Quantifying the Loss of Processed Natural Gas Within California's South Coast Air Basin Using Long-term Measurements of Ethane and Methane

Abstract. California's South Coast Air Basin (SoCAB) is a region in which the top-down methane emissions are underestimated by the bottom-up inventories. To provide insight into the sources of the discrepancy, we analyse a record of atmospheric trace gas total column abundances in the SoCAB starting in the late 1980s. The gases measured include ethane and methane and provide insight into the sources of the excess methane found in the SoCAB. The early few years of the record show a rapid decline in ethane emissions at a much faster rate than decreasing vehicle exhaust or natural gas and crude oil production can explain. Between 2010 and 2015, ethane emissions have grown gradually from 13 ± 4.5 Gg · yr−1 to 25.8 ± 3.9 Gg · yr−1, which is in contrast to the steady production of natural gas liquids over that time. Our methane emissions record begins in 2012 and shows an increase between 2012 and 2015 from 380 ± 78 Gg · yr−1 to 448 ± 91 Gg · yr−1. Since 2012, ethane to methane ratios in the natural gas withdrawn from a storage facility within the SoCAB have been increasing; these ratios are tracked in our atmospheric measurements with about half of the rate of increase. From this, we infer that about half of the excess methane in the SoCAB between 2012–2015 is attributable to losses from the natural gas infrastructure.