Development of a Spectroscopic Technique for Continuous Online Monitoring of Oxygen and Site-Specific Nitrogen Isotopic Composition of Atmospheric Nitrous Oxide

摘要

Nitrous oxide is an important greenhouse gas and ozone-depleting-substance. Its sources are diffuse and poorly characterized, complicating efforts to understand anthropogenic impacts and develop mitigation policies. Online, spectroscopic analysis of N_2O isotopic composition can provide continuous measurements at high time resolution, giving new insight into N_2O sources, sinks, and chemistry. We present a new preconcentration unit, "Stheno II", coupled to a tunable infrared laser direct absorption spectroscopy (TILDAS) instrument, to measure ambient-level variations in ~(18)O and site-specific ~(15)N N_2O isotopic composition at remote sites with a temporal resolution of <1 h. Trapping of N_2O is quantitative up to a sample size of ～4 L, with an optimal sample size of 1200-1800 mL at a sampling frequency of 28 min. Line shape variations with the partial pressure of the major matrix gases N_2/O_2 and CO_2 are measured, and show that characterization of both pressure broadening and Dicke narrowing is necessary for an optimal spectral fit. Partial pressure variations of CO_2 and bath gas result in a linear isotopic measurement offset of 2.6-6.0 ‰ mbar~(-1). Comparison of IR MS and TILDAS measurements shows that the TILDAS technique is accurate and precise, and less susceptible to interferences than IR MS measurements. Two weeks of measurements of N_2O isotopic composition from Cambridge, MA, in May 2013 are presented. The measurements show significant short-term variability in N_2O isotopic composition larger than the measurement precision, in response to meteorological parameters such as atmospheric pressure and temperature.