Abstract

As part of a closure experiment scheme, involving optical measurements of aerosol for comparison purposes of its radiative properties, a method for the generation of monodisperse crystalline ‘fibers’ made up of a highly scattering substance to visible light was applied. To simulate artificially these particles, trimethylxanthine powder (caffeine) of high purity was used for the generation of experimental samples of cylindrical-shaped aerosol with a Monodisperse Aerosol Generator (MAGE), which is an advanced condensation-type generator. The samples were characterized microphysically by applying scanning electron microscopy (SEM) with a Cambridge S360 instrument. This characterization consisted of SEM analysis of aerosol shape and dimensions in order to determine the size average and the degree of monodispersity of the samples. This study included an estimation of the fiber’s aspect ratio and therefore a numerical evaluation of the aerodynamic diameter based on the theoretical expressions given by Cox [32] for the motion of a fluid over a fiber. The results showed that the geometrically analyzed samples, representing the caffeine fibrous aerosol population generated from the MAGE, were good to account for the uniform representation of the fiber’s diameter (1.21/1.35 GSD) but, to a lesser extent, relatively acceptable to represent uniformly its length (1.44/1.48 GSD). The evaluation of the aspect ratio of the fibers indicated that they were clearly well elongated crystals. These results were compared with those obtained by other authors. It is concluded that SEM aerosol examination can be applied, in combination with an optical measurement system, to obtain experimentally the radiative properties of standard airborne particles of different shapes, sizes and substances for atmospheric and environmental studies related to radiative forcing (climate changes) and air quality.