Atmospheric nitrate is anomalously enriched in both the 18O/16O and 17O/16O isotopic ratios, with Δ17O = 20--30.8? (Δ17O = δ17O - .52 ¿ δ18O). We have modeled the seasonal variation in Δ17O by utilizing the transfer of Δ17O from ozone to HNO3 during NOx oxidation reactions and by evaluating the relative source strengths of heterogeneous and homogenous HNO3 production. The implications for new insights into the global nitrogen cycle, N2O production and paleoclimate studies are discussed.