Context <p>Atmospheric reactive nitrogen is a major issue for urban residents both as a component of air pollution and as a nutrient input to urban ecosystems. However, patterns of atmospheric reactive nitrogen concentrations within a city are not well quantified spatially or temporally.</p> Objectives <p>We tested a source-sink framework, where traffic is the key source and tree canopy is a key sink, to understand patterns of intraurban atmospheric reactive nitrogen concentrations. By using a network of low-cost, passive samplers, we aimed to measure nitrogen on a fine scale, relevant to residents.</p> Methods <p>We measured atmospheric concentration as well as throughfall nitrogen from native species in Riverside, CA. We measured atmospheric NO_X and NH₃ at different traffic levels and high and low canopy sites over 14&#xa0;months to test the roles of the hypothesized source and sink spatially and temporally.</p> Results <p>We found distinct local spatial patterns of pollution with on-road traffic leading to increased pollutants. For NO_X, at low ambient concentration we found tree canopies tended to increase concentrations and at high ambient concentrations tree canopies acted as a sink. We found that NO_X and NH₃ have an inverse relationship in seasonal concentrations, highlighting the need to understand multiple pollutants. Trees’ performance as a sink depended on both atmospheric concentration and pollutant. The results also show a concentrating effect in some cases. Nevertheless, throughfall deposition to soil under tree canopies was substantial and varied among different species, showing the potential for trees to increase deposition to local ecosystems.</p> Conclusions <p>This work shows how the landscape distribution of two dominant species of reactive nitrogen varies across seasons in a case study city. The findings support a source-sink hypothesis that affects atmospheric N concentrations and inputs to soils.</p>