High accuracy measurements of dry mole fractions of carbon dioxide and methane in humid air

Abstract

Abstract. Traditional techniques for measuring the mole fractions of greenhouse gases in the well-mixed atmosphere have required dry sample gas streams (dew point < −25 °C) to achieve the inter-laboratory compatibility goals set forth by the Global Atmosphere Watch programme of the World Meteorological Organisation (WMO/GAW) for carbon dioxide (±0.1 ppm in the Northern Hemisphere and ±0.05 ppm in the Southern Hemisphere) and methane (±2 ppb). Drying the sample gas to low levels of water vapour can be expensive, time-consuming, and/or problematic, especially at remote sites where access is difficult. Recent advances in optical measurement techniques, in particular cavity ring down spectroscopy, have led to the development of greenhouse gas analysers capable of simultaneous measurements of carbon dioxide, methane and water vapour. Unlike many older technologies, which can suffer from significant uncorrected interference from water vapour, these instruments permit accurate and precise greenhouse gas measurements that can meet the WMO/GAW inter-laboratory compatibility goals (WMO, 2011a) without drying the sample gas. In this paper, we present laboratory methodology for empirically deriving the water vapour correction factors, and we summarise a series of in-situ validation experiments comparing the measurements in humid gas streams to well-characterised dry-gas measurements. By using the manufacturer-supplied correction factors, the dry-mole fraction measurements have been demonstrated to be well within the GAW compatibility goals up to a water vapour concentration of at least 1%. By determining the correction factors for individual instruments once at the start of life, this water vapour concentration range can be extended to at least 2% over the life of the instrument, and if the correction factors are determined periodically over time, the evidence suggests that this range can be extended up to and even above 4% water vapour concentrations.

Evaluating the performance of commonly used gas analysers for methane eddy covariance flux measurements: the InGOS inter-comparison field experiment

“…This instrument does both corrections automatically during measurement using coefficients reported in Chen et al (2010) and it has been shown that the automatic correction implemented in Picarro instruments performs well (e.g. Rella et al, 2013). Thus the fully corrected G2311-f CH 4 flux is a good reference for the other instruments and there should not be any residual H 2 O effect left in G2311-f fluxes.…”

“…(4) or (5) for different gas analysers. Hiller et al (2012) several studies (Chen et al, 2010;Hiller et al, 2012;Nara et al, 2012;Rella et al 2013), and it is nowadays widely used. It is worth noting that Eq.…”

H<sub>2</sub>S interference on CO<sub>2</sub> isotopic measurements using a Picarro G1101-i cavity ring-down spectrometer

“…The instrument also measures H 2 O and CH 4 concentrations continuously because of reported cross sensitivities with CO 2 for both water vapor (Rella et al, 2013) and methane gas (Vogel et al, 2013). We used the built-in water vapor correction to correct for variable water concentrations in each sample (Rella et al, 2013).…”

Accurate measurements of carbon monoxide in humid air using the cavity ring-down spectroscopy (CRDS) technique

“…The dilution effect is proportional to the mole fractions of CO, and the pressure-broadening effect is assumed to be proportional to the mole fractions of CO based on the fact that the coefficients a and b are close to those for CO 2 . Transferring the water correction coefficients for CO 2 from one CRDS analyzer to another causes uncertainty on the order of 0.1 ppm out of 400 ppm for CO 2 Rella et al, 2013), which is at least one order of magnitude more accurate than needed for the transferability of CO water corrections for dilution and pressure broadening among different analyzers (with the WMO-recommended inter-laboratory compatibility goal of 2 ppb for CO). Furthermore, the total water vapor dilution and pressure-broadening effects for CO are relatively small (∼ 5 ppb for 2 % water vapor at a level of 200 ppb CO) when compared to the line interference effect (up to ∼ 20 ppb for the range of 0-2 % water vapor).…”

“…In fact, CRDS analyzers that measure CO 2 /CH 4 /CO/H 2 O or CO 2 /CO/H 2 O (Picarro, Inc., Santa Clara, CA, USA, the G2401 or G2302 series) in one unit are commercially available. Extensive efforts have been carried out to make accurate measurements of CO 2 and CH 4 possible without drying the sample air Winderlich et al, 2010;Nara et al, 2012;Rella et al, 2013). It is therefore beneficial to be able to accurately measure CO in humid air with the same analyzer as well.…”