Closing the budget in marine atmospheric Oxidative Capacity through the quantification of Oceanic VOC emissions (COCO-VOC)

Volatile organic compounds (VOCs) in the marine environment and the atmospheric oxidative capacity over the ocean are critical but poorly understood components of the Earth System. There are tens of thousands of different VOC species in air, and they react with the hydroxyl radical (OH) and determine the reactivity of the atmosphere - often referred to as the oxidative capacity (the ability for air to cleanse itself). The oxidative capacity is key for climate through OH oxidation of methane and for air quality (e.g. via formation of ozone). Yet ~1/4 of the observed total OH reactivity over the ocean remains unexplained. VOCs are also precursors to secondary organic aerosol (SOA), which have the potential to enable particle growth to cloud condensation nuclei (CCN). Over the ocean and far away from anthropogenic sources where aerosol concentrations are typically low, clouds are far more sensitive to changes in aerosol than over land, highlighting the essential need to understand marine VOCs. Yet to date, the number of intensive VOC studies over land outnumbers studies over the ocean by orders of magnitude.
The ocean contains a vast number of VOCs. Beyond a few well-studied gases, the inventory of these sea-air emissions is in its infancy due to the paucity of measurements. Compared to recent observations, models consistently underpredict the marine atmospheric concentrations of many VOCs, the total oxidative capacity, and marine SOA, strongly suggesting poorly constrained or unidentified oceanic emissions of VOCs. The highly uncertain VOC fluxes take the forms of "known unknowns" and "unknown unknowns". For the known unknowns, some VOCs are thought to be produced by marine biota or by photochemistry in seawater, but their oceanic emissions are poorly quantified. For the unknown unknowns, there are VOC fluxes or production pathways that we currently have little clue about, including light- or ozone-driven production from the sea surface. The sources and cycling of SOA over the background ocean are also poorly understood. While sea spray tends to dominate marine aerosol mass, SOA can be an important source of submicron particles and affect the abundance of CCN and so cloud droplets.
In this project, we will combine a) intensive and comprehensive field measurements using novel instrumentation, b) innovative laboratory studies of physicochemical/biological processes, and c) state-of-the- art modeling on multiple scales to paint an unprecedented, holistic picture of reactive carbon and OH cycling in the background marine atmosphere. Constrained by atmospheric observations of total OH reactivity and total organic carbon mass, we will substantially improve flux estimates of established VOCs, identify new VOC emission sources, and evaluate their atmospheric impact.
The fundamental questions we will address are:
1) Which VOCs exchange between the ocean and the atmosphere and what are their fluxes?
2) What are the physicochemical/biological processes that determine these fluxes?
3) What are the impacts of these marine VOCs on oxidative capacity, aerosol, clouds, and climate?
COCO-VOC will achieve a step-change in understanding of VOC and OH cycling in the background environment, thereby constraining the sensitivities of VOCs, aerosol, and the global atmospheric oxidative capacity to changes in anthropogenic and natural emissions. This work will enable more accurate predictions of chemistry and climate in the past, present, and future. The new understanding will also offer insight into potential natural climate feedback processes caused by climate- driven changes in ocean-atmosphere VOC fluxes.