The CAS was originally developed as an instrument for airborne studies of aerosol and cloud particle properties (Baumgardner et al., 2001). It uses the measurement of light scattered from individual particles to derive the diameter
assuming sphericity. Mie scattering theory is applied with the additional assumption of particle refractive index and known wavelength of the incident lightâ€”in this case, that of the laser used to illuminate the particles. The
scattered light in forward and backward cones of 4 to 12Âº and 168 to 176Âº, respectively, is collected by the optical system.
The light scattered in the forward direction is measured by two detectors, one with an optical mask that restricts scattered light from particles that are farther than 0.75 mm either side of the center of focus (COF) and one that will see light scattered from particles in all parts of the beam. A masked detector, called the
qualifier, is used to qualify particles in the Depth-of-field (DOF), where the DOF is the region Â± 0.55 mm either side of the COF. The detector without the mask, the sizer, generates a signal that iscompared to the qualifier and particles are
only accepted when the qualifying signal is larger than the sizer. Note that the beam splitter that divides the scattered light and delivers it to the qualifier and sizer is 70/30; this means 70% is delivered to the qualifier, and 30% to the sizer. This is done to ensure a sharp cut-off for particles outside the DOF.
The signals from the detectors are digitized at a sample rate of 20MHz. If the qualifier signal exceeds the forward signal, the peak amplitude is recorded from the forward and back detectors. The peak amplitudes are serially stored on a
particle by particle (PBP) basis in a buffer, along with the measured inter-arrival time (IAT), i.e. the time period that has elapsed between the current and previous particle. This IAT is recorded not as an actual time, but as the number
of 20-nanosecond clock counts accumulated during the inter-arrival period. The buffer is sent to the PADS system at the end of every sample interval. The peak amplitudes are also used to increment one of the 30 channels of a frequency distribution. The 3072 possible A/D counts are distributed among the 30 channels using a lookup table. The lower and upper threshold of each
channel represents a particular size range that is predetermined by calibration (Each particle event counts as a single increment in one of the 30 channels of the three detectors such that each sampling interval, there will be three
histograms of 30 channels each that contain the sum of all particles detected during that interval. These histograms are sent to the PADS data system at the end of the sampling interval.
The forward-sizing signal is used to derive the diameter of the particles as determined from Mie theory (see section 3.0). The signal from the back detector is used to derive information about the particleâ€™s shape or refractive index. This
is based on the fact the angular pattern of light from a particle is a function of the wavelength of light, diameter, shape and refractive index