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<p>Vehicle emissions have been identified as an important urban source of ammonia (<span class="inline-formula">NH₃</span>). However, there are large uncertainties regarding the contribution of vehicle emissions to urban <span class="inline-formula">NH₃</span> budgets, as well as the role of <span class="inline-formula">NH₃</span> in spatiotemporal fine particulate matter (<span class="inline-formula">PM_2.5</span>) formation and nitrogen (N) deposition patterns. The N stable isotopic composition (<span class="inline-formula"><i>δ</i>¹⁵N</span>) may be a useful observational constraint to track <span class="inline-formula">NH₃</span> emission sources and chemical processing, but previously reported vehicle <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> emission signatures have reported a wide range of values, indicating the need for further refinement. Here we have characterized <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> spatiotemporal variabilities from vehicle plumes in stationary and on-road measurements in the USA and China using an active <span class="inline-formula">NH₃</span> collection technique demonstrated to accurately characterize <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> on the order of hourly time resolution. Significant spatial and temporal <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> variabilities were observed and suggested to be driven by vehicle fleet composition and influences from <span class="inline-formula">NH₃</span> dry deposition on tunnel surfaces. Overall, a consistent <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> signature of <span class="inline-formula">6.6±2.1</span>&thinsp;‰ (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mi>x</mi><mo mathvariant="normal">‾</mo></mover><mo>±</mo><mn mathvariant="normal">1</mn><mi mathvariant="italic">σ</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="e41cf8a8db9b73596012c373bf3f0c65"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-11551-2020-ie00001.svg" width="33pt" height="11pt" src="acp-20-11551-2020-ie00001.png"/></svg:svg></span></span>; <span class="inline-formula"><i>n</i>=80</span>) was found in fresh vehicle plumes with fleet compositions typical of urban regions. Our recommended vehicle <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> signature is significantly different from previous reports. This difference is due to a large and consistent <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> bias of approximately <span class="inline-formula">−15.5</span>&thinsp;‰ between commonly employed passive <span class="inline-formula">NH₃</span> collection techniques and the laboratory-tested active <span class="inline-formula">NH₃</span> collection technique. This work constrains the <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> urban traffic plume signature, which has important implications for tracking vehicle <span class="inline-formula">NH₃</span> in urban-affected areas and highlights the importance of utilizing verified collection methods for accurately characterizing <span class="inline-formula"><i>δ</i>¹⁵N(NH₃)</span> values.</p>