Observations of a distinctive infra‐red spectral feature in the atmospheric spectra of polar stratospheric clouds measured by the CRISTA instrument

Abstract

Polar stratospheric cloud particles (PSCs) are known to strongly influence the infra‐red emission spectrum of the lower stratosphere in cold polar winters. The characteristics of these infra‐red features have been examined using limb sounding spectra recorded by the CRISTA experiment in August 1997 when many Antarctic PSCs were observed. A distinctive spectral feature centered at 820 cm−1 has been identified in many of these spectra, the first time that a particular band has been observed in the spectra of PSCs in the atmosphere. The feature can be attributed to the ν2 band of the NO3− ion and strongly suggests a condensed nitric acid component to the particles in the form of solid nitric acid trihydrate (NAT) or liquid ternary solution (STS). The spectral signatures belong to a definite sub‐set of the PSC observations recorded by CRISTA. The particles are observed at temperatures well above 192 K, allowing for temperature errors, and it is suggested that NAT particles are the most likely source of the spectral signature. In addition, it is shown that PSC events in which no spectral signature is present are only observed below 192 K and display a very similar HNO3‐temperature relationship to STS particles.