Characterization of the airborne aerosol inlet and transport system used during the A-LIFE aircraft field experiment
Abstract. Atmospheric aerosol particles have a profound impact on Earth’s climate by scattering and absorbing solar and terrestrial radiation and by impacting the properties of clouds. Research aircraft such as the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) Falcon are widely used to study aerosol particles in the troposphere and lower stratosphere. However, transporting a representative sample to the instrumentation inside the aircraft remains a challenge due to high airspeeds and changing ambient conditions. In particular, for high-quality coarse mode aerosol measurements, knowledge about losses or enhancements in the aerosol sampling system is crucial. In this study, we present a detailed characterization of the Falcon aerosol sampling system. Aerosol number size distributions were measured during the A-LIFE field campaign simultaneously with in-cabin and out-cabin/wing-mounted instrumentation. Sampling efficiencies were derived for different true airspeed ranges by comparing the in-cabin and the out-cabin particle number size distributions during flight sequences with a major contribution of mineral dust particles in the coarse mode size range. Additionally, experimentally derived Stokes numbers were used to calculate the cut-off diameter of the A-LIFE aerosol sampling system for different particle densities as a function of true airspeed. The results show that the velocity of the research aircraft has a major impact on the sampling of coarse mode aerosol particles with in-cabin instruments. For true airspeeds up to about 190 m s-1, aerosol particles larger than about 1 µm are depleted in the sampling system of the Falcon during the A-LIFE project. In contrast, for true airspeeds higher than 190 m s-1, an enhancement of particles up to a diameter of 4 µm is observed. For even larger particles, the enhancement effect at the inlet is still present, but inertial and gravitational particle losses in the transport system get more and more pronounced which leads to a decreasing overall sampling efficiency. In summary, aerosol particles can either be depleted or enhanced at an aerosol inlet, whereas transport in sampling lines always leads to a loss of particles. Therefore, it is important to consider both, inlet and transport efficiency, when quantifying the sampling efficiency of an aerosol sampling system.
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