Predicting and comparing fugitive methane emissions in northeast Pennsylvania using numerical weather prediction

Primarily composed of methane (CH₄), natural gas is theorized as a “bridge fuel” to stray away from the high carbon dioxide emissions released when other fossil fuels such as coal are consumed. It has since been clarified that the energy efficiency of natural gas highly depends on the leakage rate at the production level. Methane leaks from gas wells, of which methane is the primary greenhouse gas, are a major contributor to the overall carbon footprint of natural gas, methane having a Global Warming Potential 24 times larger than carbon dioxide over a 100 year period. The Marcellus shale formation, located from upstate New York and down the spine of the Appalachian Mountains, is home to thousands of unconventional and conventional wells, which are used to extract natural gas. In an effort to acquire the percentage of fugitive emissions during the production of natural gas, we used the numerical weather prediction model Weather Research and Forecasting model with the Chemistry module (WRF-Chem, Version 3.5.1) and coupled to methane emission inventories for unconventional wells, conventional wells, coal mines, industrial sources, and sources from waste management (e.g. landfills, waste water treatment plants). The emission factors used to generate the different emissions estimates were provided by the United States Environmental Protection Agency (USEPA). An aircraft campaign was then utilized to record observations of methane concentrations. Airborne measurements were compared to the simulated CH₄ concentrations using WRF-Chem in both forecast and historical modes. The purpose of this study was to evaluate our methane inventory and the ability of the WRF-Chem system to accurately capture the observed methane concentrations from natural gas leaks, primarily originating from producing wells in northeast Pennsylvania. Furthermore, we also reviewed the accuracy of the WRF-Chem system in forecast mode compared to the historical mode over ten flight days in May 2015. WRF-Chem simulation was performed using North American Regional Reanalysis (NARR) data to drive our WRF-Chem model (WRF-Historical). Results show that the simulated methane concentrations are consistent with the aircraft measurements, which suggests that our emission inventory across northeast Pennsylvania is reasonable. Further analysis shows large enhancements of methane from the vast coal mines in southwest Pennsylvania and eastern Ohio, which is especially prominent given certain meteorological conditions. The data from WRF-Historical showed a correction of inaccuracies noted in WRF-Forecast; transport and concentration errors were normally adjusted for. When comparing the two model outputs, there was also a correlation regarding failure to agree to the flight observations. After the analysis, it is evident fugitive methane emissions due to unconventional wells are low and agree with previous estimations from prior studies. More interestingly, enhancements due to the coal mines in southwest Pennsylvania and east Ohio have an influence on the area comparable to that of unconventional wells, which suggests that the characterization of the regional background concentrations is of major importance over the northeastern region of the Marcellus shale.