Cities are concentrated areas of CO₂ emissions and have become the foci of policies for mitigation actions. However, atmospheric measurement networks suitable for evaluating urban emissions over time are scarce. Here we present a unique long-term (decadal) record of CO₂ mole fractions from five sites across Utah’s metropolitan Salt Lake Valley. We examine “excess” CO₂ above background conditions resulting from local emissions and meteorological conditions. We ascribe CO₂ trends to changes in emissions, since we did not find long-term trends in atmospheric mixing proxies. Three contrasting CO₂ trends emerged across urban types: negative trends at a residential-industrial site, positive trends at a site surrounded by rapid suburban growth, and relatively constant CO₂ over time at multiple sites in the established, residential, and commercial urban core. Analysis of population within the atmospheric footprints of the different sites reveals approximately equal increases in population influencing the observed CO₂, implying a nonlinear relationship with CO₂ emissions: Population growth in rural areas that experienced suburban development was associated with increasing emissions while population growth in the developed urban core was associated with stable emissions. Four state-of-the-art global-scale emission inventories also have a nonlinear relationship with population density across the city; however, in contrast to our observations, they all have nearly constant emissions over time. Our results indicate that decadal scale changes in urban CO₂ emissions are detectable through monitoring networks and constitute a valuable approach to evaluate emission inventories and studies of urban carbon cycles.

城市是CO ₂排放的集中区域，已成为缓解行动政策的重点。但是，缺乏适合于随时间评估城市排放的大气测量网络。在这里，我们呈现了来自犹他州大都会盐湖谷地区五个地点的CO ₂摩尔分数的独特长期（年代际）记录。我们研究了本地排放和气象条件导致的超出背景条件的“过量” CO ₂。我们没有将CO ₂趋势归因于排放变化，因为我们没有发现大气混合代理的长期趋势。三种对比的CO ₂各种城市类型都出现了趋势：住宅-工业用地呈负趋势，郊区郊区快速增长所围绕的带正趋势，以及已建立，住宅和商业城市核心中多个位置的CO ₂随时间相对恒定。对不同地点的大气足迹内的人口进行分析后，发现影响观测到的CO _2的人口增加大致相等，这意味着与CO _2呈非线性关系排放：经历郊区发展的农村地区人口增长与排放增加有关，而发达城市核心地区的人口增长与稳定排放有关。四个最新的全球排放清单也与整个城市的人口密度呈非线性关系。但是，与我们的观察相反，它们随时间推移几乎都具有恒定的排放量。我们的结果表明，可以通过监测网络检测到城市CO ₂排放的年代际规模变化，这是评估排放清单和研究城市碳循环的一种有价值的方法。