Abstract
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 ((1)O(2)) 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.