Heterogeneous interaction of SiO₂with N₂O₅: Aerosol flow tube and single particle optical levitation-Raman spectroscopy studies

Silica (SiO₂) is an important mineral present in atmospheric mineral dust particles, and the heterogeneous reaction of N₂O₅on atmospheric aerosol is one of the major pathways to remove nitrogen oxides from the atmosphere. The heterogeneous reaction of N₂O₅with SiO₂has 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 SiO₂particles with N₂O₅at 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 SiO₂particles, 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 SiO₂particles were optically levitated and exposed to a continuous flow of N₂O₅at 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 SiO₂particles 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 N₂O₅at RH of 3between the gas and adsorbed phases. The atmospheric implications of this work are discussed.