Source partitioning of methane emissions and its seasonality in the U.S. Midwest

Authors: CHEN, ZICHONG - University Of Minnesota; GRIFFIS, TIMOTHY - University Of Minnesota; Baker, John; MILLET, DYLAN - University Of Minnesota; WOOD, JEFFREY - University Of Missouri; DLUGOKENCKY, EDWARD - National Oceanic & Atmospheric Administration (NOAA); ANDREWS, ARLYN - National Oceanic & Atmospheric Administration (NOAA); HU, CHENG - University Of Minnesota; KOLKA, RANDALL - Us Forest Service (FS)

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2018
Publication Date: 3/14/2018
Citation: Chen, Z., Griffis, T.J., Baker, J.M., Millet, D.B., Wood, J.D., Dlugokencky, E.J., Andrews, A.E., Hu, C., Kolka, R.K. 2018. Source partitioning of methane emissions and its seasonality in the U.S. Midwest. Journal of Geophysical Research-Biogeosciences. 123(2):646-659. https://doi.org/10.1002/2017JG004356.
DOI: https://doi.org/10.1002/2017JG004356

Interpretive Summary: After CO2, methane is the most significant greenhouse gas, and it's global concentration is increasing rapidly. However, the relative importance of different sources is not well understood. We used a novel data set of regional flux measurements derived from concentration gradients measured continuously over a year on a tall (185 m) radio tower together with inverse modeling to separate the methane budget into natural (e.g. wetland) sources and anthropogenic (e.g. oil & gas, livestock, waste) from June 2016 to September 2017 The landscape as a whole was found to generally be a methane source and was rarely a net sink. The results also showed that the anthropogenic source was about 1.5 times greater than estimated from the EPA inventory, due primarily to underestimation of the livestock and oil/gas sources. Wetland emissions were found to be the second largest source, accounting for slightly more than a third of emissions. The seasonal variation of total emissions peaked in August, primarily due to the wetland contribution. Emissions from anthropogenic sources had only weak seasonality. These results will help to identify where and when there is the most potential to mitigate methane emissions.

Technical Abstract: The methane (CH4) budget and its source partitioning are poorly constrained in the Midwestern, United States. We used tall tower (185 m) aerodynamic flux measurements and atmospheric scale factor Bayesian inversions (SFBI) to constrain the monthly budget and to partition the total budget into natural (e.g. wetlands) and anthropogenic (e.g. livestock, waste, and natural gas) sources for the period June 2016 to September 2017. Aerodynamic flux observations indicated that the landscape was a CH4 source with a mean annual CH4 flux of +13.7 ± 0.34 CH4 nmol m-2 s-1 and was rarely a net sink. The SFBI analyses revealed a mean annual source of +12.3 ± 2.6 CH4 nmol m-2 s-1. Flux partitioning revealed that the anthropogenic source (7.8 Tg CH4 yr-1) was 1.5 times greater than the bottom-up gridded EPA inventory, in which livestock and oil/gas sources were underestimated by 1.8- and 1.3-fold, respectively. Wetland emissions (4.0 Tg CH4 yr-1) were the second largest source, accounting for 34% of the total budget. The temporal variability of total CH4 emissions was dominated by wetlands with peak emissions occurring in August. In contrast, emissions from oil/gas and other anthropogenic sources showed relatively weak seasonality.