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.

Variation and distribution of naphthenic acids in Dalian Bay sediment

Previous studies that have investigated marine oil pollution have predominantly focused on petroleum hydrocarbons. Naphthenic acids (NAs), in contrast, are toxins that are less well studied. Following the Dalian oil spill accident, monitoring surveys were carried out to investigate NAs in Dalian Bay sediments. Concentrations of NAs were between 14.826 and 34.279 mg kg^-1, with acyclic and motorcycle carboxylic acids (43.28% and 35.12%, respectively) being the dominant components. NAs were 10-30 times more abundant than polycyclic aromatic hydrocarbons (PAHs) in the sediment. Further correlation analysis showed the abundance of NAs was highly correlated with total PAH levels (0.705, p < 0.01, n = 24) and total oil (0.485, p < 0.05, n = 24), indicating that NAs may be a potential marker for oil pollution in coastal sediments. The present study may help to expand the scope of marine environmental monitoring and provide guidance for the remediation of marine pollutants.