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Within the California South Coast Air Basin (SoCAB), X-CO2 varies significantly due to atmospheric dynamics and the nonuniform distribution of sources. X-CO2 measurements within the basin have seasonal variation compared to the " background" due primarily to dynamics, or the origins of air masses coming into the basin. We observe basin- background differences that are in close agreement for three observing systems: Total Carbon Column Observing Network (TCCON) 2.3 +/- 1.2 ppm, Orbiting Carbon Observatory- 2 (OCO-2) 2.4 +/- 1.5 ppm, and Greenhouse gases Observing Satellite 2.4 +/- 1.6 ppm(errors are 1 sigma). We further observe persistent significant differences (+/- 0.9 ppm) in X-CO2 between two TCCON sites located only 9 km apart within the SoCAB. We estimate that 20% (+/-1 sigma confidence interval (CI): 0%, 58%) of the variance is explained by a difference in elevation using a full physics and emissions model and 36% (+/- 1 sigma CI: 10%, 101%) using a simple, fixed mixed layer model. This effect arises in the presence of a sharp gradient in any species (here we focus on (CO2)) between the mixed layer (ML) and free troposphere. Column differences between nearby locations arise when the change in elevation is greater than the change in ML height. This affects the fraction of atmosphere that is in the ML above each site. We show that such topographic effects produce significant variation in X-CO2 across the SoCAB as well.

Plain Language Summary Cities persistently have elevated carbon dioxide (CO2) levels as compared to surrounding regions. Within a city CO2 levels can also vary significantly at different locations for reasons such as more CO2 being emitted in some parts than others. Elevated column CO2 levels in the South Coast Air Basin (SoCAB) are in agreement for three observation systems (two satellite and one ground- based) systems and vary with regional wind patterns throughout the year. In Pasadena, California, within the SoCAB, a significant fraction (about 25%) of variation in the column-averaged CO2 can be explained by differences in surface altitude. This is important to understand so that all variations in column CO2 within an urban region are not mistakenly interpreted as being from CO2 surface fluxes.