Evaporation of ammonium nitrate particles containing ammonium sulfate

Abstract

Single solid particles with initial composition about 80% NH4NO3 by number, 20% (NH4)2SO4 are levitated in an electric quadrupole trap at T = 25°C. Particle mass is measured continually over periods of up to 8 h as NH3 and HNO3 evaporate and are removed. For particles in vacuum the HNO3 effective pressure αP(Torr), where a is the sticking coefficient, is measured to be log (αP) = {logical and with double overbar}n3 = -1anxn, where x is the NH4NO3 mole fraction and a-1 = -3.48 × 10-3, a0 = -7.17, a1 = 1.28, a2 = -1.67 and a3 = 2.60 with an estimated uncertainty in αP of 40% of this value. At x = 1 this gives, by extrapolation, P = (1.09 ±0.44) × 10-5 Torr α = 1, in good agreement with that calculated from thermodynamics, P = 9.46 × 10-6 Torr. For particles in water vapor, at humidities below the lowest deliquescence point at 62%, the evaporation rate increases. No mass increase is detected when the water vapor is introduced. For r.h. > 40% the evaporation rate is independent of NH4NO3 mole fraction until that falls below about 0.2. The results suggest that NH4+ and NO3- ions are solvated in an absorbed water layer with the solvation controlling evaporation. In a warm atmosphere without NH3 and HNO3 solid mixed NO3--SO42- particles would lose most of their NO3- in 1-h, according to our results. © 1988.