Evaluating a Bottom-Up Inventory of Oil and Natural Gas Emissions with OMI and TROPOMI Satellite Retrievals

Over the past two decades, US oil and natural gas extraction and production has grown. Associated with the production of oil and natural gas are emissions of greenhouse gases and ozone precursors, including methane (CH4), non-methane volatile organic compounds (NMVOCs), and nitrogen oxides (NOx). Prior evaluations of bottom-up emission inventories with field observations suggest significant uncertainties with quantifying emissions from oil and natural gas activity. Here we build upon a previously developed Fuel-based Oil and Gas (FOG) inventory constructed utilizing bottom-up methodologies to estimate NOx from drilling rigs, compressor stations, artificial lifts, dehydrators, and heaters. We expand FOG to cover the continental US and add co-emitted CH₄ and NMVOCs. We assess NOx emissions in FOG utilizing historical trends of NO₂ from the Aura Ozone Monitoring Instrument (OMI). The satellite trends analysis provides insights in the distribution of NOx emissions between the exploration-phase (e.g., drilling) and production-phase across individual basins. For CH4, we assess FOG with mass balance estimates derived from the NOAA P3 aircraft during the Shale Oil and Natural Gas Nexus Study (SONGNEX). We then model NOx and CH4 emissions from FOG in the Weather Research Forecasting with Chemistry (WRF-Chem) model and evaluate with satellite retrievals of NO2 and CH4 from the Sentinel 5P Tropospheric Ozone Monitoring Instrument (TROPOMI). Lastly, NMVOCs are estimated by ratio to methane utilizing whole air samples taken on the NOAA P-3 aircraft and analyzed by gas chromatography-mass spectrometry. The composition and emissions of NMVOCs are strongly influenced by whether a basin is wet (i.e., oil dominated) or dry (i.e., natural gas dominated). Based on FOG, we preliminarily estimate ~0.3 Tg of NOx and ~13 Tg of methane from onshore oil and natural exploration and production. These values are generally consistent to officially reported inventories, and within -25% and +30% for NOx and methane, respectively.