To guide development of greenhouse gases (GHGs) reduction strategies, California has developed state-wide GHGs emission inventories (EI). ARB has supported research to improve our GHGs EI, developing regional and source specific information for some GHGs. For example, ARB has supported methane and nitrous oxide (N2O) research programs, including measurements at towers and inverse modeling. In this project, proton transfer mass spectrometry (PTR-MS) was used to continuously measure a suite of volatile organic compounds (VOCs) at 5 elevations on a tall tower in Walnut Grove from summer 2012 through early fall 2013. The VOC data and continuous carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) measurements were analyzed to develop source apportionments for CH4 and N2O. Their results confirm that dairies and livestock are the largest regional sources contributing to CH4 emissions, accounting for 55 - 90 percent of total emissions over different seasons. N2O agriculture emissions accounted for about 80 - 90 percent of the observed enhancements during fertilizer use in the spring and summer but declined to about 20 percent of the observed enhancements in late fall season when crops are harvested. In contrast, N2O emissions from the dairy and livestock source were relatively constant across seasons, accounting for more than 80 percent of the total enhancements in fall and winter, and less when agricultural emissions were larger. Likely because of the rural location, the contribution of motor vehicles and hydrocarbon extraction emissions was not significant in the regional inventory observed at Walnut Grove. This is a significant difference from the state-wide GHG EI which includes urban emissions. However, Walnut Grove Tower measurements represent a regional assessment of emissions as opposed to state-wide emission inventories. Nevertheless, for GHG sources which have significant seasonal or process variation in emissions, the findings highlight the importance of multi-month measurements to validate the inventory.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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