Long-term atmospheric emissions for the Coal Oil Point natural marine hydrocarbon seep field, offshore California

Abstract

Abstract. In this study, we present a novel approach for assessing nearshore seepage atmospheric emissions through modeling of air quality station data, specifically a Gaussian plume inversion model. A total of 3 decades of air quality station meteorology and total hydrocarbon concentration, THC, data were analyzed to study emissions from the Coal Oil Point marine seep field offshore California. THC in the seep field directions was significantly elevated and Gaussian with respect to wind direction, θ. An inversion model of the seep field, θ-resolved anomaly, THC′(θ)-derived atmospheric emissions is given. The model inversion is for the far field, which was satisfied by gridding the sonar seepage and treating each grid cell as a separate Gaussian plume. This assumption was validated by offshore in situ data that showed major seep area plumes were Gaussian. Plume total carbon, TC (TC = THC + carbon dioxide, CO2, + carbon monoxide), 18 % was CO2 and 82 % was THC; 85 % of THC was CH4. These compositions were similar to the seabed composition, demonstrating efficient vertical plume transport of dissolved seep gases. Air samples also measured atmospheric alkane plume composition. The inversion model used observed winds and derived the 3-decade-average (1990–2021) field-wide atmospheric emissions of 83 400 ± 12 000 m3 THC d−1 (27 Gg THC yr−1 based on 19.6 g mol−1 for THC). Based on a 50 : 50 air-to-seawater partitioning, this implies seabed emissions of 167 000 m3 THC d−1. Based on atmospheric plume composition, C1–C6 alkane emissions were 19, 1.3, 2.5, 2.2, 1.1, and 0.15 Gg yr−1, respectively. The spatially averaged CH4 emissions over the ∼ 6.3 km2 of 25 × 25 m2 bins with sonar values above noise were 5.7 µM m−2 s−1. The approach can be extended to derive emissions from other dispersed sources such as landfills, industrial sites, or terrestrial seepage if source locations are constrained spatially.

Measuring Floating Thick Seep Oil from the Coal Oil Point Marine Hydrocarbon Seep Field by Quantitative Thermal Oil Slick Remote Sensing

“…The Hornafius, Quigley and Luyendyk [22] estimate was similar to the 1970 estimate of 50-70 bbl oil day −1 based on aerial imagery and assumed oil slick thickness by Allen, et al [23]. Notably, though, gas emissions were at a cyclical minimum in 1995 [24] and peaked in 2010, declining since [25], suggesting the potential for significant cyclical trends in oil emissions.…”

“…The study focused on oil slicks from the COP seep field in the northern Santa Barbara Channel, California. The COP seep field is located in water from a few meters to ~85 m deep (oily seepage arises from deeper than 20 m), extending to ~3 km offshore and covering 6.3 km 2 [14,25]. The COP seep field comprises many focused seep areas-regions of high spatial vent density, separated by larger areas with sparse to no seepage [53].…”

Decadal cyclical geological atmospheric emissions for a major marine seep field, offshore Coal Oil Point, Southern California

“…The remainder is primarily CO 2 , which is more soluble than CH 4 and thus rapidly evades the bubbles, decreasing to trace levels at the sea surface 27 . The atmospheric plume of the major seep area, Trilogy Seep, found THC was 88.5% CH 4 22 . Plume CO 2 enhancement (12 ppm) was 25% that of CH 4 (50 ppm), i.e., similar to seabed CO 2 bubble enhancement, demonstrating efficient upwelling transport and evasion 22 .…”

“…The atmospheric plume of the major seep area, Trilogy Seep, found THC was 88.5% CH 4 22 . Plume CO 2 enhancement (12 ppm) was 25% that of CH 4 (50 ppm), i.e., similar to seabed CO 2 bubble enhancement, demonstrating efficient upwelling transport and evasion 22 . Based on a 91% CH 4 :NMHC composition, the Hornafius et al 6 estimate implies 36 Gg CH 4 year −1 .…”

“…Seabed emissions based on sonar data were estimated at 150,000 m 3 day −1 for 1994–1996 6 . Recently, Leifer et al 22 derived the 30-year average E B at 164,000 m 3 day −1 for 1990–2020 using an inversion model and a 50:50 ocean/atmosphere partition based on a study by Clark et al 23 .…”