Photoenhanced heterogeneous reaction of O3 with humic acid: Focus on O3 uptake and changes in the composition and optical property

Highlights: • The O₃ uptake coefficient on humic acid (HA) was greatly enhanced by the light. • The O₃ uptake coefficient on HA under irradiation relied on environmental factors. • Hydrophobic and hydrophilic groups of HA were consumed during the ozonization. • The mass absorption efficiency of HA decreased during the ozonization. • The absorption Ångström exponent of HA aged by O₃ showed a slight increase. Heterogeneous photochemical reaction of O₃ with humic acid (HA) under simulated sunlight was performed using a flow tube reactor coupled to an O₃ analyzer at ambient pressure. It was confirmed that light significantly enhanced the uptake of O₃ on HA. The initial uptake coefficient (γ_i) and the steady-state uptake coefficient (γ_ss) of O₃ under irradiation increased by 1.6 and 3.8 times compared to those in the dark, respectively. The γ_i and γ_ss on HA varied in the range of 0.76–2.77 × 10⁻⁵ and 1.50–9.55 × 10⁻⁶, respectively, which were dependent on various environmental factors including HA mass, total irradiance, initial O₃ concentration, O₂ content, temperature, relative humidity (RH) and HA solution pH. Both γ_i and γ_ss showed linear dependence on the total irradiance (0–2.07 × 10¹⁶ photons/(cm²⋅s)) of the light source, and increased with the HA mass (0–3.2 μg/cm²), temperature (278–298 K) and HA solution pH (4.0–9.6). However, they showed negative correlations with the initial O₃ concentration and O₂ content. The γ_i remained constant in the RH range of 7%–60%, while γ_ss exhibited the maximum value at RH = 20%. During the ozonization of HA under irradiation, some functional groups were consumed, including CH₂, CH₃, aromatic CC, OH, CO, COOH and COO⁻. HA aged by O₃ exhibited a decrease in the mass absorption efficiency (MAE) and a small increase in the absorption Ångström exponent between 300 and 600 nm wavelength (AAE_300,600), which was ascribed to changes in the composition of HA during the photochemical ozonization process.