A kinetic and product study of the hydrolysis of ClONO₂ on type Ia polar stratospheric cloud materials at 185 K

A Knudsen cell reactor equipped with FTIR reflection-absorption spectroscopy (FTIR-RAS) has been used to study the hydrolysis of chlorine nitrate (ClONO₂) on thin nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD) films. The reaction efficiencies and condensed-phase products have been measured for a variety of water partial pressures at 185 K. The reaction efficiencies for ClONO₂ hydrolysis on NAT and NAD were of similar magnitude and increased from γ = 0.0004 to γ= 0.007 as the water partial pressure was increased from 5 × 10^-6 to 1.4 × 10^-4 Torr. Although these values agree well with literature values at 90% relative humidity, significant differences are observed at lower relative humidities. Using our results at 185 K along with previous studies at 191 and 202 K, we suggest that this reaction may have a temperature dependence that is not currently addressed in atmospheric models. In the condensed phase, we observed the water dependence by the formation of progressively water-rich surface layers. The amount of water incorporated onto the NAT and NAD films increased with increasing relative humidity, suggesting that ClONO₂ hydrolysis occurs most efficiently on a water-rich hydrate surface. For water saturation ratios S ≥ 1.5, nucleation and growth of crystalline ice was observed to occur in the aqueous HNO₃ surface layer on top of the reactant NAD and NAT films.