title: Coral reefs as a source of dimethylsulfide (DMS) and the influence on the atmosphere of the Great Barrier Reef
Creators: name: Jackson, Rebecca
Publisher: name: Griffith University
Year: 2022
Subjects: subject: coral reef
subject: dimethylsulfide (DMS)
subject: sulfate
subject: aerosol
subject: ACCESS
subject: temperature
subject: climate change
subject: CMIP6

description: Coral reefs are important regional sources of natural atmospheric sulfur through stressinduced emissions of dimethylsulfide (DMS). Marine DMS is a source of non-sea salt sulfate, which can influence the microphysical properties of aerosols and low-level clouds. It has been hypothesised that emissions of DMS from coral reefs may influence aerosol nucleation and growth to cloud condensation nuclei, and increase the lifetime and albedo of low-level clouds to shade and cool the coral reef below. However, the source-strength of coral reefs to the atmospheric sulfur budget and the influence on aerosols, clouds and the Earth's radiative balance is uncertain. This thesis investigates the Great Barrier Reef (GBR), Australia, as a source of DMS and aims to determine whether coral reef-derived DMS can influence regional aerosolcloud processes. A combination of field and remotely sensed observations and climate model simulations are used to address these research objectives. A significant link is identified between remotely sensed proxies of coral physiological stress, atmospheric DMS (DMSa), fine-mode aerosol optical depth (AOD) and particle number concentration (0.5-2.5 [microns]) in the GBR. However, a 'tipping-point' in the observed relationship occurred during mass coral bleaching events, where a decline in DMSa and AOD occurred when the coral thermal bleaching threshold was approached. These relationships may reflect a coral reef-derived source of biogenic aerosol, that can weaken when corals experience high levels of oxidative stress and bleaching. The GBR is an important regional source of DMS, releasing 0.03-0.05 Tg yr-1 of DMS (equivalent to 0.015-0.025 Tg yr-1 S). To derive this estimate, sea surface DMS (DMSw) concentration is parameterised as a function of sea surface temperature and irradiance using observational data in the GBR. The parameterisation is used to calculate the first climatology of DMSw and DMS sea-air flux from the GBR using remotely sensed observations. Current DMS sea-air flux parameterisations do not account for direct coral-air DMS flux, which is an important, albeit intermittent source of DMSa above the background oceanic signal. In this thesis, direct coral-air DMS flux is accounted for by adding a laboratory-based estimate to the calculated sea-air flux, scaled by the percentage of coral reef cover. Assuming that DMS sea-air flux is uniform across coral reef regions, global tropical coral reefs could emit 0.08 Tg yr-1 of DMS (equivalent to 0.04 Tg yr-1 S). While this represents only ~0.2% of global sea-air flux estimates (17.7- 34.6 Tg yr-1 S as DMS), it is a disproportionate volume released from 0.1% of the ocean surface. The influence of coral reef-derived DMS on the atmosphere over north-eastern Australia is investigated by incorporating the GBR DMSw climatology and coral-air DMS flux estimate into the Australian Community Climate and Earth-System Simulator Coupled Model (ACCESS-CM2). Inclusion of coral reef-derived DMS increased the concentration of atmospheric DMS by approximately 150% and gas-phase sulfur dioxide and sulfuric acid by up to 18% over the GBR. The findings demonstrate that DMS emissions from the GBR are an important source of sulfate aerosol precursors, with potentially important implications for local-scale aerosol-cloud interactions. However, no significant influence on modelled aerosol sulfate mass or number concentration was detected, even with a reduction in anthropogenic sulfur dioxide emissions, indicating that coral reef-derived DMS may not significantly influence the regional atmosphere at temporal and spatial scales that are resolved by global climate models. The research presented in this thesis demonstrates that the GBR is an important regional source of DMS and contributor to the atmospheric sulfur budget. While DMS emissions from the GBR may not play an important role in the regional radiative balance, it is expected that a stronger response may occur at local, sub-daily time scales, as observational studies have suggested. Understanding the influence of natural aerosols is key to reducing uncertainty in our estimates of radiative forcing, particularly in coral reefs which are threatened by ongoing climate change. Given the global initiatives to shift towards renewable energy, aerosol and cloud processes may become more sensitive to changes in natural aerosol sources in future. However, further research is required to understand the complex aerosol-cloud system and how it responds to changes in anthropogenic and natural emissions.