Photochemical formation of peroxides and fluorescence characteristics of the water-soluble fraction of bulk aerosols collected in Okinawa, Japan

Photochemical reactions of dissolved organic compounds, fluorescent dissolved organic matter (FDOM) in particular, are implicated as an important source of peroxides, but their contribution to overall peroxide formation is not well understood. We studied the photochemical formation of peroxides (hydrogen peroxide and organic peroxides), together with changes in fluorescent properties of water-soluble fraction (WSF) solutions of bulk aerosols (n=28) collected in Okinawa, Japan. Monochromatic wavelengths of 313, 334, 366, and 405 nm were used to examine the samples, and the changes in peroxide concentrations and fluorescence intensities (FIs) during these illuminations were measured. For many samples, two peaks of fluorescence were found in the WSF solutions at excitation/emission wavelengths (Ex/Em) of 250–275/375–455 and 300–320/400–440 nm, which can be signatures for fulvic acid-like compounds. Several samples collected between November and April (winter samples) showed another fluorescence peak at 260–290/305–345 nm, which could be due to the presence of aromatic amines. As illumination time increased, the peroxide concentrations increased and the FI changed, but not uniformly. The FI and peroxide photo-production rates were only weakly correlated (R<0.36), and the correlation between FI and dissolved organic carbon (DOC) concentrations was also weak (R<0.75). These results suggest that the photochemistry of FDOM may not comprise a dominant pathway to form peroxides in the WSF solutions, but that peroxides are formed as a result of complex processes in the WSF solutions of aerosols collected in Okinawa.