Size effect of hematite and corundum inclusions on the efflorescence relative humidities of aqueous ammonium nitrate particles

Abstract

The influence of relative humidity (RH) and mineral dust inclusions on the physical state of NH4NO3/H2O particles is investigated through infrared aerosol flow tube experiments. Neat submicron aqueous particles do not effloresce, while those bearing mineral inclusions ranging from 150 to 400 nm in diameter effloresce between 3 and 10% RH at 298 K. NH4NO3/H2O coatings are achieved on hematite (α-Fe2O3), corundum (α-Al2O3), mullite (Al6Si2O13), and amorphous silica (am-SiO2) through a HNO3 vaporization-condensation technique followed by neutralization with NH3. The extent of internal mixing in the aerosol ranges from 7 to 27% depending on the mineral dust diameter. The size-dependent efflorescence results are successfully rationalized within the framework of an active-site model, and the physical parameters obtained from the model optimization are consistent with previous work on the efflorescence of (NH4)2SO4/H2O by these mineral dusts. Evidence is presented for a mechanism of heterogeneous nucleation that proceeds by chemisorption of nitrate at the surface of the oxide minerals followed by epitaxial germ formation of crystalline NH4NO3, all of which is driven by the saturation of the aqueous solution with respect to the solid.