Fossil Versus Nonfossil CO Sources in the US: New Airborne Constraints From ACT-America and GEM

Carbon monoxide (CO) is an ozone precursor, oxidant sink, and widely used pollution tracer. The importance of anthropogenic versus other CO sources in the US is uncertain. Here, we interpret extensive airborne measurements with an atmospheric model to constrain US fossil and nonfossil CO sources. Measurements reveal a low bias in the simulated CO background and a 30% overestimate of US fossil CO emissions in the 2016 National Emissions Inventory. After optimization we apply the model for source partitioning. During summer, regional fossil sources account for just 9%-16% of the sampled boundary layer CO, and 32%-38% of the North American enhancement-complicating use of CO as a fossil fuel tracer. The remainder predominantly reflects biogenic hydrocarbon oxidation plus fires. Fossil sources account for less domain-wide spatial variability at this time than nonfossil and background contributions. The regional fossil contribution rises in other seasons, and drives ambient variability downwind of urban areas.

Automated summary

This study used extensive airborne measurements and an atmospheric model to investigate fossil and nonfossil CO sources in the US, finding a low bias in the simulated CO background. Fossil sources account for a small proportion of CO in the air during summer, while nonfossil and biogenic sources play a larger role. In other seasons, fossil sources drive significant variability downwind of urban areas.