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

- The reactions kinetics of soot particles with O3 and O3/SO2 are studied as a function of RHs.
- Water vapor significantly promotes the proceeding of the heterogeneous reactions.
- Reaction rates and uptake coefficients at different RHs are the first reported.
- The possible reaction mechanism is proposed for the first time.

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 (CO) 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.

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