Tracking nitrous oxide emission processes at a suburban site with semicontinuous, in situ measurements of isotopic composition

The isotopic composition of atmospheric nitrous oxide (N2O) was measured semicontinuously, at similar to 35 min frequency in intermittent periods of 1-6 days over one and a half years, using preconcentration coupled to a quantum cascade laser spectrometer at the suburban site of Dubendorf, Switzerland. The achieved measurement repeatability was 0.08%, 0.11%, and 0.10% for delta O-18, site preference, and delta N-15(bulk) respectively, which is better than or equal to standard flask sampling-based isotope ratio mass spectrometry performance. The observed mean diurnal cycle reflected the buildup of N2O from isotopically light sources on an isotopically heavy tropospheric background. The measurements were used to determine the source isotopic composition, which varied significantly compared to chemical and meteorological parameters monitored at the site. FLEXPART-COSMO transport modeling in combination with modified Emissions Database for Global Atmospheric Research inventory emissions was used to model N2O mole fractions at the site. Additionally, isotopic signatures were estimated for different source categories using literature data and used to simulate N2O isotopic composition over the measurement period. The model was able to capture variability in N2O mole fraction well, but simulations of isotopic composition showed little agreement with observations. In particular, measured source isotopic composition exhibited one magnitude larger variability than simulated, clearly indicating that the range of isotopic source signatures estimated from literature significantly underestimates true variability of source signatures. Source delta O-18 signature was found to be the most sensitive tracer for urban/ industry versus agricultural N2O. delta N-15(bulk) and site preference may provide more insight into microbial and chemical emission processes than partitioning of anthropogenic source categories.