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 (NOx) emissions. However, it remains uncertain to the actual contribution of the non-fossil fuel NOx to total NOx emissions. Natural N isotopes of NO3- in precipitation (delta N-15(w-NO3-)) have been widely employed for tracing atmospheric NOx sources. Here, we compiled global delta N-15(w-NO3-) observations to evaluate the relative importance of fossil and non-fossil fuel NOx emissions. We found that regional differences in human activities directly influenced spatial-temporal patterns of delta N-15(w-NO3-) variations. Further, isotope mass-balance and bottom-up calculations suggest that the non-fossil fuel NOx accounts for 557% of total NOx 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 NOx emissions and provide direct evidence for making strategies on mitigating atmospheric NOx pollution. This study investigates in the importance of non-fossil fuel NOx emissions in the surface-earth-nitrogen cycle. The study shows how changes of regional human activities directly influence delta N-15 signatures of deposited NOx to terrestrial environments and that emissions have largely been underestimated.