Long-term monitoring of a marine geologic hydrocarbon source by a coastal air pollution station in Southern California

Abstract

Hourly total hydrocarbon (THC) data, spanning 1990–2008 from a California air pollution station located near the Coal Oil Point (COP) seep field, were analyzed and clearly showed geologic CH 4 emissions as the dominant local source. Annual COP emissions are conservatively estimated as 0.015 Tg CH 4 year −1 and represent a natural and concentrated geologic methane source (24 m 3 m −2 day −1 gas flux at some active seeps, Clark et al., 2010). For a sense of the scale and potential importance to the regional Southern California methane budget, COP emits an amount equivalent to 8% of the estimated Los Angeles County anthropogenic emissions. Station THC measurements near COP showed a strong wind dependency with elevated levels closely correlated with a sonar-derived spatial distribution of seep field emissions. THC varied seasonally, with a maximum in January and minimum in July and a peak-to-peak amplitude of 0.24 ppm. The seasonal signal was more readily apparent midday ( R 2 = 0.69 harmonic fit), compared to nighttime and morning ( R 2 < 0.45). The bimodal diel THC pattern consisted of seasonally-modulated peaks in the morning and evening. THC temporal and spatial trends were consistent with both transport and source emission variations. Long-term, annual seep field emissions consistently decreased on a field-wide basis until the late 1990s, before increasing consistently, most likely as a function of underlying geologic processes. This study demonstrates the value of municipal air quality monitoring stations for insight into local greenhouse gas sources and highlights the non-negligible and variable contribution from marine geologic seepage.