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Abstract

Particle concentration measurements with underwing probes on aircraft are impacted by air compression upstream of the instrument body as a function of flight velocity. In particular for fast-flying aircraft the necessity arises to account for compression of the air sample volume. Hence, a correction procedure is needed to invert measured particle number concentrations to ambient conditions that is commonly applicable for different instruments to gain comparable results. In the compression region where the detection of particles occurs (i.e. under factual measurement conditions), pressure and temperature of the air sample are increased compared to ambient (undisturbed) conditions in certain distance away from the aircraft. Conventional procedures for scaling the measured number densities to ambient conditions presume that the particle penetration speed through the instruments' detection area equals the aircraft speed (True Air Speed, TAS). However, particle imaging instruments equipped with pitot-tubes measuring the Probe Air Speed (PAS) of each underwing probe reveal PAS values systematically below those of the TAS. We conclude that the deviation between PAS and TAS is mainly caused by the compression of the probed air sample. From measurements during two missions in 2014 with the German Gulfstream G-550 (HALO – High Altitude LOng range) research aircraft we develop a procedure to correct the measured particle concentration to ambient conditions using a thermodynamic approach. With the provided equation the corresponding concentration correction factor ξ is applicable to the high frequency measurements of each underwing probe which is equipped with its own air speed sensor (e.g. a pitot-tube). ξ-values of 1 to 0.85 are calculated for air speeds (i.e. TAS) between 60 and 260 m s−1. From HALO data it is found that ξ does not significantly vary between the different deployed instruments. Thus, for the current HALO underwing probe configuration a parameterisation of ξ as a function of TAS is provided for instances if PAS measurements are lacking. The ξ-correction yields higher ambient particle concentration by about 15–25 % compared to conventional procedures – an improvement which can be considered as significant for many research applications. The calculated ξ-values are specifically related to the considered HALO underwing probe arrangement and may differ for other aircraft or instrument geometries. Moreover, the ξ-correction may not cover all impacts originating from high flight velocities and from interferences between the instruments and, e.g., the aircraft wings and/or fuselage. Consequently, it is important that PAS (as a function of TAS) is individually measured by each probe deployed underneath the wings of a fast-flying aircraft.