Air-sea exchange of volatile organic compounds (VOCs): a missing link between the sea surface carbon pool and the reactive carbon in the atmosphere

Volatile organic compounds (VOCs) affect the oxidative capacity of the atmosphere, which largely controls the removal of short-lived climate forcers (e.g. methane and ozone). VOCs also leads to the formation of organic aerosols, which directly and indirectly affects the radiative balance of the atmosphere. Airborne observations of VOCs in the vastness of remote troposphere, especially oxygenated VOCs (OVOCs) such as acetaldehyde and acetone, are often underestimated by current chemistry-climate models. Ocean plays a key role in the budget of OVOCs in the remote regions, yet the air-sea exchange of OVOCs remains poorly understood, especially the role of ocean biogeochemistry.

In this work, an online air-sea exchange framework has been developed for the Community Earth System Model (CESM2). A simple satellite-based ocean biogeochemistry scheme has been adopted to represent the sources and sinks of OVOCs in the seawater. The modeled seawater concentrations and oceanic fluxes of acetaldehyde and acetone have been compared to all previous measurements available in the literature. The modeled spatial and vertical distributions of these OVOCs in the atmosphere have been evaluated using airborne measurements obtained during the recent NASA-sponsored, multi-year, nearly pole-to-pole campaign, Atmospheric Tomography Mission (ATom). We show that the air-sea exchange module greatly improves the model performance in the marine boundary layer, yet in the free troposphere a substantial fraction of the observed OVOCs still remain unexplained. We further show that this carbon pool in the atmosphere inferred from these measurements cannot be all explained by current knowledge of the chemical evolution of organic aerosols, of ocean or continental origin. The ocean is one of the largest carbon reservoirs on the surface of the Earth, and dissolved organic materials (DOM) are known to undergo photolysis, producing a wide spectrum of compounds in the seawater, from very long-lived CO₂, CO, to short-lived (O)VOCs, and even radicals. Moreover, studies have found that many (O)VOCs are enriched in the sea surface monolayer (SML) relative to the bulk seawater. We hypothesize that there is a missing link between the reactive carbon pool in the atmosphere and the ocean biogeochemistry.