On the Sources of Methane to the Los Angeles Atmosphere

We use historical and new atmospheric trace gas observations to refine the estimated source of methane (CH₄) emitted into California’s South Coast Air Basin (the larger Los Angeles metropolitan region). Referenced to the California Air Resources Board (CARB) CO emissions inventory, total CH₄ emissions are 0.44 ± 0.15 Tg each year. To investigate the possible contribution of fossil fuel emissions, we use ambient air observations of methane (CH₄), ethane (C₂H₆), and carbon monoxide (CO), together with measured C₂H₆ to CH₄ enhancement ratios in the Los Angeles natural gas supply. The observed atmospheric C₂H₆ to CH₄ ratio during the ARCTAS (2008) and CalNex (2010) aircraft campaigns is similar to the ratio of these gases in the natural gas supplied to the basin during both these campaigns. Thus, at the upper limit (assuming that the only major source of atmospheric C₂H₆ is fugitive emissions from the natural gas infrastructure) these data are consistent with the attribution of most (0.39 ± 0.15 Tg yr^–1) of the excess CH₄ in the basin to uncombusted losses from the natural gas system (approximately 2.5–6% of natural gas delivered to basin customers). However, there are other sources of C₂H₆ in the region. In particular, emissions of C₂H₆ (and CH₄) from natural gas seeps as well as those associated with petroleum production, both of which are poorly known, will reduce the inferred contribution of the natural gas infrastructure to the total CH₄ emissions, potentially significantly. This study highlights both the value and challenges associated with the use of ethane as a tracer for fugitive emissions from the natural gas production and distribution system.