Abstract
Abstract. This review article compiles the characteristics of the gas chlorine nitrate and discusses its role in atmospheric chemistry. Chlorine nitrate is a reservoir of both stratospheric chlorine and nitrogen. It is formed by a termolecular reaction of ClO and NO2. Sink processes include gas-phase chemistry, photo-dissociation, and heterogeneous chemistry on aerosols. The latter sink is particularly important in the context of polar spring stratospheric chlorine activation. ClONO2 has vibrational–rotational bands in the infrared, notably at 779, 809, 1293, and 1735 cm−1, which are used for remote sensing of ClONO2 in the atmosphere. Mid-infrared emission and absorption spectroscopy have long been the only concepts for atmospheric ClONO2 measurements. More recently, fluorescence and mass spectroscopic in situ techniques have been developed. Global ClONO2 distributions have a maximum at polar winter latitudes at about 20–30 km altitude, where mixing ratios can exceed 2 ppbv. The annual cycle is most pronounced in the polar stratosphere, where ClONO2 concentrations are an indicator of chlorine activation and de-activation.
Highlights
The species NO3Cl was first discovered by Martin and Jacobson in 1955 and called “nitroxyl chloride” (Martin and Jacobsen, 1955; Martin, 1958)
Emission spectroscopy was developed as an alternative observational technique (Fischer et al, 1983; Brasunas et al, 1986), and the first measurements of nighttime profiles of ClONO2 were reported by von Clarmann et al (1993), who used measurements recorded by a balloon-borne limb infrared emission spectrometer
While chlorine monoxide (ClO) is a radical which is directly involved in ozone destruction, the resulting ClONO2 is harmless for the ozone layer until the chlorine atoms are released again through heterogeneous reactions on polar stratospheric clouds in the polar winter vortex
Summary
The species NO3Cl was first discovered by Martin and Jacobson in 1955 and called “nitroxyl chloride” (Martin and Jacobsen, 1955; Martin, 1958). In the literature of atmospheric sciences, this species is usually written ClONO2 or ClNO3 and called “chlorine nitrate”, it can be challenged if this species is rightly called a “nitrate”. It is known as “chloro nitrate”, “nitryl hypochlorite”, or “nitroxyl chloride”. It is a stratospheric species and acts as a reservoir of both reactive chlorine and nitrogen. In polar spring it is involved in heterogenic reactions in the stratosphere that release active chlorine, which destroys ozone
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