Investigating Elevated Concentrations of Dimethyl Sulfide in the Coachella Valley

Dimethyl sulfide (DMS) is primarily emitted to the atmosphere by marine environments and agriculture, and is important to study because its oxidation has been shown to contribute to the formation of sulfate aerosols. Whole air samples were collected in Southern California during the 2017 NASA Student Airborne Research Program and an average DMS concentration of 12 ± 11 pptv was observed during flights over land. However, elevated DMS concentrations as high as 63 pptv were observed over an area of dense agriculture in the northern area of the Coachella Valley. Wind trajectories originating from the Los Angeles Basin were determined using the NOAA HYSPLIT model. Since DMS has a short lifetime of 1 day, results indicated that despite HYSPLIT data originating from LA, the DMS was emitted locally. Comparisons to local marine trace gases imply that the elevated DMS was not solely emitted from the nearby Salton Sea, and likely also had terrestrial sources. Further analysis using ground sample comparisons between DMS with beta-pinene excluded citrus farms in the area as a potential source. Carbon dioxide from samples in the Coachella Valley was tested against an urban tracer and results indicated that the carbon dioxide was actually biogenic; therefore, it was used as a tracer for the fermentation process from the local vineyards. However, a weak correlation between carbon dioxide and ethanol ruled out the vineyards as a source as well. Further investigations into potential land use change in the region confirmed an increase in crop acreage for aquaculture, which tripled in size in the area between 2014 and 2016. A notable increase was also observed in DMS concentrations during different NASA SARP campaigns over the Coachella Valley, from 10 ± 8 pptv in 2014 to 30 ± 20 pptv in 2017. Ultimately, findings confirmed that the elevated DMS was influenced by both the Salton Sea and the local agriculture. Ruling out the unlikely sources provided a better understanding of terrestrial emissions, which helps give insight into the understanding of the impact of DMS on sulfate aerosol formation.