Nitrogen isotope signatures of oxidized nitrogen species from biomass burning

Because nitrogen isotopes (delta 15N) of emitted nitric oxide (e-NO) from combustion-related sources (mainly fossil fuels and biomasses) remain unknown, the delta 15N values of emitted N oxides (e-NOx, the sum of emitted N dioxide (e-NO2) and e-NO) have long been employed in the isotopic tracing of nitrate (NO3-) in the atmosphere, causing substantial uncertainties in the isotope differentiation of source contributions. Here we measured concentrations and delta 15N values of e-NOx and NO3- from the burning of four biomass materials (corn, wheat, poplar, and pine) in northern China, based on which we calculated concentrations and delta 15N values of the corresponding e-NO, e-NO2, initially produced NO (i-NO) and NO2 (i-NO2) before emissions and oxidizations. We found that 58 +/- 12% of i -NO emitted as e-NO and 42 +/- 10% was oxidized to i-NO2, 77 +/- 7% of i-NO2 emitted as e-NO2 and 23 +/- 3% was oxidized to NO3-, and the mean delta 15Ne-NO (-10.2 +/- 6.0%o) was distinctly lower than the delta 15Ne-NOx (-2.3 +/- 3.9%o) and delta 15Ne-NO2 (11.8 +/- 3.6%o). Based on the delta 15Ni-NO (-0.2 +/- 3.2%o) and delta 15Ni-NO2 (13.5 +/- 3.8%o), we obtained the theoretical N isotope fractionations for the transformations from i-NO to i-NO2 (23.8 +/- 14.3%o) and from i-NO2 to NO3- (8.0 +/- 11.3%o) during biomass burning. These results filled the long-standing knowledge gap of species-specific delta 15N signatures of NOx produced by biomass burning, expanded the understanding of N isotope fractionations of N oxidization under burning processes, and provided critical parameters for isotope source apportionment of NOx and NO3- in the atmosphere.

Automated summary

In this study, concentrations and δ15N values of emitted NOx and NO3- from biomass burning were measured in northern China. The study filled the knowledge gap of species-specific δ15N signatures of NOx produced by biomass burning, expanded the understanding of N isotope fractionations during burning processes, and provided critical parameters for isotope source apportionment of NOx and NO3- in the atmosphere.