A Mid-IR, Wavelength-Scanned, Cavity Ring-Down Spectrometer for Continuous Trace N2O and Nitrogen Isotope Measurements

Nitrous oxide (N2O) is an important trace atmospheric gas with both a greenhouse effect and a role in ozone depletion. The globally averaged surface abundance of N2O was 314 ppb in 1998, corresponding to a global burden of 1510 TgN. The atmospheric burden of nitrous oxide continues to increase by about 0.25%/yr. The detailed impact of N2O cannot, however, be accurately assessed or mitigated as there is currently no quantitative analysis tool for N2O (and isotopes of nitrogen) that combines the requisite precision (sub-ppbv) with the rugged simplicity, low-drift and hands-free operation necessary for real-time field studies at unattended monitoring stations. In fact, the only laboratory tool capable of delivering this sensitivity - gas chromatography - is slow, relatively complex and labor intensive. Further, the utility of in-situ atmospheric nitrogen isotope analysis is practically nonexistent due to the impracticality of using isotope ratio mass spectrometry equipment in the field. We are addressing these needs by extending the capabilities of Picarro’s wavelength-scanned, cavity ring-down spectroscopy (WS-CRDS) instrumentation to cover N2O and nitrogen isotopes. These portable gas and isotope analyzers currently utilize one or more tunable, narrowband, near-infrared lasers to provide ppbv-level detection of several gases including CO2, CH4, H2O and various isotopes. However, detection of N2O at the ppbv level requires the use of longer, mid-infrared wavelengths. The advent of compact, tunable, quantum cascade lasers (QCLs) operating at room temperature in the mid-IR has recently enabled analyzers incorporating them to reach these sensitivities. Moreover, the long effective optical path length - over 15km - of WS-CRDS and corresponding high signal-to-noise ratio inherent in this time-based measurement means that high precision can be achieved with simple thermoelectric cooling of the laser and detector. We report the performance of this first-generation WS-CRDS-based analyzer utilizing a QCL with a center wavelength near 4.55 μm. Based on the field performance of existing WS-CRDS-based greenhouse gas analyzers and the known extinction coefficients for the target mid-IR N2O spectral lines, the targeted precision is <0.1 ppbv in only seconds of data acquisition time. Nitrogen isotopes in N2O can also be analyzed at the sub-permil level with this instrument. The analyzer is designed for continuous use both in the field and in the laboratory. Because this small-footprint instrument does not require frequent calibration and maintains high linearity, precision, and accuracy over changing environmental conditions, the analyzer offers low operating costs and thereby enables high-density field deployment. The ability to measure N2O and nitrogen isotopes with both high accuracy and high precision reduces the uncertainty in determination of terrestrial sources and sinks of this important greenhouse gas. Such knowledge is needed to improve predictive models that lead to a better understanding of the human contribution to global warming.