Photochemical production of hydrogen peroxide in size-fractionated Southern California coastal waters

Hydrogen peroxide (H2O2) photochemical production was measured in bulk and size-fractionated surf zone and source waters (Orange County, California, USA). Post-irradiation (60 min; 300 W ozone-free xenon lamp), maximum H2O2 concentrations were ∼10 000 nM (source) and ∼1500 nM (surf zone). Average initial hydrogen peroxide production rates (HPPR) were higher in bulk source waters (11 ± 7.0 nM s−1) than the surf zone (2.5 ± 1 nM s−1). A linear relationship was observed between non-purgeable dissolved organic carbon and absorbance coefficient (m−1 (300 nm)). HPPR increased with increasing absorbance coefficient for bulk and size-fractionated source waters, consistent with photochemical production from CDOM. However, HPPR varied significantly (5×) for surf zone samples with the same absorbance coefficients, even though optical properties suggested CDOM from salt marsh source waters dominates the surf zone. To compare samples with varying CDOM levels, apparent quantum yields (Φ) for H2O2 photochemical production were calculated. Source waters showed no significant difference in Φ between bulk, large (>1000 Da (>1 kDa)) and small (<1 kDa) size fractions, suggesting H2O2 production efficiency is homogeneously distributed across CDOM size. However, surf zone waters had significantly higher Φ than source (bulk 0.086 ± 0.04 vs. 0.034 ± 0.013; <1 kDa 0.183 ± 0.012 vs. 0.027 ± 0.018; >1 kDa 0.151 ± 0.090 vs. 0.016 ± 0.009), suggesting additional production from non-CDOM sources. H2O2 photochemical production was significant for intertidal beach sand and senescent kelp (sunlight; ∼42 nM h−1 vs. ∼5 nM h−1), on the order of CDOM production rates previously measured in coastal and oceanic waters. This is the first study of H2O2 photochemical production in size-fractionated coastal waters showing significant production from non-CDOM sources in the surf zone.