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Discussion papers | Copyright
https://doi.org/10.5194/amt-2018-274
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Sep 2018

Research article | 07 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

A decade of CH4, CO and N2O in situ measurements at Lauder, New Zealand: assessing the long-term performance of a Fourier transform infra-red trace gas and isotope analyser

Dan Smale1, Vanessa Sherlock1, David W. T. Griffith2, Rowena Moss1, Gordon Brailsford1, Sylvia Nichol1, and Michael Kotkamp1 Dan Smale et al.
  • 1National Institute of Water and Atmospheric Research Ltd (NIWA), New Zealand
  • 2Centre for Atmospheric Chemistry, University of Wollongong, Australia

Abstract. We present a ten-year (Jan 2007–Dec 2016) time series of continuous in situ measurements of methane (CH4), carbon monoxide (CO) and nitrous oxide (N2O) made by an in situ Fourier transform infra-red trace gas and isotope analyser (FTIR) operated at Lauder, New Zealand (45.04S, 169.68E, 370mAMSL). Being the longest continuous deployed operational FTIR system of this type, we are in an ideal position to perform a practical evaluation of multi-year performance of the analyser. The operational methodology, measurement precision, reproducibility, accuracy and instrument reliability are reported.

We find the FTIR has a measurement repeatability of the order of 0.37ppb (1-sigma standard deviation) for CH4, 0.31ppb for CO and 0.12ppb for N2O. Regular target cylinder measurements provide a reproducibility estimate of 1.19ppb for CH4, 0.74ppb for CO and 0.27ppb for N2O. FTIR measurements are compared to co-located ambient air flask samples acquired at Lauder since May 2009, which allows a long-term assessment of the FTIR data set across annual and seasonal composition changes. Comparing FTIR and co-located flask measurements show that the bias (FTIR minus flask) for CH4 of −1.02ppb±2.61 and CO of −0.43ppb±1.60 are within the Global Atmospheric Watch (GAW) recommended compatibility goals of 2ppb. The N2O FTIR flask bias of −0.01ppb±0.77 is within the GAW recommended compatibility goals of 0.1ppb should be viewed as a serendipitous result due to the large standard deviation along with known systematic differences in the measurement sets. Uncertainty budgets for each gas are also constructed based upon instrument precision, reproducibility and accuracy. In the case of CH4, systematic uncertainty dominates whilst for CO and N2O it is comparable to the random uncertainty component.

The long-term instrument stability, precision estimates and flask comparison results indicate the FTIR CH4 and CO time series meet the GAW compatibility recommendations across multiple years of operation, (and instrument changes), and is sufficient to capture annual trends and seasonal cycles observed at Lauder. The differences between FTIR and flask N2O measurements need to be reconciled. Trend analysis of the ten-year time series captures seasonal cycles, the secular upward trend of CH4 and N2O. The CH4 and CO time series have the required precision and accuracy at a high enough temporal resolution to be used in inversion models in a data sparse region of the world.

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We present a ten-year (Jan 2007–Dec 2016) time series of continuous in situ measurements of methane, carbon monoxide and nitrous oxide made by an in situ Fourier transform infra-red trace gas and isotope analyser operated at Lauder, New Zealand. We perform a practical evaluation of multi-year performance of the analyser and report on operational methodology, measurement precision, reproducibility, accuracy and instrument reliability.
We present a ten-year (Jan 2007–Dec 2016) time series of continuous in situ measurements of...
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