Geologic seepage of methane and light (C2-C5) 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 μg/h of i-pentane, and 5.3 ± 1.9 μg/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 C2 – C5 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.

中文翻译：

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洛杉矶拉布雷亚焦油坑中甲烷和轻烷烃的宏观渗流

在洛杉矶的拉布雷亚焦油坑测量了甲烷和轻质 (C2-C5) 烷烃的地质渗漏。使用带有不锈钢罐的助焊剂室收集样品并使用气相色谱法进行分析。各个渗漏点的平均渗流率为 970 ± 330 毫克/小时的甲烷、14.0 ± 5.5 毫克/小时的乙烷、9.1 ± 3.7 毫克/小时的丙烷、3.7 ± 1.6 毫克/小时的异丁烷、0.33 ± 0.16 毫克/h 的正丁烷、260 ± 120 μg/h 的异戊烷和 5.3 ± 1.9 μg/h 的正戊烷，而最大渗流率超过 17 g/h 的甲烷、270 mg/h 的乙烷、190丙烷毫克/小时、异丁烷 95 毫克/小时、正丁烷 10 毫克/小时、异戊烷 7 毫克/小时和正戊烷 0.1 毫克/小时。由于采样方法的原因，这些绝对通量具有与其相关的额外未知量的误差，并且应被视为排放的下限。收集的样品显示通常是干燥的气体，相对于轻质烷烃，甲烷排放量较高。发现焦油坑的总体排放量不仅来自活跃的地质渗漏，还来自现场沥青的除气。使用本研究中观察到的受采样方法引入的误差影响可忽略不计的气体比率，拉布雷亚地区的 C2-C5 烷烃的每日排放量估计为 4.7 ± 1.6 Mg，占总量的 2-3%整个洛杉矶地区的排放量。