Global emissions of nitric oxide (NO) have increased dramatically over the last century primarily due to human activity. Atmospheric NO is oxidized rapidly to nitrogen dioxide (NO2) in the troposphere, and these compounds (collectively referred to NOx) affect tropospheric ozone (O3) production, secondary organic aerosol formation, the atmospheric lifetimes of carbon dioxide and methane, and can cause ecosystem acidification and eutrophication. The global inventory of NO emissions is poorly constrained with a large portion of the uncertainty attributed to soil NO emissions that result from microbial nitrification and denitrification processes. There is a growing interest in partitioning NOx sources using natural abundance stable nitrogen (N) isotopes (d15N) of atmospheric nitrogen oxides (e.g., dry and wet nitrate (NO3-) deposition), given the findings that different NOx sources may have distinct N isotopic signatures. Although it has been suggested that natural sources of NOx have lower d15N-NOx values than anthropogenic sources, soil d15N-NO is largely unknown mainly due to the diffuse nature, low concentrations, and high reactivity of soil-emitted NO. Here, we present the development of a novel dynamic flux chamber (DFC) system capable of simultaneously measuring soil NO fluxes and collecting NO for d15N-NO measurements. The system couples a widely used flow-through soil chamber with a NO collection train, in which NO can be converted to NO2 through O3 titration in a Teflon reaction coil (length 260 cm, i.d. 9.5 mm), followed by NO2 collection in a 20% (v/v) triethanolamine (TEA) solution as nitrite (NO2-) and NO3-. d15N-NO can then be effectively determined by d15N analysis of collected NO2- and NO3- using the denitrifier method. The reaction time of the reaction coil was determined experimentally by sampling zero air with a constant NO concentration (100 ppbv) and varying the O3 concentration. Subsequent numerical model calculations including reactions between O3, NO, and NO2 indicate that the conversion efficiency is quantitative (>99%) over a 10 to 100 ppbv NO mixing ratio range. The recovery of NO in the TEA solution was determined by NO2- and NO3- concentration measurements and was found to be 99.7±3.0% on average where the chamber NO mixing ratio ranged from 10 to 100 ppbv. This indicates that the N isotopic fractionation resulting from NO conversion to NO2 and NO2 collection should be minimized. Samples collected were neutralized with 12N HCl to pH 8.0 8.5 before being measured using the denitrifier method. Because it is likely that soil-emitted NO has very low d15N-NO values, a blank-matching strategy was adopted to match both injected N amount and solution volume between the samples and standards to minimize the interference from trace amounts of N inherent to the denitrifier medium and the TEA solution. A set of working NO2- standards spanning -1.8 to -79.6 was used to calibrate all collection samples. The DFC system has been evaluated in the laboratory through collection of NO from an analytical NO tank under various conditions mimicking soil NO emissions. The resulting accuracy and precision of d15N-NO analysis was ±0.86, better than that of methods previously published for collection of NOx from anthropogenic sources (e.g., ±1). A preliminary measurement of d15N-NO of surface soils sampled from an urban riparian site resulted in a soil d15N-NO value of -56, suggesting that soil d15N-NO could be lower than previously assumed (e.g., -49 ~ -28). The successful development of the DFC system will allow a thorough evaluation of the role of soil-emitted NO in the reactive N cycle and further investigations on NO sources and microbial pathways in natural and fertilized soils.
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