Organic molecular markers in PM2.5 on a tropical island in the South China Sea: Temporal variations, sources, and process implications

Abstract

Organic aerosols (OA) in marine environments can be derived from various origins. However, fundamental temporal variations of marine OA are lacking, leading to an insufficient understanding of their sources and atmospheric processes. In this study, numerous organic molecular markers in fine aerosols were measured on a tropical island in the South China Sea (SCS) for one year to examine their occurrence, temporal variations, sources, and atmospheric transport. Differences in the temporal variations of these compounds, depending on their input pathways were observed. The carbon preference index indicated that diesel and gasoline exhausts are the main sources of n-alkanes. The backward trajectory showed that the occurrence of levoglucosan (the dominant anhydrosugar) was associated largely with air masses from East Asia delivered by the monsoon. Glucose and galactose in the aerosols mainly originated from biogenic sources in the tropical regions. The temporal pattern of stearic acid and palmitic acid, the dominant fatty acids (FAs), was highly monsoon-dependent and associated with atmospheric inputs. Sea salt aerosols (SSA) were considered the primary origin of the FAs. Four main sources (biomass burning, biogenic organic aerosols, sea salt aerosols, and vehicle and vessel exhaust) of these markers at this island were identified by principal component analysis. The Concentration Weighted Trajectory (CWT) model revealed that the source regions were mainly associated with the northeast and southwest directions of the island as well as from local emissions. Specific ratios and relationships of/between the organic molecular markers in the aerosols also implied strong compound-selective atmospheric loss, which needs to be validated.