Seasonal and diurnal variability in methane emissions from California dairy facilities partially explain mismatch between inventory and atmospheric observations: implications for reducing emissions

Dairy facilities produce nearly half of all methane emissions in California. Methane emissions vary seasonally and diurnally, but measurements are taken infrequently and extrapolated to an entire year. The state has mandated reductions in methane emissions by 40% of 1990 levels by 2030. To monitor these reductions, it is critical to account for seasonal and diurnal cycles in atmospheric measurements and inventories. Here, we partition a recently developed facility-scale annual inventory of dairy methane emissions to monthly and hourly estimates, which we compare with published atmospheric measurements. Our model incorporates the impact of temperature on methanogenesis reaction rates and the impact of temperature, precipitation, and evapotranspiration on animal management. We find that spring has the highest emissions, due to the carry-over of volatile solids from the winter. Further, the seasonality of emissions and volatile solids build up means changing the timing of annual cleanout of manure from manure lagoons from October to February could reduce the annual emissions by nearly . We also find that afternoon measurements overestimate daily, monthly, and annual emissions by on average 72%. Most atmospheric measurements are higher than inventory estimates by a factor of ~2, so our results partially explain the mismatch between inventory and measurements. Our results imply that considering seasonality and diurnal variability of methane emissions from dairy facilities can help us interpret and extrapolate beyond our measurements to more accurately compare with inventories.