Alexander Berner Gregory A. Wetherbee Sheila F. Murphy Ruth C. Heindel J. David Felix 2023 <div id="abs0010" class="abstract author" lang="en"><div id="abssec0010"><p id="abspara0010">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<span>&nbsp;</span>sources to WUI ecosystems, we measured the concentration and isotopic composition (δ¹⁵N–NH₃) of ambient NH_3(g)<span>&nbsp;</span>from April to October 2018 across a five-site urban to rural gradient in the corridor. The urban sites had higher NH₃<span>&nbsp;</span>concentrations and δ¹⁵N–NH₃<span>&nbsp;</span>values than the rural/suburban sites. Based on isotope mixing models, NH₃<span>&nbsp;</span>emission source contributions for all sites were fertilizer (12&nbsp;±&nbsp;5.7%), livestock waste (18&nbsp;±&nbsp;12%), vehicles (37&nbsp;±&nbsp;23%), and biomass burning (34&nbsp;±&nbsp;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<span>&nbsp;</span>emitted from wildfires. Vehicle emissions contributed the greatest amount of NH₃<span>&nbsp;</span>(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<span>&nbsp;</span>would have been attributed to vehicle emissions at the WUI site and urban sites, respectively. NH₃<span>&nbsp;</span>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<span>&nbsp;</span>emissions.</p></div></div><div id="abs0015" class="abstract graphical"><br></div> application/pdf 10.1016/j.envpol.2022.120537 en Elsevier Nitrogen isotopes indicate vehicle emissions and biomass burning dominate ambient ammonia across Colorado's Front Range urban corridor article