The mass accommodation coefficient of ammonium nitrate aerosol

The mass transfer rate of pure ammonium nitrate between the aerosol and gas phases was quantified experimentally by the use of the tandem differential mobility analyzer/scanning mobility particle sizer (TDMA/SMPS) technique. Ammonium nitrate particles 80–220 nm in diameter evaporated in purified air in a laminar flow reactor under temperatures of 20–27°C and relative humidities in the vicinity of 10%. The evaporation rates were calculated by comparing the initial and final size distributions. A theoretical expression of the evaporation rate incorporating the Kelvin effect and the effect of relative humidity on the equilibrium constant is developed. The measurements were consistent with the theoretical predictions but there was evidence of a small kinetic resistance to the mass transfer rate. The discrepancy can be explained by a mass accommodation coefficient ranging from 0.8 to 0.5 as temperature increases from 20–27°C. The corresponding timescale of evaporation for submicron NH4NO3 particles in the atmosphere is of the order of a few seconds to 20 min.