Effects of potassium nitrate on the solid phase transitions of ammonium nitrate particles

Ammonium nitrate (NH4NO3) is a common constituent of atmospheric particulate pollutants. It exists in five stable polymorphic forms, designated as phases V, IV, III, II and I, below its melting point of 170 °C. In atmospheric research, very little attention has been paid to the solid phase transitions of NH4NO3 because phase IV NH4NO3 particles are stable over a wide range of tropospheric temperatures. Potassium nitrate (KNO3) is often found to co-exist with NH4NO3 in atmospheric aerosols, and it can change the phase transition behaviors of solid NH4NO3 particles. In this study, we investigated the effects of KNO3 on the solid phase transitions of NH4NO3 particles using in situ microscopic Raman spectroscopy. Both the transition path and transition temperature of NH4NO3 single particles (40–700 μm) depend on the KNO3 mass percentage and the particle size. With the addition of KNO3, the IV→II transition, which appears at 52 °C for pure NH4NO3 particles, is replaced by the IV→III transition. The KNO3 mass percentage required for this change in transition path increases with decreasing particle size and the transition temperature decreases with increasing KNO3 mass percentage. At a relatively high mass percentage of KNO3 (⩾7.4 wt%), the KNO3/NH4NO3 mixed particles undergo the IV→III transition under ambient temperatures, or even crystallize directly in phase III from droplets with a further increase in the mass percentage of KNO3. Submicron KNO3/NH4NO3 particles crystallize to phase IV at low KNO3 mass percentages (⩽5.7 wt%) but to phase III at higher KNO3 mass percentages (⩾7.4 wt%). These results suggest that atmospheric solid NH4NO3 particles may exist in phase III and the phase transitions should not be ignored in atmospheric chemical models.