Improving the accuracy of particle concentration measurements of an optical particle counter (UCASS) for balloon soundings

Since the Earth's energy balance is also influenced by aerosols and cloud droplets, knowledge concerning their size, number and vertical distribution is essential. To enable frequent, continuous, and cost-effective observations, a balloon-borne optical particle counter (“Universal Cloud and Aerosol Sounding System” (UCASS)) was developed by the University of Hertfordshire (UK). Hitherto, GPS or pressure-based measurements of the balloon’s ascent rate have been used to calculate the air’s flow velocity and volume flow rate through the UCASS, from which aerosol and cloud droplet concentrations were obtained. However, it appeared reasonable to modify the UCASS set-up by directly measuring the flow velocity in the immediate vicinity of the particle detection region within the UCASS with the aid of a thermal flow sensor (TFS), such that the volume flow within the UCASS can be measured continuously and in real time.

Consequently, a modification of the UCASS instrument has been conducted, including an internal TFS within the instrument for a more accurate determination of the probed (analyzed) air volume. This study shows that the TFS, located in a UCASS extending housing, has negligible influence on the flow velocity in the detection region within the UCASS. Field tests (in the framework of “TPChange”, DFG TRR301) have demonstrated that the ascent rates derived from GPS and pressure rarely match the TFS-based ascent rates and deviate by up to 30 %. Laboratory experiments show that with an isoaxial flow (between 2 and 8 m/s) towards the UACSS, the flow velocity within the UCASS is generally increased by ~11.3 % compared to the external flow velocity. Only if the angle of attack of the UCASS is changed to values between 20°-30°, the flow velocities within the UCASS correspond approximately to the external flow. In contrast to GPS and pressure-based ascent rates, the TFS-measured volume flow within the UCASS allows for obtaining true volume flow rates despite flow distortions (caused by the UCASS housing) and in particular the deflection of the UCASS body from an isoaxial orientation. In this way, the UCASS extension including the TFS represents an improvement of the UCASS measurements in the sense of more accurate recordings of volume flows and, thus, particle concentrations up to 7.5 km altitude.