - 47.0 mm

Accuracy: ±20% (Diameter)

±16% (Concentration)

3b. Primary Output

RAF Parameter Name Plain Language Name Description

AFS01-15 Channels 1-15 - 15 channels of accumulated counts

FSTRB Total Strobes Total Particles in depth of field

FRST Fast Resets Total Particles outside depth of field

FACT Activity Fraction of sample period that probe was active

3c. Derived Output

RAF Parameter Name Plain Language Name Description

CONCF Concentration # of droplets per unit volume - number per cubic centimeter

PLWCF Liquid Water Content Total droplet mass -

grams per cubic meter

DBARF Average Diameter Arithmetic average of droplet size - micrometers

where n_i is the number of droplets detected in size channel i, d_i is the diameter represented by channel i, and V is the sample volume measured in a given sample period.

4. Data Interpretation

The FSSP-100 was developed as a cloud droplet measurement instrument. The size that is determined by the FSSP assumes that the scattered light detected is from a spherical, liquid droplet of refractive index 1.33. The size distributions produced from these measurements must be viewed with great caution when in clouds containing mixtures of water and ice, since ice particles will not be correctly sized due to their different refractive index and non-spherical shapes.

A secondary caution is when looking at size distributions when precipitation sized drops are presents. These are suspected of colliding with the sample inlet and causing spurious satellite droplets.

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 droplet sizing instrument, not a liquid water content probe. Since the liquid water content is derived by integrating the size distribution, uncertainties in the size measurement lead to root sum squared accuracies in liquid water content a factor of three higher.