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Published December 5, 2023 | Published
Journal Article Open

Methane Emissions from Dairy Operations in California's San Joaquin Valley Evaluated Using Airborne Flux Measurements


State inventories indicate that dairy operations account for nearly half of California's methane budget. Recent analyses suggest, however, that these emissions may be underestimated, complicating efforts to develop emission reduction strategies. Here, we report estimates of dairy methane emissions in the southern San Joaquin Valley (SJV) of California in June 2021 using airborne flux measurements. We find average dairy methane fluxes of 512 ± 178 mg m⁻² h⁻¹ from a region of 300+ dairies near Visalia, CA using a combination of eddy covariance and mass balance-based techniques, corresponding to 118 ± 41 kg dairy⁻¹ h⁻¹. These values estimated during our June campaign are 39 ± 48% larger than annual average estimates from the recently developed VISTA-CA inventory. We observed notable increases in emissions with temperature. Our estimates align well with inventory predictions when parametrizations for the temperature dependence of emissions are applied. Our measurements further demonstrate that the VISTA-CA emission inventory is considerably more accurate than the EPA GHG-I inventory in this region. Source apportionment analyses confirm that dairy operations produce the majority of methane emissions in the southern SJV (∼65%). Fugitive oil and gas (O&G) sources account for the remaining ∼35%. Our results support the accuracy of the process-based models used to develop dairy emission inventories and highlight the need for additional investigation of the meteorological dependence of these emissions.


Copyright and License

© 2023 American Chemical Society.


We would like to thank Greg Cooper, Bryce Kujat, and George Loudakis for their dedicated mission support during RECAP-CA. We would also like to thank Timothy Lueker, Jooil Kim, and Ralph Keeling for assistance with the Picarro G2401-m calibrations at the Scripps Institution of Oceanography. This research was funded by the following sources: California Air Resources Board Contract numbers 20RD003 and 20AQP012, NOAA Climate Program Office's Atmospheric Chemistry, Carbon Cycle, and Climate program, grant number NA22OAR4310540 [UCB]/NA22OAR4310541 [AD], Office of Naval Research Defense University Research Instrumentation Program grant number N00014-19-1-2108. We thank the Resnick Sustainability Institute at Caltech for funding the purchase of the Picarro G2401-m.

Conflict of Interest

The authors declare no competing financial interest.


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Additional details

December 13, 2023
December 13, 2023