Macroseepage of methane and light alkanes at the La Brea tar pits in Los Angeles

作者：Weber Derek; Marquez Bertha A; Taylor Charnise; Raya Paul; Contreras Paul; Howard David; Nwachuku Ikenna; Fusco Katie; Morales Fernanda; Doezema Lambert A^*

来源：Journal of Atmospheric Chemistry, 2017, 74(3): 339-356.

DOI：10.1007/s10874-016-9346-4

• Light alkanes
• NMHCs
• Methane
• Macroseepage
• Geologic seepage

Geologic seepage of methane and light (C-2-C-5) alkanes was measured at the La Brea Tar Pits in Los Angeles. Samples were collected using flux chambers with stainless steel canisters and analyzed using gas chromatography. Average seepage rates from individual seepage sites were 970 +/- 330 mg/h of methane, 14.0 +/- 5.5 mg/h of ethane, 9.1 +/- 3.7 mg/h of propane, 3.7 +/- 1.6 mg/h of i-butane, 0.33 +/- 0.16 mg/h of n-butane, 260 +/- 120 mu g/h of i-pentane, and 5.3 +/- 1.9 mu g/h of n-pentane, while maximum seepage rates exceeded 17 g/h of methane, 270 mg/h of ethane, 190 mg/h of propane, 95 mg/h of i-butane, 10 mg/h of n-butane, 7 mg/h of i-pentane, and 0.1 mg/h of n-pentane. These absolute fluxes have an additional unknown amount of error associated with them due to sampling methodology, and should be taken as the lower limit of emissions. Samples collected revealed generally dry gas, with high methane emissions relative to the light alkanes. Overall emissions from the tar pits were found to come not only from the active geologic seepage, but also from the outgassing of the standing asphalt at the site. Using the gas ratios, which are negligibly affected by errors introduced by sampling methodology, observed in this study, daily emissions of C-2 - C-5 alkanes from the La Brea area were estimated to be 4.7 +/- 1.6 Mg, which represents 2-3 % of total emissions in the entire Los Angeles region.

• 出版日期2017-9