Ocean Biogeochemistry Control on the Marine Emissions of Halogenated Very Short-Lived Ozone-Depleting Substances: a Bottom-up Framework for Chemistry-Climate Models powered by Machine-Learning

Halogenated very short-lived substances (VSLS) are important precursors of the reactive halogen chemistry, affecting the ozone budget in the stratosphere and the troposphere. Brominated VSLS are naturally produced in the ocean by phytoplanktons and emitted into the atmosphere, and current oceanic emission inventories vary dramatically. Most chemistry-climate models use prescribed oceanic emissions (often derived from limited observations), which do not respond to changes in local conditions. A process-level representation of the bi-directional oceanic emissions of trace gases remains challenging, mainly because the ocean biogeochemical processes controlling the natural synthesis of these compounds in the seawater remain poorly understood. We present a new global oceanic emission inventory of Br-VSLS (bromoform and dibromomethane), considering the physical forcing in the ocean and the atmosphere, as well as the ocean biogeochemistry control. An observation-based machine-learning emulator was developed to couple the ocean biogeochemistry with the air-sea exchange. The predicted surface seawater concentrations and the surface atmospheric mixing ratios of Br-VSLS are evaluated with long-term, global-scale observations. This new bottom-up oceanic emission inventory of Br-VSLS is more skillful than the widely used top-down approaches for representing the seasonal/spatial variations and the annual means of atmospheric concentrations. The new approach improves the model predictability for the coupled earth system model, and can be used as a basis for investigating the future ocean emissions and feedbacks under climate change. Lastly, this model framework can be used for other compounds of interest, such as dimethyl sulfide (DMS) and volatile organic compounds.