Size-Resolved Sea Spray Aerosol Particles Studied by Vibrational

We present vibrational sum frequency generation (SFG) spectra of the external surfaces and the internal interfaces of size-selected sea spray aerosol ...
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Size-Resolved Sea Spray Aerosol Particles Studied by Vibrational Sum Frequency Generation Carlena J. Ebben,† Andrew P. Ault,‡ Matthew J. Ruppel,§ Olivia S. Ryder,§ Timothy H. Bertram,§ Vicki H. Grassian,*,‡ Kimberly A. Prather,*,§,∥ and Franz M. Geiger*,† †

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States § Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States ∥ Scripps Institution of Oceanography, La Jolla, California 92093, United States ‡

S Supporting Information *

ABSTRACT: We present vibrational sum frequency generation (SFG) spectra of the external surfaces and the internal interfaces of size-selected sea spray aerosol (SSA) particles generated at the wave flume of the Scripps Hydraulics Laboratory. Our findings support SSA particle models that invoke the presence of surfactants in the topmost particle layer and indicate that the alkyl chains of surfactant-rich SSA particles are likely to be disordered. Specifically, the SFG spectra suggest that across the range of sizes studied, surfactant-rich SSA particles contain CH oscillators that are subject to molecular orientation distributions that are broader than the narrow molecular distribution functions associated with well-ordered and well-aligned alkyl chains. This result is consistent with the interpretation that the permeability of organic layers at SSA particle surfaces to small reactive and nonreactive molecules may be substantial, allowing for much more exchange between reactive and nonreactive species in the gas or the condensed phase than previously thought. The SFG data also suggest that a one-component model is likely to be insufficient for describing the SFG responses of the SSA particles. Finally, the similarity of the SFG spectra obtained from the wave flume microlayer and 150 nm-sized SSA particles suggests that the SFG active CH oscillators in the topmost layer of the wave flume and the particle accumulation mode may be in similar chemical environments. Needs for additional research activities are discussed in the context of the results presented.

I. INTRODUCTION Sea spray aerosol (SSA) particles are a major component of aerosols in the atmosphere.1−3 These particles are an important part of the global sulfur and halogen cycles, may scatter or absorb the sun’s radiation, and may also influence cloud formation.1,4,5 SSA particles are therefore believed to have an important influence on climate-relevant processes. The formation of SSA particles has been studied for decades, and we note here Blanchard’s seminal work.6 Laboratory model studies3,7−12 and theoretical13,14 investigations on NaCl substrates or NaCl-derived aerosol particles, as well as infrared,15−17 Raman,18 and mass spectroscopic19,20 studies of field-derived particles, suggest the following: surfaces of natural SSA particles are likely to be important for regulating (1) the oxidative capacity in the marine boundary layer9 and (2) climate-relevant properties, such as the propensity of marine aerosol particles to act as cloud condensation nuclei or to nucleate ice crystals.21 While these studies are important for quantifying climate-relevant properties of SSA particles, their shortcomings from a surface science point of view are 2-fold: (1) there generally exists a sensitivity limitation for obtaining signals from the surface-bound species in a nondestructive way unless large amounts of sample are available (Raman and IR © 2013 American Chemical Society

spectroscopy); and (2) high sensitivities for studying the chemical composition of SSA particles are achievable mainly when the particles are destroyed in the process, and even then, the detected species are those that are volatile enough for identification (mass spectrometry). The focus in this work is the application of vibrational sum frequency generation (SFG) at ambient pressure and temperature conditions to sub-μg amounts of size-selected SSA particles. The use of SFG is beneficial in this case due to its high selectivity for environments where symmetry is broken, making it highly complementary to aerosol analysis methods that report on both bulk and surface chemical composition. The fact that the SFG signals from the oscillators on a given particle are produced coherently allows for the analysis of nano-22 to sub-fg23 amounts of sample. Yet, one technique alone does not provide a complete description of SSA particles. It is therefore advantageous to work in a collaborative team where the sample preparation, collection, characterization, and Received: February 25, 2013 Revised: July 2, 2013 Published: July 2, 2013 6589

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0.6 Hz over an artificial beach consisting of a panel placed at an angle of 30° from the bottom of the flume, in order to generate SSA particles. In this work, we focus on two periods of the intensive experiment and study particles collected before and after the addition of bacteria and phytoplankton to the wave flume. The SSA particle samples were collected on 3 and 14 November 2011, corresponding to dates before and after the addition of bacteria and phytoplankton to the flume. B. Collection of SSA Particles. SSA particles were sampled in several size fractions, namely, 150 nm, 1.0 μm, 2.5 μm, and the entire PM1 size fraction. Particles were collected on Teflon filters using a home-built PM1 sampler described previously27 and a micro-orifice uniform-deposit impactor (MOUDI), which size-separates particles based on their aerodynamic diameter.28 Particle size fractions are defined according to the 50% aerodynamic diameter cutoff of each impactor stage, which means that, for example, the 1 μm impactor stage will collect particles over a log-normal distribution containing particles that are both larger and smaller than 1 μm. Sampling methods using a PM1 sampler or a MOUDI result in low loadings of particles and typically produce just a few micrograms on a given filter or stage when the sample is collected for several hours. Sampling by the MOUDI lasted for 5 and 12 h for the experiments carried out on 3 and 14 November 2011, respectively. The masses on the filter stages from the 3 November 2011 experiments were estimated, to within an order of magnitude, to be ∼100 ng, 25 ng, and 0.5 ng for the 2.5, the 1.0, and the 0.15 μm size fractions, respectively, while the masses on the filter stages from the 14 November 2011 experiments were estimated to be approximately twice as large. After collecting the particle samples, the filters were placed in labeled Petri slide holders that were sealed with Teflon tape and placed in a freezer on site. Once sampling was complete, the samples were placed in a cooler with several ice packs and shipped overnight from Scripps to Northwestern University, where the samples were stored in a freezer at −12 °C prior to analysis by optical microscopy and SFG. Samples collected on various additional substrates were shipped to the University of Iowa, where they were analyzed by Raman microspectroscopy, optical microscopy, ATR-FTIR, SEM, and TEM. The experimental details of the Raman microspectroscopy and the scanning probe studies are described elsewhere.18,29 We emphasize that our results are applicable to the particle material collected, stored, and probed under the conditions stated here. Control studies (please see Supporting Information) indicate that wet vs dry samples obtained from the wave flume microlayer produce the same number of SFG peaks and similar spectral lineshapes. This finding seems to indicate that the off-line analysis by SFG of wave flume samples may be of relevance to understanding SSA particles under wet conditions. Yet, changes in aerosol particle size, size distribution, surface composition, and extent of hydration are not controlled for when SSA particles are removed from the wave flume and used in SFG experiments. C. Vibrational Sum Frequency Generation from Particles. SFG signals are generally taken to vanish if oscillators are located in centrosymmetric environments having length scales that are much less than the SFG coherence length.30 The SFG coherence length is typically several hundred nm at the signal wavelengths (∼650 nm) used here. SFG signals from macroscopically flat systems are typically produced over Å- to nm-thick regions, as shown in the pioneering molecular dynamics-coupled SFG work by Walker and Richmond.31 Our application of nonlinear optics to study

