Nitrogen isotopes indicate vehicle emissions and biomass burning dominate ambient ammonia across Colorado's Front Range urban corridor

Urban ammonia (NH₃) emissions contribute to poor local air quality and can be transported to rural landscapes, impacting sensitive ecosystems. The Colorado Front Range urban corridor encompasses the Denver Metropolitan Area, rural farmland/rangeland and montane forest between the city and the Rocky Mountains. Reactive nitrogen emissions from the corridor are partly responsible for increased N deposition to the wildland-urban interface (WUI) in this region. To determine the significance of individual NH₃ sources to WUI ecosystems, we measured the concentration and isotopic composition (δ¹⁵N–NH₃) of ambient NH_3(g) from April to October 2018 across a five-site urban to rural gradient in the corridor. The urban sites had higher NH₃ concentrations and δ¹⁵N–NH₃ values than the rural/suburban sites. Based on isotope mixing models, NH₃ emission source contributions for all sites were fertilizer (12 ± 5.7%), livestock waste (18 ± 12%), vehicles (37 ± 23%), and biomass burning (34 ± 20%). Vehicle contributions were consistent across all months with an average of 35% and summer months showed a peak in biomass burning contributions (40%). As wildfires are projected to increase due to climate change, we stress a need for constraints on the isotopic signature of NH₃ emitted from wildfires. Vehicle emissions contributed the greatest amount of NH₃ (40%) at the urban sites while rural/suburban sites had higher agricultural contributions (41%). Had 2018 not had an anomalously high wildfire season, 46% and 60% of the NH₃ would have been attributed to vehicle emissions at the WUI site and urban sites, respectively. NH₃ emissions have historically been ascribed to agricultural activities but these findings illustrate the universal significance of vehicle emissions and the potential for sustained wildfire activity to be a primary contributor to NH₃. Air quality (e.g., particulate matter) and nitrogen deposition reduction plans may benefit by including management practices that address vehicle NH₃ emissions.