NH₃-promoted heterogeneous reaction of SO₂ to sulfate on α-Fe₂O₃ particles with coexistence of NO₂ under different relative humidities

Heterogeneous reactions play an essential role in the research area of formation processes of secondary aerosols. In this study, heterogeneous reactions of NH₃/SO₂/NO₂ with α-Fe₂O₃ particles at different relative humidities (RHs) were investigated by using a gas-flow system combined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results show that the presence of NH₃ leads to the rapid conversion of SO₂ to sulfate as a result of the proton transfer function and neutralization on the α-Fe₂O₃ particles in dry condition. NH₃ can also promote the hydrolysis of NO₂ dimers to HONO formation and result in the formation of more sulfate on the particle surface in humid conditions. On the other hand, water vapor plays both positive and negative roles in the uptake of SO₂ during the reaction processes. When the RH increases from 0% to 62%, the formation rate of sulfate and uptake coefficient for SO₂ in the initial stage increase about 3-fold. However, due to the inhibition effect of water layer between trace gases and active surface, the above values of reaction kinetic decrease about 21% when the RH increases from 62% to 72%. Our results may provide insight into the effects of NH₃ and RH for the converting SO₂ into sulfate on α-Fe₂O₃ particles, which supply basic data for the formation of secondary particles in field studies of atmospheric environment.