Continuous field measurements of delta C-13-CO2 and trace gases by FTIR spectroscopy

Continuous analysis of the C-13/C-12 ratio of atmospheric CO2 (C-13-CO2) is a powerful tool to quantify CO2 flux strengths of the two major ecosystem processes assimilation and respiration. Traditional laboratory techniques such as isotope ratio mass spectrometry (IRMS) in combination with flask sampling are subject to technical limitations that do not allow to fully characterising variations of atmospheric C-13-CO2 at all relevant timescales. In our study, we demonstrate the strength of Fourier transform infrared (FTIR) spectroscopy in combination with a PLS-based calibration strategy for online analysis of C-13-CO2 in ambient air. The ability of the instrument to measure C-13-CO2 was tested on a grassland field-site and compared with standard laboratory-based IRMS measurements made on field-collected flask samples. Both methods were in excellent agreement, with an average difference of 0.4 (n = 81). Simultaneously, other important trace gases such as CO, N2O and CH4 were analysed by FTIR spectroscopy.