Measurements of Δ¹⁴C and CO₂ can cleanly separate biogenic and fossil contributions to CO₂ enhancements above background. Our measurements of these tracers in air around Los Angeles in 2015 reveal high values of fossil CO₂ and a significant and seasonally varying contribution of CO₂ from the urban biosphere. The biogenic CO₂ is composed of sources such as biofuel combustion and human metabolism and an urban biospheric component likely originating from urban vegetation, including turf and trees. The urban biospheric component is a source in winter and a sink in summer, with an estimated amplitude of 4.3 parts per million (ppm), equivalent to 33% of the observed annual mean fossil fuel contribution of 13 ppm. While the timing of the net carbon sink is out of phase with wintertime rainfall and the sink seasonality of Southern California Mediterranean ecosystems (which show maximum uptake in spring), it is in phase with the seasonal cycle of urban water usage, suggesting that irrigated urban vegetation drives the biospheric signal we observe. Although 2015 was very dry, the biospheric seasonality we observe is similar to the 2006–2015 mean derived from an independent Δ¹⁴C record in the Los Angeles area, indicating that 2015 biospheric exchange was not highly anomalous. The presence of a large and seasonally varying biospheric signal even in the relatively dry climate of Los Angeles implies that atmospheric estimates of fossil fuel–CO₂ emissions in other, potentially wetter, urban areas will be biased in the absence of reliable methods to separate fossil and biogenic CO₂.

Δ的测量¹⁴ C和CO ₂可以生物和化石贡献干净地分离，以CO ₂的改进上述背景。我们在2015年对洛杉矶周围空气中这些示踪剂的测量表明，化石CO _2的价值很高，并且城市生物圈中CO _2的贡献在季节性和显着变化。生物CO ₂由生物燃料燃烧和人类新陈代谢等来源以及可能源自城市植被（包括草皮和树木）的城市生物圈组成。城市生物圈的组成部分是冬季的来源，夏季则是下沉的地方，估计振幅为百万分之4.3（ppm），相当于观测到的年平均化石燃料贡献量（13 ppm）的33％。尽管净碳汇的时间与冬季降雨和南加州地中海生态系统的汇季节（在春季显示最大吸收）不同，但与城市用水的季节性周期是不同步的，这表明灌溉城市植被驱动着我们观察到的生物圈信号。尽管2015年非常干旱，但我们观察到的生物圈季节与2006-2015年的平均值相似，该平均值来自独立的Δ洛杉矶地区的¹⁴ C记录，表明2015年的生物圈交换不是高度异常。即使在洛杉矶相对干燥的气候中，也存在着较大且季节性变化的生物圈信号，这意味着如果没有可靠的方法来分离化石，则大气中对其他可能湿润的城市地区化石燃料-CO ₂排放的估计将有偏差。和生物CO ₂。