Top-down constraints on anthropogenic CO2 emissions within an agricultural-urban landscape

Authors: HU, CHENG - University Of Minnesota; GRIFFIS, TIMOTHY - University Of Minnesota; LEE, XUHUI - Yale University; MILLET, DYLAN - University Of Minnesota; CHEN, ZICHONG - University Of Minnesota; Baker, John; XIAO, KE - University Of Minnesota

Submitted to: Journal of Geophysical Research: Atmospheres
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2015
Publication Date: 5/16/2018
Citation: Hu, C., Griffis, T.J., Lee, X., Millet, D.B., Chen, Z., Baker, J.M., Xiao, K. 2018. Top-down constraints on anthropogenic CO2 emissions within an agricultural-urban landscape. Journal of Geophysical Research Atmospheres. 123(9):4674-4694. doi: 10.1029/2017JD027881.
DOI: https://doi.org/10.1029/2017JD027881

Interpretive Summary: One of the challenges in trying to reduce atmospheric CO2 concentrations is determining the relative contributions of the various anthropogenic sources. We have used a combination of CO2 measurements from the top of a tall radio tower, carbon isotope ratio measurements, and inverse modeling to more accurately estimate the relative contributions of different CO2 sources within a heterogeneous region that includes both agricultural lands and urban/suburban land within the a major metropolitan area, Minneapolis-St. Paul. The results indicate that anthropogenic emissions enhanced the background CO2 concentration by approximately 7 micromol per mol over the period 2008-2010. Oil production from nearby refineries was the largest contributor to anthropogenic emissions, responsible for 42.5% of that total, with energy (power production) accounting for nearly 20% and residential emissions approximately 15%. Carbon isotope data were used to further divide these emssions into their constituent fossil fuel sources, revealing that fuel oil and natural gas were responsible for 83% of the total CO2 emissions, with the largest portion orignating from natural gas. These results will be helpful indetermining where improvements in efficiency will provide the greatest reduction in carbon emissions, and will also be useful in interpreting subsequent measurements of changes in atmospheric CO2.

Technical Abstract: Anthropogenic carbon dioxide (CO2) emissions dominate the atmospheric greenhouse gas radiative forcing budget. However, these emissions are poorly constrained at the regional 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 mmol mol-1 annual CO2 enhancements (i.e. departures from the background values) in 2008, 2009, and 2010, respectively. Oil production, the energy industry, and residential emissions contributed 2.9 (42.5%), 1.4 (19.8%), and 1.1 mmol mol-1 (15.8%) of the total anthropogenic enhancement over the three-year period. The total anthropogenic signal was further partitioned into CO2 emissions 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 83% of the anthropogenic CO2 sources. Here, the a posteriori CO2 emission from natural gas was 60.4 ± 24.0% (a priori 20.0%), while the a posteriori CO2 emission from fuel oil was 25.2 ± 3.8% (a priori 50.8%) during winter— suggesting a more important role of residential heating. The modeled carbon isotope ratio of the CO2 source (-30.6 ± 0.4‰) was relatively more enriched in 13C-CO2 compared to that derived from Keeling plot analyses (-37.5‰ to -35.0‰), supporting that natural gas consumption was underestimated for this region.