Distribution, Sources, and Environmental Impact Assessment of Monocyclic Aromatic Hydrocarbons (BTEX) in the Western South China Sea and the Pearl River Estuary

Abstract

Monocyclic aromatic hydrocarbons (MAHs), represented by benzene, toluene, ethylbenzene, m/p-xylene, and o-xylene (BTEX), are key components of volatile organic compounds (VOCs). These compounds play crucial roles in the formation of secondary organic aerosols and ozone. BTEX emissions from oceans contribute to localized atmospheric hydroxyl radical reactions and accelerate the formation of secondary organic aerosols. Additionally, as small lipophilic molecules, BTEX can easily penetrate living organisms, leading to unavoidable ecological risks in BTEX-contaminated environments. Due to its proximity to the economically developed Pearl River Delta (PRD) region, the western South China Sea experiences frequent economic activities, particularly fisheries and oil and gas exploration. Rapid development in the PRD region has also established the western South China Sea as a vital shipping channel. Consequently, industrial development and human activities exert pressure on the ecological environment, leading to increased water pollution and risks to fishery resources. However, limited observational data hinder the comprehensive understanding of the sources, spatial distribution, and environmental impact of BTEX in this region. To address this gap, an in-situ investigation was conducted in the western South China Sea and the Pearl River Estuary. Surface and bottom seawater samples were collected from 39 sites, and atmospheric samples were obtained from 16 sites. Temperature and salinity were measured directly during sampling using a CTD. BTEX in seawater samples was analyzed using automatic purge-trap gas chromatography-mass spectrometry (GC-MS), and sea-air fluxes were calculated using Donald Mackay's fugacity model. The ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAFP) of BTEX were also evaluated. The average concentrations of benzene, toluene, ethylbenzene, m/p-xylene, and o-xylene in surface seawater were (12.6±6.3), (79.5±92.8), (10.3±9.6), (21.6±24.1), and (13.4±14.6) ng/L, respectively, showing similar distribution trends. In bottom seawater, these concentrations were (11.2±7.0), (58.0±33.3), (8.2±7.7), (17.3±19.4), and (8.8±9.4) ng/L, respectively. BTEX concentrations in both surface and bottom seawater were consistent with previously reported levels in the nearshore waters of Dalian and the Yangtze River estuary. Significant positive correlations were observed between benzene and ethylbenzene and between m/p-xylene and o-xylene in seawater, suggesting analogous source-sink processes. These compounds are influenced by atmospheric deposition, offshore transportation activities, drilling platforms, and ocean currents. Furthermore, regions with high BTEX concentrations coincided with areas of frequent marine transportation activities, highlighting their impact on marine pollution. The concentrations of BTEX measured in this study were below the acceptable limits established by the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA), indicating no immediate threat to marine organisms. Sea-air fluxes of benzene, toluene, ethylbenzene, m/p-xylene, and o-xylene ranged from 8.6–43.8, 71.2–849.4, 4.0–78.9, 1.7–189.0, and 1.1–112.4 g/(km²·d), demonstrating the release of BTEX from the ocean to the atmosphere. Comparison with other coastal areas revealed lower sea-air fluxes in this region, attributed to lower sea surface wind speeds. The mean atmospheric concentrations of benzene, toluene, ethylbenzene, m/p-xylene, and o-xylene were (0.31±0.20), (0.33±0.22), (0.39±0.44), (0.47±0.44), and (0.46±0.46) μg/m3, respectively. Atmospheric BTEX concentrations decreased significantly from inshore to offshore areas, influenced predominantly by continental air masses and volatile transport emissions from surface seawater. The OFP and SOAFP calculations showed that m/p-xylene contributed the most to their formation, necessitating stricter control. Toluene was also identified as a significant contributor to ozone and secondary organic aerosol pollution, consistent with findings from inland areas. However, OFP and SOAFP values in the western South China Sea and the Pearl River Estuary were much lower than those reported in inland regions. This study provides critical data for estimating BTEX emissions and assessing their environmental effects in the western South China Sea and the Pearl River Estuary.

Keywords

• btex
• sea-air exchange
• environmental effects
• western south china sea