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| Titel |
Modified cavity attenuated phase shift (CAPS) method for airborne aerosol light extinction measurement |
| VerfasserIn |
Julia Perim de Faria, Ulrich Bundke, Andrew Freedman, Andreas Petzold |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106403
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| Publikation (Nr.) |
 EGU/EGU2015-6073.pdf |
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| Zusammenfassung |
| Monitoring the direct impact of aerosol particles on climate requires the consideration of at
least two major factors: the aerosol single-scattering albedo, defined as the relation between
the amount of energy scattered and extinguished by an ensemble of aerosol particles; and the
aerosol optical depth, calculated from the integral of the particle extinction coefficient over
the thickness of the measured aerosol layer. Remote sensing networks for measuring
these aerosol parameters on a regular basis are well in place (e.g., AERONET,
ACTRIS), whereas the regular in situ measurement of vertical profiles of atmospheric
aerosol optical properties remains still an important challenge in quantifying climate
change.
The European Research Infrastructure IAGOS (In-service Aircraft for a Global Observing
System; www.iagos.org) responds to the increasing requests for long-term, routine in situ
observational data by using commercial passenger aircraft as measurement platform.
However, scientific instrumentation for the measurement of atmospheric constituents
requires major modifications before being deployable aboard in-service passenger
aircraft.
Recently, a compact and robust family of optical instruments based on the cavity attenuated
phase shift (CAPS) technique has become available for measuring aerosol light extinction. In
particular, the CAPS PMex particle optical extinction monitor has demonstrated sensitivity of
less than 2 Mm-1 in 1 second sampling period; with a 60 s averaging time, a detection limit
of less than 0.3 Mm-1 can be achieved. While this technique was successfully deployed for
ground-based atmospheric measurements under various conditions, its suitability
for operation aboard aircraft in the free and upper free troposphere still has to be
demonstrated.
Here, we report on the modifications of a CAPS PMex instrument for measuring aerosol light
extinction on aircraft, and subsequent laboratory tests for evaluating the modified
instrument prototype: (1) In a first set of tests, the robustness of the method was
demonstrated down to pressure levels below 200 hPa, using air and CO2 as test gases.
Rayleigh scattering cross-section values for both gases deviated by less than 5 %
from literature data for all investigated pressure levels.(2) The measurement of
aerosol particles at lower pressure levels required the modification of the air flow
handling. A new flow scheme using mass flow controllers and a revised vacuum
pump set-up was developed and successfully tested. The overall reduction of the
instrument noise level to values less than 0.15 Mm-1 was achieved. (3) Polydisperse
laboratory-generated ammonium sulphate particles and monodisperse polystyrene latex
spheres where used to evaluate the instrument operation for the pressure range from
1000 hPa to less than 200 hPa against an optical particle counter. Reference aerosol
extinction coefficients were calculated from measured size distributions, using Mie
theory. We found less than 10 % deviation between the CAPS PMex instrument
response and calcuated extinction coefficients over the investigated pressure range. |
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