Aerodyne Research, Inc.


CAPS PMex Monitor  ("the Extinction Monitor")


PRINCIPLE OF OPERATION (largely described by Aerodyne)


The Cavity Attenuated Phase Shift extinction (CAPS PMex) monitor as deployed in DISCOVER-AQ operates as an optical extinction spectrometer, utilizing a visible light-emitting diode (LED) as a light source, a sample cell incorporating two high reflectivity mirrors centered at the wavelength of the LED and a vacuum photodiode detector.

Unlike a conventional absorption spectrometer, in which the concentration of the absorbing species is determined by measuring the attenuation of light, the CAPS monitor relies on measuring the average time spent by the light within the sample cell. The presence of particles causes this average time to decrease as the particle concentration increases because of the propensity of particles to scatter or absorb light.    Of course, given the high velocity of light (0.3 m per nanosecond) and the very low concentrations of particles typically found in the ambient environment, a very long path length within the sample cell is required in order to accurately measure this change in time. This is accomplished by forming the ends of the sample cell with high reflectivity mirrors (R ≥ 0.9998), a configuration which provides an optical path length on the order of 1-2 kilometers. In the absence of particles, the photon lifetime within the cell is on the order of several microseconds, a magnitude which is readily measured with high accuracy.

Figure 1. Waveforms of LED (top) and signal at detector (bottom)


In CAPS-based instruments, in order to measure the average time spent by the light within the sample cell, the LED output is square wave modulated and directed into the back of the first reflective mirror. The light beam passes through the absorption cell, out of the back mirror and into a detector where it appears as a distorted waveform which is characterized by a phase shift with respect to the initial modulation. (See Figure 1.)

This distortion is the result of an exponential decay of the light within the cell as it leaks out through the mirrors. The amount of that phase shift (θ) is a function of fixed instrument properties - cell length, mirror reflectivity, and modulation frequency – and of the presence of aerosols using the following relationship:

cot θ = cot θ0 + cε/2πf

where cot is the cotangent, c is the speed of light, f is the LED modulation frequency, T and P are the sample temperature and pressure respectively, ε is the absolute extinction of light caused by the presence of aerosols. The term cot θ0 is obtained from a periodic baseline measurement (using particle-free air). Thus,

ε = [cot θ - cot θ0] * (ω/c)

Note that this measurement provides an absolute value, requiring no calibration – all the other quantities in the above expression are known. However, it should be noted that this value is averaged over the effective spectral output of the light reaching the detector, a function of the particular LED, bandpass filter and mirror reflectivity.

The configuration of the monitor is quite simple and is shown below in schematic form in Figure 2. The high reflectivity mirrors are directly attached to the ends of the sample cell, forming the optical cavity which provides for the concentration measurement, The LED, optical filter and appropriate focusing optics are attached directly to the sample cell. The light emanating from the cell is directed into a vacuum photodiode where the resultant signal is integrated, digitized and sent to a PC- 104 format computer where all subsequent data processing takes place. The sample cell contains both pressure and temperature sensors which allow for both accurate correction because of transient changes in the air Rayleigh scattering.

PMex Schematic


MEASUREMENT SPECIFICATIONS

Measurement Range:             0-4000 Mm-1

Resolution:                             0.1 Mm-1

Precision (3σ, 1s):                 < 3.5 Mm-1

Time Response:                     < 2 seconds

Baseline Drift:                        automated baseline provided

Span Drift:                             negligible

PHYSICAL SPECIFICATIONS

Sample Flow Rate:                 0.85 liters per minute

Purge Flow:                            Provided internally

Cell Pressure:                          ambient

Cell Temperature:                 ~5 above ambient

Power Usage:                         < 50 Watts (@120 VAC) Weight: < 50 Watts (@120 VAC)

Size:                                        12 kg ~65 cm x 43 cm x 23 cm (L x W x H) 19” rack mount, 5U, 24” deep]