Seawater sampling and Experimental Set up.\u00a0 Seawater was gravity filtered directly from the Niskin bottle through a 0.2-\u03bcm POLYCAP 75 AS Nylon filter (Whatman) into two 2 L Qorpak glass bottles previously rinsed by Milli Q water and muffled at 550 \u02daC for 8 h. Each bottle was filled leaving 3\u20135 mL headspace, sealed with a Teflon-lined silicone screw cap, and stored at 4 \u00b0C 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 \u03bcm-filtered seawater sample was pneumatically pushed through 1/8\u201d 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.\u00a0 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.\u00a0 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.).\u00a0 A 10 nm bandwidth was used for irradiations <330 nm and a 20 nm bandwidth was used for longer wavelengths.\u00a0 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.\u00a0 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 \u00b0C for all irradiations.
\nCarbonyl Determination. A 2.2 mL aliquot of the irradiated seawater sample or dark control was added to a 20 \u00b5L aliquot of the 2,4-dinitrophenylhydrazine (DNPH) reagent in a ~2.2 mL Qorpak vial that was capped tightly with no headspace.\u00a0 The lid of the cap contained a Teflon-lined silicone septum.\u00a0 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\u00d7100 mm Nova-Pak cartridge with 4 \u03bcm 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 \u02daC.\u00a0 The sample analysis time was 37 min.
\nAQY 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.
Apparent Quantum Yields for the Photochemical Formation of Carbonyl Compounds in Seawater from the R/V Endeavor EN589 in the Northwest Atlantic Ocean from September to October 2016.
Data processing was done in Excel
\nBCO-DMO processing notes:
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