Isotopic composition of passively collected nitrogen dioxide emissions: Vehicle, soil and livestock source signatures

Quantifying contributions of local and regional NOx emission sources is an important initial step towards accurately assessing improvements in NOx emission reduction efforts. Current global NOx inventories report large uncertainties in contributions of some NOx sources, especially diffuse sources (e.g. lightning and soil NOx). Examining the isotopic composition of NOx and its oxidation products (NOy) is one approach to further constrain contributions from these sources. While natural and anthropogenically-derived NOx emissions are reported to have relatively distinct δ15N values that could aid NOx source apportionment studies, existing δ15N–NOx source data is limited and variable collection approaches have been employed. To build on existing δ15N–NOx source data, inexpensive and easily deployable passive samplers were used to collect nitrogen dioxide (NO2) emissions and its oxidation product, nitric acid (HNO3), from multiple emission sources including livestock waste, fertilized soils, and vehicles. The resulting isotope data provides evidence that passive samplers can be used across a range of environmental conditions with widely varying NO2 concentrations and NO2 isotopic compositions. Using this approach, we report the first δ15N and δ18O–NO2 of livestock waste emissions, as well as the first measurements of δ18O–NO2 from biogenic soil and vehicle emissions. We observe the highest δ15N–NO2 values to date of vehicle emissions and investigate potential fractionations associated with oxidation and equilibrium processes. The large differences reported here between δ15N–NO2 values from fossil fuel-based sources and microbially-produced sources allows for identification and possible quantification of source contributions to ambient NOx concentrations.