Development of ballon-borne laser scattering micro-particle counting System

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A laser type aerosol sonde was developed to be mounted on an observation balloon to measure the airborne particle size to 0.1 to 5 mum for polystirene latex spheres (PSL) and enable it to endure atmospheric pressure variation from 1013 hPa (at the ground surface) to 10 hPa (at an altiude of 30 to 35 km), design and experimental investigations regarding the signal-to-noise (S/N) ratio, laser power variation, and sampling flow rate variation were carried out. As a results, an external mirror type He-Ne laser was chosen. Its configuration is characterized by a reinforced laser tube being hermetically sealed at normal atmospheric pressure and having a control system for the supply current. This made the laser power fluctuation a mere 0.7% against the above-stated atmospheric pressure variation. A gear pump was specially chosen as an aerosol suction pump to effect a constant sampling flow rate ratio under the ambient atmospheric pressure variation, and the ratio was hold to 0.5% by speed control of the pumpmotor. A special nebulizer for use in a vacuum was developed to experimentally ascertain the response curve of the aerosol sonde over the full range of normal atmospheric pressure dowm to 10 hPa. The response curve together with the S/N ratio was evaluated by both the peak channel and the standard deviation channel of the pulse height analyzer (PHA) histogram. Using this nebulizer, it was ascertained that both channel values were kept nearly constant against the atmospheric pressure variation and that the response curve well satisfied the Mie scattering equation. It was also demonstrated that 0.109 mum PSL particles could be detected with a relative standard deviation value in a PHA histogram of 42% and an S/N ratio of 2.5-3.