Observations of Methane Emissions from California Dairies from Ground and Space: New Top-Down Constraints at Regional Scales

California has made reducing methane (CH₄) from its dairy industry, which accounts for nearly 50% of its inventoried CH₄ emissions, a key part of its climate change mitigation plan. However, in situ atmospheric measurement-based estimates suggest that the state-wide dairy source may be underestimated by up to a factor of 2.  Furthermore, emissions at the local scales important to mitigation policy (10’s km) are very uncertain. Ground based measurements of atmospheric column averaged methane concentrations (XCH₄) can provide useful constraints on local to regional methane fluxes. Additionally, the high spatial and temporal resolution XCH₄ observations by the space-based TROPOMI and GOSAT instruments provide an excellent opportunity to measure CH₄ fluxes at these scales. We report field measurements of XCH₄ gradients across the dairy intensive region (500 dairies, 330 Gg/yr CH4 emissions inventory) in the San Joaquin Valley (SJV) using two EM27/SUN solar spectrometers. With our EM27’s we observed several days of sustained downwind-upwind XCH₄ enhancements of over 40 ppb, placing these signals well above TROPOMI and GOSAT’s precision level and among the highest reported XCH₄ enhancements. We compare TROPOMI and GOSAT spatial XCH₄ enhancements to our EM27 data to validate it in an area of high signal and to demonstrate its utility for observing localized sources. We also use GOSAT and TROPOMI’s data to characterize the wider SJV’s CH₄ sources and to fill in temporal gaps between our field campaigns. Finally, we perform inverse optimizations using WRF-STILT simulations demonstrating that top-down observationally constrained dairy emissions are a factor of 2 larger than reported inventories.  This work illustrates how ground and space-based measurements can complement each other to improve our understanding of CH₄ sources at scales relevant to mitigation policy.