Rapid and Reproducible Characterization of the Wavelength Dependence of Aquatic Photochemical Reactions Using Light-Emitting Diodes

Arguably, the largest knowledge gap in the aquatic photochemistry discipline is the wavelength dependence of sunlight-driven reaction rates in surface waters. Here, we introduce a new light-emitting diode (LED)-based approach to directly quantify the wavelength dependence of aquatic photochemical reaction rates. The LEDs generate narrow-banded, spatially uniform light at five wavelengths (275, 309, 348, 369, and 406 nm), with irradiances that are stable and easily adjusted to desired levels. Strong agreement was observed between irradiance measurements in each LED reactor using chemical actinometry and spectroradiometry. Apparent quantum yield (AQY) spectra of photochemical oxygen consumption by Suwannee River organic matter were determined four times across a six-month period. The shape and magnitude of the AQY spectra were highly reproducible, as indicated by strong exponential fits (R-2 >= 0.98) and low variability in oxidation rates across the four trials (coefficient of variation = similar to 10%). This LED-based approach is cost effective, high throughput, and portable, allowing a broader community to study the wavelength dependence of aquatic photochemical processes in more detail than was previously possible. We anticipate that this approach will substantially advance our understanding of the wavelength dependence of photochemical reactions in surface waters and improve the accuracy of kinetic models.