Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem-atmosphere CO2 exchange

The stable isotope content of atmospheric CO2 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 CO2 at atmospheric mole fractions (350-700 mumol mol(-1)) and isotopic abundance (delta(13)C of -6 to - 16parts per thousand). Using infrared absorption, the TDL system determines the mole fractions of (CO2)-C-12 and (CO2)-C-13 independently, rather than their ratio as in mass spectrometry (MS). The ability of the instrument to measure isotope ratios (delta(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 delta(13)C of 1.77parts per thousand between the TDL and MS measurements, and the standard deviation of the error (MS - TDL) was 0.35parts per thousand (n = 82). Removal of two outliers improved this standard deviation to 0.25parts per thousand (n = 80). After removing the offset, 62 out of 82 samples had absolute differences less than 0.3parts per thousand. 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 CO2 mixed with air rather than nitrogen. Based on these results we estimate the precision for delta(13)C to be 0.25parts per thousand for our sampling scheme. A similar comparison with flask-based measurements of CO2 mole fraction ((CO2)-C-12 + (CO2)-C-13) made with a calibrated infrared gas analyzer indicated a TDL precision of 0.4% (1.6 mumol mol(-1) at 400 mumol mol(-1)). The TDL was used to investigate the vertical and temporal variation in the carbon isotope content of respired CO2 (delta(13) C-R) from the grassland. Measurements of delta(13)C of CO2 in air were made during four separate nights at 1 and 60 cm height above ground. 313 CR did not vary with height, but it did vary from one night to the next. Hourly measurements of 313 CR showed it changed as much as 6.4parts per thousand (-29.1 +/- 0.4 to -22.7 +/- 0.8parts per thousand) in a single night. Temporal changes in delta(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. (C) 2003 Elsevier Science B.V. All rights reserved.