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| Titel |
Analysis of the PKT correction for direct CO2 flux measurements over the ocean |
| VerfasserIn |
S. Landwehr, S. D. Miller, M. J. Smith, E. S. Saltzman, B. Ward |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 7 ; Nr. 14, no. 7 (2014-04-04), S.3361-3372 |
| Datensatznummer |
250118563
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| Publikation (Nr.) |
 copernicus.org/acp-14-3361-2014.pdf |
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| Zusammenfassung |
| Eddy covariance measurements of air–sea CO2 fluxes can be
affected by cross-sensitivities of the CO2 measurement to
water vapour, resulting in order-of-magnitude biases. Well-established
causes for these biases are (i) cross-sensitivity of the
broadband non-dispersive infrared sensors due to band-broadening and
spectral overlap (commercial sensors typically correct for this) and
(ii) the effect of air density fluctuations (removed by determining
the dry air CO2 mixing ratio). Another bias related to water
vapour fluctuations has recently been observed with open-path sensors,
attributed to sea salt build-up and water films on sensor optics. Two
very different approaches have been used to deal with these water
vapour-related biases. Miller et al. (2010) employed a membrane drier to
physically eliminate 97% of the water vapour fluctuations in the
sample air before it entered a closed-path gas analyser.
Prytherch et al. (2010a) employed the empirical (Peter K. Taylor,
PKT) post-processing correction to correct open-path sensor data. In
this paper, we test these methods side by side using data from the
Surface Ocean Aerosol Production (SOAP) experiment in the Southern
Ocean. The air–sea CO2 flux was directly measured with four
closed-path analysers, two of which were positioned down-stream of
a membrane dryer. The CO2 fluxes from the two dried gas
analysers matched each other and were in general agreement with common
parameterisations. The flux estimates from the un-dried sensors agreed
with the dried sensors only during periods with low latent heat flux
(≤7 W m−2). When latent heat flux was higher,
CO2 flux estimates from the un-dried sensors exhibited large
scatter and an order-of-magnitude bias. Applying the PKT correction
to the flux data from the un-dried analysers did not remove the bias
when compared to the data from the dried gas analyser. The results of
this study demonstrate the validity of measuring CO2 fluxes
using a pre-dried air stream and show that the PKT correction is not
valid for the correction of CO2 fluxes. |
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