California Air Resources Board

Principal Investigator/Author: Frank Mitloehner

Contractor: University of California, Davis

Contract Number: 17RD017

Project Status: Completed

Relevant CARB Programs: Climate Change

Topic Areas: Dairy & Livestock, Greenhouse Gas Emissions

Research Summary:

Methane (CH4) emissions from dairies represent nearly half of all CH4 emissions in California, with dairy manure accounting for 25 percent (9.88 MMTCO2e) and enteric fermentation accounting for 20 percent (7.81 MMTCO2e). Most dairy farms in California utilize manure lagoons, in which organic matter in manure undergoes a biochemical degradation process that creates CH4. Alternative Manure Management Program (AMMP) practices are considered a cost-effective set of solutions to reduce CH4 emissions; however, data on the real-world effectiveness of such technologies is limited. This project aimed at evaluating the effectiveness of AMMP practices on the emissions of GHG and other criteria pollutants by conducting real-world emissions measurements on four dairies after installation of AMMP practices (post-AMMP) and comparing the results to those of the pre-AMMP. It also applied regional chemical transport models to determine the effect of extreme dairy emissions reductions on the SJV airshed-wide ozone (O3) and PM2.5 concentrations in the coming years. Although it was expected that the post-AMMP CH4 emissions would be lower than the pre-AMMP emissions, as AMMP practices divert volatile solids away from anaerobic conditions, measurement results indicated higher or similar emissions for post-AMMP. Multiple factors may have contributed to this observation: 1) Changes in manure characteristics from pre- to post-AMMP conditions that are not related to AMMP installation; 2) Changes in other confounding factors that were not captured during the measurement period; and 3) Improper execution of the AMMP technologies on dairies. The regional chemical transport modeling revealed the changes in airshed-wide O3 and PM2.5 concentrations in the SJV under an extreme dairy emissions reduction scenario against Business As Usual (BAU) and future clean energy scenarios. Since future scenarios for AMMP did not show a measurable impact on the SJV air quality, a dairy digester scenario model run was added as well. Additional manure management options were explored to gain a comprehensive understanding of potential future air quality challenges. The results suggested the need for an AMMP execution follow-up plan, highlighted the importance of long-term measurements in evaluating real-world emissions, and provided insight into the effects of extreme emissions reductions from the dairy sector on the SJV air quality that can help facilitate the development of the future State Implementation Plans (SIP).

Keywords: methane (CH4), emissions, dairies, methane emissions, dairy manure, manure, enteric fermentation, dairy farm, alternative manure management program (AMMP), greenhouse gas, ozone (O3), particulate matter (PM2.5), San Joaquin Valley (SJV), digester, air quality