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

Leifer, Ira; Melton, Christopher; Blake, Donald R.

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, inline-formulaθ. An inversion model of the seep field, inline-formulaθ-resolved anomaly, THCinline-formula(inline-formulaθ)-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 inline-formula= THC inline-formula+ carbon dioxide, COinline-formula2, inline-formula+ carbon monoxide), 18 % was COinline-formula2 and 82 % was THC; 85 % of THC was CHinline-formula4. 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 inline-formula± 12 000 minline-formula3 THC dinline-formula−1 (27 Gg THC yrinline-formula−1 based on 19.6 g molinline-formula−1 for THC). Based on a 50 : 50 air-to-seawater partitioning, this implies seabed emissions of 167 000 minline-formula3 THC dinline-formula−1. Based on atmospheric plume composition, Cinline-formula1–Cinline-formula6 alkane emissions were 19, 1.3, 2.5, 2.2, 1.1, and 0.15 Gg yrinline-formula−1, respectively. The spatially averaged CHinline-formula4 emissions over the inline-formula∼ 6.3 kminline-formula2 of 25 inline-formula× 25 minline-formula2 bins with sonar values above noise were 5.7 inline-formulaµM minline-formula−2 sinline-formula−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.

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Leifer, Ira / Melton, Christopher / Blake, Donald R.: Long-term atmospheric emissions for the Coal Oil Point natural marine hydrocarbon seep field, offshore California. 2021. Copernicus Publications.

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