Emissions of organic carbon and methane from petroleum and dairy operations in California's San Joaquin Valley

摘要

Petroleum and dairy operations are prominent sources of gas-phase organiccompounds in California's San Joaquin Valley. It is essential to understandthe emissions and air quality impacts of these relatively understudiedsources, especially for oil/gas operations in light of increasing USproduction. Ground site measurements in Bakersfield and regional aircraftmeasurements of reactive gas-phase organic compounds and methane were partof the CalNex (California Research at the Nexus of Air Quality and ClimateChange) project to determine the sources contributing to regional gas-phaseorganic carbon emissions. Using a combination of near-source and downwinddata, we assess the composition and magnitude of emissions, and provideaverage source profiles. To examine the spatial distribution of emissions inthe San Joaquin Valley, we developed a statistical modeling method using ground-based data and theFLEXPART-WRF transport and meteorological model. Wepresent evidence for large sources of paraffinic hydrocarbons from petroleumoperations and oxygenated compounds from dairy (and other cattle)operations. In addition to the small straight-chain alkanes typicallyassociated with petroleum operations, we observed a wide range of branchedand cyclic alkanes, most of which have limited previous in situ measurementsor characterization in petroleum operation emissions. Observed dairyemissions were dominated by ethanol, methanol, acetic acid, and methane.Dairy operations were responsible for the vast majority of methane emissionsin the San Joaquin Valley; observations of methane were well correlated withnon-vehicular ethanol, and multiple assessments of the spatial distributionof emissions in the San Joaquin Valley highlight the dominance of dairyoperations for methane emissions. The petroleum operations source profilewas developed using the composition of non-methane hydrocarbons in unrefinednatural gas associated with crude oil. The observed source profile isconsistent with fugitive emissions of condensate during storage orprocessing of associated gas following extraction and methane separation.Aircraft observations of concentration hotspots near oil wells and dairiesare consistent with the statistical source footprint determined via ourFLEXPART-WRF-based modeling method and ground-based data. We quantitativelycompared our observations at Bakersfield to the California Air ResourcesBoard emission inventory and find consistency for relative emission rates ofreactive organic gases between the aforementioned sources and motor vehiclesin the region. We estimate that petroleum and dairy operations eachcomprised 22% of anthropogenic non-methane organic carbon at Bakersfieldand were each responsible for 8–13% of potential precursors to ozone.Yet, their direct impacts as potential secondary organic aerosol (SOA)precursors were estimated to be minor for the source profiles observed inthe San Joaquin Valley.