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| Titel |
Correction for a measurement artifact of the Multi-Angle Absorption Photometer (MAAP) at high black carbon mass concentration levels |
| VerfasserIn |
A.-P. Hyvärinen, V. Vakkari, L. Laakso, R. K. Hooda, V. P. Sharma, T. S. Panwar, J. P. Beukes, P. G. Zyl, M. Josipovic, R. M. Garland, M. O. Andreae, U. Pöschl, 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 ; 6, no. 1 ; Nr. 6, no. 1 (2013-01-11), S.81-90 |
| Datensatznummer |
250017372
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| Publikation (Nr.) |
 copernicus.org/amt-6-81-2013.pdf |
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| Zusammenfassung |
| The Multi-Angle Absorption Photometer (MAAP) is a widely-used instrument for
aerosol black carbon (BC) measurements. In this paper, we show correction methods
for an artifact found to affect the instrument accuracy in environments
characterized by high black carbon concentrations. The artifact occurs after
a filter spot change – as BC mass is accumulated on a fresh filter spot,
the attenuation of the light (raw signal) is weaker than anticipated. This
causes a sudden decrease, followed by a gradual increase in measured BC
concentration. The artifact is present in the data when the BC concentration
exceeds ~3 μg m−3 at the typical MAAP flow rate of
16.7 L min−1 or 1 m3 h−1. The artifact is caused
by erroneous dark counts in the photodetector measuring the transmitted
light, in combination with an instrument internal averaging procedure of the
photodetector raw signals. It was found that, in addition to the erroneous
temporal response of the data, concentrations higher than 9 μg m−3
(at the flow rate of 16.7 L min−1) are underestimated by the
MAAP. The underestimation increases with increasing BC accumulation rate. At
a flow rate of 16.7 L min−1 and concentration of about 24 μg m−3
(BC accumulation rate ~0.4 μg min−1), the underestimation is about 30%. There are two ways
of overcoming the MAAP artifact. One method is by logging the raw signal of
the 165° photomultiplier measuring the reflected light from the
filter spot. As this signal is not affected by the artifact, it can be
converted to approximately correct absorption and BC values. However, as the
typical print formats of the MAAP do not give the reflected signal as an
output, a semi-empirical correction method was developed based on laboratory
experiments to correct for the results in the post-processing phase. The
correction function was applied to three MAAP datasets from Gual Pahari
(India), Beijing (China), and Welgegund (South Africa). In Beijing, the
results could also be compared against a photoacoustic spectrometer (PAS).
The correction improved the quality of all three MAAP datasets
substantially, even though the individual instruments operated at different
flow rates and in different environments. |
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