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 (δ15Nw-NO3-) have been widely employed for tracing atmospheric NOx sources. Here, we compiled global δ15Nw-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 δ15Nw-NO3- variations. Further, isotope mass-balance and bottom-up calculations suggest that the non-fossil fuel NOx accounts for 55 ± 7% 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.