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

The uptake of HNO 3 on γ-Fe 2 O 3 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 HNO 3 saturation coverage (θ) and initial uptake coefficient (γ), the specific surface area of γ-Fe 2 O 3 was measured in situ using water as an adsorbate. The saturation coverage of HNO 3 on γ-Fe 2 O 3 was found to be θ = (1.1 ± 0.9) × 10 14 molecules/cm 2 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, HNO 3 was observed to bind to the surface in the form of nitrate ions. The significance of this reaction to tropospheric chemistry is discussed.