The HALO Submicrometer Aerosol Inlet (HASI): Design concept and first characterization

The HALO Submicrometer Aerosol Inlet (HASI) was specifically designed for HALO with the aim to provide up to 30 l/min sample air flow (divided over four forward facing sample lines) to aerosol instruments mounted inside the aircraft cabin. HASI samples the air on the top of the fuselage outside of the aircraft boundary layer. The air stream is aligned in the inlet and deaccelerated in three stages by roughly a factor of 40 using a front shroud, a main diffusor and smaller diffusers at the tips of four sample tubes which protrude into the deaccelerated air stream. In addition, there is a backward-facing sample tube installed for optional use. The design concept is to allow regulating the sample air flow in each of the four forward-facing sample lines to achieve isokinetic sampling conditions according to the actual speed of the aircraft. Pumps and bypass lines for regulation of the air flow are part of the Aerosol Measurement System (AMETYST) cabin rack on HALO operated by DLR. The geometric design of the inlet should prevent large cloud droplets and ice crystals from entering the sample lines directly. Judging from the first measurements of different instruments using HASI during the ML-CIRRUS and ACRIDICON-CHUVA missions with HALO, it appears that condensation particle counter (CPC) measurements of interstitial aerosol in liquid clouds are affected by artefacts while in ice clouds there is no indication of any measurement problem. Selected examples of other measurement results for initial experimental characterization of the inlet will be discussed. In order to achieve better isokinetic sampling, further numerical flow simulations of the final inlet design for different aircraft altitudes and speeds are required to establish a final regulation of the sample air flow in each of the four sample lines based on pressure and temperature sensors in the inlet and aircraft data