Abstract
The marginal sea ice zone has been identified as a source of different climate active gases to the atmosphere due to its unique biogeochemistry. However, it remains highly undersampled and the impact of changes in sea ice concentration on the distributions of these gases is poorly understood. To address this, we present measurements of dissolved methanol, acetone, acetaldehyde, dimethyl sulfide and isoprene in the sea ice zone of the Canadian Arctic from the surface down to 60 m. The measurements were made using a Segmented Flow Coil Equilibrator coupled to a Proton Transfer Reaction Mass Spectrometer. These gases varied in concentrations with depth, with the highest concentrations generally observed near the surface. Underway (3–4 m) measurements showed broadly higher concentrations in partial sea ice cover compared to ice-free waters. The large number of depth profiles at different sea ice coverages enables proposition of the likely dominant production processes of these compounds in this area. Methanol concentrations appear to be controlled by specific biological consumption processes. Acetone and acetaldehyde concentrations are influenced by the penetration depth of light and the mixed layer depth, implying dominant photochemical sources in this area. Dimethyl sulfide and isoprene both display higher surface concentrations in partial sea ice coverage compared to ice-free waters due to ice edge blooms. Dimethyl sulfide concentrations sometimes display a subsurface maximum in ice -free conditions, while isoprene displays more reliably a subsurface maximum. Surface gas concentrations were used to estimate their air – sea fluxes. Despite obvious in situ production, we estimate that the sea ice zone is absorbing methanol and acetone from the atmosphere. In contrast, DMS and isoprene are consistently emitted from the ocean, with marked episodes of high emissions during ice-free conditions, suggesting that these gases are produced in ice-covered areas and emitted once the ice has melted. Our measurements show that the seawater concentrations and air-sea fluxes of these gases are clearly impacted by sea ice concentration. These novel measurements and insights will allow us to better constrain the cycling of these gases in the polar regions and their effect on the oxidative capacity and aerosol budget in the Arctic atmosphere.
Highlights
The Arctic is an important part of the global climate system and is warming faster than the rest of the world (Dai et al, 2019)
Six casts are highlighted in a separate figure along with more detailed auxiliary data to show absolute concentrations and allow comparisons between casts (Figure 7). These profiles are chosen from careful examinations of the overview plots as they; (i) represent the typical effect of sea ice on these compounds, (ii) present higher vertical resolution near the surface and (iii) contain acetaldehyde concentrations which could not be determined for all profiles
To the best of our knowledge, these represent the first measurements of seawater concentrations of methanol, acetone, acetaldehyde and isoprene in the Canadian Arctic Archipelago
Summary
The Arctic is an important part of the global climate system and is warming faster than the rest of the world (Dai et al, 2019). Based on atmospheric measurements and correlations with DMS, the Canadian Arctic sea ice zone has been suspected to be a sink for methanol and acetone (Sjostedt et al, 2012), and a source for other oxygenated VOCs produced in the sea surface microlayer from photochemical activity (Mungall et al, 2017, 2018). We present depth profile (0-60 m) and underway (~5 m) seawater measurements of methanol, acetone, acetaldehyde, DMS and isoprene in the Canadian Arctic during boreal summer (July-August 2017). These data enable assessment of the air – sea fluxes of these gases. We investigate the impact of sea ice concentration on the dissolved concentrations of these VOCs
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