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.