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.

抽象的

含有硫酸铵的大气气溶胶在固相中具有几种形态类型。实验室实验被用来重现通常在大气样品中发现的颗粒形状，以阐明影响这些不同颗粒形状的因素。在不同的相对湿度（RH）条件下制备了硫酸铵颗粒和草酸混合颗粒，并使用透射电子显微镜（TEM）对其进行了观察。从水溶液产生后，使用扩散干燥器干燥的大多数硫酸铵颗粒是球形或球形单元簇。然而，在大气压力下将TEM样品暴露于59％RH和75％RH之后，一些颗粒变为具有平行直线作为颗粒周长的圆形长方形。在59％RH和75％RH下从球形变为圆形的矩形颗粒的数量分数分别在3 h时分别为6％和52％，在1天时分别为18％和66％。实验过程中的形态变化表明，硫酸铵结晶在相对稳定的相对湿度下发生。还发现硫酸铵与草酸盐共存时，圆形到矩形的形态发生了变化。这些结果表明，球形或复杂形状的含硫酸盐的气溶胶可以根据颗粒的相对RH条件逐渐改变为具有更规则结构的形状。在大气气溶胶样品中经常发现球形和圆形的矩形硫酸盐颗粒。