Measuring molecular singlet oxygen (¹O₂*) from atmospheric photosensitizers: Intercomparison of techniques, irradiation setups, data analysis and protocol recommendations

Abstract. Molecular singlet oxygen (¹O₂*) is the first excited state of molecular oxygen O₂ and can be formed through indirect photochemistry during irradiation of chromophoric organic matter. Once formed in the particle and droplet phases in the atmosphere, ¹O₂* can be a competitive oxidant in the photochemical processing of organic matter. Yet, as more researchers study the atmospheric photochemistry of ¹O₂*, it is useful to establish protocols by evaluating and comparing experimental setups across laboratories. Here, we present ¹O₂* measurements from four photosensitizing molecules in four photoreactor setups at three research institutions, including two xenon lamps of different strengths and two multi-bulb UVA + UVB broadband systems. The production of ¹O₂* was investigated from perinaphthenone, lignin, and juglone, which are photosensitizers with atmospherically relevant light absorbing moieties, as well as from Rose Bengal, a standard photosensitizer. Two chemical actinometers, 2-nitrobenzaldehyde and p-nitroanisole/pyridine, were used to quantify photon fluxes and calculate rates of light absorbance for photosensitizers for each photoreactor. We compared two commonly used ¹O₂* quantification methods, chemical probe method using furfuryl alcohol, as well as direct ¹O₂* phosphorescence detection at 1270 nm. Rates of light absorbance across experimental setups for each photosensitizer ranged between 0.2 and 62 × 10^-5 mol_photons L^-1 s^-1, while ¹O₂* steady-state concentrations ranged between 0.01 and 129 × 10^-11 M. Despite order of magnitude differences in rate of light absorbance and ¹O₂* steady state concentrations, normalizing to ¹O₂* quantum yields showed good inter-laboratory agreement but only for perinaphthenone (94 ± 9 % – 112 ± 17 %) and for Rose Bengal (67 ± 15 % – 87 % ± 5 %). ¹O₂* quantum yields for lignin and juglone increased with decreasing irradiation wavelength, consistent with a wavelength-dependence. Finally, we make five recommendations to improve the accuracy and reproducibility of ¹O₂* measurements. These include considering wavelength-dependent quantum yields, avoiding suppression of ¹O₂*, controlling and reporting photoreactor temperature, considering light scattering from nanoparticles, and conducting control experiments. These recommendations will help standardize ¹O₂* measurements in studying photochemical processing of atmospheric aerosols and droplets.