Feasibility of 13CO2 eddy covariance flux measurements using pulsed quantum cascade tunable infrared laser differential absorb

Better quantification of atmosphere-ecosystem exchange of the isotopologues of CO2 could substantially improve our ability to probe underlying physiological and ecological mechanisms controlling ecosystem carbon exchange, but the ability to make long-term continuous measurements of the isotopic composition of exchange fluxes has been limited by measurement difficulties. Quantum cascade (QC) lasers are a new generation of infrared light sources that offer increased stability and power for absorption spectroscopy applications, including the measurement of atmospheric CO2 isotope ratios, and promise substantial improvements over existing instruments: smaller size, increased robustness, and most significantly for remote or long-term field deployments, no need for cryogenic cooling of laser or detectors. In this paper, we used simulations to test whether the performance of a prototype pulsed QC laser-based isotope-ratio absorption spectrometer (and plausible improvements thereon) is sufficient for making direct eddy covariance measurements of the isotopic composition of CO2 fluxes above a mid-latitude temperate forest (Harvard Forest, in central Massachusetts, USA). We found that the simulated isoflux measurement error associated with the prototype instrument is not more than ~1.5 to 2 times larger than the irreducible “meteorological” noise inherently associated with turbulent flux measurements above this ecosystem (daytime measurement error SD of ~60% of flux versus meteorological noise of 30-40% for instantaneous half-hour fluxes), and that plausible instrument improvements could increase precision to the point where measurement error is comparable to or less than the meteorological noise (measurement error SD reduced to ~ 25% of halfhourly flux; and to ~10% or less of mean 10-day mean hourly flux). This suggests that QCLbased isotope ratio absorption spectroscopy should be able to approach the precision at which sensor error is not a limiting factor in measuring CO2 isotope fluxes via eddy covariance.