Coupling Aqueous Phase Chemical Actinometry with EPR Spectroscopy: An Approach for Probing Photochemical Processes at the Air–Sea Interface

Coupling investigations of the photochemistry of ambient samples with the detection of photoproduced radicals is a field of ongoing interest. Herein, we investigated the use of chemical actinometers to account for the photon flux in in situ electron paramagnetic resonance (EPR) spectroscopy experiments (I_EPR) and correct it for solar light exposure. The developed method is based on the production of singlet oxygen (¹O₂) and its detection in spin-trapping with TEMP-OH in the photosensitization of Protoporphyrin IX (PPIX) and rose bengal (RB). Similar I_EPR values were achieved in both PPIX and RB systems, with values ranging from (1.3 ± 0.2) × 10^-8 ≤ I_EPR ≤ (1.6 ± 0.3) × 10^-8 mol photons L^-1 s^-1. TEMP-OH protonation was shown to interfere with the results at pH < 2. The aggregation effects of PPIX were investigated, and solutions were shown to be stabilized by the presence of TEMP-OH. The photochemical activity of the sea-surface microlayer (SML) samples was subsequently probed in in situ EPR experiments for the first time and corrected for the equivalent solar photochemical activity. The average results of the sunlight-induced oxidant formation rate were (3.8 ± 0.5) 10^-8 M s^-1, demonstrating the pronounced photochemical activity present in SML samples.