Nitrogen isotope characteristics and importance of NO_x from biomass burning in China

Biomass burning is a primary source of atmospheric nitrogen oxide (NO_x), however, the lack of isotopic fingerprints from biomass burning limits their use in tracing atmospheric nitrate (NO₃^-) and NO_x. A total of 25 biomass fuels from 10 provinces and regions in China were collected, and the δ¹⁵N values of biomass fuels (δ¹⁵N-biomass) and δ¹⁵N-NO_x values of biomass burning (δ¹⁵N-NO_x values of BB, open burning, and rural cooking stove burning) were determined. The δ¹⁵N-NO_x values of open burning and rural cooking stove burning ranged from -0.8 ‰ to 11.6 ‰ and 0.8 ‰ to 9.5 ‰, respectively, indicating a significant linear relation with δ¹⁵N-biomass. Based on the measured δ¹⁵N-NO_x values of BB and biomass burning emission inventory data, the δ¹⁵N-NO_x values of BB in different provinces and regions of China were calculated using the δ¹⁵N-NO_x model, with a mean value of 5.0 ± 1.8 ‰. The spatial variations in the estimated δ¹⁵N-NO_x values of BB in China were mainly controlled by the differences in the δ¹⁵N-NO_x values and the proportions of NO_x emissions from various straw burning activities in provinces and regions of China. Furthermore, by using the combined local emissions of biomass burning with regional transportations of NO_x based on air-mass backward trajectories, we established an improved δ¹⁵N-NO_x model and obtained more accurate δ¹⁵N-NO_x values of BB in regions (2.3 ‰ to 8.4 ‰). By utilising the reported δ¹⁵N-NO_x values of precipitation and particulate matter from 21 cities in China and the more accurate δ¹⁵N-NO_x values of BB, the NO_x contributions from four sources (mobile sources, coal combustion, biomass burning, and microbial N cycle) at the national scale were estimated using a Bayesian model. The significant contributions of biomass burning (20.9 % to 44.3 %) to NO_x emissions were revealed, which is vital for controlling NO_x emissions in China.