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| Titel |
Particle sizing calibration with refractive index correction for light scattering optical particle counters and impacts upon PCASP and CDP data collected during the Fennec campaign |
| VerfasserIn |
P. D. Rosenberg, A. R. Dean, P. I. Williams, J. R. Dorsey, A. Minikin, M. A. Pickering, A. Petzold |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 5, no. 5 ; Nr. 5, no. 5 (2012-05-21), S.1147-1163 |
| Datensatznummer |
250002870
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| Publikation (Nr.) |
 copernicus.org/amt-5-1147-2012.pdf |
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| Zusammenfassung |
Optical particle counters (OPCs) are used regularly for atmospheric
research, measuring particle scattering cross sections to generate particle
size distribution histograms. This manuscript presents two methods for
calibrating OPCs with case studies based on a Passive Cavity Aerosol
Spectrometer Probe (PCASP) and a Cloud Droplet Probe (CDP), both of which
are operated on the Facility for Airborne Atmospheric Measurements BAe-146
research aircraft.
A probability density function based method is provided for modification of
the OPC bin boundaries when the scattering properties of measured particles
are different to those of the calibration particles due to differences in
refractive index or shape. This method provides mean diameters and widths
for OPC bins based upon Mie-Lorenz theory or any other particle scattering
theory, without the need for smoothing, despite the highly nonlinear and
non-monotonic relationship between particle size and scattering cross
section. By calibrating an OPC in terms of its scattering cross section the
optical properties correction can be applied with minimal information loss,
and performing correction in this manner provides traceable and transparent
uncertainty propagation throughout the whole process.
Analysis of multiple calibrations has shown that for the PCASP the bin
centres differ by up to 30% from the manufacturer's nominal
values and can change by up to approximately 20% when routine
maintenance is performed. The CDP has been found to be less sensitive than
the manufacturer's specification with differences in sizing of between
1.6 ± 0.8 μm and 4.7 ± 1.8 μm for one
flight. Over the course of the Fennec project in the Sahara the variability
of calibration was less than the calibration uncertainty in 6 out of 7
calibrations performed.
As would be expected from Mie-Lorenz theory, the impact of the refractive
index corrections has been found to be largest for absorbing materials and
the impact on Saharan dust measurements made as part of the Fennec project
has been found to be up to a factor of 3 for the largest particles measured
by CDP with diameters of approximately 120 μm.
In an example case, using the calibration and refractive index corrections
presented in this work allowed Saharan dust measurement from the PCASP, CDP
and a Cloud Imaging Probe to agree within the uncertainty of the
calibration. The agreement when using only the manufacturer's specification
was poor.
Software tools have been developed to perform these calibrations and
corrections and are now available as open source resources for the community
via the SourceForge repository. |
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