Dairy sector greenhouse gas and ammonia emissions estimates based on seasonal measurements from 200 farms in California

We report facility-scale emissions measurements of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ammonia (NH3) from over 200 farms in California, representing 2.4% of the total dairy cow population of the United States. The dairy industry—and its 270 million cattle—are responsible for a significant proportion of global greenhouse gas and reactive nitrogen emissions. Open-path laser spectrometers mounted on top of an electric vehicle were used to conduct measurements downwind of facilities. Emission rates and associated uncertainties for each facility were quantified using in-plume observations and an inverse Gaussian plume atmospheric dispersion model with Bayesian inference. A subset of 53 farms were revisited quarterly to investigate seasonal variability and changes in management practices. 24 farms were sampled multiple times a week on different days for select seasons in order to capture inter-day variability. We partnered with 6 farms for intensive 24-hour measurements to study diurnal variability in emissions.  Measurements of emissions were compared with national and state emissions inventories, and they were also evaluated against bottom-up estimates using activity data from the Vista-CA database. The median of the top-down CH4 emissions agreed with the median of facility-level bottom-up emissions to within 8%. Measured N2O emissions were 2.5 times higher than current inventories. N2O was responsible for about 15% of total greenhouse gas emissions in terms of CO2-equivalents (CO2e). NH3 was 40% lower than the values indicated by the 2017 US EPA national emissions inventory (NEI).  For N2O, we found that emissions were dominated by large emission events (≈30 kg N2O hr-1) with high spatial and temporal variability. These events are potentially not captured by shorter measurement campaigns that focus only on a few farms. The top 10% largest N2O emission events were responsible for 58% of total N2O emissions. Focused studies on such events may elucidate potential opportunities for reduction of N2O emissions.  For CH4, 14 out of the 207 dairies that we sampled had verified anaerobic digestion systems installed for manure management. The effectiveness of these systems for reducing methane emissions will be evaluated by comparing emissions from farms with digesters to emissions from farms without digesters.