Dimethylsulfide photolysis rates and apparent quantum yields in Bering Sea seawater

Wavelength dependence and modeled rates of dimethylsulfide (DMS) photolysis were determined in seawater from two Bering Sea stations in August 2001. Monochromatic irradiations were employed to determine wavelength-dependent apparent quantum yields (AQY_DMS(λ)). AQY _DMS(λ) scaled to 1 nM DMS decreased exponentially with increasing wavelength, ranging from 3.8×10^-6 at 290 nm to 4.7×10^-8 at 400 nm. No appreciable loss of DMS was observed in dark controls or at visible wavelengths (λ≥400 nm). DMS photolysis rates were calculated using experimentally determined AQY_DMS(λ ), spectral chromophoric dissolved organic matter (CDOM) absorption coefficients (a_CDOM(λ)), and simulated spectral scalar irradiance derived from a coupled atmosphere-ocean radiative transfer model. DMS photolysis rate estimates indicated that ∼72-78% of DMS photolysis was observed in the UVA (320-400 nm) region, with a maximum response near 330-340 nm. Results from monochromatic and polychromatic irradiations agreed well, with the latter showing ∼71% DMS photolysis in the UVA and ∼29% attributed to UVB (280-320 nm). This supports the assertion that DMS photolysis is primarily driven by UV radiation. Turnover rate constants for DMS photolysis and sea-to-air fluxes in the upper 20 m of the water column were comparable, ranging between 0.02 and 0.11 d^-1 and 0.13 and 0.26 d^-1, respectively, in the late summer non-bloom phase of the Bering Sea.