Abstract
Abstract. Important information about the biogeochemical cycle of nitrous oxide (N2O) can be obtained by measuring its three main isotopic species, 14N15N16O, 15N14N16O, and 14N14N16O, and the respective site-specific relative isotope ratio differences δ15Nα and δ15Nβ. Absorption laser spectroscopy in the mid-infrared is a direct method for the analysis of the 15N isotopic composition of N2O, yet not sensitive enough for atmospheric N2O mixing ratios (320 ppb). To enable a fully-automated high precision analysis of N2O isotopic species at ambient mixing ratios, we built and optimized a liquid nitrogen-free preconcentration unit to be coupled to a quantum cascade laser (QCL) based spectrometer. During standard operation 10 l of ambient air are preconcentrated on a HayeSep D trap and desorbed in 50 ml of synthetic air. Rigorous tests were conducted, using FTIR, quantum cascade laser absorption spectroscopy (QCLAS), GC-FID and component-specific ozone and oxygen analysers to investigate recovery rates, conservation of isotopic signatures and spectral interferences after preconcentration. We achieve quantitative N2O recovery of >99% with only minor, statistically not significant isotopic fractionation and no relevant spectral interferences from other atmospheric constituents. The developed preconcentration unit also has the potential to be applied to other trace gases and their isotopic composition.
Highlights
Nitrous oxide (N2O) is currently the most important anthropogenic ozone depleting substance and exerts the second largest global warming potential weighted (GWP) emission of non-CO2 greenhouse gases (Ravishankara et al, 2009).Since pre-industrial times, N2O mixing ratios in the troposphere increased from 270 ppb to the current level of 320 ppb at a rate of 0.26% a−1
We recently demonstrated a precision of 0.5‰ for δ15N at N2O mixing ratios of 90 ppm (Wachter et al, 2008) using a pulsed quantum cascade laser (QCL) emitting at 4.6 μm
Our results indicate that the coupling of preconcentration and laser spectroscopy may be feasible for the measurement of mixing ratios and isotopic composition of other trace gases
Summary
Nitrous oxide (N2O) is currently the most important anthropogenic ozone depleting substance and exerts the second largest global warming potential weighted (GWP) emission of non-CO2 greenhouse gases (Ravishankara et al, 2009). The only technique for N2O isotopic measurements at ambient mixing ratios is laboratory-based isotoperatio mass-spectrometry (IRMS) in combination with flasksampling and cryogenic preconcentration. Mohn et al.: A liquid nitrogen-free preconcentration unit high power output (Faist et al, 1994) The application of these light sources for high-precision isotope ratio measurement has been recently demonstrated for CO2 by Nelson et al (2008) and Tuzson et al (2008a, b). Atmospheric N2O mixing ratios (320 ppb), are more than two orders of magnitude lower and are currently not accessible for direct isotopic analysis by laser spectroscopy For these reasons we suggest to combine laser spectroscopy with preconcentration techniques to increase N2O mixing ratios from ambient levels to ≈60 ppm. Our results indicate that the coupling of preconcentration and laser spectroscopy may be feasible for the measurement of mixing ratios and isotopic composition of other trace gases (e.g. many VOCs, CH4, CO)
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