Effect of estimations of ultraviolet absorption spectra of chromophoric dissolved organic matter on the uncertainty of photochemical production calculations

[1] The UV-visible absorption spectrum of chromophoric dissolved organic matter (CDOM) is crucial for accurate calculation of photochemical reaction rates in the ocean. The literature contains considerable variability for quantifying CDOM absorption spectra, and it is unclear how these different approaches affect subsequent photochemical calculations. Using 128 surface ocean samples collected during coastal transects from Texas to Maine, we examine the ability of four simple models to reconstruct the measured UV absorption spectra and examine the accuracy of photochemical production calculations made using the reconstructed spectra. Three exponential models are based on determination of a spectral slope coefficient (SSC) over distinct wavelength ranges (412-560 nm, 290-412 nm, and 290-650 nm) and one is based on absorption at only 412 nm. Including UV wavelengths to determine SSC resulted in the reconstruction of UV absorption spectra with high accuracy, underestimating measured absorption integrated over the UV by only -1.5% to -4.3% at worst for the 128 samples. The 412 nm model estimated UV-integrated absorption ranged between -4.3% to + 6.5% of measured spectra for coastal stations. The 412-560 nm SSC model underestimated measured UV absorption at all wavelengths by up to 60%. A spectral correction factor based on the average percent underestimation for all samples was found to improve reconstruction of UV absorption and photochemical estimates. Without the correction factor, photoproduction from this model underestimates values calculated from measured UV spectra ranging from -13% to -20% for coastal stations. Corrected estimates improve this to between -1.4% and + 6.8%.