Methane emissions from oil and gas platforms in the Bohai Sea, China

Although oil and gas explorations contribute to atmospheric methane (CH₄) emissions, their impact and influence along the shelf seas of China remain poorly understood. From 2012 to 2017, we conducted four ship-based surveys of CH₄ in the seawater column and boundary layer of the Bohai Sea, China, and further measured CO₂ and several meteorological parameters. The average observed CH₄ mixing ratios in the boundary layer and its concentrations in seawater column were 1950 ± 46 ppb in November 2012 (dissolved CH₄ was not observed in this survey), 2222 ± 109 ppb and 13.0 ± 5.9 nmol/L in August 2014, 2014 ± 20 ppb and 5.4 ± 1.4 nmol/L in February 2017, and 1958 ± 25 ppb and 5.3 ± 3.8 nmol/L in May 2017, respectively. The results demonstrated that the CH₄ emissions from the oil and gas platforms accounted for approximately 72.5 ± 27.0% of the increase in the background atmospheric CH₄ in the local area. The remaining emissions were attributed to land-sea air mass transportation. Conversely, the influence of the air-sea exchange was negligible, measuring within the 10^-3 ppb range. For carbon balance calibration, the mean flaring efficiency of the oil-associated gas based on the enhancement of CO₂ (ΔCO₂) and enhancement sum of CO₂ and CH₄ (ΔCO₂ + ΔCH₄) was 98.5 ± 0.5%. Furthermore, the CH₄ emission rate from the oil and gas platforms was 0.026 ± 0.017 Tg/year, which was approximately 7.2 times greater than the sea-to-air CH₄ flux over the entire Bohai Sea area. Thus, oil and gas platforms must be recognized as important artificial hotspot sources of atmospheric CH₄ in the Bohai Sea.

Impact of air-sea exchange on the spatial distribution of atmospheric methane in the Dalian Bay and adjacent coastal area, China

To date, the impact of air-sea exchange on spatial distribution of atmospheric methane (CH₄ hereafter) remains less understood in the coastal areas of China. Here we measured the dissolved and atmospheric CH₄ in the Dalian Bay and adjacent area in July and August 2014, respectively. Results showed that the study area was a net significant source of atmospheric CH₄, with a mean sea-to-air CH₄ flux of 170.6 ± 149.5 μmol/(m²·day). We optimized a method to accurately quantify the elevated atmospheric CH₄ mole fraction (△CH₄ hereafter) caused by air-sea exchange. The calculated △CH₄ in the study area ranged from 15.4 to 102.1 nmol/mol, 1.5-10.2 nmol/mol, and 0.03-0.22 nmol/mol at the mixing height of 1, 10, and 471 m, respectively. The △CH₄ mole fractions caused by air-sea exchange were positive with sea-to-air CH₄ flux and in situ observed atmospheric CH₄ mole fraction, while negative with altitude. Under the standard conditions, we defined 50.8 μmol/(m²·day) as the criteria value of sea-to-air CH₄ flux which could result in a detectable elevation atmospheric CH₄ mole fraction at a height of 10 m.