Air-sea interactions of semi-volatile organic compounds in the tropical environment of Southeast Asia

Division of Environmental Science and Engineering National University of Singapore, Singapore 117576

Abstract

Major urban and industrial centers increase loadings of semi-volatile organic compounds (SVOCs) to proximate sea waters through riverine transport, atmospheric deposition via dry particle deposition, wet deposition, and air-sea gas exchange. In addition to acting as sinks for SVOCs, oceans can act as sources of SVOCs to coastal atmospheres and play important roles in the global biogeochemistry of SVOCs. Particle-sorbed SVOCs can settle to the ocean surface by dry particle deposition, a uni-directional advective transport process from the atmosphere to the water, the removal rate by which is a function of the physical and chemical properties of the aerosols and bound pollutants, meteorological conditions and surface characteristics. In addition, SVOCs are removed from the atmosphere and transported to the waters by precipitation scavenging of atmospheric vapors and particles, which are incorporated into the rain within or below the clouds. After SVOCs are deposited into the bulk seawater, water-column partitioning can affect the distribution of pollutants between the dissolved aqueous and the solid phases and eventually impact the fate of these compounds in oceans. Other than the abovementioned processes, air-sea exchange can make SVOCs diffuse across the air-sea interface; however, the sea surface microlayer (SML), a unique compartment at the air-sea boundary defined operationally as the upper millimeter (1 ∼ 1000 μm) of the sea surface, has large storage capacity to delay the transport of SVOCs across the interface.

This article reports the dry particle deposition and wet deposition of selected SVOCs based on an extensive set of yearly data collected in Singapore. Singapore, a representative country of Southeast Asia (SEA), is a small but highly developed island with dense industrial parks in the Southwestern part, where the terrestrial sources affect the surrounding coasts. In this study, Singapore’s Southern coastline was chosen during the Northeast monsoon season to evaluate if this coastal area acts a sink or source for selected SVOCs via air-sea diffusive exchange as well as to investigate the SML enrichment effect.