Concentrations and Photochemistry of Acetaldehyde, Glyoxal, and Methylglyoxal in the Northwest Atlantic Ocean.

The photochemical production and degradation of acetaldehyde, glyoxal, and methylglyoxal along with spatiotemporal variations in their concentrations were investigated in the Northwest Atlantic Ocean from September to October 2016. Surface seawater concentrations did not exhibit day-night differences and ranged from 1.0-7.1, 1.4-4.8, and 0.25-2.8 nmol L-1 for acetaldehyde, glyoxal, and methylglyoxal, respectively. Higher glyoxal and methylglyoxal concentrations were observed in biologically productive seawater from Georges Bank and coastal Rhode Island compared to the oligotrophic Sargasso Sea, whereas no differences were seen in acetaldehyde concentrations among these stations. Carbonyl photoproduction rates in surface seawater ranged from 0.35-0.79, 0.06-0.2, and 0.02-0.07 nmol L-1 h-1 for acetaldehyde, glyoxal, and methylglyoxal, respectively. Methylglyoxal slowly photodegraded in seawater (∼0.001-0.03 nmol L-1 h-1), whereas acetaldehyde and glyoxal were photochemically stable. Photochemical sources explained from ∼7 to 53% of the estimated total production of acetaldehyde in the surface mixed layer; a similar estimate could not be determined for glyoxal or methylglyoxal, since several processes have not been quantified that potentially affect their concentrations. Our results suggest that acetaldehyde is likely supersaturated in surface seawater relative to its typical atmospheric concentrations, whereas glyoxal and methylglyoxal are significantly undersaturated.