Effect of relative humidity on O₃ and NO₂ oxidation of SO₂ on α-Al₂O₃ particles

Heterogeneous reactions of SO₂/O₃ and SO₂/NO₂ with α-Al₂O₃ particles at different RHs were investigated using a gas-flow system combined with microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the trace gas O₃ or NO₂ leads to rapid conversion of SO₂ to sulfate on the surface of α-Al₂O₃ particles in initial stage and then conversion rate decreases in the following stages. The rate of sulfate formation and uptake coefficient (γ) for SO₂ in the two systems as a function of relative humidity (RH) are determined for the first time, which are all strongly enhanced more than seven-fold as the RH increase from 15% to 95% in initial stage for SO₂/O₃ and SO₂/NO₂. Moreover, the γ in the system of SO₂/O₃ on α-Al₂O₃ particles is more than 3-fold than that of SO₂/NO₂ for the similar RH condition. Our results may be broadly applicable to understand the effects of RH and trace gases (e.g., O₃, NO₂) for the converting SO₂ into sulfate on the mineral dust, which supply basic data for atmospheric chemistry modeling studies.