Ammonium nitrate (NH_4NO_3) is a common constituent of atmospheric participate pollutants. It exists in five stable polymorphic forms, designated as phases Ⅴ,Ⅳ,Ⅲ,Ⅱ and Ⅰ, below its melting point of 170℃. In atmospheric research, very little attention has been paid to the solid phase transitions of NH_4NO_3 because phase Ⅳ NH_4NO_3 particles are stable over a wide range of tropospheric temperatures. Potassium nitrate (KNO_3) is often found to co-exist with NH_4NO_3 in atmospheric aerosols, and it can change the phase transition behaviors of solid NH_4NO_3 particles. In this study, we investigated the effects of KNO_3 on the solid phase transitions of NH_4NO_3 particles using in situ microscopic Raman spectroscopy. Both the transition path and transition temperature of NH_4NO_3 single particles (40-700 μm) depend on the KNO_3 mass percentage and the particle size. With the addition of KNO_3, the Ⅳ→Ⅱ transition, which appears at 52℃ for pure NH_4NO_3 particles, is replaced by the Ⅳ→Ⅲ transition. The KNO_3 mass percentage required for this change in transition path increases with decreasing particle size and the transition temperature decreases with increasing KNO_3 mass percentage. At a relatively high mass percentage of KNO_3 (7.4wt%), the KNO_3/NH_4NO_3 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 KNO_3. Submicron KNO_3/NH_4NO_3 particles crystallize to phase Ⅳ at low KNO_3 mass percentages (≤ 5.7 wt %) but to phase Ⅲ at higher KNO_3 mass percentages (≥7.4wt%). These results suggest that atmospheric solid NH4NO_3 particles may exist in phase Ⅲ and the phase transitions should not be ignored in atmospheric chemical models.
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