The adsorption of nitrogen oxides on crystalline ice

[摘要] The partitioning of nitrogen oxides between ice and air is of importance tothe ozone budget in the upper troposphere. In the present study, adsorptionof nitrogen oxides on ice was investigated at atmospheric pressure usingachromatographic technique with radioactively labelled nitrogen oxides at lowconcentrations. The measured retentions solely depended on molecular adsorption and were not influenced by dimerisation, formation of encapsulatedhydrates on the ice surface, dissociation of the acids, nor by migration intoa quasi-liquid layer or grain boundaries. Based on the chromatographic retention and the model of thermo-chromatography, the standard adsorptionenthalpy of -20 kJ mol^-1 for NO, -22kJ mol^-1 for NO₂,-30kJ mol^-1 for peroxyacetyl nitrate, -32kJ mol^-1 forHON} and -44 kJ mol^-1 for HNO₃ was calculated. To perform those calculationswithin the model of thermo-chromatography, the standard adsorption entropywas calculated based on statistical thermodynamics. In this work, two different choices of standard states were applied, and consequently differentvalues of the standard adsorption entropy, of either between -39 kJ mol^-1 and-45kJ mol^-1, or -164 kJ mol^-1 and -169 kJ mol^-1for each nitrogen oxide were derived. The standard adsorption enthalpy was identical for both standardadsorption entropies and thus shown to be independent of the choice of standard state. A brief outlook on environmental implications of our findingsindicates that adsorption on ice might be an important removal process ofHNO₃. In addition, it might be of some importance for HONO and peroxyacetyl nitrate and irrelevant forNO and NO₂.

[学科分类] 大气科学