data interpretation are performed in an integrated manner and using an array of aerosol analysis techniques. For this present work, SSA particles were prepared under controlled conditions from waves breaking inside the wave flume of the Scripps Institution of Oceanography (SIO) Hydraulics Laboratory that was filled with natural seawater taken from the Pacific Ocean off Scripps Pier during the Center for Aerosol Impacts on Climate and the Environment (CAICE) 2011 intensive experiment.24 Sampling and complete characterization of SSA particles took place before and after the addition of bacteria and phytoplankton to the flume. Following sizeselection, the particles were deposited onto collection filters for off-line analysis by SFG, Raman microspectroscopy, optical, scanning, and transmission electron microscopy (SEM and TEM), and scanning transmission X-ray microscopy (STXM), as described in detail elsewhere.18,24,25 As described below, the wave tank allows us to bring complexity from the real ocean into the laboratory while controlling for a variety of exogenous parameters. Yet, even when many aspects of the size, bulk chemical composition, and bulk phase of SSA particles are known and even when interfaceselective probes, such as SFG, are applied to SSA particles, it is challenging to study SSA particle surfaces and their chemistry and physics because the exact chemical composition of these multicomponent materials is not known. Compounding this difficulty is the many interfaces or multiphase problem5 posed by SSA particles. As shown, for instance, by Buseck and coworkers in the mid-1990s,26 field-derived SSA particles may be confined not only by an external surface but may also contain internal interfaces, depending on conditions inside and outside the particle as well as their origin and history. This many interfaces problem is important because SSA particles containing fully miscible material are in a homogeneous phase state and present just one external surface to the gas phase, where heterogeneous physical and chemical processes may occur. In contrast, SSA particles containing a collection of different immiscible material in the same or different phases can be thought of as being in a mixed phase state. Such particles present not just an external surface to the gas phase but contain also internal interfaces. In that case, heterogeneous processes may occur at both external surfaces and internal interfaces. Here, we combine what we know about the chemical composition and phase state of the SSA particles prepared at the wave flume with the SFG spectroscopic responses produced by CH oscillators located at the particle surfaces and interfaces.

II. EXPERIMENTAL SECTION A. Particle Generation at the SIO Hydraulics Laboratory. SSA particles were collected at the Hydraulics Laboratory of the SIO during the CAICE 2011 intensive experiment, which took place in November 2011 and is described in detail elsewhere.24 Briefly, the intensive utilized a 33 m long sealed glass wave flume approximately one meter in height and depth, which was filled to approximately half a meter with seawater pumped from approximately four meters above the ocean floor at the end of the SIO pier. Seawater passed through two No. 12 crystal sand bed filters to remove large biological material and was delivered directly to the wave flume. The total organic carbon (TOC) content was held at approximately 70 μM at the beginning of the experiment and exceeded 300 μM after the addition of biological material. Breaking waves were produced inside the flume by a hydraulic paddle located at the end. These waves broke at a frequency of 6590

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Figure 1. (A) Representative aerosol particle concentrations and (B) average particle size distribution for the periods studied here (light blue), along with the log normal fit of the data (black). (C) Optical image (100× magnification) of 2.5 μm size fraction of SSA particles collected on a quartz substrate during the experiments carried out on 14 November 2011. Please see text for details. Scale bar = 10 μm. (D) SEM image (adapted from ref 32) of an SSA particle in the 0.56−1.0 μm size fraction collected on 3 November 2011 indicating the presence of internal and external interfaces, marked by the white lines in the lower image.

densities ranging from 100 to 150 particles cm−3, which are typical for the conditions employed at the wave flume. Nevertheless, the approach presented in this work is of practical use for aerosol analysis in that it allows for the investigation of aerosol particles deposited at low densities on the same impactor materials and sample filters that are used in the field for off-line analysis by other methods. In addition, while the SFG intensity scales, in principle, linearly with the number of particles in the laser spot for particles of the same size, it scales quadratically with the number of SFG-active oscillators associated with each particle, which, along with the coherent nature of the signal generation process, provides some significant signal intensity advantages over other vibrational spectroscopic methods.

