Heterogeneous Reactions of Gaseous Hydrogen Peroxide on Clean and Coated Mineral Particles: Dependence on Relative Humidity and Surface Coverage

Hydrogen peroxide (H2O2) is a significant atmospheric oxidant, playing an important role in secondary sulfate formation and HOx radical chemistry. Recent studies have shown that heterogeneous reactions on atmospheric aerosol particles seem to be an important sink for gaseous H2O2. The data concerning the kinetics and mechanisms of these reactions, however, are quite scarce so far. Here we investigated, for the first time, the heterogeneous reactions of gaseous H2O2 on the surface of clean and coated silica and alumina particles, two major components of mineral dust particles, as a function of relative humidity (RH) and surface coverage of coatings using transmission-Fourier Transform Infrared (T-FTIR) spectroscopy and online high-performance liquid chromatography (HPLC). It is found that H2O2 molecularly adsorbs on SiO2, and a small amount of molecularly adsorbed H2O2 decomposes due to its thermal instability. For α-Al2O3, catalytic decomposition of H2O2 evidently occurs, but there is also a small amount of H2O2 molecularly adsorbed on the particle surface. The measured uptake coefficients of H2O2 on both particles largely decrease with increasing RH. Pretreatment of the alumina surfaces with gaseous SO2 or HNO3 to simulate atmospheric aging of mineral particles has a strong impact on its reactivity toward H2O2. On SO2-processed particles, the presence of adsorbed S(IV) appears to enhance the intrinsic reactivity of the alumina surface and the uptake of H2O2 increases compared to that on unprocessed particles, in particular, at high RH, whereas the alumina surface is significantly deactivated when the S(IV) is completely transformed to S(VI), and the measured uptake of H2O2 apparently decreased. For HNO3-processed particles, the presence of nitrate coatings seems to decrease or increase the reactivity of the alumina particles toward H2O2, with a strong dependence on RH and surface nitrate coverage. For example, as the surface nitrate coverage increases, the uptake of H2O2 decreases at low RH (3%) but linearly increases at high RH (75-92%). The mechanism for heterogeneous reactions of H2O2 with these mineral particles was discussed, as well as its potential implications on the tropospheric chemistry. The results of our study suggest that the reactivity of mineral dust particles toward H2O2 and maybe other atmospheric trace gases will depend on the chemical nature and content of the coating and thus will vary considerably during atmospheric transport.