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

Abstract

The isotopic composition of atmospheric nitrous oxide (N2O) was measured semicontinuously, at ∼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 Dübendorf, Switzerland. The achieved measurement repeatability was 0.08‰, 0.11‰, and 0.10‰ for δ18O, site preference, and δ15Nbulk 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 δ18O signature was found to be the most sensitive tracer for urban/industry versus agricultural N2O. δ15Nbulk and site preference may provide more insight into microbial and chemical emission processes than partitioning of anthropogenic source categories.

Attribution of N<sub>2</sub>O sources in a grassland soil with laser spectroscopy based isotopocule analysis

“…Accordingly, average daytime N2O concentrations, δ 15 (Toyoda et al, 2013)). Observed changes in N2O concentrations and isotopic composition at night are within the range of previous studies from agricultural sites (Wolf et al, 2015;Toyoda et al, 2011), but clearly higher than variations measured at 13 m or 95 m above ground in an urban or suburban environment (Harris et al, 2014;Harris et al, 2017).…”

“…Data processing was conducted as previously described by Harris et al (2017) using Matlab (MathWorks, Inc., USA). Abundances of the four isotopocules ( 14 N 14 N 16 O, 14 N 15 N 16 O, 15 N 14 N 16 O and 14 N 14 N 18 O) were obtained with TDL Wintel (Aerodyne Research Inc., Billerica, USA), and isotope ratios were drift-corrected for changes observed in S1.…”

“…For comparison, the source signatures were also calculated with the Miller and Tans (2003) approach. An in-depth description of the implementation of the Miller-Tans method is provided by Harris et al (2017). In brief, first, a baseline is determined by 40 averaging the data points in the lowest 5 % of the diurnal N2O concentrations with a 5-day moving window (see SI Figure 3).…”

“…CC BY 4.0 License. Mohn et al, 2013;Winther et al, 2018), and more recently to analyse diurnal and seasonal isotopic variations in ambient N2O (Mohn et al, 2012;Toyoda et al, 2013;Wolf et al, 2015;Harris et al, 2017). The isotopic composition of N2O emitted from soils can be extracted from ambient air measurements using traditional two end-member mixing models, i.e.…”

Attribution of N<sub>2</sub>O sources in a grassland soil with laser spectroscopy based isotopocule analysis

“…The fractionation factors ε 18 O/ε 15 N bulk , ε 18 O/εSP, and ε 15 N bulk /εSP have been determined in a number of incubation experiments, and it has been suggested that their ratios (2.4, 2.8, and 1.2, respectively) may be indicators of N 2 O reduction (Koba et al, 2009). It has to be mentioned, however, that fractionation factors may deviate depending on environmental conditions (Koster et al, 2013) or even over the course of a single experiment due to multiple reaction steps involved (Haslun et al, 2018). Furthermore, δ 18 O-N 2 O of denitrification is affected by oxygen exchange between reaction intermediates (NO − 3 , NO − 2 ) and soil water as a function of WFPS (Well et al, 2008;Kool et al, 2011).…”