Wavelength- and Temperature-Dependent Apparent Quantum Yields for Photochemical Production of Carbonyl Compounds in the North Pacific Ocean.

Abstract

Photolysis of dissolved organic matter is the main source of carbonyl compounds in sunlit seawater, but rates and photoefficiences are poorly constrained. Wavelength- and temperature-dependent apparent quantum yields (AQYs) were determined for photochemical production of acetaldehyde, glyoxal, and methylglyoxal in North Pacific Ocean seawater. Wavelength-dependent AQYs at 20 °C decreased exponentially with increasing wavelength between 290 and 380 nm, from 1.29 × 10-4 to 4.12 × 10-6, 2.52 × 10-5 to 6.89 × 10-7, and 4.37 × 10-6 to 1.25 × 10-7 mol (mol quanta)-1 for acetaldehyde, glyoxal, and methylglyoxal, respectively. AQYs decreased after 6 h irradiation at 310 nm, possibly due to depletion of photochemical precursors or carbonyl photolysis. Average activation energies (95% CI) for photochemical production at 320 nm were 9.31 (±9.3), 26.0 (±7.5), and 34.7 (±12.8) kJ mol-1 for acetaldehyde, glyoxal, and methylglyoxal, respectively. The peak response for photochemical production rates in surface seawater was ∼325 nm, with ∼30% contribution from UV-B and ∼70% from UV-A. Computed noontime wavelength-integrated photoproduction rates were 0.5-0.8, 0.04-0.2, and 0.03-0.06 nmol L-1 h-1 for acetaldehyde, glyoxal, and methylglyoxal, respectively, under cloudless conditions in August. Results can be used to determine regional-scale photochemical production rates for these compounds in the surface ocean.