UC Irvine

Short lived greenhouse gases are an immediate target for reductions in climate policy planning. To understand to what extent legislation to limit greenhouse gas emissions is working, monitoring projects are necessary. In this work, whole air samples were collected downwind of the Los Angeles megacity over the course of two years between June 2014 and May 2016. Samples were analyzed via gas chromatography instruments to quantify over 70 volatile organic compounds with limits of detection on the single part per trillion level.

Unlike larger hydrocarbons, the daytime average mixing ratio for both methane and ethane did not decrease in the region since a previous study in 2007 suggestive of continued natural gas leakage. Using monthly ethane to methane enhancement ratios and natural gas composition of gas delivered to the region showed that 72 ± 16% of all excess methane emitted in the SoCAB was due to natural gas leakage. This result is significantly higher than the fraction due to natural gas in previous bottom-up inventories in the region.

Positive matrix factorization was applied to the data set to aid in apportionment of hydrocarbons to various sources in the region. A seven factor result was obtained in which natural gas, oil and gas refining, biogenic, industrial sources, BTEX, gasoline liquids and tailpipe emissions were matched to source profiles. Via this analysis, 59% (57-65%) of the methane seen in the SOCAB was due to the leakage of unburned natural gas. This agrees well with the result obtained using the composition of delivered gas and two recent studies of the region using other data sets.

Finally, several classes of refrigerants which are potent greenhouse gases were studied. The emission of banned chlorofluorocarbons was noted to have completely stopped in the region. Hydrofluorochlorocarbons are currently being phased out and so emissions estimates were calculated and compared to older data sets to show that regulations are working to limit their emission. HCFC-142b emissions were noted to have dramatically stopped during the sampling period as the result of a phase down rule that went into place on January 1, 2015.