For smaller scale, focused studies, it is beneficial to have a generation mechanism that can be easily manipulated to probe fine level variations and can be effectively reproduced at numerous locations.  The design of the  MART system accomplishes these goals by using the lessons learned during experimentation with the glass walled wave channel to properly reproduce the properties of sea spray.  There are currently three iterations of the MART  tank system.

1. The first generation model for smaller experimental systems.
2. A larger version for increased particle production capabilities.
3. A duplicate of the larger tank optimized for measurements of the physical properties of the wave simulation system.

The ATOFMS is a mass spectrometer developed by the Prather Research Group that is capable of measuring the size and composition of individual aerosol particles, returning the results in real time.  Particles are introduced to the instrument where they are intercepted by an ultraviolet (266 nm) laser pulse, which causes the components of the particle to vaporize and ionize.  The ATOFMS is equipped with two time-of-flight mass spectrometers: both the positive and negative ions produced by photo-ionization can be detected.  These unique instruments can help determine whether chemicals are mixed together in the same particle (‘internally mixed’) or if the various compounds detected are in different particles (‘externally mixed’).  This concept of mixing state is important in the cloud forming potential of atmospheric aerosols, for instance, and techniques that measure individual particles, like ATOFMS, are uniquely suited to resolve aerosol mixing state.

CIMS uses chemical reactions to produce molecular ions, which can then be identified by their mass-to-charge ratio. This technique is used by CAICE investigators to understand the gas-to-particle partitioning of trace gases of atmospheric importance.  Natural volatile organic compounds (VOCs), which are detected with great sensitivity by CIMS, are released from the ocean surface and are important to secondary aerosol formation. Urban pollutants (e.g., N2O5) react on sea salt particle surfaces and impact atmospheric oxidation in the morning hours; the reaction kinetics of N2O5 uptake are studied by the Bertram Research Group using flow-tube reactor coupled to CIMS.

For purposes of elucidating complex organic samples, perhaps the most powerful mass spectrometry tool is the Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer.  When coupled to a linear ion trap, this type of instrument is capable of doing MS-MS experiments, in which an ion is isolated by its mass-to-charge ratio, and then fractured by bombardment with neutral molecules (usually helium atoms), permitting more detailed characterization of molecular structures.  In collaboration with the Dorrestein Research Group, CAICE researchers have been using this technique to understand the detailed nature of the organic chemical composition of aerosol and seawater samples.  Interfaced with an array of possible ionization sources, including electrospray ionization (ESI), desorption electrospray (DESI), and nanospray ionization, allowing optimal control of sample introduction and soft ionization.

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.

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.