Laboratory and Field Measurements of the Nitrogen Isotopic Composition of NOx
Abstract:The nitrogen isotopic composition of nitrogen oxides (NOx = NO + NO2) has been measured from several NOx emissions sources in prior studies. These measurements have utilized a variety of methods for collecting the NOx as nitrate or nitrite for isotopic analysis, but none of these methods have been verified for complete conversion of NOx. Less than 100% conversion can result in isotopic fractionations. We present a method for accurately measuring the nitrogen isotopic composition of NOx using a .25 M KMnO4 and 0.5 M NaOH solution. Based on laboratory tests, this technique has been found to collect all NOx passed through under a variety of conditions (e.g., air flow rate, NOx concentration, temperature, humidity), allowing for diagnosis of δ15N-NOx without correction for fractionation. The precision across the entire analytic technique is 1.5‰. This active collection method is advantageous for collecting NOx over short time scales in environments with highly variable NOx sources and concentrations. The major drawback of the NaOH/KMnO4 method is a significant nitrate background found in the KMnO4, but this background is consistent and can be easily accounted for. We aim to use this method to provide more robust constraints on the isotopic signatures of NOx emissions from different sources.
Initial results will be presented from lab- and field-based collections of NOx emissions. Emissions from a diesel engine were measured in a laboratory smog chamber and yielded δ15N values with a mean of -18.0‰ (n = 5, 1σ = 0.97‰). Measurements of δ15N-NOx were also made on a rooftop between two highways in Providence, RI. The values ranged from -7.7 to -0.63‰ for different time periods sampled, with excellent reproducibility in side-by-side collections. Additionally, the NaOH/KMnO4 was deployed in a laboratory study of biomass burning (FLAME4) to analyze the nitrogen isotopic composition of NOx produced from the burning of variety of materials (e.g. trees, agricultural products, grasses). The δ15N of NOx ranged from -7 to +12‰, with a very strong relationship between the δ15N of the starting biomass and the δ15N of NOx produced. Finally, a comparison of the active KMnO4/NaOH method with passive diffusion NO2 and NOx collectors (Ogawa brand), conducted both in ambient urban air and in a diesel smog chamber, will be discussed.