synthetic and natural aerosol particles and their surfaces, ranging from semiconductors32 to noble-metal nanoparticles33 to nanostructured surfactants34,35 to size-resolved secondary organic aerosol particles from tropical and boreal forests,22,27,36 has been described in the literature. The approach is based on one first presented by Roke et al.,37−40 which, in turn, is the vibrational spectroscopic form of earlier nonlinear optical scattering studies by second harmonic generation of Eisenthal and co-workers.41 While the nonlinear optical scattering approaches of suspended particles reported by Eisenthal and co-workers and Roke and co-workers are appropriate for studying colloidal samples with particle densities exceeding 106 particles cm−3, there exists currently no SFG-based method to study samples containing aerosol particles suspended in air at 6591

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D. Vibrational Sum Frequency Generation from Multiphase Particles. Within the electric dipole approximation,42 a single oscillator within a centrosymmetric bulk phase does not produce SFG signals. Provided a nonzero average net orientation distribution, oscillators of species adsorbed at the internal interface between two immiscible bulk phases inside a given particle or at the interface between air and the external particle surface would produce SFG signals provided that the materials are transparent at the infrared and visible frequencies employed and provided that they are separated by distances approaching the SFG coherence length (see, for instance, work on nonlinear optical signal generation from vesicles30 or symmetric lipid bilayers,42,43 which is discussed in much more detail in the literature). As shown below, we are applying SFG spectroscopy to SSA particles in various mixing states that may contain aqueous, liquid organic, and insoluble material, i.e., a many-interface and/or multiphase problem. Attributing the origin of the SFG response is particularly challenging in such a situation. However, the following two observations suggest that, to within a first-order approximation, internal interfaces are unlikely to contribute significantly to the detected SFG response in SSA particles: (1) SFG signal intensities obtained under the same signal collection conditions from α-pinenederived secondary organic aerosol (SOA) particles prepared with and without ammonium sulfate seed particles22 are similar for comparable filter loadings (20 to 200 μg, respectively) and particles sizes (50 to 90 nm, respectively, see Supporting Information for data). (2) As shown in section III.A, the SFG signal intensities obtained under the same signal collection conditions from the SSA particles studied here are quite comparable for conditions in which their mixing state varies significantly at similar mass loadings and particles sizes. The exact quantification of the percentage of SFG signal intensity that is detected from external surfaces vs internal interfaces of a complex, mixed-phase particle will require the synthesis of meaningful model systems having different particle architectures and chemical compositions for analysis by the various methods appropriate for studying SSA particles. As discussed in section V, this goal is the subject of future studies from our groups. E. Laser and Detection System. The ssp polarization combination that is used here to study the aerosol particles on the impactor stages and the PM1 filters with SFG employs upconverter and infrared light fields that are plane-polarized parallel (s) and perpendicular (p) to the collection filter surface, respectively, and detects the component of the SFG signal that is polarized perpendicularly to the collection filter surface. While ssp-polarized SFG spectra generally probe the components of vibrational modes that are oriented perpendicular to a surface,43 that interpretation applies to the flat surfaces of the laboratory frame and not to the surfaces of the aerosol particles, as we describe in our recent work on secondary organic aerosol particles.27 The SFG experiments44−48 are based on a standard broadband 120-fs infrared optical parametric amplifier operating at a 1 kHz repetition rate. As described in our prior work,46 the SFG spectra are collected using a hybrid scanning/ broadband method first applied by Walker and co-workers,43 in which the broadband IR light field probes the oscillators of interest at the interface at various center frequencies with comparable IR power over a frequency range between 2700 and 3200 cm−1 with about 3 to 4 spectral slices that are each between 100 and 150 cm−1 broad. Each spectral slice is

normalized to the SFG response from a gold sample to account for the distribution of IR energy and calibrated to the methyl CH stretches of a polystyrene spectroscopic standard. Optical damage is minimized by limiting the incident pulse energies to about 1 μJ for the visible beam and to around 1.5 μJ for the IR beam while limiting the foci to approximately 30 μm in diameter.

III. RESULTS A. Number Densities and Phase State of the Particles. Figure 1A shows that the particle counts in the headspace of the wave flume are stable at a given time. Background concentrations upstream of the wave-breaking beach were typically between 10 and 20 particles cm−3. The overall size distribution of the SSA particles (Figure 1B) was obtained by merging data obtained via a scanning mobility particle sizer (SMPS; TSI 3936) and an aerodynamic particle sizer (APS; TSI 3321) and by assuming that particles in the SMPS size range are spherical and particles in the APS size range have an effective density of 1.8, an experimentally determined quantity for sea spray particle density. The discontinuity in the full distribution observed at ∼600 nm is a result of the merging of these two data sets and not indicative of an actual jump in the distribution. For a given wave break, the size distribution peaks near 200 nm and extends out into the supermicrometer size range. The Raman microspectroscopy, SEM, TEM, and STXM studies, which are described in detail elsewhere,18,29 show that the particles are composed of inorganic and organic matter. From those measurements, we estimate that the percentage of particles containing a single phase in the 0.15 μm size range is around 40−50% when background seawater is used and 70− 90% after addition of bacteria and phytoplankton. In contrast, more than 90% of particles in the 1.0 and 2.5 μm size ranges are estimated to contain separate phases before and after the addition of bacteria and phytoplankton to the flume. Figure 1C shows a representative optical image at 100× magnification of SSA particles in the 2.5 μm size fraction prior to the addition of bacteria and phytoplankton. The image shows a population of particles composed of crystalline material surrounded by a thin film, shown by energy dispersive X-ray (EDX) spectroscopy maps to be mainly NaCl and organic material, respectively. Figure 1D shows an optical image of such a particle, indicating the presence of internal and external interfaces. There is also a population of spherical particles containing few to no inclusions, which SEM/EDX and STXM/NEXAFS show to be composed mainly of organic material primarily below 300 nm.24,29 Raman microspectroscopy of the organic portion of the SSA particles studied here are consistent with the presence of several surfactant species for which reference spectra were taken.18 Specifically, the Raman analysis indicates that the most abundant organic species are present as a combination of lipopolysaccharides (LPS), long organic chains (such as those found within sodium dodecyl sulfate, which was used in that work as a surfactant proxy), and glycine, all of which contain aliphatic moieties. B. ATR-FTIR, Raman, and SFG Spectra from SSA Particles Collected on the Same Teflon Substrate. Figure 2 shows the sensitivity advantage that the nonlinear methods, discussed here for probing the surfaces of SSA particles, have over linear spectroscopies. Specifically for this comparison, we took a Teflon substrate that contained SSA particles in the one micrometer size fraction collected on 3 November 2011 and 6592

