What causes δ15N variations in atmospheric nitrate, NOx, and NOy: N sources or N chemistry?

Understanding the sources and chemistry of NOy in the atmosphere is key for comprehending tropospheric chemistry. The δ15N values of important NOy compounds (nitrate, nitric acid, HONO, and NO2) show a wide spatial and temporal range. There are currently two hypotheses that attempt to explain the observed variability. The first is the "source hypothesis" that assigns the δ15N variation to changes in the relative importance of nitric oxide emissions from various sources, such as denitrification, fossil fuel combustion, and lightning. If true, the observed changes in the δ15N values of NOy may be a new way of constraining the NOx budget from local to regional scales. The second hypothesis is the "chemistry effect" that ascribes the NOy δ15N variations to photolytic, kinetic, and equilibrium isotope effects that occur during the partitioning of NOx into various NOy pools. In this case NOy δ15N variations may be a new approach to understanding NOx oxidation chemistry and how it changes in different environments and during different times to the day, year, or century. Our recent observations, experiments, and modeling suggests that the chemistry effect is the cause of temporal δ15N oscillations that occur on timescales from hours to months to decades, while the "source hypothesis" may be important for baseline shifts resulting regional differences in NOy δ15N oscillation mean values.