| Monocyclic aromatic hydrocarbons (MAHs), represented by benzene, toluene, ethylbenzene, m/p-xylene and o-xylene (i.e., BTEX), are the key components of volatile organic compounds (VOCs), which have attracted much attention because of their important roles in the formation of secondary organic aerosols and ozone. Emissions of BTEX from the oceans contribute to localized atmospheric hydroxyl radical reactions and accelerate the formation of secondary organic aerosols. In addition, as a lipophilic small molecule, BTEX can easily penetrate into living organisms, and organisms exposed to BTEX-contaminated environments are exposed to unavoidable ecological risks. Due to its proximity to the economically developed Pearl River Delta (PRD) region, the western South China Sea are characterized by frequent economic activities, especially in the areas of fisheries, oil and gas exploration, etc. The rapid development of the PRD region has made the western South China Sea an important shipping channel. As a result of industrial development and human activities, the ecological environment of the western South China Sea and the Pearl River Estuary is under pressure, with water pollution becoming increasingly serious and fishery resources at risk of decline. However, due to the lack of sufficient observational data, the source and spatial distribution characteristics of BTEX in the western South China Sea and the Pearl River Estuary and its impact on the environment still need to be further studied. Consequently, this study conducted an in-situ investigation in the western South China Sea and the Pearl River Estuary, the spatial distributions, sea- air fluxes, and environmental effects of BTEX in the area were investigated in this study. Surface and bottom seawater samples were collected from 39 sites, and atmospheric samples were collected from 16 sites. Temperature and salinity data of the sample are measured directly by the CTD at the time of sampling. The BTEX of seawater samples were analyzed by automatic purge-trap gas chromatography-mass spectrometry (GC-MS), and Donald Mackay's fugacity model was used to compute the sea-air fluxes of BTEX. The ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAFP) of BTEX were also calculated. The average concentrations of benzene, toluene, ethylbenzene, m/p-xylene and o-xylene in the surface seawater were 12.6 ± 6.3、79.5 ± 92.8、10.3 ± 9.6、21.6 ± 24.1、13.4 ± 14.6 ng L-1, respectively, with similar distribution trends. The average concentrations of the five pollutants in the bottom seawater were 11.2 ± 7.0、58.0 ± 33.3、8.2 ± 7.7、17.3 ± 19.4、8.8 ± 9.4 ng L-1. Concentrations of BTEX in surface and bottom seawater in the western South China Sea and the Pearl River Estuary were comparable to the range of BTEX concentrations in the nearshore waters of Dalian and the estuary of the Yangtze River, which were previously observed. Significant positive correlation between benzene and ethylbenzene, m/p-xylene and o-xylene was observed in the seawater, indicating that these compounds share analogous source-sink processes in seawater. Both can be affected by atmospheric deposition, offshore transportation activities, drilling platforms, and the movement of ocean currents. Meanwhile, the observation results show that the area of high BTEX concentration coincides well with the area of frequent activities, which reveals the influence of marine transportation activities on the pollution of the marine environment. The concentrations of BTEX measured in this study were lower than the acceptable concentrations of BTEX in water established by the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA), it was considered that the present concentrations of BTEX in seawater in the study area do not pose a direct existential threat to marine organisms. The sea-air fluxes of benzene, toluene, ethylbenzene, m/p-xylene, and o-xylene ranged 8.6–43.8, 71.2–849.4, 4.0–78.9, 1.7–189.0, and 1.1–112.4 g km-2 d-1, exhibited a release of BTEX from the ocean to the atmosphere. Meanwhile, we compared the sea-air flux of BTEX between other coastal area and our study and found that the lower sea-air flux of BTEX in this study could be attributed to the lower sea surface wind speed. The mean concentrations of atmospheric benzene, toluene, ethylbenzene, m/p-xylene, and o-xylene were 89.1 ± 56.5, 79.3 ± 53.4, 82.3 ± 92.2, 99.8 ± 93.6, and 96.3 ± 97.3 pptv, respectively. Overall, the concentrations of atmospheric BTEX decreased significantly from the inshore to the offshore. The atmospheric BTEX over the investigated study area are predominantly influenced by continental air masses and also regulated by volatile transport emissions from the surface seawater. Calculation results of the OFP and SOAFP of BTEX show that m/p-xylene contributes the most to its generation, which needs to be controlled more intensively. Besides, toluene was also an important ozone and secondary organic aerosol pollution source, and similar results have been studied in many inland areas. Different from the inland areas, the OFP and SOAFP of BTEX in the western South China Sea and the Pearl River Estuary are much lower than those in inland areas. This study provides important data support for the emission estimation of BTEX and its environmental effect assessment in the western South China Sea and the Pearl River Estuary. |
