The adsorption enthalpy of nitrogen oxides on crystalline ice

The partitioning of nitrogen oxides between ice and air is important to the ozone budget in the upper troposphere. In the present study, the adsorption of nitrogen oxides on ice was investigated at atmospheric pressure using a chromatographic technique with low concentrations of radioactively labelled nitrogen oxides. The measured retentions solely depended on molecular adsorption and were not influenced by dimerisation, formation of encapsulated hydrates on the ice surface, dissociation of the acids, nor by migration into a quasi-liquid layer or grain boundaries. Based on the chromatographic retention and the model of thermochromatography, the adsorption enthalpies of −20 kJ mol−1 for NO, −22 kJ mol−1 for NO2, −30 kJ mol−1 for peroxyacetyl nitrate, −32 kJ mol−1 for HONO and −44 kJ mol−1 for HNO3 were calculated. To assess the adsorption enthalpies, standard adsorption entropies were calculated based on statistical thermodynamics. In this work, the use of two different standard states was demonstrated. Consequently different values of the standard adsorption entropy, of either between −39 J (K mol)−1 and −45 J (K mol)−1, or −164 J (K mol)−1 and −169 J (K mol)−1 for each nitrogen oxide were deduced. The adsorption enthalpy derived from the measurements, was independent of the choice of standard state. A brief outlook on environmental implications of our findings indicates that adsorption on ice might be an important removal process of HNO3. In addition, it might be of some importance for HONO and peroxyacetyl nitrate and irrelevant for NO and NO2.