Evaluation of Photoacoustic Infrared Spectroscopy for Simultaneous Measurement of N2O and CO2 Gas Concentrations and Fluxes At the Soil Surface.

Photoacoustic infrared spectroscopy (PAS) measurement of N2O and CO2
flux at the soil surface is gaining widespread popularity due to portability
and ease-of-operation. However, the ability of PAS to measure N2O with
accuracy and precision similar to gas chromatography (GC) methods is
uncertain due to large overlap in N2O, CO2, and H2O absorbance spectra
combined with the large range in analyte concentrations (~380 PPM CO2
vs. ~0.320 PPM N2O). We tested the ability of six PAS units to
simultaneously measure N2O and CO2 gas concentrations and fluxes with
accuracy and precision similar to two GC units. We also evaluated H2O
vapor and CO2 interferences with N2O measurement. Among the
individual PAS units, NIST-certified concentrations of N2O (0.7148 PPM)
and CO2 (473.45 PPM) in ‘breathing air’ were measured within 0.5-8.8%
and 3.1-8.6% of the known concentrations, respectively. Coefficients of
variation (CV) for repeated measurements of the NIST-certified gas with
each PAS unit ranged from 1.20-2.52% for N2O and 0.46-0.96% for CO2.
The two GC units measured the NIST-certified N2O concentration within
2.68 and 6.73% with CVs for repeated measurements of 1.97 and 5.18%,
respectively. High water vapor (~26,600 PPM) and CO2 concentrations
(~4500 PPM) did not interfere with N2O measurement across the
concentration range typically observed in static flux chambers at the soil
surface. On average, N2O fluxes measured with the six PAS were 4.7%
higher than one GC, and 9.9% lower than the second GC.