Investigating novel reactive organic gases emitted from the ocean

The sea surface microlayer (SML) acts as a gateway for molecules to enter the atmosphere or ocean phase and plays a particularly important role in the production and removal of climatically active marine trace gases including volatile organic compounds (VOCs). Unaccounted-for and/or underestimated marine VOCs have been attributed as a major factor in the poor understanding of missing OH reactivity in the marine atmosphere.

Additionally, some OVOCs have high potential to form secondary organic aerosol and hence impact climate.

Abiotic (non-biological) emission of VOCs from the organic-enriched SML can potentially occur via three different mechanisms: heterogeneous oxidation, direct photooxidation, or photosensitized oxidation. The laboratory studies investigating these mechanisms have generally used commercial photosensitizing agents and SML proxies and it is not yet clear how representative they are of the dilute and chemically complex SML.

Additionally, most of the more complex and higher molecular weight gas phase VOCs arising from these abiotic mechanisms have not been monitored in the marine atmosphere. The advent of highly sensitive chemical ionisation mass spectrometry (CIMS) techniques, which are well suited to measure polar or acidic volatile organic compounds, bring new capabilities for detection of these gases in the marine atmosphere.

This project will use a new VOCUS mass spectrometer (Tofwerk) in two different modes of ionisation (PTR-TOF and iodide-CIMS) in order to capture the oceanic emissions of a wide range of VOCs with different levels of oxygenation and polarity. The project will encompass both laboratory and field work. In the laboratory, experiments will be performed using a photochemical flow cell which will simulate sunlit (and dark) conditions of the troposphere with or without the presence of ozone.

The experiments will utilise real oceanic SML samples, rather than commercial proxies, and will use a combination of different treatments to quantify the contribution of biological versus abiotic production routes to VOCs. Liquid phase measurements of the SML, including surfactants and dissolved organic carbon will aid identification of the mechanisms of VOC production.

In the field, there will be opportunities to deploy the VOCUS on a research ship during 2025 and 2026 as part of NERC-funded programs. The new understanding gained from this work will then be used to improve our representation of important atmospheric processes in models.