Imee Su Martinez

Northwestern University, Evanston, IL, United States

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Publications (8)19.9 Total impact

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    ABSTRACT: 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.
    The Journal of Physical Chemistry A 06/2012; 116(32):8271-90. · 2.77 Impact Factor
  • Imee Su Martinez, Cherry Santos, Steven Baldelli
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    ABSTRACT: Sum frequency generation, surface potential, and surface tension measurements have been combined on the pure ionic-liquid-gas interface for 1-alkyl-3-methylimidazolium alkylsulfate ionic liquids. The results show that surface potential of the ionic liquid generally increases as the alkyl chain on the cation or anion increases in length. This is due to the increased ordering of the surface dipole, mostly coming from the terminal methyl group of the alkyl chain. Both sum frequency generation spectroscopy and surface potential measurements suggest that the charged components, that is, the aromatic ring and the sulfate, occupy nearly the same plane at the surface for all ionic liquids studied herein.
    ChemPhysChem 04/2012; 13(7):1818-24. · 3.35 Impact Factor
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    ABSTRACT: Atmospheric organic aerosol particles from boreal and tropical forest environments were studied during two recent field campaigns, namely HUMPPA-COPEC-2010, conducted in Southern Finland, and AMAZE-08, conducted in the central Amazon basin. Here, we show how the chemical identity of the aerosol particle phase sampled there can be evaluated directly and without the need for solid phase extraction, reconstitution, or other sample manipulation with two table-top laser spectroscopic methods that probe sub-nanogram amounts of particle material collected on impactor and filter samples. The particles are studied using vibrational sum frequency generation (SFG) and second harmonic generation (SHG) imaging microscopy, followed by comparison to SFG and SHG responses obtained from synthetic organic aerosol particles prepared at the Harvard Environmental Chamber and to hydrocarbon reference compounds. 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 α-pinene-derived secondary organic aerosol (SOA) particles are an appropriate model for submicron-sized SOA particles formed in air that is rich in monoterpenes provided that the oxidation chemistry is dominated by ozone. This conclusion is supported by size-invariant SFG spectra obtained from aerosol particle filters collected during a nucleation event that occurred during HUMPPA-COPEC 2010. If the isoprene concentration dominates the chemical composition of organic compounds in tropical forest air, then the hydrocarbon component of submicron SOA particles is not simply well-represented by that of isoprene-derived model SOA particles but is more complex. Throughout the climate-relevant size range of the fine mode, however, we find that the chemical composition of the SOA particle material from such air is size invariant, suggesting that the particle growth does not change the chemical composition of the hydrocarbon component of the particles in a significant way. Complementary SHG imaging studies show that submicron organic aerosol particles collected during HUMPPA-COPEC 2010 and AMAZE-08 are often chiral. The chiral responses are found to always originate from aerosol particle material on the filter, but not all aerosol particle material produces chiral signals, which is discussed in the context of aerosol particle phase, possible climate-related properties, and the use of chirality as a tracer for source apportionment.
    AGU Fall Meeting Abstracts. 12/2011;
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    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 O3 concentration levels can be described in terms of their hydrocarbon content as being similar to α-pinene-derived model secondary organic 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 well-represented 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.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 10/2011; 11(20):10317-10329. · 5.51 Impact Factor
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    ABSTRACT: 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.
    Physical Chemistry Chemical Physics 07/2011; 13(26):12114-22. · 3.83 Impact Factor
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    ABSTRACT: Nonlinear optical imaging is a powerful method for observing bulk and interfacial phenomena in time and space. Here, we present a step-by-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.
    Optical Materials Express 04/2011; 1(1):57-66. · 2.92 Impact Factor
  • Imee Su Martinez, Steven Baldelli
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    ABSTRACT: The characterization of three ionic liquids [BMIM][BF4], [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.
    Journal of Physical Chemistry C - J PHYS CHEM C. 06/2010; 114(26).
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    Imee Su Martinez, Steven Baldelli
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    ABSTRACT: Novel glass cells were designed and built to be able to perform surface potential and surface tension measurements in a contained environment. The cells can withstand pressures of approximately 1 x 10(-6) Torr, providing a reasonable level of control in terms of the amounts of volatile contaminants during experimentation. The measurements can take several hours; thus the cells help maintain the integrity of the sample in the course of the experiment. To test for the feasibility of the cell design, calibration measurements were performed. For the surface potential cell, the modified TREK 6000B-7C probe exhibited performance comparable to its unmodified counterpart. The correlation measurements between applied potential on the test surface and the measured potential showed R-values very close to 1 as well as standard deviation values of less than 1. Results also demonstrate improved measurement values for experiments performed in vacuum. The surface tension cell, on the other hand, which was used to perform the pendant drop method, was tested on common liquids and showed percentage errors of 0.5% when compared to literature values. The fabricated cells redefine measurements using classical surface techniques, providing unique and novel methods of sample preparation, premeasurement preparation, and sample analysis at highly beneficial expenditure cost.
    The Review of scientific instruments 04/2010; 81(4):044101. · 1.52 Impact Factor