Real refractive indices of infrared‐characterized nitric‐acid/ice films: Implications for optical measurements of polar stratospheric clouds

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Abstract
The infrared spectra of nitric‐acid/ice films representative of polar stratospheric clouds (PSCs) were collected with simultaneous optical interference measurements to determine the real refractive indices at λ = 632 nm. Ice and amorphous nitric‐acid/ice films were prepared by condensation of water and nitric acid vapors onto a wedged Al2O3substrate. The real refractive indices of these films were determined from the optical interference of a reflected helium‐neon laser during film growth. The indices of the amorphous films varied smoothly fromn= 1.30 for ice ton= 1.49 for nitric acid, similar to observations in previous work. We were unable to obtain the refractive index of crystalline films during adsorption because of optical scattering caused by surface roughness. Therefore crystalline nitric acid hydrate films were prepared by annealing amorphous nitric‐acid/ice films. Further heating caused desorption of the crystalline hydrate films. During desorption, the refractive indices for ice, NAM (nitric acid monohydrate), α‐ and β‐NAT (nitric acid trihydrate) films were measured using the optical interference technique. In agreement with earlier data, the real refractive indices for ice and NAM determined in desorption weren= 1.30±0.01 andn= 1.53±0.03, respectively. The real refractive indices for α‐ and β‐NAT were found to ben= 1.51±0.01 andn≥ 1.46, respectively. Our measurements also suggest that the shape of crystalline nitric acid particles may depend on whether they nucleate from the liquid or by vapor deposition. If confirmed by future studies, this observation may provide a means of distinguishing the nucleation mechanism of crystalline PSCs.

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