Hydroxyl radical-induced photochemical formation of dicarboxylic acids from unsaturated fatty acid (oleic acid) in aqueous solution

In this study, we assess under laboratory controlled conditions the direct and hydroxyl radical (OH)-induced photochemical production of low molecular weight (LMW) dicarboxylic acids and related compounds (C2–C9) (DCAs) from oleic acid (cis-9-octadecenoic, Δ9C18) in aqueous solution. Nitrate (NO3−)-amended and unamended oleate solutions were irradiated under ultraviolet-B radiation (UV-B, 313 nm) for 5 h, with NO3− being the source of OH. The OH-induced photochemical production of DCAs (C2di–C9di) (170 ± 26 nM h−1) was much higher than that induced by the direct effect of UV-B (33 ± 22 nM h−1), accounting for approximately 85% of the total (direct + OH-induced) photochemical production of DCAs (C2di–C9di) (198 ± 15 nM h−1). Azelaic acid (C9di) was the dominant photoproduct (comprising 63 and 44% of DCAs in the direct and OH-induced photochemical production, respectively) followed by C8di, C7di and C6di, whereas shorter chain compounds (C2di–C5di) were minor produced species. Using our estimate of OH photoproduction (P-OH in nM h−1), the production of C9di from 50 μM of oleic acid was evaluated at ∼45 nM (nM OH)−1.