A new method and application for determining the nitrogen isotopic composition of NOx

Abstract:

Atmospheric nitrogen oxides (NO_x = NO + NO₂) play key roles in atmospheric chemistry, air quality, and radiative forcing, and contribute to nitric acid deposition. Sources of NO_x include both natural and anthropogenic emissions, which vary significantly in space and time. NO_x isotopic signatures offer a potentially valuable tool to trace source impacts on atmospheric chemistry and regional acid deposition. Previous work on NO_x isotopic signatures suggests large ranges in values, even from the same emission source, as well as overlapping ranges amongst different sources, making it difficult to use the isotopic composition as a quantitative tracer of source influences. These prior measurements have utilized a variety of methods for collecting the NO_x as nitrate or nitrite for isotopic analysis, and testing of some of these methods (including active and passive collections) reveal inconsistencies in efficiency of collection, as well as issues related to changes in conditions such as humidity, temperature, and NO_x fluxes. A recently developed method allows for accurately measuring the nitrogen isotopic composition of NO_x (NO_x = NO + NO₂) after capturing the NO_x in a potassium permanganate/sodium hydroxide solution as nitrate (Fibiger et al., Anal. Chem., 2014). The method has been thoroughly tested in the laboratory and field, and efficiently collects NO and NO₂ under a variety of conditions. There are several advantages to collecting NO_x actively, including the ability to collect over minutes to hourly time scales, and the ability to collect in environments with highly variable NO_x sources and concentrations. Challenges include a nitrate background present in potassium permanganate (solid and liquid forms), accurately deriving ambient NO_x concentrations based upon flow rate and solution concentrations above this variable background, and potential interferences from other nitrogen species. This method was designed to collect NO_x in environments with very different emission source loadings in an effort to isotopically characterize NO_x sources. Results to date suggest very different values, and less variability than previous work, particularly for vehicle emissions. Ultimately, we aim to determine whether the influence of NO_x sources can be quantitatively tracked in the environment.