Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem–atmosphere CO 2 exchange

Abstract

The stable isotope content of atmospheric CO 2 provides information about ecosystem carbon–water relations and biosphere–atmosphere carbon exchange. Virtually every isotope study within these fields has required air sample collection at remote locations followed by isotope analysis at a laboratory. This requirement severely limits sampling frequency and experiment duration. In this paper, we evaluate a tunable diode laser absorption spectrometer (TDL) for measuring the carbon isotope content of CO 2 at atmospheric mole fractions (350–700 μmol mol −1) and isotopic abundance ( δ 13 C of −6 to −16‰). Using infrared absorption, the TDL system determines the mole fractions of 12 CO 2 and 13 CO 2 independently, rather than their ratio as in mass spectrometry (MS). The ability of the instrument to measure isotope ratios ( δ 13 C ) was tested outdoors in a grassland and compared to standard laboratory-based MS measurements made on field-collected flask samples. The TDL was operated at a sampling flow rate of 230 ml min −1 and a sampling interval of 2 min for two intake heights. There was a consistent offset for δ 13 C of 1.77‰ between the TDL and MS measurements, and the standard deviation of the error (MS−TDL) was 0.35‰ ( n=82). Removal of two outliers improved this standard deviation to 0.25‰ ( n=80). After removing the offset, 62 out of 82 samples had absolute differences less than 0.3‰. Subsequent laboratory experiments indicated that the TDL/MS offset was caused by pressure broadening, and can be avoided in the future by calibrating the TDL with CO 2 mixed with air rather than nitrogen. Based on these results we estimate the precision for δ 13 C to be 0.25‰ for our sampling scheme. A similar comparison with flask-based measurements of CO 2 mole fraction ( 12 CO 2+ 13 CO 2 ) made with a calibrated infrared gas analyzer indicated a TDL precision of 0.4% (1.6 μmol mol −1 at 400 μmol mol −1). The TDL was used to investigate the vertical and temporal variation in the carbon isotope content of respired CO 2 ( δ 13 C R ) from the grassland. Measurements of δ 13 C of CO 2 in air were made during four separate nights at 1 and 60 cm height above ground. δ 13 C R did not vary with height, but it did vary from one night to the next. Hourly measurements of δ 13 C R showed it changed as much as 6.4‰ (−29.1±0.4 to −22.7±0.8‰) in a single night. Temporal changes in δ 13 C R during the night have not been reported in prior studies. Such observations could provide a new way to investigate temporal dynamics of the carbon substrates utilized for ecosystem respiration.