Nitrogen isotope characteristics and importance of NO from biomass burning in China.

Biomass burning is a primary source of atmospheric nitrogen oxide (NO), however, the lack of isotopic fingerprints from biomass burning limits their use in tracing atmospheric nitrate (NO) and NO. A total of 25 biomass fuels from 10 provinces and regions in China were collected, and the δN values of biomass fuels (δN-biomass) and δN-NO values of biomass burning (δN-NO values of BB, open burning, and rural cooking stove burning) were determined. The δN-NO 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 δN-biomass. Based on the measured δN-NO values of BB and biomass burning emission inventory data, the δN-NO values of BB in different provinces and regions of China were calculated using the δN-NO model, with a mean value of 5.0 ± 1.8 ‰. The spatial variations in the estimated δN-NO values of BB in China were mainly controlled by the differences in the δN-NO values and the proportions of NO 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 based on air-mass backward trajectories, we established an improved δN-NO model and obtained more accurate δN-NO values of BB in regions (2.3 ‰ to 8.4 ‰). By utilising the reported δN-NO values of precipitation and particulate matter from 21 cities in China and the more accurate δN-NO values of BB, the NO 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 emissions were revealed, which is vital for controlling NO emissions in China.