Improving the accuracy in particle concentration measurements of a balloon-borne optical particle counter, UCASS

Abstract. For balloon-borne detection of aerosols and cloud droplets (diameter 0.4 < Dp < 40 µm), a passive-flow Universal Cloud and Aerosol Sounding System (UCASS) was used, whose sample flow rate is conventionally derived from balloon ascent rates using GPS or pressure measurements. Improvements are achieved by implementing thermal flow sensors (TFSs) 94 mm downstream of the UCASS detection region for continuously measuring true UCASS sample flow velocities. UCASS-mounted TFSs were calibrated during wind tunnel experiments at up to 10 m s−1, and under various angles of attack (AOAs), as these vary during actual balloon ascents. It was found that the TFS calibration is determined with sufficient precision using three calibration points at tunnel flows of ∼ 2, 5, and 8 m s−1, simplifying efficient TFS upgrades of numerous UCASSs. In iso-axial alignment, UCASS flows are accelerated (by ∼ 11.3 %) compared to tunnel flows (at 2–8 m s−1). In-flight comparisons up to 7.5 km in height revealed that UCASS sample flows rarely match the balloon's ascent rate. Laboratory experiments show that equality (vGPS=vTFS) is achieved only at AOA ≠ 0°, potentially affecting the UCASS internal flow pattern and particle transmission efficiency in flight. To minimise errors in calculated UCASS-based particle number concentrations, real-time measurements of the true UCASS flow velocity are recommended.