Influence of relative humidity on heterogeneous reactions of O₃ and O₃/SO₂ with soot particles: Potential for environmental and health effects

The heterogeneous reactions of soot particles with O₃ and the mixture of O₃ and SO₂ were studied as a function of relative humidities (RHs). The reactions were followed in real time using microscopic Fourier transform infrared (micro-FTIR) spectrometer to obtain kinetic data. The results show that the ketone (C[dbnd]O) group is the main product of the O₃/soot reaction, and the sulfate is identified on the surface of soot particles in the presence of O₃/SO₂. Both reactions are sensitive to RHs and surrounding water significantly promotes the proceeding of the heterogeneous reactions. For the O₃/soot reaction, the pseudo-first-order rate constant increases from 3.2 × 10⁻⁴ s⁻¹ to 7.1 × 10⁻⁴ s⁻¹ with increasing RH in the range of 1%–82%. When O₃ and SO₂ exist simultaneously during the reaction, the reaction rate and uptake coefficient are all enhanced by about an order of magnitude as the RH increases from 1% to 83%. The high productions of the ketone and sulfate on soot surface are of highly hydrophilic, which play a key role in environmental effect under humid environment. The possible reaction mechanism speculates that products of aromatic carbonyls and dihydrofuran species on soot particles will be more harmful to human health.