Quantifying the loss of processed natural gas within California's South Coast Air Basin using long-term measurements of ethane and methane

Abstract

Abstract. Methane emissions inventories for Southern California's South Coast Air Basin (SoCAB) have underestimated emissions from atmospheric measurements. To provide insight into the sources of the discrepancy, we analyze records of atmospheric trace gas total column abundances in the SoCAB starting in the late 1980s to produce annual estimates of the ethane emissions from 1989 to 2015 and methane emissions from 2007 to 2015. The first decade of measurements shows a rapid decline in ethane emissions coincident with decreasing natural gas and crude oil production in the basin. Between 2010 and 2015, however, ethane emissions have grown gradually from about 13 ± 5 to about 23 ± 3 Gg yr −1 , despite the steady production of natural gas and oil over that time period. The methane emissions record begins with 1 year of measurements in 2007 and continuous measurements from 2011 to 2016 and shows little trend over time, with an average emission rate of 413 ± 86 Gg yr −1 . Since 2012, ethane to methane ratios in the natural gas withdrawn from a storage facility within the SoCAB have been increasing by 0.62 ± 0.05 % yr −1 , consistent with the ratios measured in the delivered gas. Our atmospheric measurements also show an increase in these ratios but with a slope of 0.36 ± 0.08 % yr −1 , or 58 ± 13 % of the slope calculated from the withdrawn gas. From this, we infer that more than half of the excess methane in the SoCAB between 2012 and 2015 is attributable to losses from the natural gas infrastructure.

Southern California Megacity CO<sub>2</sub>, CH<sub>4</sub>, and CO flux estimates using remote sensing and a Lagrangian model

“…Several previous studies have discussed the SoCAB X CO2 , X CH4 , and X CO enhancements from local anthropogenic activity (Wunch et al, 2009;Kort et al, 2012;Janardanan et al, 2016;Wunch et al, 2016;Hedelius et al, 2017a;Schwandner et al, 2017). There have also been several studies which have discussed enhancements noted from the CLARS (California Laboratory…”

“…Previous studies Wunch et al, 2009Wunch et al, , 2016 have noted that the Pasadena area is a good receptor site for the basin, so tracer-tracer ratios observed there should 10 approximately correlate with emission ratios. If the ratios are significantly different it could highlight an error in the inversion scheme, or the a priori assumption of sources.…”

“…7). For the first test, we filter out data where the observations differ from the Tracer-tracer relations were used for the TCCON (Wunch et al, 2009(Wunch et al, , 2016, CLARS (Wong et al, 2015, and in situ observations (Hsu et al, 2010;Wennberg et al, 2012). model above a threshold.…”

“…Harvard-EPA inventory (Maasakkers et al, 2016 Wunch et al (2016), and the rest of the emissions were distributed based on population which was assumed to correspond with the January 2017 Suomi NPP nightlights (15 arcseconds). An average monthly trend was included based on results of Wong et al (2016), and emissions were assumed to be constant on a monthly timescale.…”

“…We estimate emission ratios using the solar zenith angle (SZA) anomaly method described by Wunch et al (2009Wunch et al ( , 2016, as well as from the average enhancement compared with AFRC or the Pasadena:Lancaster gradient ratio. Errors are assumed to equal the standard deviation of all the data, and a linear fit is made using the methods of York et al (2004).…”