Global Measurement of Nitrous Oxide Stable Isotopes Using Cavity Ring-Down Spectroscopy

Nitrous oxide continues to increase in the atmosphere mainly due to heightened microbial production from fertilized agricultural systems. Soil microorganism processes are spatiotemporally heterogeneous, limiting our ability to constrain the anthropogenic influence on N2O production at a global scale. The intramolecular position of 15N (β position 15N14N16O versus α position 14N15N16O) in addition to δ15Nbulk-N2O can aid in our understanding of both the biological controls and stratospheric influence of tropospheric N2O. A subset of 22 sites from the NOAA Global Monitoring Division Cooperative Sampling Network is being measured in order to describe the global distribution and seasonality of N2O isotopocules. Simultaneous and continuous measurement of N2O mole fraction, δ15Nbulk-N2O, δ15Nα-N2O, and δ15Nβ-N2O is conducted using the Picarro G5101-i wavelength-scanned cavity ring-down spectrometer coupled with a quantum cascade laser capable of the mid-infrared wavelength detection needed for N2O. While isotopic differences within and between sites are observed, long term measurement uncertainties of 0.7‰, 0.8‰, and 1.3‰ for δ15Nbulk, δ15Nα, and δ15Nβ respectively, limit our ability to detect tropospheric trends. Applying additional correction factors for environmental conditions and molecular interference may help to reduce these uncertainties. Due to the lack of isotopic reference material for N2O, we have developed an isotopic calibration technique using trace additions of δ15Nα and δ15Nβ to our reference gas at the ambient mole fraction needed for laser based isotopic measurement.