Preprints
https://doi.org/10.5194/amt-2017-174
https://doi.org/10.5194/amt-2017-174
07 Jun 2017
 | 07 Jun 2017
Status: this preprint was under review for the journal AMT but the revision was not accepted.

An improved water correction function for Picarro greenhouse gas analyzers

Friedemann Reum, Christoph Gerbig, Jost V. Lavric, Chris W. Rella, and Mathias Göckede

Abstract. Measurements of dry air mole fractions of atmospheric greenhouse gases are widely used in inverse models of atmospheric tracer transport to quantify the sources and sinks of the gases. The measurements have to be calibrated to a common scale to avoid bias in the inferred fluxes. The World Meteorological Organization (WMO) has set requirements for the inter-laboratory compatibility of atmospheric greenhouse gas measurements to ±0.1 ppm for CO2 (Southern hemisphere ±0.05 ppm) and to ±2 ppb for CH4. An established series of devices for measurements of greenhouse gas (GHG) mole fractions are the trace gas analyzers manufactured by Picarro, Inc. These have been shown to deliver dry air mole fractions with accuracies within the WMO goals when trace gas signals are measured in wet air and the effects of water vapor are corrected for. Here, we report for the first time on sensitivity of the pressure inside the measurement cavity of Picarro GHG analyzers to water vapor. This sensitivity induces biases in the inferred dry air mole fractions of CO2 and CH4 if they are obtained using the traditional water correction function. To correct for the pressure effect, we add a pressure-related term to the traditional water correction function, and consider differences between the traditional and enhanced water correction function to be biases of the traditional model. The effect primarily affects low water vapor mole fractions from about 0.05 to about 0.5 %, a domain that has gone undersampled in previous studies of the water correction for Picarro GHG analyzers. We observed biases up to about 40 % of the WMO tolerances (80 % for CO2 in the southern hemisphere). The magnitude of the effect varied across instruments and appeared to be negligible for some, and our experimental results were more robust for CH4 than for CO2. Thus, correction coefficients should be determined for each analyzer individually. Applying our enhanced water correction function improves the accuracy of measurements of dry air mole fractions of CO2 and CH4 in humid air with Picarro GHG analyzers on a scale important for keeping the measurement accuracy within the WMO requirements.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Friedemann Reum, Christoph Gerbig, Jost V. Lavric, Chris W. Rella, and Mathias Göckede
 
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Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement
Friedemann Reum, Christoph Gerbig, Jost V. Lavric, Chris W. Rella, and Mathias Göckede
Friedemann Reum, Christoph Gerbig, Jost V. Lavric, Chris W. Rella, and Mathias Göckede

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Latest update: 08 Dec 2024
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Short summary
High-accuracy observations of atmospheric CO2 and CH4 levels, which are vital for quantifying sources and sinks of these gases, are often obtained using Picarro greenhouse gas analyzers. These require a correction for the effects of water vapor. We report biases in CO2 and CH4 levels obtained using the traditional water correction for Picarro analyzers related to pressure changes in the optical cavity and mainly affecting measurements at low water vapor mole fractions, and how to correct them.