Nitric acid partitioning in cirrus clouds: a synopsis based on field, laboratory and model studies

Abstract. From a synopsis of field, laboratory and model studies at T>205 K as well as from the field experiments POLSTAR at T<205 K we derive a general picture of the partitioning of nitric acid (HNO₃) in cirrus clouds and a new hypothesis on the uptake of HNO₃ on ice particles:

A substantial part of nitric acid remains in the gas phase under cirrus cloud conditions. The HNO₃ removed from the gas phase is distributed between interstitial aerosol and ice particles in dependence on the temperature and ice surface, respectively. In cold cirrus clouds with small ice surface areas (T <205 K) the partitioning is strongly in favour of interstitial ternary solution particles while in warmer cirrus clouds with large ice surface areas the uptake on ice dominates. Consequently, denitrification via sedimenting ice particles may occur only in the -more frequently occurring- warm cirrus clouds

The HNO₃ coverage on ice is found to be different for ice particles and ice films. On ice films the coverage can increase with decreasing temperature from about 0.1 to 0.8 monolayer, while that on ice particles is found to decrease with temperature and P_HNO3 from 0.1 to 0.001 monolayer. An HNO₃ uptake behaviour following dissociative Langmuir isotherms where the coverage decreases for descending temperatures may explain the observations for ice particles

From a comparison of the HNO₃ measurements with model calculations it is found that (i) the global model of Lawrence and Crutzen (1998) overestimates the HNO₃ partitioning in favour of the ice particles (ii) the Langmuir surface chemistry model of Tabazadeh et al. (1999) overestimates HNO₃ coverages for temperatures ≤210 K More appropriate coverages are calculated when implementing in that model a temperature dependent function for the adsorption free energy (ΔG_ads (T)), which is empirically derived from the coverage measurements.