Abstract
This work presents an overview of a unique set of surface ocean dimethylsulfide (DMS) measurements from four shipboard field campaigns conducted during the North Atlantic Aerosol and Marine Ecosystem Study (NAAMES) project. Variations in surface seawater DMS are discussed in relation to biological and physical observations. Results are considered at a range of timescales (seasons to days) and spatial scales (regional to sub-mesoscale). Elevated DMS concentrations are generally associated with greater biological productivity, although chlorophyll a (Chl) only explains a small fraction of the DMS variability (15%). Physical factors that determine the location of oceanic temperature fronts and depth of vertical mixing have an important influence on seawater DMS concentrations during all seasons. The interplay of biomass and physics influences DMS concentrations at regional/seasonal scales and at smaller spatial and shorter temporal scales. Seawater DMS measurements are compared with the global seawater DMS climatology and predictions made using a recently published algorithm and by a neural network model. The climatology is successful at capturing the seasonal progression in average seawater DMS, but does not reproduce the shorter spatial/temporal scale variability. The input terms common to the algorithm and neural network approaches are biological (Chl) and physical (mixed layer depth, photosynthetically active radiation, seawater temperature). Both models predict the seasonal North Atlantic average seawater DMS trends better than the climatology. However, DMS concentrations tend to be under-predicted and the episodic occurrence of higher DMS concentrations is poorly predicted. The choice of climatological seawater DMS product makes a substantial impact on the estimated DMS flux into the North Atlantic atmosphere. These results suggest that additional input terms are needed to improve the predictive capability of current state-of-the-art approaches to estimating seawater DMS.
Highlights
The surface oceans are universally supersaturated with DMS relative to the overlying atmosphere, inducing a flux of approximately 28 Tg S yr −1 into the atmosphere (Lana et al, 2011)
Four cruises in the North Atlantic during the North Atlantic Aerosol and Marine Ecosystem Study (NAAMES) project provide a large amount of surface seawater DMS data throughout the different stages of the seasonal phytoplankton bloom
Elevated seawater DMS levels were observed during the seasons of high biological productivity (March/April, May, and September) and lower levels were observed during November
Summary
The surface oceans are universally supersaturated with DMS relative to the overlying atmosphere, inducing a flux of approximately 28 Tg S yr −1 into the atmosphere (Lana et al, 2011). The sea-to-air flux of DMS is the largest biological source of sulfur to the marine atmosphere and the principle precursor of non-sea-salt sulfate in marine aerosols. These non-sea-salt sulfate aerosols are a significant contributor to cloud condensation nuclei (CCN) number and influence cloud radiative properties (Charlson et al, 1987). Previous observations show large seasonal changes in North Atlantic surface ocean DMS concentrations (Lana et al, 2011) and DMS-derived sulfate is a significant component of the North Atlantic mass of submicron marine aerosol (Sanchez et al, 2018; Quinn et al, 2019). Documenting the linkages between surface ocean biological activity and overlying aerosol/cloud properties with direct observations is challenging in the highly dynamic North Atlantic environment. The mechanistic links between atmospheric DMS and new particle formation are uncertain (see Quinn and Bates, 2011), given the recent identification of novel DMS oxidation products (Veres et al, 2020)
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