The characterization of three ionic liquids [BMIM][BF 4 ], [BMIM][DCA], and [BMIM][MS] having a common cation and with anions of varying sizes and shapes was performed with three complementary surface techniques: sum frequency generation-polarization mapping, surface tension measurement, and surface potential measurement. Custom vacuum cells were designed for each technique to be able to perform measurements in a highly controlled environment minimizing the presence of water and other contaminants, which may compromise measured values. SFG results show evidence of having anions and cations present on the surface with the butyl chain of the cation positioned toward the gas phase and the imidazolium ring mostly parallel to the surface plane. Results from the surface potential measurements reveal the relative positions of the ions where the anions are located at a slightly lower plane compared to the cations. Observed values from the surface tension measurements denote surface intermolecular interactions indicative of both van der Waals and Coulombic interactions suggesting the presence of alkyl chains as well as ions on the surface. A model on the gas-liquid interface of ionic liquids is described based on the concurring results from these three surface characterization techniques, as well as current literature.

This article summarizes and compares the analysis of the surfaces of natural aerosol particles from three different forest environments by vibrational sum frequency generation. The experiments were carried out directly on filter and impactor substrates, without the need for sample preconcentration, manipulation, or destruction. We discuss the important first steps leading to secondary organic aerosol (SOA) particle nucleation and growth from terpene oxidation by showing that, as viewed by coherent vibrational spectroscopy, the chemical composition of the surface region of aerosol particles having sizes of 1 μm and lower appears to be close to size-invariant. We also discuss the concept of molecular chirality as a chemical marker that could be useful for quantifying how chemical constituents in the SOA gas phase and the SOA particle phase are related in time. Finally, we describe how the combination of multiple disciplines, such as aerosol science, advanced vibrational spectroscopy, meteorology, and chemistry can be highly informative when studying particles collected during atmospheric chemistry field campaigns, such as those carried out during HUMPPA-COPEC-2010, AMAZE-08, or BEARPEX-2009, and when they are compared to results from synthetic model systems such as particles from the Harvard Environmental Chamber (HEC). Discussions regarding the future of SOA chemical analysis approaches are given in the context of providing a path toward detailed spectroscopic assignments of SOA particle precursors and constituents and to fast-forward, in terms of mechanistic studies, through the SOA particle formation process.

Abstract. We present the vibrational sum frequency generation spectra of organic particles collected in a boreal forest in Finland and a tropical forest in Brazil. These spectra are compared to those of secondary organic material produced in the Harvard Environmental Chamber. By comparing coherent vibrational spectra of a variety of terpene and olefin reference compounds, along with the secondary organic material synthesized in the environmental chamber, we show that submicron aerosol particles sampled in Southern Finland during HUMPPA-COPEC-2010 are composed to a large degree of material similar in chemical composition to synthetic α-pinene-derived material. For material collected in Brazil as part of AMAZE-08, the organic component is found to be chemically complex in the coarse mode but highly uniform in the fine mode. When combined with histogram analyses of the isoprene and monoterpene abundance recorded during the HUMPPA-COPEC-2010 and AMAZE-08 campaigns, the findings presented here indicate that if air is rich in monoterpenes, submicron-sized secondary aerosol particles that form under normal OH and O 3 concentration levels can be described in terms of their hydrocarbon content as being similar to α-pinene-derived model secondary orCorrespondence to: F. M. Geiger (geigerf@chem.northwestern.edu) ganic aerosol particles. If the isoprene concentration dominates the chemical composition of organic compounds in forest air, then the hydrocarbon component of secondary organic material in the submicron size range is not simply wellrepresented by that of isoprene-derived model secondary organic aerosol particles but is more complex. Throughout the climate-relevant size range of the fine mode, however, we find that the chemical composition of the secondary organic particle material from such air is invariant with size, suggesting that the particle growth does not change the chemical composition of the hydrocarbon component of the particles in a significant way.

In this perspectives article, we reflect upon the existence of chirality in atmospheric aerosol particles. We then show that organic particles collected at a field site in the central Amazon Basin under pristine background conditions during the wet and dry seasons consist of chiral secondary organic material. We show how the chiral response from the aerosol particles can be imaged directly without the need for sample dissolution, solvent extraction, or sample preconcentration. By comparing the chiral-response images with optical images, we show that chiral responses always originate from particles on the filter, but not all aerosol particles produce chiral signals. The intensity of the chiral signal produced by the size resolved particles strongly indicates the presence of chiral secondary organic material in the particle. Finally, we discuss the implications of our findings on chiral atmospheric aerosol particles in terms of climate-related properties and source apportionment.

Nonlinear optical imaging is a powerful method for observing bulk and interfacial phenomena in time and space. Here, we present a stepby-step description of how to carry out second harmonic generation imaging with a kHz amplifier laser system and demonstrate its applicability for SHG microscopy studies of highly size-resolved colloidal CdSe quantum dots having radii of 1-2 nm deposited on glass slides. It is found that not all quantum dots are SHG active, which suggests that environmental effects and particle distributions are important for SHG activity of quantum dots.