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shows that while the SFG and the Raman spectra indicate the presence of CH oscillators, the ATR approach is not sensitive enough to detect them. While the sensitivity of infrared spectroscopy is usually understood to be higher than that of Raman spectroscopy, we can rationalize the differences in the ATR vs the Raman responses by realizing that their lightfields sample to different depths (factor of 10 or more greater penetration for Raman spectroscopy) of the porous Teflon filters, whose exterior and interior contain SSA particles. C. SFG Responses from SSA Particles in a Given Size Fraction. Figure 3A shows how we collect the SFG spectra of SSA particles discussed in this work: we press a given sample filter containing SSA particles in a certain size regime between a fused silica window and a sample support under conditions approaching total internal reflection, which boosts nonlinear optical signal intensities.49 Control studies shown in the Supporting Information indicate little to no SFG signal intensity is detected from air/Teflon filters containing SSA particles. This finding is consistent with low SSA particle surface loadings and/or poor optical reflectivities of Teflon at the relevant optical and infrared frequencies applied here. Images obtained using optical microscopy show the distribution of particles from a given sample in the supermicrometer mode across the filter surface before and after pressing a window on the samples (Figure 3B′,B″). Before the silica window is placed on the surface of the sample filter, the particles are somewhat evenly distributed, while after the window is placed on the sample, the particles tend to clump together as expected.

Figure 2. Teflon substrate containing SSA particles in the onemicrometer size fraction collected on 3 November 2011 probed by vibrational SFG spectroscopy (blue, ssp-polarization), Raman microspectroscopy (green), and attenuated total reflectance (ATR) infrared spectroscopy (red). The black spectrum is the background spectrum collected by ATR from a blank Teflon filter. Please see text for details.

probed it using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR with a Germanium crystal), Raman microspectroscopy, and vibrational SFG. Figure 2

Figure 3. (A) Schematic of the experimental setup, which utilizes internal reflection geometry to analyze samples at the fused silica/particle filter sample interface. (B) Optical microscope images of a sample substrate before (B′) and after (B″) a fused silica window is pressed on the surface. (C) A Teflon filter substrate that was used for aerosol particle collection from the wave flume after bacteria and phytoplankton were added and onto which then was pressed a fused silica window for vibrational SFG analysis, as well as the individual ssp-polarized vibrational SFG spectra that were collected from four different spots on the sample. (D) ssp-Polarized SFG spectrum of 150 nm-sized aerosol particles collected after addition of bacteria and phytoplankton while centering the incident IR frequency at 3100 cm−1 with 150 cm−1 bandwidth and the corresponding gold spectrum (light green) that indicates the input IR power. Spectra offset for clarity. 6593

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We analyzed several spots per particle filter sample in order to assess possible variability in the SFG responses from these heterogeneous samples. Figure 3C shows a particle filter from the 150 nm size fraction of our MOUDI sampler and the corresponding SFG spectra from four locations on the filter. Even though the samples studied here were collected in the highly complex environment of a wave flume, their SFG spectra appear to be remarkably similar and reproducible. The Supporting Information shows that the same is observed for the larger size fractions as well. On the basis of this finding, we present here the averages of the SFG spectra we collected from various spots on each filter sample. We note that we find that changes in the SFG spectral shapes we observe from SSA particle surfaces collected under the various conditions described here depend on order-of-magnitude changes in particle size (i.e., 100 nm vs ∼1 μm) and the amount and type of biological material present in the flume. The slight changes in particle number density and size observed during a given experiment, such as those shown in Figure 1A, are thus expected to produce negligible changes in the spectral shapes of the SFG spectra presented here. While we cannot yet assess whether each SSA particle present on the filter sample produces SFG signals that would indicate the presence of molecular surface species, we find that all areas on the particle-containing samples that we investigated with our 30 μm sized laser spot show signal intensity at 2850, 2880, 2920, and 2950 cm−1, to varying degree of spectral resolution, which, as we will show below, depends on which size fraction is studied. These frequencies are generally indicative of the symmetric and asymmetric stretches of methylene and methyl groups as well as Fermi resonances.50−56 While significant signal intensity exists around 2900 cm−1, spectral features are not particularly well resolved in that frequency region. Finally, when tuning the incident IR frequency specifically to above 3000 cm−1 (Figure 3D), we find that the samples produce no SFG responses that would indicate the presence of aromatic and olefinic CH stretches at the particle surfaces. Instead, the SFG spectra exhibit weak and broad SFG responses toward the 3200 cm−1 region, typically associated with the presence of OH stretching bands of water. This finding is not surprising given that the particles are mixed with hygroscopic salts such as MgCl2, which have been shown to retain water even at low relative humidity. D. Control Studies. Control experiments were performed to assess the contributions of three possible external sources of our signal: CH oscillators at the surface of bare Teflon filter substrates, contamination from the air within the Hydraulics Laboratory, and contamination from the pump oil or hydraulic fluid used in the instrumentation. All of the spectra were recorded under identical spectral acquisition conditions. Particles sampled for these experiments were collected in the size range of 1 μm and below (PM1 size fraction). The top spectrum displayed in Figure 4 shows that filters containing material in the PM1 size range that were sampled for 3.5 h inside the operating wave flume produce SFG intensities at 2850, 2880, 2920, and 2950 cm−1, with a signal-tonoise ratio of 16 at 2950 cm−1. The PM1 sample includes all particle sizes below one micrometer as well as the tail end of the supermicrometer size fraction, whereas the samples collected using the various MOUDI stages contain a small fraction of the total size distribution. While we do not expect the SFG spectra of the PM1 and the MOUDI samples to be

