Top-Down Constraints on Anthropogenic CO₂ Emissions Within an Agricultural-Urban Landscape

Anthropogenic carbon dioxide (CO₂) emissions dominate the atmospheric greenhouse gas radiative forcing budget. However, these emissions are poorly constrained at the regional (10²–10⁶ km²) and seasonal scales. Here we use a combination of tall tower CO₂ mixing ratio and carbon isotope ratio observations and inverse modeling techniques to constrain anthropogenic CO₂ emissions within a highly heterogeneous agricultural landscape near Saint Paul, Minnesota, in the Upper Midwestern United States. The analyses indicate that anthropogenic emissions contributed 6.6, 6.8, and 7.4 μmol/mol annual CO₂ enhancements (i.e., departures from the background values) in 2008, 2009, and 2010, respectively. Oil refinery, the energy industry (power and heat generation), and residential emissions (home heating and cooking) contributed 2.9 (42.5%), 1.4 (19.8%), and 1.1 μmol/mol (15.8%) of the total anthropogenic enhancement over the 3-year period according to a priori inventories. The total anthropogenic signal was further partitioned into CO₂ emissions derived from fuel oil, natural gas, coal, gasoline, and diesel consumption using inverse modeling and carbon isotope ratio analyses. The results indicate that fuel oil and natural gas consumption accounted for 52.5% of the anthropogenic CO₂ sources in winter. Here the a posteriori CO₂ emission from natural gas was 79.0 ± 4.1% (a priori 20.0%) and accounted for 63% of the total CO₂ enhancement including both biological and anthropogenic sources. The a posteriori CO₂ emission from fuel oil was 8.4 ± 3.8% (a priori 32.5%)—suggesting a more important role of residential heating in winter. The modeled carbon isotope ratio of the CO₂ source (δ¹³C_s, −29.3 ± 0.4‰) was relatively more enriched in ¹³C-CO₂ compared to that derived from Miller-Tans plot analyses (−35.5‰ to −34.8‰), supporting that natural gas consumption was underestimated for this region.