Toward Using Space-Based CO2 and NO2 Observations to Estimate Urban CO2 Emissions

Abstract:

Urban CO₂ emissions are a major and dynamic source of fossil fuel CO₂ emissions, and observational methods of constraining these urban emissions are critical for addressing uncertainties in the global carbon budget. Observations of species co-emitted with CO₂ are increasingly being used to constrain CO₂ emissions, as they can complement existing CO₂ observations. In particular, observations of NO_x, which is co-emitted with CO₂ during high-temperature fossil fuel combustion, could improve our ability to reliably disentangle and attribute sources of fossil fuel CO₂. In this study, we work toward estimating urban CO₂ emissions in cities like Buenos Aires, Melbourne, and Tsukuba using pseudo-CO₂ fields empirically derived from space-based observations of urban CO₂ and NO₂. Rather than rely on prior knowledge of CO₂:NO_x emission ratios, our method depends on empirically quantifying relationships between observations of XCO₂ from the Orbiting Carbon Observatory-3 (OCO-3) instrument aboard the ISS and tropospheric NO₂ observations from the Tropospheric Monitoring Instrument (TROPOMI) aboard the S5P satellite. These relationships are then applied to TROPOMI NO₂ data to generate pseudo-CO₂ fields for time periods that have no co-located CO₂ observations. Inverse methods are then used on the pseudo-CO₂ fields to estimate CO₂ emissions. We evaluate the utility of this method using co-located simulations of CO₂ and NO₂ and discuss the representativeness of the generated pseudo-CO₂ fields. Given the relative abundance and resolution of space-based NO₂ observations compared to CO₂, we highlight the potential our new method has on the constraint of urban CO₂ emissions.