Figure 4. ssp-Polarized vibrational SFG spectra of bare Teflon (bottom), a lab air sample collected using a PM1 cyclone for 3.5 h (second from bottom), Inland FF-TW pump oil dropcast onto a fused silica window (third from bottom), and a PM1 particle sample collected using the same PM1 cyclone for the same amount of time in the operating wave flume (top). All of the spectra were recorded under identical spectral acquisition conditions. Spectra are offset for clarity.

exactly the same, we show below that 2.5 μm-sized particle samples collected prior to addition of bacteria and phytoplankton exhibit SFG responses that are comparable to those obtained from particles sampled using the PM1 cyclone collected after the addition of this material. The second SFG spectrum from the top in Figure 4 assesses whether pump oil, used in the vacuum instrumentation and the hydraulics of the wave flume, contributed to the SFG signals obtained from the aerosol particles. To this end, we recorded, in internal reflection geometry, the vibrational SFG spectrum of a fused silica window onto which Inland FF-TW pump oil had been dropcast. This spectrum exhibits significantly different SFG line shapes and intensities when compared to the SFG spectra of the PM1 aerosol particles. Specifically, the SFG spectrum obtained from the pump oil shows poorly resolved features above 2900 cm−1 and a maximum signal-to-noise ratio of 11 at 2880 cm−1. For additional comparison, the SFG spectrum obtained from freshly dropcast material obtained by skimming the sea surface microlayer (vide infra), also collected in internal reflection geometry, shows a maximum signal-tonoise ratio of 29 at 2950 cm−1. Figure 4 also shows the SFG spectrum of a filter sampling particles in the size range of 1 μm and below (PM1 size fraction) for three hours outside of the wave flume but inside the Scripps Hydraulics Laboratory. While this SFG spectrum exhibits some minor SFG signal intensity at frequencies typical of C−H stretches, which is not surprising given that the hydraulics laboratory houses multiple mechanical pumps that were operating at the time of the campaign, the signal intensities are significantly lower than those obtained from the SFG spectra of the PM1 size fraction sampled inside the operating wave flume. Finally, the bottom SFG spectrum displayed in Figure 4 shows that the Teflon filter surfaces yield negligible SFG responses over the spectral integration times applied here. This finding is consistent with the low to 6594

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Figure 5. ssp-Polarized vibrational SFG spectra of the average of up to four ssp-polarized vibrational SFG spectra of 2.5 μm (A) 1.0 μm (B), and 150 nm (C) sized aerosol particles collected at the Scripps wave flume before (dark blue) and after (green) the addition of bacteria and phytoplankton, and of the 180−320 nm size fraction of particles prepared using synthetic sea salt in the presence of dissolved lipopolysaccharides (bottom, gray, spectrum in panel C). Spectra are offset for clarity. Please see text for details.

2232 g/mol. The LPS used was derived from Escherichia coli 0111:B4 (Sigma-Aldrich, L4130) and contained between 1 and 10% protein impurities. Pure LPS was stored at 5 °C, according to manufacturer storage instructions, until being added directly to the MART system. The MART system was equipped with a nitrogen line, allowing for 6 sLpm of N2 to purge the headspace of the tank continuously during the experiment. Aerosol particles were collected using a MOUDI sampler fitted with substrates allowing for the separate collection of sub- (180−320 nm) and supermicrometer (1.8−3.2 μm) particles. Postcollection at UCSD, particles were stored under nitrogen and placed in a freezer until analysis at Northwestern. Figure 5C shows the ssp-polarized SFG spectrum obtained from the 180−320 nm size fraction of the LPS-derived SSA particles, normalized to the signal-to-noise ratio in the frequency region that yields negligible SFG responses, along with those discussed in the previous section. The comparison of the SFG intensity ratios at 2880 and 2950 cm−1 for the LPSderived SSA particles vs those collected at the waveflume indicates that LPS is unlikely to be the chief contributor to the SFG spectrum obtained from the largest fraction of waveflumederived SSA particles once chemical complexity in the form of bacteria and phytoplankton are added to the wave flume. Figure 5C also shows that 180−320 nm sized LPS-containing SSA particles produce SFG responses that are similar to those obtained from tank-derived SSA particles in the 150 nm size range before or after the addition of chemical complexity in the form of bacteria and phytoplankton to the tank. Yet, we caution here that we cannot definitively rule out the presence of LPS in the large-sized particles collected under those conditions nor definitely confirm the presence of LPS in the small particle size fractions. G. SFG Responses from Material Skimmed off the Wave Flume Surface Microlayer. Following the addition of bacteria and phytoplankton to the wave flume, a water sample was skimmed from the top centimeter of the water in the wave flume using a glass vial in order to sample the organic-enriched surface microlayer of the wave flume. This sample was frozen until analysis. After the sample was thawed and allowed to settle, a pipet was used to collect a small amount of liquid from close to the very top of the vial. Two drops of the sample were placed on the surface of a clean fused silica window. After approximately one hour, the vibrational SFG spectrum was recorded in internal reflection mode. The spectrum is due to the CH oscillators left on the optical window following

