Seawater sampling and Experimental Set up. Seawater was gravity filtered directly from the Niskin bottle through a 0.2-μm POLYCAP 75 AS Nylon filter (Whatman) into two 2 L Qorpak glass bottles previously rinsed by Milli Q water and muffled at 550 ˚C for 8 h. Each bottle was filled leaving 3–5 mL headspace, sealed with a Teflon-lined silicone screw cap, and stored at 4 °C in the dark until analyzed in Syracuse, NY. POLYCAP filters were cleaned prior to use by alternating rinses of acetonitrile and Milli Q water followed by extensive flushing with Milli Q. Prior to irradiation experiments, a Milli Q sample or 0.2 μm-filtered seawater sample was pneumatically pushed through 1/8” O.D. Teflon tubing with ultra-high purity helium (99.999%) into a rectangular quartz cell (4 mL capacity, 1 cm pathlength, Spectrocell, Inc.) for at least 10 min at a flow rate of 2 mL min-1. The quartz cell was periodically inverted to remove residual air bubbles. The quartz cell was sealed with a screw cap containing a Teflon-lined silicone septum insert. Once the quartz cell was filled with a sample, it was placed into an enclosed temperature-controlled cell holder equipped with a stirrer. All irradiations were performed using a model QP-SX10001, 1000 W xenon lamp (Superior Quartz Products, Inc.) along with a GM 252 monochromator (Spectral Energy, Corp.). A 10 nm bandwidth was used for irradiations <330 nm and a 20 nm bandwidth was used for longer wavelengths. A longpass filter with a 307 nm cutoff (50% transmission at 307 nm) was placed in the optical path directly after the monochromator for wavelengths >330 nm. Irradiation times varied from 1 to 36 h and were chosen depending on the wavelength of the irradiation and the absorbance of the seawater sample. Dark controls were incubated in the cell holder for up to 36 h. Except when noted, the cell holder temperature was set at 20 °C for all irradiations.

Carbonyl Determination. A 2.2 mL aliquot of the irradiated seawater sample or dark control was added to a 20 L aliquot of the 2,4-dinitrophenylhydrazine (DNPH) reagent in a ~2.2 mL Qorpak vial that was capped tightly with no headspace. The lid of the cap contained a Teflon-lined silicone septum. All samples were reacted at room temperature for a minimum of 12 h and a maximum of 48 h. Derivatized standards (Sigma-Aldrich), dark controls, and samples were analyzed using a Shimadzu Prominence high performance liquid chromatography (HPLC) system with a model SPD-20A/V UV-Vis absorbance detector set in dual wavelength mode at 371 and 435 nm. The HPLC column consisted of a Waters 8×100 mm Nova-Pak cartridge with 4 μm C18 packing placed in a Waters RCM radial compression cartridge holder (Waters Associates, Milford, MA). The mobile phase consisted of solvent A (Milli Q) and solvent B (acetonitrile). The elution program was isocratic at 30% B for 3 min, 30 to 55% B in 5 min, isocratic at 55% B for 2 min, 55 to 90% B in 6 min, isocratic at 90% B for 5 min, 90% B to 30% B in 1 min, followed by column equilibration to the initial mobile phase composition for 15 min. All samples were injected using a 1.25 mL injection loop. The flow rate was 1.5 mL min-1 and the column oven temperature was 40 ˚C. The sample analysis time was 37 min.

AQY method. AQY was calculated by dividing the moles of carbonyl compounds produced by the moles of photons absorbed by seawater. Refer to Zhu & Kieber (2018, 2019) for details regarding AQY determination, seawater absorptivity, and photon flux of the monochromatic irradiation.

aqy.csv