(53d) The Effects of Humidity Cycling on a Range of Aerosol Particles

Atmospheric aerosols can influence the Earth’s climate both directly and indirectly. The magnitude of these effects strongly depends on their size, morphology, and their interaction with water, yet our knowledge of them is still limited. An investigation of these characteristics is essential to both assess the impacts of aerosols as well as their laboratory generation methods. Different techniques have been used to study these characteristics of particles; nevertheless, these studies did not extend their analysis to the effects of different drying rates or humidity cycling. In this work, we investigated the impact on the size and morphology of a few inorganics, amino acids and dicarboxylic acids nanoparticles through the use of a scanning mobility particle sizer and an atomic force microscope (AFM) combined with Nafion rehumidification of the original dry aerosol stream. Our hypothesis was confirmed that some pure chemical species exhibited a change in their size and morphology after the humidity cycling process while others did not. In other words, some chemicals formed different types of crystals and/or morphologies depending on the drying and rewetting process. Particularly, the even carbon number dicarboxylic acid particles (C4 and C6) did not change while the odd ones (C3, C5 and C7) did. Both glutamic acid and L-leucine results showed that their size stayed the same. However, AFM images of glutamic acid showed changes in their morphology. Finally, CaCl2 and NH4Cl exhibited no change while NH4NO3 results indicated that the particles might have crystallized into two forms after RH cycling and redrying.

Pre-selected dry particles were also collected using a PIXE impactor and analyzed under the AFM. For one chemical species, a bimodal size distribution was produced from fast drying of dilute, atomized aerosol. The AFM image of the 28 nm mode of dry particles shows mostly spherical particles while the 110 nm mode image shows a mixture of rounded-shape and sharp- edged particles. The humidity cycled particles were also collected and analyzed under the AFM. These particles consistently produced 50 nm unimodal size distributions. In other words, the particles crystallize to different forms when they are rehumidified and redried, re-arranging their structures and forming only one type of crystal. Interestingly, this effect occurred both when size-selected 28 nm or 110 nm particles were individually sent through the rehumidification and redrying process - the resulting mode was always 50 nm. We will present our conclusion that the 28 and 110 nm modes were simply different mobility shapes of the same total volume that were able to restructure to a common 50 nm mobility size upon exposure to a humid atmosphere. These results have implications for all laboratory aerosol generation methods, interpretations from these laboratory studies, and atmospheric aerosol processing transformations.