The Passive Cavity Aerosol Spectrometer Probe (PCASP) is an instrument used to make airborne aerosol size measurements within 15 size intervals or channels. The aerosol size spectrum is recorded at 1 Hz frequency by a data acquisition system. To collect size information, the PCASP uses a combination reflecting-refracting imaging system. The high-efficiency light gathering system allows the instrument to size particles from 3.00 µm to as small as 0.10 µm. The PCASP has a size resolution in its smallest channels of 0.02 µm and can size particles up to an altitude of 40,000 feet.

A calibration is generally performed once at the beginning of a field project. To do so, one must send aerosols of a known size through the PCASP. Ideally, a uniform peak in one channel should occur, however the peak typically will not be contained within a single channel, but instead will be a Gaussian curve centered on the peak channel. Using the counts in the peak channel, pre-peak channel, and post-peak channel, the average peak channel can be determined for that known aerosol size. Periodically, throughout a project, calibration checks should be performed to ensure the measurements are accurate and the instrument is performing within the specifications.

In order to realistically compare data from different locations, it is imperative that the instrument be calibrated for each project. In addition to being calibrated, calibration checks should be done throughout the project. A great way to understand PCASP data is to look at it in comparison to other data sets. By comparing measurements from different places, one can pick out trends and/or anomalies. PCASP measurements from recent field projects in North Dakota, Saudi Arabia, and Mali were analyzed to identify similarities and differences in the aerosol size spectrum. These similarities and differences are important to understanding cloud properties and precipitation processes.