Laboratory and Field Measurements of the Nitrogen Isotopic Composition of NOx

Abstract:

The nitrogen isotopic composition of nitrogen oxides (NO_x = NO + NO₂) has been measured from several NO_x emissions sources in prior studies. These measurements have utilized a variety of methods for collecting the NO_x as nitrate or nitrite for isotopic analysis, but none of these methods have been verified for complete conversion of NO_x. Less than 100% conversion can result in isotopic fractionations. We present a method for accurately measuring the nitrogen isotopic composition of NO_x using a .25 M KMnO₄ and 0.5 M NaOH solution. Based on laboratory tests, this technique has been found to collect all NO_x passed through under a variety of conditions (e.g., air flow rate, NO_x concentration, temperature, humidity), allowing for diagnosis of δ¹⁵N-NO_x without correction for fractionation. The precision across the entire analytic technique is 1.5‰. This active collection method is advantageous for collecting NO_x over short time scales in environments with highly variable NO_x 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 NO_x emissions from different sources.

Initial results will be presented from lab- and field-based collections of NO_x 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-NO_x 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 NO_x produced from the burning of variety of materials (e.g. trees, agricultural products, grasses). The δ¹⁵N of NO_x ranged from -7 to +12‰, with a very strong relationship between the δ¹⁵N of the starting biomass and the δ¹⁵N of NO_x produced. Finally, a comparison of the active KMnO₄/NaOH method with passive diffusion NO₂ and NO_x collectors (Ogawa brand), conducted both in ambient urban air and in a diesel smog chamber, will be discussed.