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

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,. An inversion model of the seep field,-resolved anomaly, THC0(/-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 (TCDTHCCcarbon dioxide, CO2, Ccarbon 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 40012 000m3 THC d-1 (27 GgTHC yr-1 based on 19.6 g mol-1 for THC). Based on a 50 : 50 airto-seawater partitioning, this implies seabed emissions of 167 000m3 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 25-25m2 bins with sonar values above noise were 5.7?Mm-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.