negligible abundance of CH bonds in Teflon. We conclude that signal contributions from adventitious carbon material that might be present on aerosol particle surfaces in the form of pump oil and exogenous volatile organic compounds in the lab air are minor. E. SFG Responses from SSA Particles in Different Size Fractions. As mentioned above, all particle samples studied here produce SFG signal intensities at 2850, 2880, 2920, and 2950 cm−1 to varying degree of spectral resolution. Specifically, when the wave flume was operated using only filtered seawater from offshore the Scripps Pier, we find the SFG responses of the surfaces of collected 1.0 and 2.5 μm particles to be dominated by vibrational resonances at 2880 and 2950 cm−1 (Figure 5A,B). When chemical complexity in the form of bacteria and phytoplankton is added to the wave flume, we find that the ratio of the SFG intensity of the vibrational modes at 2880 and 2950 cm−1 decreases from approximately 1:2 to approximately 1:1 in the 1.0 and 2.5 μm particles. In addition to these two spectral features, the SFG responses of the surfaces of particles having aerodynamic diameters of 150 nm also exhibit vibrational resonances at 2850 and 2920 cm−1 (Figure 5C), attributed to the symmetric and asymmetric methylene CH stretches, respectively. However, unlike the 1.0 and 2.5 μm particles, the line shapes of the 150 nm particles SFG responses do not change significantly when bacteria and phytoplankton were added to the flume. F. Comparison to LPS. Given the possible presence of LPS in the SSA particles, as described briefly in section IIIA, we carried out SFG experiments on SSA particles prepared in the presence of LPS in synthetic seawater using a newly developed Marine Aerosol Reference Tank (MART) system,57 which allows for the production of an accurate size distribution relevant for ocean conditions. Briefly, the 210 L MART system is filled with 127 L of synthetic seawater, which is then pumped into a horizontal column that allows an intermittently plunging sheet to form, which produces a bubble plume that closely approximates that of real ocean waves. Here, the MART system was filled with synthetic seawater (Milli-Q water and sea salt mix, Sigma-Aldrich, S9883) to achieve a chloride concentration of 545 mM, which is comparable to ocean conditions. In order to mimic dissolved organics in the ocean, present at approximately 70 μM, the system was subsequently doped with 200 mg of lipopolysaccharides (LPS), resulting in a minimum of 70 μM carbon based on a lipid A headgroup and keto-deoxyoctulosonate residue combined molecular weight of 6595

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Figure 6. (A) ssp-Polarized SFG spectra of 2.5 μm (top, green) and 150 nm (middle, green) sized aerosol particles collected after the addition of bacteria and phytoplankton to the Scripps wave flume and freshly dropcast material obtained by skimming the wave flume surface microlayer on the same day (light blue), all normalized to the maximum SFG signal intensity at 2950 cm−1. (B) Comparison of the ssp-polarized SFG spectrum of dropcast material from the wave flume (light blue) to an SFG spectrum collected from the Baltic Sea surface nanolayer (black), as well as water samples collected at depths of 1 m (dark gray) and 25 m (light gray), by Friedrichs and co-workers. Spectra are offset for clarity. Please see text for details.

could be due to the presence of cis-configured methylene groups, even though the spectral responses could also be due to exogenous line shape effects. As mentioned in section IIIE, on the basis of the SFG spectra shown in Figure 5C, we cannot definitely confirm the presence of LPS in the small particle size fractions, but if large quantities of LPS molecules were present on the particle surfaces, we would expect them to produce SFG spectra similar to those shown at the bottom of Figure 5C. B. Order and Disorder of the CH Oscillators. Provided that the SFG responses at 2850 and 2920 cm−1 indicate the presence of cis-configured methylene oscillators, the spectra shown in Figure 5 suggest that if alkyl chains are present at the interface, they are not well ordered. As mentioned in the previous section, alkyl chains produce vibrational SFG responses at 2880 and 2950 cm−1, attributable to the methyl symmetric and asymmetric stretches, with little to no contributions from the methylene symmetric and asymmetric stretches at 2850 and 2920 or 2900 cm−1.50,53,64 With this information at hand, we hypothesize here that the surfaces of the size-selected SSA particles studied in this work contain alkyl chains in various states of disorder and not well-ordered alkyl chains like those one would expect to find in a well-formed selfassembled monolayer.72 Furthermore, if attractive chain−chain interactions are important, then the SFG spectra would be consistent with the presence of patches of disordered alkyl chains, with other portions of the particle much less populated by alkyl chains. The larger footprint and space associated with disordered alkyl chains are likely to allow for diffusion of gas phase oxidants through the aerosol particle surface into the particle. C. CH Oscillators in the Wave Flume Surface Microlayer and on Aerosol Particle Surfaces. Figure 6A shows that the spectral features from the wave flume surface microlayer sample are similar to those of the 150 nm particles collected on the same day at the wave flume, although the methylene CH stretches at 2850 and 2920 cm−1 are somewhat more resolved in the wave flume surface microlayer sample than in the 150 nm particle sample. The spectral similarity is not necessarily surprising since particles that are ejected into the atmosphere through the breaking of waves are thought to be formed from bubbles, which scavenge organic material as they pass through this organic-enriched surface layer. The

evaporation of most of the liquid phase over an hour. The SFG spectrum displays responses at 2850, 2880, 2920, and 2950 cm−1, and the SFG intensity ratio at 2880 and 2950 cm−1 is approximately 1:2 (Figure 6). After the sample was allowed to dry further overnight, a second SFG spectrum was collected (Supporting Information). The signal intensities increased by more than a factor of 2 but the line shapes and peak ratios did not change.

