Morphological change of solid ammonium sulfate particles below the deliquescence relative humidity: Experimental reproduction of atmospheric sulfate particle shapes

Abstract

Atmospheric aerosols containing ammonium sulfate have several morphological types in the solid phase. Laboratory experiments were used to reproduce the particle shapes commonly found in atmospheric samples to elucidate factors affecting these different particle shapes. Ammonium sulfate particles and mixed particles with oxalate were prepared under different relative humidity (RH) conditions and were observed using transmission electron microscopy (TEM). Most ammonium sulfate particles dried using a diffusion dryer after generation from aqueous solution were spherical or clusters of spherical units. However, after exposure of the TEM samples to 59% RH and 75% RH at atmospheric pressure, some particles changed to a rotundate-rectangular shape with parallel straight lines as the particle perimeter. Number fractions of particles that had changed to rotundate-rectangular shape from a spherical shape at 59% RH and 75% RH were, respectively, 6% and 52% at 3 h, and 18% and 66% at 1 day. Morphological changes during the experiment suggest that ammonium sulfate crystallization occurred under metastable RH. Morphological changes to rotundate-rectangular shapes were also found for ammonium sulfate co-existing with oxalate. These results indicate that spherical or complexly shaped sulfate-containing aerosols can change gradually to shapes having more regular structures according to a particle’s experienced RH condition. Both spherical and rotundate-rectangular shapes of sulfate particles have often been found in atmospheric aerosol samples. The morphological change of ammonium sulfate found in this study will be useful to infer the optical properties of atmospheric particles and the thermodynamic process on a solid particle surface.