Nitrogen oxides (NOx = NO + NO2) are emitted from a variety of natural and anthropogenic sources and, upon oxidation, are eventually deposited as nitrate. While efforts have been made to constrain the importance of various NOx sources, there is still significant disagreement between various inventories and models. In addition, nitrate deposition (from oxidation of NOx to HNO3) is difficult to trace back to particular NOx sources. One tool that may help constrain NOx sources is the nitrogen isotopic composition (δ15N). While there have been some measurements of δ15N from some NOx sources, source isotopic signatures are, currently, not well constrained. There have been variations in both measurement method and source details, making it difficult to evaluate variability within and between sources. The major sources missing from our current δ15N-NOx inventory are North American automobile emissions and biomass burning. Biomass burning is widespread and can have great impacts on atmospheric chemistry through the release of aerosols and trace gases. It can occur naturally, through lightning strikes and volcanic eruptions, or it can be anthropogenically derived through agricultural or land-clearing activities. We will present the δ15N of NOx from biomass burning was measured during a laboratory-based experiment (FLAME4) at the USDA fire laboratory in Missoula, MT. The measurement method was laboratory-verified for complete conversion of NOx to NO3- to avoid fractionation with isotopic analysis. The δ15N-NOx produced by a variety of biomass types ranged from -7 to +12‰ (vs. air N2). The variation in δ15N is due primarily to variations in the biomass types burned. Additionally, there were two methods for sampling emissions: stack fires where emissions were samples within a few seconds of production and chamber fires where the emissions were held in a room for 1-2 hours. The δ15N of the starting biomass is the primary control on δ15N-NOx produced. For the stack emissions, δ15N-NOx = 0.39 x δ15N-biomass +1.0 and for chamber fires, δ15N-NOx = 0.95 x δ15N-biomass + 1.7. With increasing constraints on source isotopic signatures, including a signature for various types of biomass burning, we may be able to use δ15N of NOx to trace NOx sources and/or nitrate deposition.

Isotopic variability of nitrogen oxides (NOx) from biomass burning

249th National Meeting (2015)

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