Abstract. We used 50 kHz sonar data to estimate natural hydrocarbon emission rates from the 18 km 2 marine seep field offshore from Coal Oil Point, Santa Barbara, California. The hydrocarbon gas emission rate is 1.7 + 0.3 x 105 m3d -1 (including gas captured by a subsea seep containment device) and •he associated oil emission rate is 1.6 + 0.2 x 104 Ld -l (100 barrels d-l). The nonmethane hydrocarbon emission rate from the gas seepage is 35 + 7 td -1 and a large source of air pollution in Santa Barbara County. Our estimate is equal to twice the emission rate fi-om all the on-road vehicle traffic in the county.

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