Automated chamber technique for gaseous flux measurements: Evaluation of a photoacoustic infrared spectrometer-trace gas analyzer

摘要

Experiments were made in order to evaluate the accuracy and sensitivity of a photoacoustic infrared trace gas analyzer (TGA) in conjunction with an automatic opening and closing chamber system developed for near-continuous (2 min intervals) soil gaseous flux measurements. Humidity interference tests on N_2O, CH_4, and CO_2 concentrations measured by the TGA were carried out, and the results showed a linear interference, with correction factors of 3 * 10~(-5)x, 1.9 * 10~(-3)x and 4.4 * 10~(-3)x (x = H_2O vapor ppm), respectively. CO_2 interference on N_2O and CH_4 signals were also linear, with average correction factors of 2.8 * 10~(-4)x and 6 * 10~(-5)x (x = CO_2 ppm), respectively. Laboratory intercomparisons between the TGA and GC measurements of N_2O and CH_4 standards showed good agreement (R~2 > 0.993), indicating the accuracy of the TGA for measurement of these gases at concentrations up to 100 and 40 ppm N_2O and CH_4, respectively. The relatively rapid measurement time for up to five gases simultaneously in 2 min, linearity, and ease of operation of the TGA represent major advantages compared to gas chromatography (GC). The automated chamber system provides a continuous measurement of fluxes with minimum disturbance to the soil environment enclosed by the chamber and provides the means, for example, of quantifying diurnal variability. In situ measurements of N_2O-N and CH_4-C fluxes with a sensitivity <10 g ha~(-1) d~(-1) (11.6 ng m~(-2) s~(-1)), as well as of CO_2 and water vapor (H_2O), can be measured by the TGA when used with the automated system, and fluxes at background levels (i.g., from unfertilized soils) can be determined.