Heterogeneous interaction of SiO₂ with N₂O₅: aerosol flow tube and single particle levitation–Raman spectroscopy studies

Silica (SiO₂) is an important mineral present in atmospheric mineral dust particles, and the heterogeneous reaction of N₂O₅ on atmospheric aerosol is one of the major pathways to remove nitrogen oxides from the atmosphere. The heterogeneous reaction of N₂O₅ with SiO₂ has only been investigated by two studies previously, and the reported uptake coefficients differ by a factor of >10. In this work two complementary laboratory techniques were used to study the heterogeneous reaction of SiO₂ particles with N₂O₅ at room temperature and at different relative humidities (RHs). The uptake coefficients of N₂O₅, γ(N₂O₅), were determined to be (7.2 ± 0.6) × 10^–3 (1σ) at 7% RH and (5.3 ± 0.8) × 10^–3 (1σ) at 40% RH for SiO₂ particles, using the aerosol flow tube technique. We show that γ(N₂O₅) determined in this work can be reconciled with the two previous studies by accounting for the difference in geometric and BET derived aerosol surface areas. To probe the particle phase chemistry, individual micrometer sized SiO₂ particles were optically levitated and exposed to a continuous flow of N₂O₅ at different RHs, and the composition of levitated particles was monitored online using Raman spectroscopy. This study represents the first investigation into the heterogeneous reactions of levitated individual SiO₂ particles as a surrogate for mineral dust. Relative humidity was found to play a critical role: while no significant change of particle composition was observed by Raman spectroscopy during exposure to N₂O₅ at RH of <2%, increasing the RH led to the formation of nitrate species on the particle surface which could be completely removed after decreasing the RH back to <2%. This can be explained by the partitioning of HNO₃ between the gas and adsorbed phases. The atmospheric implications of this work are discussed.