An Analysis of Errors Associated with the In Situ Measurement of Aerosol Size and Concentration with Optical Particle Counters

Miao Qun, University of Wyoming

Calibrations and experiments were conducted with the UW OPC (Optical Particle Counter) to investigate some aspects of the uncertainties in number concentration, particle size, and particle index of refraction measurements. An attempt was also made to evaluate a method of refractive index deduction of particles passing through the OPC.

The flow rate in the OPC remains stable at different pressure levels and can be calibrated based on the bag test results. Two kinds of flow meters were studied under laboratory and real flight conditions to examine whether these flow meters are suitable for real-time monitoring of the flow rate in the OPC operations. The CE flow meters are considered not suitable for the OPC operations because of the strong dependence of flow rate on the pressure and the difficulty of flow rate calibration due to the dependence of the discharge coefficient on the Reynolds number. Although PF flow meters provided much more reasonable measurements in lab, it had more complicated and different patterns compared to the laboratory results in a flight and needs further investigations. The broadening effect was studied by using a pulse height analyzer and Labview. According to the particle tests, the particle size uncertainty due to the broadening effect is relatively smaller for bigger particles, such as the uncertainty of 4 % for 2.00 mm radius particles and 8 % for 1.01 mm radius particles. A method of index of refraction deduction by analyzing the ratio of counter responses between the phototubes at 40° and 74° was investigated at a preliminary stage. More large particles of different refractive indices are needed to complete an evaluation of this method.