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

Donald R Blake,Jean-François L Blavier,Nicholas Vizenor,Geoffrey C Toon,Coleen M Roehl,Debra Wunch,Jacob K Hedelius,Katherine M Saad,Paul O Wennberg

doi:10.5194/acp-16-14091-2016

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 Since 2012, ethane to methane ratios in the natural gas withdrawn from a storage facility within the SoCAB have been increasing by 0.62 ± 0.05 % yr−1, consistent with the ratios measured in the delivered gas.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show an increase in these ratios but with a slope of 0.36 ± 0.08 % yr−1, or 58 ± 13 % of the slope calculated from the withdrawn gas.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.

Highlights

Anthropogenic sources of the potent greenhouse gas methane (CH4) constitute about 60 % of the global total CH4 emissions, or nearly 350 Tg CH4 yr−1 (Saunois et al, 2016)
The fourth instrument, which is located about 10 km from JPL at the California Institute of Technology (Caltech), is part of the Total Carbon Column Observing Network (TCCON) and has been measuring ethane, methane, and other trace gases with high temporal frequency since September 2012 (Wennberg et al, 2014b)
We calculate that ethane emissions declined rapidly until the mid-1990s, coincident with the decline in Los Angeles Basin production of natural gas and crude oil, but the absolute abundances are inconsistent with recent estimates of natural gas emissions from the South Coast Air Basin (SoCAB) oil and gas production

Summary

Introduction

Anthropogenic sources of the potent greenhouse gas methane (CH4) constitute about 60 % of the global total CH4 emissions, or nearly 350 Tg CH4 yr−1 (Saunois et al, 2016). Southern California’s South Coast Air Basin (SoCAB) has been the focus of several studies. These studies have quantified the emissions from the basin and generally find that the SoCAB emissions are higher than the reported inventories (Wunch et al, 2009; Hsu et al, 2010; TownsendSmall et al, 2012; Wennberg et al, 2012; Peischl et al, 2013; Wong et al, 2015, 2016; Hopkins et al, 2016). Urban emissions within the basin have long residence times and, under prevailing wind conditions, have strong and predictable diurnal flow: out to the ocean at night and inland during the day

Methods

Results

Conclusion