Characterization of reactive photoinduced species in rainwater.

Rainfall is a highly effective and important carrier that can remove a majority of aerosol mass into land and marine ecosystems. The photochemically formed reactive species in the rainwater are likely dominant oxidants for organic and inorganic substances. Here, we collected rainwater samples from Oct. 2016 to Dec. 2016 in CUG campus (Wuhan, Hubei, China) and measured their formation rates, lifetimes, steady-state concentrations, and apparent quantum yields of reactive photoinduced species, including hydroxyl radical (HO•), H2O2, singlet oxygen (1O2), and chromophoric dissolved organic matter triplet state (3CDOM*) in the laboratory. Results showed that rainwater samples contained photochemical sources, like DOM, nitrate, heavy metals, etc. Quantification of HO• showed that rHO• (the photogeneration rate of HO•) were in the range of 1.05 × 10-10-4.56 × 10-10 M s-1, and [•OH]ss (the steady-state concentrations of OH•) were of 4.06 × 10-18-2.97 × 10-17 M for the three samples. Further investigations revealed that 10-24% of r•OH was attributed to nitrate photolysis, suggesting DOM was possibly the prevailing source of HO•. Apparent quantum yields of H2O2 (ΦH2O2) correlated negatively with E2/E3 (the ratio of absorption at 250 and 365 nm), while Φ1O2 and Φ3CDOM* increased with elevated E2/E3.