NASA's Orbiting Carbon Observatory-3 (OCO-3) was designed to support the quantification and monitoring of anthropogenic CO₂ emissions. Its Snapshot Area Map (SAM) and target mode measurements provide an innovative dataset for carbon studies on sub-city scales. Unlike any other current space-based instrument, OCO-3 has the ability to scan large contiguous areas of emission hot spots like cities, power plants, and volcanoes. These measurements result in dense, fine-scale spatial maps of column averaged dry-air mole fractions of carbon dioxide (X_CO2). For the first time, we present and analyze X_CO2 distributions over the Los Angeles megacity (LA) derived from OCO-3 SAM and target mode observations. Urban X_CO2 enhancements range from 0 − 6 ppm (median enhancements ≃ 2 ppm) relative to a clean background and show excellent agreement with nearby ground-based TCCON measurements of X_CO2. OCO-3's dense observations reveal intra-urban variations of X_CO2 over the city that have never been observed from space before. The spatial variations are mainly driven by the complex fossil fuel emission patterns and meteorological conditions in the LA Basin and are in good agreement with those from co-located TROPOMI measurements of co-emitted NO₂. Differences between measured and simulated X_CO2 enhancements from two models (WRF-Chem and X-STILT) are typically below 1 ppm with larger differences for some sub regions. Both models capture the observed intra-urban X_CO2 gradients. Further, OCO-3's multi-swath measurements capture about three times as much of the city emissions compared to single-swath overpasses. OCO-3's frequent target and SAM mode observations will pave the way to constrain urban emissions at finer, sub-city scales.

NASA的轨道碳观测站3（OCO-3）旨在支持对人为CO ₂排放的量化和监测。其快照区域图（SAM）和目标模式测量结果为郊区城市的碳研究提供了创新的数据集。与当前的任何其他天基仪器不同，OCO-3能够扫描大面积连续的发射热点区域，例如城市，发电厂和火山。这些测量结果产生了二氧化碳的平均柱干空气摩尔分数（X _{CO 2}）的密集，精细的空间图。首次，我们介绍并分析了从OCO-3 SAM和目标模式观测值得出的洛杉矶大城市（LA）上的X _{CO 2}分布。都市X _{CO 2}相对于干净的背景，增强范围为0 − 6 ppm（中值增强≃2 ppm），并且与附近基于地面的X _{CO 2} TCCON测量值显示出极好的一致性。OCO-3的密集观测结果揭示了城市中X _{CO 2}的城市内部变化，这是以前从未从太空中观察到的。空间变化主要是由LA盆地中复杂的化石燃料排放模式和气象条件驱动的，并且与共同排放的NO _2的TROPOMI测量结果相吻合。实测X _{CO 2}与模拟X _{CO 2}之间的差异两种模型（WRF-Chem和X-STILT）的增强通常低于1 ppm，某些子区域的差异更大。这两个模型都捕获了观察到的城市内部X _{CO 2}梯度。此外，OCO-3的多段测量可捕获到的城市排放量是单段立交桥的三倍。OCO-3对目标和SAM模式的频繁观察将为在更细的副城市规模上限制城市排放铺平道路。