Importance of atmospheric aging in reactivity of mineral dust aerosol: a case study of heterogeneous reaction of gaseous hydrogen peroxide on processed mineral particles

Abstract. Atmospheric aging and processing appears to alter physical and chemical properties of mineral dust aerosol and thus its role as reactive surface in the troposphere. Yet, previous studies in the atmosphere have mainly focused on the clean surfaces of mineral dust aerosol, and the reactivity of aged mineral aerosol toward atmospheric trace gases is still poorly recognized. This work presents the first laboratory investigation of heterogeneous reactions of gaseous hydrogen peroxide (H₂O₂), an important atmospheric oxidant, on the surface of HNO₃ and SO₂-processed alumina particles as surrogates of mineral dust aerosol aged by acidic trace gases as a function of relative humidity (RH) and surface coverage of coatings. Pretreatment of the alumina surfaces with HNO₃ and SO₂ has a strong impact on its reactivity toward H₂O₂ uptake. On HNO₃-processed particles, because of the dual role of the nitrate coating in modifying the reactivity of the particle surface, namely blocking oxide active sites but altering surface hygroscopicity, H₂O₂ uptake seems to decrease in some cases whereas increase in other cases, largely depending on RH and surface coverage of nitrate. On SO₂-processed particles, the presence of adsorbed S(IV) species appears to enhance the intrinsic reactivity of the alumina particles due to its affinity for H₂O₂, and the uptake of H₂O₂ increases by 40–80% in the range of RH from 25% to 92% relative to the unprocessed particles. However, when S(IV) is completely oxidized to S(VI), the alumina surface is significantly deactivated and the measured uptake of H₂O₂ decreased markedly. The mechanisms for heterogeneous reactions of H₂O₂ with these processed particles are discussed, as well as its potential implications on tropospheric chemistry. The results of our study suggest that the reactivity of mineral dust aerosol toward H₂O₂ and maybe other atmospheric trace gases will depend on the chemical nature and coverage of the coatings as well as ambient RH, and thus will vary considerably in different polluted atmosphere, which should be taken into account in current atmospheric models.