Heterogeneous Reaction of Gaseous Nitric Acid on γ-Phase Iron(III) Oxide

The uptake of HNO3 on γ-Fe2O3 particles that model atmospheric mineral dust has been investigated using FT-IR spectroscopy and mass spectrometry to probe the condensed and gas phases, respectively. To accurately calculate the HNO3 saturation coverage (θ) and initial uptake coefficient (γ), the specific surface area of γ-Fe2O3 was measured in situ using water as an adsorbate. The saturation coverage of HNO3 on γ-Fe2O3 was found to be θ = (1.1 ± 0.9) × 1014 molecules/cm2 and the initial uptake coefficient was found to be γ = (1.5 ± 1.0) × 10-5 at 297 K and P HNO 3 = 1 × 10-5 Torr. These values were slightly larger at 220 K for the same partial pressure of nitric acid. Spectroscopically, HNO3 was observed to bind to the surface in the form of nitrate ions. The significance of this reaction to tropospheric chemistry is discussed.