T

he three-channel integrating nephelometer, TSI-3563, is a high-sensitivity device capable of detecting the scattering properties of aerosol particles.  It is designed for long-term monitoring of visual range and air quality in both ground-based and airborne uses, specifically for studies of radiative forcing of the Earth's climate by aerosol particles and studies of atmospheric visual air quality.  The nephelometer detects by measuring the amount of light, or photons, scattered by the aerosol and then subtracts light scattered by the walls of the measurement chamber, light scattered by the atmospheric gasses present (Rayleigh scattering), and the electronic noise inherent to the detector system.  It may also be used after a pretreatment step, such as heating, humidification or segregation by size.

The light scattering coefficient is a highly variable aerosol property.  This instrument measures the angular integral of light scattering that yields the quantity called the scattering coefficient, used in the Beer-Lambert Law to calculate total light extinction.

System Functions:

The instrument works in the following manner.  A small blower continuously draws in an aerosol sample (eg. atmospheric air that has been reduced to 40% RH by heating before inlet.)  The sample is illuminated by a halogen lamp directed through an optical light pipe.  The sample volume is viewed by three photomultiplier tubes (PMT) through a set of aperture plates (see cut-away diagram below.)  Aerosol scattering is viewed against a backdrop of a very efficient light trap, where all internal surfaces are coated to give a very light low scatter.  Dichroic filters split and redirect the scattered light into three bandpass filters, red, green and blue.

A constantly rotating disc, the reference chopper,  interrupts the beam going to the photomultiplier (PMT) tubes.  The chopper has three separate areas to provide three types of signal detection.  The first area gives a measure of the light scattering signal passing through a notch in the chopper.

The second area blocks all light and gives a measurement of the PMT dark current, and is subtracted from the first signal.  The third is translucent and illuminated by the lamp, providing a measure of the light illumination intensity, used to compensate for light source variation.

In backscatted mode, a backscatter shutter rotates under the lamp to block sample illumination in the 7 to 90° range.  When light is blocked, only light scatered in the backward direction is detected by the PMTs.  The backscatter signal can be subtracted from the forward scattered signal data.

Cross-Sectional View of TSI 3563 Nephelometer

System Description:

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