Effects of water vapor dilution on trace gas flux, and practical correction methods

Abstract

The effect of water vapor dilution on CH 4 flux was examined here using dynamic chamber measurements at the forest floor of a black spruce forest in interior Alaska. CH 4 and CO 2 concentrations were differently diluted by increased water vapor in each chamber. After correction for water vapor dilution was applied, source (positive) CH 4 flux was enhanced, whereas sink (negative) flux was reduced, and magnitude changed depending on the condition of water vapor.Several methods were examined to practically correct water vapor dilution for fluxes previously measured. Numerical correction for all data sampled at high frequency intervals (DAE-method) is highly recommended. Water vapor dilution correction for ensemble-averaged data (SA-method) is also applicable, though not recommended in case of long-term average data for accurate flux determination, especially for data obtained at irregular source strength plots. When evapotranspiration is measured in parallel to CH 4 flux, the removal of the water vapor increment due to evapotranspiration in the chamber is another option for correction (HCE-method). This method resulted in similar accuracy to the DAEmethod, as long as continuously sampled data were applicable.The application of the correction for water vapor dilution to previous tower-based flux data (measured over a wet sedge tundra in Barrow throughout the month of August, 2000) also resulted in an increase in source (positive) flux of 6.6% on average.This study concludes that either accurate flux must be determined using mixing ratio, or a water vapor dilution correction must be applied using simultaneously measured water vapor and trace gases.

Pore Distances of Particulate Organic Matter to Predict N2o Emissions from Intact Soil at Moist Conditions

“…The gas concentrations were monitored continuously with a scan rate of one second, using an infrared spectrometer QCLTILDAS (Aerodyne Research). Mixing ratio of the N 2 O was corrected on water vapor dilution and on broadening effects (Harazono et al, 2015;Deng et al, 2017). The duration of the measurement for each cell was 10 min.…”

Capturing temporal heterogeneity in soil nitrous oxide fluxes with a robust and low-cost automated chamber apparatus

“…The correction under a more moderate water vapor difference between inlet and outlet (<12.6 millimoles mole -1 ) that spans >97% of observed differences is approximately half the impact of this extreme example (0.09 umol m -2 s -1 ). Unrelated to its impacts on instrument performance, water vapor can also impact flux measurements by dilution (Harazono et al, 2015). Given 365 an average water vapor difference between inlet and outlet of 1.8 mmol mole -1 and maximum of 36.4 mmol mole -1 , impacts of dilution on measured fluxes would also be small: typically <0.18% and as much as 3.6%.…”

Adaptation and performance assessment of a quantum and interband cascade laser spectrometer for simultaneous airborne in situ observation of CH&lt;sub&gt;4&lt;/sub&gt;, C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt;, CO&lt;sub&gt;2&lt;/sub&gt;, CO and N&lt;sub&gt;2&lt;/sub&gt;O

“…It digests the sample cell pressure and temperature measurements to generate a synthetic spectrum based on line-by-line parameters from the HITRAN2012 and HITRAN2016 (Rothman et al, 2013;Gordon et al, 2017) database using a conventional Voigt profile approach. Ethane line-by-line data have been taken from high-resolution Fourier-transform infrared spectra due to deficiencies in the HITRAN data for this particular specieswavenumber combination (Harrison et al, 2010). The computation of the Voigt profile has been adopted from Abrarov and Quine (2015).…”