Electron Microscopy with Energy Dispersive X-ray Spectroscopy
The ability for an aerosol particle to accomodate water or react with trace gases in the atmosphere is key to its influence on atmospheric chemistry and climate. While some instruments, like ATOFMS, have been designed to analyze many individual particles in a short amount of time, the level of detail in the chemical composition is not always sufficient for fully understanding their climatic properties. Spatial information is key, and can be provided by scanning (SEM) or transmission electron microscopy (TEM) coupled with energy dispersive x-ray spectroscopy (EDX). This technique can provide not only information on the shape of aerosol particles, but also spatial relationships between the compounds that comprise those particles.
The power of optical microscopy is multiplied when coupled to techniques that can chemically speciate the object being magnified. Raman microspectroscopy reveals information about the composition of single aerosol particles by probing vibrations in chemical bonds. This technique can give very specific information about the nature of organic material in aerosol particles, the existence and chemistry of which can play a controlling role in the influence of aerosols on the global climate.
Ault, A.P., et al. “Raman Microspectroscopy and Vibrational Sum Frequency Generation Spectroscopy as Probes of Bulk and Surface Compositions of Size-Resolved Sea Spray Aerosol Particles,” Phys. Chem. Chem. Phys., 2013, 15, 6206-6214.
Atomic Force Microscopy
One of the primary issues with electron microscopy techniques for the analysis of aerosol particles lies in the necessity of operating under high vacuum conditions which can fundamentally alter the properties of aerosol particles from their natural state. Atomic force microscopy is one possible method for probing the physical properties of collected aerosol specimens. In addition to morphological properties, differences in the relative mechanical properties can be determined during the acquisition of an image and subsequently used as an indirect chemical probe. Beyond analysis under ambient conditions, atomic force microscopes can easily be outfitted for detailed relative humidity and reactive gas studies to further probe processes of chemical importance under environmentally applicable conditions.