Human activities release greenhouse gases and air pollutants into the atmosphere, causing global climate change and other widespread impacts on the Earth System. These emissions are concentrated in cities, where most humans live and where most transportation, energy generation and consumption occur. While many cities are taking action to reduce their emissions, verifying the success of such efforts is difficult, especially at local scales in complex urban environments. Without reliable monitoring of urban emissions trends, it is uncertain whether attempted solutions are effective and our ability to steer climate policy is limited.In my dissertation, I provide novel information about spatial and temporal patterns of anthropogenic gas emissions and our capacity to monitor emissions in urban environments. In my first study, I used a mobile laboratory to measure on-road carbon monoxide (CO) and carbon dioxide (CO2). The ratio of these two gases (CO/CO2) is a useful metric for assessing the success of regulations intended to reduce air pollutant emissions from vehicles. The results show that California’s policies and technological advancements have made the Los Angeles traffic fleet more efficient. However, combustion efficiency worsened during the COVID-19 pandemic and in Salt Lake City, likely because of changes to traffic conditions and fleet composition that offset progress in reducing vehicle CO emissions. In Chapter 2, I focus on quantifying fossil fuel CO2 (ffCO2) emission reductions that occurred during the COVID-19 pandemic using the mobile measurements from Chapter 1 and a community-sourced dataset of plant radiocarbon (14C). These two datasets reveal a significant reduction in ffCO2 in California’s urban areas in 2020 due to social distancing measures imposed by the pandemic. Furthermore, ffCO2 emissions rebounded to pre-pandemic levels by 2021, but not uniformly, with some areas taking longer to return to “normal” than others. The study demonstrated the capacity for plant 14C samples to capture ffCO2 emission reductions with shifts in human behaviors. This implied that plant sampling could be an informative and accessible tool for ffCO2 monitoring in cities that lack CO2 monitoring infrastructure as climate change mitigation policies take effect. This motivated us to conduct further tests comparing ffCO2 patterns indicated by plant 14C analysis with more established CO2 monitoring approaches. In Chapter 3, I collected turfgrass 14C samples along an urban to rural gradient in Southern California alongside measurements of in situ surface CO2 and remotely sensed total column CO2. The ffCO2 patterns indicated by each of these metrics agreed well with each other, suggesting that plant 14C analysis can independently provide similar quality information about urban ffCO2 emissions, but at a finer resolution since it is more operationally feasible to sample plants. In conjunction with surface CO2, plant 14C can also provide insight on biogenic fluxes in CO2, but more work is needed to inform nature-based climate solutions in cities. Future work should use the insights in these studies to monitor trends in urban ffCO2 emissions and guide policymakers during the transition away from fossil fuels.