Development of a Cloud Particle Sensor for Balloon-Borne Observations

Clouds play key roles in the Earth's radiation budget and thus in the climate system. The microphysical properties such as number density, size, and shape affect the radiative properties of the atmosphere. “In-situ” measurements of clouds are still a technical challenge, and there are few sensors suitable for routine balloon-born observations.

A cloud particle sensor (CPS) has been developed to measure cloud-particle number density, size distribution, and the particle phase (i.e., liquid water or ice). This is a low-cost and light-weighted (~200 g, including batteries) particle counter based on a pollen sensor used in an in-house air purifier. The CPS consists of a light source with a linear-polarization laser diode and two detectors, one detecting the scattered light intensity directly and the other detecting the intensity through a polarization plate to estimate the degree of polarization by particles. The number of counts, intensity of the scattered light, and the degree of polarization correspond to the particle number density, size, and the phase, respectively. The detection limit for small particles was estimated as ~0.5 ìm by using reference polystyrene micro-particles.

We have conducted several test flights in Japan and in Indonesia since November 2012. The results showed that the CPS can detect both ice (cirrus) clouds in the upper troposphere and water clouds in the lower troposphere, and discriminate the phase of the particles by the degree of polarization. However, there are still two major problems to be solved. One is that the estimation of particle size, particularly for ice particles, needs more laboratory experiments using reference particles. The other is a particle-overlapping issue in the sampling volume, which gives a limitation for counting the number of particles correctly in dense clouds. The estimated current limit is ~ 1 /cm3. For the latter we need further redesigning of the sensor to measure wider range of number density that the sensor encounters during a tropospheric flight.