Seasonal trends of Δ17O, δ18O, and δ15N in atmospheric NO3- of the Midwestern United States

Archived National Atmospheric Deposition Program samples collected from eight sites between 2001 and 2003 in the Midwestern United States were analyzed for nitrate isotopes (Δ¹⁷O, δ¹⁸O and δ¹⁵N). Variations in all three isotope abundances show significant seasonal trends. Both nitrate Δ¹⁷O and δ¹⁸O values were elevated in the winter (Δ¹⁷O: ~ +28-33%; δ¹⁸O: ~ +85-95%) relative to summer (Δ¹⁷: ~ +21-25%; δ¹⁸O: ~ +55-70%). This is likely related to changes in atmospheric chemistry with changing seasons. Using photochemical models to investigate how formation pathways of NO₃^- in the atmosphere change with season can provide a link between the chemistry of the atmosphere and the observed isotope variations. The partitioning between different formation pathways of atmospheric NO₃^- is highly dependent on temperature and sunlight. Colder temperatures and less sunlight favor the N₂O₅ heterogeneous hydrolysis pathway, while warm temperatures and abundant light favor the NO₂+OH pathway. Each pathway has a different degree of ozone oxidation and results in different oxygen isotope values in the production of atmospheric NO₃^-. δ¹⁵N exhibits the same seasonal trend (~ +10 to -5% in the winter and summer, respectively), however whether these trends are due to changing chemistry or source apportionment is unclear. Changing chemistry, particularly in polluted areas, could account for the δ¹⁵N trends we observe as per Freyer et al. (1991), however changing emission source could also have an effect. Atmospheric chemistry models can help to de-convolute the interpretations of these trends by taking a closer look at the specific chemistry responsible for the formation of atmospheric NO₃^-. Interpretations of these results will be aided by models such as the Community Multiscale Air Quality model amended to account for isotope effects during photochemical processing in the atmosphere.