Thermodynamic correction of particle concentrations measured by underwing probes on fast-flying aircraft
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 correctio...
| Main Authors: | , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-10-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | http://www.atmos-meas-tech.net/9/5135/2016/amt-9-5135-2016.pdf |
| Summary: | 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 to 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 air volume probed per time interval is determined by 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 <i>G-550</i> (<i>HALO</i> – 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 <i>ξ</i> is applicable to the high-frequency measurements of
the underwing probes, each of which is equipped with its own air speed sensor
(e.g. a pitot tube). <i>ξ</i> values of 1 to 0.85 are calculated for air speeds
(i.e. TAS) between 60 and 250 m s<sup>−1</sup>. For different instruments at
individual wing position the calculated <i>ξ</i> values exhibit strong
consistency, which allows for a parameterisation of <i>ξ</i> as a function of
TAS for the current <i>HALO</i> underwing probe configuration. The ability
of cloud particles to adopt changes of air speed between ambient and
measurement conditions depends on the cloud particles' inertia as a function
of particle size (diameter <i>D</i><sub>p</sub>). The suggested inertia correction
factor <i>μ</i> (<i>D</i><sub>p</sub>) for liquid cloud drops ranges between 1 (for
<i>D</i><sub>p</sub> < 70 µm) and 0.8 (for
100 µm < <i>D</i><sub>p</sub> < 225 µm) but
it needs to be applied carefully with respect to the particles' phase and
nature. The correction of measured concentration by both factors, <i>ξ</i> and
<i>μ</i> (<i>D</i><sub>p</sub>), yields higher ambient particle concentration by about
10–25 % compared to conventional procedures – an improvement which can
be considered as significant for many research applications. The calculated
<i>ξ</i> values are specifically related to the considered <i>HALO</i>
underwing probe arrangement and may differ for other aircraft. Moreover,
suggested corrections 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. |
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| ISSN: | 1867-1381 1867-8548 |