Characterizing Local Methane Enhancements at the Southern Great Plains ARM Site in Oklahoma: Meteorological Influences and Source Attribution

Understanding the factors that lead to methane concentration enhancements is significant for refining emission inventories, improving atmospheric modeling, and assessing methane's role in climate change. Our study indicates significant methane enhancements at the Southern Great Plains (SGP) ARM site in Oklahoma, where daily averages are up to 615 ppb higher compared to other locations, and hourly peaks can exceed 4600 ppb particularly during nighttime and early morning. We focus on analyzing both meteorological conditions and local methane sources to understand their relationship with the enhanced methane concentrations at the SGP ARM site.

Our results suggest that high methane concentrations at SGP ARM site are associated with stable atmospheric conditions, including potential temperature inversions over 0.05 K/m, shallow atmospheric boundary height, and calm wind conditions. These conditions notably restrict vertical air mixing, trapping methane close to the surface and resulting in the observed spikes in concentration. Furthermore, high methane concentrations have been frequently observed when winds blown from approximately 150 and 300 degrees, directions pointing to open range cattle, natural gas pipeline, and a large animal feeding operation (AFO). Based on observations from our June field campaign, methane concentrations downwind of the AFO (5.8 km from the ARM site) exceeded 6000 ppb on an early morning – we estimate using the mass balance method that the emission rate from the AFO surpasses 100 kg•hr^-1. This emission rate is likely an underestimate, as the animal feeding operation had a higher cattle population before our observations. Additional sources contributing to the elevated methane levels at the SGP ARM site include other nearby cattle (~ 1km from the SGP ARM site) and leaks from a nearby natural gas pipeline (same location as the AFO).

By integrating the impacts of meteorological stability and local emission dynamics, our study not only enhances the understanding of methane distribution at the SGP ARM site but also contributes to broader discussions on methane's role in climate dynamics.