Important contributions of non-fossil fuel nitrogen oxides emissions

Since the industrial revolution, it has been assumed that fossil-fuel combustions dominate increasing nitrogen oxide (NO x ) emissions. However, it remains uncertain to the actual contribution of the non-fossil fuel NO x to total NO x emissions. Natural N isotopes of NO 3 − in precipitation (δ 15 N w-NO3− ) have been widely employed for tracing atmospheric NO x sources. Here, we compiled global δ 15 N w-NO3− observations to evaluate the relative importance of fossil and non-fossil fuel NO x emissions. We found that regional differences in human activities directly influenced spatial-temporal patterns of δ 15 N w-NO3− variations. Further, isotope mass-balance and bottom-up calculations suggest that the non-fossil fuel NO x accounts for 55 ± 7% of total NO x emissions, reaching up to 21.6 ± 16.6Mt yr −1 in East Asia, 7.4 ± 5.5Mt yr −1 in Europe, and 21.8 ± 18.5Mt yr −1 in North America, respectively. These results reveal the importance of non-fossil fuel NO x emissions and provide direct evidence for making strategies on mitigating atmospheric NO x pollution. This study investigates in the importance of non-fossil fuel NO x emissions in the surface-earth-nitrogen cycle. The study shows how changes of regional human activities directly influence δ 15 N signatures of deposited NO x to terrestrial environments and that emissions have largely been underestimated.