W. M. Keck Institute for Space Studies Postdoctoral Fellowship Final Report

We use historical and new atmospheric trace gas observationsto refine the estimated source of methane (CH4) emitted into California’sSouth Coast Air Basin (the larger Los Angeles metropolitan region).Referenced to the California Air Resources Board (CARB) CO emissionsinventory, total CH4 emissions are 0.44 ± 0.15 Tg each year. To investigatethe possible contribution of fossil fuel emissions, we use ambient airobservations of methane (CH4), ethane (C2H6), and carbon monoxide (CO),together with measured C2H6 to CH4 enhancement ratios in the Los Angelesnatural gas supply. The observed atmospheric C2H6 to CH4 ratio during theARCTAS (2008) and CalNex (2010) aircraft campaigns is similar to the ratioof these gases in the natural gas supplied to the basin during both thesecampaigns. Thus, at the upper limit (assuming that the only major source ofatmospheric C2H6 is fugitive emissions from the natural gas infrastructure)these data are consistent with the attribution of most (0.39 ± 0.15 Tgyr−1) of the excess CH4 in the basin to uncombusted lossesfrom the natural gas system (approximately 2.5−6% of natural gas delivered to basin customers). However, there are othersources of C2H6 in the region. In particular, emissions of C2H6 (and CH4) from natural gas seeps as well as those associated withpetroleum production, both of which are poorly known, will reduce the inferred contribution of the natural gas infrastructure tothe total CH4 emissions, potentially significantly. This study highlights both the value and challenges associated with the use ofethane as a tracer for fugitive emissions from the natural gas production and distribution system. ■ INTRODUCTIONFive to six hundred teragrams (Tg) of methane (CH4) arecurrently released into the atmosphere each year. Since 1750,the atmospheric abundance of CH4 has increased from ∼700 to1800 ppb, yielding an increase in the globally averaged radiativeforcing of ∼0.5 W m−2, or nearly 1/3 of the total estimatedchange. The large change in the abundance of CH4 has likelyalso altered the concentrations of atmospheric oxidants such asozone and the hydroxyl radical. While the total CH4 budgetand its trend are well constrained by atmospheric data recorded in situ or from air trapped in polar ice and snow, the individual contributions from its many sources (agriculture, natural wetlands, landfill gas release, energy production, and biomassburning) remain uncertain.Based on inventory analysis, or bottom-up methods, the United States Environmental Protection Agency (USEPA) estimates that US anthropogenic emissions of CH4 to theatmosphere in 2009 were 32 Tg. Top-down estimates usingmeasurements of atmospheric CH4 over the US suggest thisnumber is likely too low by 20% or more. Even using the lowerUSEPA number, CH4 accounts for approximately 10% of all USgreenhouse gas (GHG) emissions under EPA’s assumption that CH4 has a 100-year radiative forcing 21 times that of CO2 bymass (∼ 12% using IPCC’s estimate of 25).One of the largest sources of CH4 in the US are fugitiveemissions from natural gas production and use (estimated to be10 Tg or approximately 3% of the total gas produced).Because CH4 has such a large radiative forcing relative to CO2,relatively small losses of CH4 to the atmosphere cansubstantially increase the GHG forcing associated with this sector (e.g., 11% fugitive emission (mol/mol) doubles the 100Received: March 23, 2012 Revised: July 19, 2012 Accepted: August 1, 2012 Published: August 1, 2012Article