Local Atmospheric Pollution Driven by Traffic Sources, with Limited Protections by Tree Canopy Sinks

Abstract Context 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. Objectives We developed 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. Methods We measured atmospheric concentration as well as throughfall nitrogen from native species in Riverside, CA .We measured atmospheric NOX and NH₃ at different traffic levels and high and low canopy sites over 14 months to test the roles of the hypothesized source and sink spatially and temporally. Results We found that NOX and NH₃ have an inverse relationship in seasonal concentrations, highlighting the need to understand multiple pollutants. We found distinct local spatial patterns of pollution with on-road traffic leading to increased pollutants but did not find that tree canopy was consistently a sink. Trees’ performance as a sink depended on 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. Conclusions This work highlights the need to address pollution directly, where traffic has a substantial role. We further need to evaluate assumptions of the ecosystem services trees can consistently provide.