IV. DISCUSSION A. CH Oscillators at the Aerosol Particle Surfaces. Surfaces containing long and extended alkyl chains generally produce SFG responses between 2950 and 2940 cm−1, attributable to the Fermi resonance of the CH3 symmetric stretch with a CH3 bending overtone and the CH3 asymmetric stretch,58−60 respectively, and an SFG response at 2880 cm−1, which is attributable to the CH3 symmetric stretch.60−62 Given these tell-tale vibrational signatures and given the welldocumented presence of methyl-terminated alkyl chains in marine surfactants,63 we tentatively assign the vibrational modes at 2950 and 2880 cm−1 that are seen in the SFG spectra shown in Figure 5 to methyl asymmetric/Fermi resonances and symmetric CH stretches, respectively. In general, methylene groups in aliphatic chains give rise to vibrational SFG signatures at 2850 and 2920 cm−1.43,51,62,64−70 Adjacent pairs of methylene groups that form long chain aliphatic molecules can be either cis- or trans-configured. If they are trans-configured, the motions of the methylene oscillators, described by their vibrational transition dipole moments, are opposite in phase and cancel, resulting in very small to negligible contributions to ssp-polarized SFG spectra.65 Intense SFG responses from methylene CH stretches can be observed in ssp-polarization when the adjacent pairs of methylene groups are cis-configured, which results in the in-phase addition of the transition dipole moments of the methylene oscillators. A large number density of cis-configured methylene groups along an aliphatic carbon chain generally indicates a high degree of molecular disorder, whereas well-aligned, zigzag shaped aliphatic chains would contain only trans-configured methylene groups.71 Given this information, the spectral intensity seen at 2850 and 2920 cm−1 of the SFG spectra obtained from the SSA particles collected at the wave flume suggests that those signals 6596

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similarity of the SFG spectra obtained from the wave flume surface microlayer and the 150 nm sized aerosol particles above it suggests that the CH oscillators located at the surfaces of the aerosol particles may be in similar chemical environments to those that are found in the sea surface microlayer. Figure 6A also shows that the SFG response of the wave flume surface microlayer sample does not match those of 2.5 μm particles collected on the same day. This observation suggests that a different mechanism exists for connecting organic material in the wave flume microlayer with that on the surfaces of those larger particles as compared to the smaller particles. We are not the only group who has applied vibrational SFG spectroscopy to natural marine samples. Specifically, Friedrichs and co-workers73,74 studied samples obtained in the Baltic Sea (Figure 6B) by SFG. While aerosol particles were not analyzed in that study, the Friedrichs group pioneered the use of SFG spectroscopy, albeit with a ps scanning system as opposed to our fs broadband system, to analyze the very top nanolayer of the sea surface microlayer, with surface water samples collected using a screen sampling technique. Friedrichs and co-workers also collected water samples from depths of 1 and 25 m in the Baltic Sea (Figure 6B), which show negligible signal, indicating the enrichment of organic material in the surface layer. Compared to our wave flume surface microlayer sample, the Baltic Sea surface nanolayer sample exhibits somewhat better resolved SFG signals in the symmetric CH stretching region. Both samples exhibit spectral intensity at frequencies approaching 2800 cm−1, which the Friedrichs group interpreted to be due to the presence of CH oscillators in carbohydrate head groups that might be present in the sea surface nanolayer.74 Both samples exhibit vibrational SFG resonances at 2850, 2880, 2920, and 2950 cm−1. The well-resolved methylene peaks in these spectra indicate that both samples contain organic species that are part of broad molecular orientation distributions. Yet, while these results suggest the presence of disordered organic species in both samples, we find that the ratio between the spectral peaks at 2880 and 2950 cm−1 is 1:2 for the dropcast wave flume surface microlayer sample; the SFG spectrum of the Baltic Sea surface nanolayer sample exhibits the inverse ratio, namely, 2:1. The differences in the spectra are attributable to the fact that the samples are from two different parts of the world (Pacific Ocean vs Baltic Sea) that were collected in different ways. In addition, the wave flume surface microlayer sample was obtained from filtered seawater collected off the Scripps Pier with added bacteria and phytoplankton. The Baltic Sea sample was not filtered, and no additional biological material had been added to it. Furthermore, the SFG spectra were recorded with femtosecond broadband vs picosecond scanning laser systems, respectively, and the sample geometries are arranged perpendicularly from one another. Finally, the spectral differences in the SFG spectra of the CAICE and the Baltic Sea samples can be attributed to the fact that the CAICE sample was dropcast while the Baltic Sea sample was recorded directly at the liquid/air interface following sample collection in the field and transport to their laboratory in Kiel, Germany. Despite the spectral differences noted here, which could be due to several factors, the organic species present in these two samples may be in similar molecular environments. D. Implications for Atmospheric Chemistry. As mentioned in the introduction, the wave tank allows us to bring complexity from the real ocean into the laboratory while controlling for a variety of exogenous parameters. Yet, even

under such controlled conditions, the exact chemical composition of SSA particles and their phase states are not known. The best experiment would be an online SFG spectroscopic study of individual particles or collections thereof, which are currently in the design phase, but not yet realized. In the absence of such a challenging experiment, our results thus far allow us to make the following conclusions and predictions about SSA particles collected at the SIO wave flume: (1) Some SSA particle models invoke the presence of surfactants in the topmost particle layer.75,76 The SFG results presented here support such models and allow for some refinement, namely, that the alkyl chains are likely to be disordered. This property may allow for the diffusion of reactive species between the aerosol gas and particle phases even in surfactant-rich SSA particles having particle sizes for which the Kelvin effect, i.e., size-dependent changes in the surface tension,77 is not relevant. SSA particles containing surface active species of biological origin, such as LPS, may then be involved in much more exchange of reactive and nonreactive species in the gas or the condensed phase than previously thought. (2) The comparison of the SFG spectra obtained from the SSA particle samples studied here in the various size ranges with SFG spectra obtained from air/water or air/fused silica interfaces containing oleic and palmitic acid78,79 or various model phospholipids35,80,81 suggests that one-component models consisting of fatty acids or phospholipids are not sufficient for describing the SFG responses of the CAICE samples. This finding is similar to the conclusions made by Friedrichs and co-workers73,74 for the case of the sea surface nanolayer sampled in the Baltic Sea and studied using SFG, but we show here that this concept also applies to SSA particles. To elaborate, fatty acids78,79 and phospholipids80,81 at the vapor/ D2O interface exhibit little to negligible SFG intensity at 2920 cm−1, while the SFG spectra of the particle samples collected during CAICE 2011 all exhibit substantial SFG intensity at that frequency. In addition, the ratio of the SFG peaks at 2880 and 2950 cm−1 in the SFG spectra of the phospholipids is 2:1, whereas the 150 nm and 2.5 μm sized particles collected during CAICE 2011 exhibit a ratio of 1:2 (the latter of which decreases to 1:1 after bacteria and phytoplankton were added to the flume). Furthermore, SFG spectra of 1,2-distearoyl-sn-glycero3-phosphocholine bilayers on fused silica80 exhibit negligible SFG intensity at 2920 cm−1 and a ratio of the SFG peaks at 2880 and 2950 cm−1 of 1.2:1. (3) While it has been known that the transfer of organic material into macroscopic bubbles produced during wave action is a function of bubble size,6 the SFG spectra presented here indicate that such a size-dependent mechanism is likely to operate at the nanoscale level of sea spray aerosol particles as well: the SFG responses obtained from a sample collected from the wave flume surface microlayer do not match those of 1.0 and 2.5 μm particles collected on the same day, whereas the SFG responses from particles collected in the 150 nm size range do.

