First Measurements of the Nitrogen Isotopic Composition of NOx from Biomass Burning

The nitrogen isotopic composition (δ 15 N) of NO x (NO + NO 2 ) was measured during the fourth Fire Lab at Missoula Experiment (FLAME-4). The δ 15 N-NO x produced by burning a variety of biomass types ranged from −7 to +12‰ (vs air N 2 ). In the laboratory experiments, two types of emissions were sampled: “stack” fires where the emissions were measured within a few seconds of production from the fire and “chamber” fires where the emissions were held in a room for 1–2 h and sampled continuously. For both types of emissions sampled, the primary control on δ 15 N-NO x is the δ 15 N of the biomass burned (δ 15 N-biomass), although differences were found for δ 15 N-NO x between the two types of fires. For the stack emissions, δ 15 N-NO x = 0.41 × δ 15 N-biomass +1.0 (R 2 = 0.83, p -value <0.001) and for the chamber fires, δ 15 N-NO x = 0.98 × δ 15 N-biomass +1.7 (R 2 = 0.94, p -value <0.001). While a large range of δ 15 N-NO x values were observed, the strong relationship between δ 15 N-NO x and δ 15 N-biomass suggests that in any given environment, the δ 15 N-NO x can be predicted.