Abstract
Abstract. In this investigation we analyze two common optical configurations to retrieve CO2 total column amounts from solar absorption infrared spectra. The noise errors using either a KBr or a CaF2 beam splitter, a main component of a Fourier transform infrared spectrometer (FTIR), are quantified in order to assess the relative precisions of the measurements. The configuration using a CaF2 beam splitter, as deployed by the instruments which contribute to the Total Carbon Column Observing Network (TCCON), shows a slightly better precision. However, we show that the precisions in XCO2 ( = 0.2095 ⋅ Total Column CO2Total Column O2) retrieved from > 96 % of the spectra measured with a KBr beam splitter fall well below 0.2 %. A bias in XCO2 (KBr − CaF2) of +0.56 ± 0.25 ppm was found when using an independent data set as reference. This value, which corresponds to +0.14 ± 0.064 %, is slightly larger than the mean precisions obtained. A 3-year XCO2 time series from FTIR measurements at the high-altitude site of Altzomoni in central Mexico presents clear annual and diurnal cycles, and a trend of +2.2 ppm yr−1 could be determined.
Highlights
During the last decades, carbon dioxide (CO2) has exceeded the pre-industrial levels by about 40 % mainly due to fossil fuel combustion and land use change (Hartmann et al, 2013), contributing more than any other anthropogenic gas to the positive total radiative forcing of the Earth and becoming the most important anthropogenic greenhouse gas (Myhre et al, 2013)
Total Carbon Column Observing Network (TCCON) is a network of Fourier transform infrared spectrometers (FTIRs) that record solar absorption spectra in the near-infrared (NIR, 3300–13 000 cm−1) spectral region in order to retrieve column-averaged dry-air mole fractions of CO2 (XCO2 ) and other molecules that absorb in the NIR (Wunch et al, 2011)
Measurements of CO2 from space have been done by many satellite missions like ACE (Foucher et al, 2011), AIRS (Chahine et al, 2008), IASI (Crevoisier et al, 2009), TES (Kulawik et al, 2010), SCIAMACHY (Reuter et al, 2011), GOSAT (Kuze et al, 2009) and OCO-2 (Wunch et al, 2017), with the last three missions relying on TCCON data for validation
Summary
Received: 24 December 2016 – Discussion started: 11 January 2017 Revised: 28 May 2017 – Accepted: 30 May 2017 – Published: 6 July 2017
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