V. NEEDS FOR ADDITIONAL RESEARCH While we have interpreted the results presented here to produce a refined working model of the surfaces and interfaces of SSA particles, we also stated that there are many unknowns that remain to be addressed. Given that there is no silver bullet to solve complexity problems, addressing these unknowns will 6597

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success and impact. Furthermore, combining research directions 1 and 2 will allow for a quantification of mass transport through the disordered surfactant layers that are likely to separate the various bulk phases present in externally mixed SSA particles and lead to refined parametrization approaches such as the one first proposed by Russell and Seinfeld.86

require integrated experimental and computational approaches. In this section, we briefly discuss two new scientific directions that arise from our study and highlight needs for additional research directions for studying SSA particles. Research Direction (1): As stated in the introduction, the exact chemical composition of SSA particles and how this exact composition changes during the SSA particle lifecycle are not known. Work carried out during CAICE 2011 has identified several surfactants that are possibly present in the SSA particles prepared at the wave flume (lipopolysaccharides (LPS), sodium dodecyl sulfate, and glycine), and the results present in this current work are consistent with the presence of LPS on the surfaces of the smaller size fractions of the SSA particles. However, SFG spectroscopy alone cannot definitely verify the presence of LPS at the SSA particle surfaces. Clearly, more experiments are needed that report on the identity and spatial distribution of organic species in and on SSA particles. The challenge in this work is to carry out such studies under ambient conditions, i.e., without pumping on the sample or cooling it. Spectroscopies that probe vibrational modes while having the appropriate sensitivities and limits of detection (for instance, STXM, Raman microspectroscopy, or SFG spectroscopy) as well as mass spectroscopic approaches will play important roles to reach this goal, but only if those results can be compared to those obtained from spectral libraries of model compounds and their mixtures, present in various phase states. Being still far away from building atomistic models of the SSA particle surface or the interfaces within mixed-phase SSA particles, we also know now that such models need to allow for the exploration of many configurations that can be sampled in a disordered system. Computational studies, via coarse grained, molecular dynamics, or Monte Carlo approaches, will play a key role in the quantification of how important molecular disorder is in the physical and chemical transformations involving SSA particles. One such approach,82,83 focusing on the hydrogen bond dynamics and vibrational spectroscopy of bulk water, will be an important starting point for interpreting the experimental results discussed here. Research Direction (2): As pointed out in the introduction and also in section II.D, it is challenging to study the heterogeneous chemistry of mixed-phase SSA particles because of the multiphase problem highlighted by Ravishankara5 and also Rudich et al.84 Upon further reflection, this problem is really a many interfaces problem. In section III.A, we state that SSA particles below 300 nm include a population of spherical particles, mainly consisting of organic material, that contain little to no inclusions, but we also provide estimates for how many of the particles we sample contain phase-separated liquid and/or solid material under experimental conditions. In section II.D, we examine SFG responses from SSA particle samples containing predominantly mixed-phases with those obtained from mainly single-phase particle samples and state that internal interfaces are unlikely to contribute significantly to the detected SFG response in SSA particles. Yet, to quantify what percentage of the detected SFG signal intensity originates from external surfaces vs internal interfaces of such complex, mixed-phase particles will require the synthesis of meaningful model systems having different particle architectures and chemical compositions. A reasonable starting point would be the synthesis of core−shell and also onion-like particles, readily achieved using colloid synthesis. Such SSA particle models will build on the important single-component fatty acid systems that were first employed by Moise and Rudich,85 with much

VI. CONCLUSIONS In conclusion, we have identified CH oscillators in samples of filter-deposited sea spray aerosol particles collected at the wave flume of the Scripps Hydraulics Laboratory during the CAICE 2011 campaign. Vibrational SFG spectra show the presence of methyl and methylene groups, as well as OH oscillators and differ from SFG spectra of common phospholipids at interfaces. These findings support SSA particle models that invoke the presence of surfactants in the topmost particle layer75,76 and indicate that the alkyl chains of surfactant-rich SSA particles are disordered. Specifically, the SFG spectra shown here suggest that across the range of sizes studied here, surfactant-rich SSA particles contain CH oscillators that are subject to molecular orientation distributions that are broader than the narrow molecular distribution functions associated with well-ordered and well-aligned alkyl chains. This result is then consistent with the interpretation that the permeability of organic layers at SSA particle surfaces to small reactive and nonreactive molecules may be substantial, allowing for much more exchange between reactive and nonreactive species in the gas or the condensed phase than previously thought. The SFG data presented here also suggests that a one-component model is not sufficient for describing the SFG responses of the SSA particles collected during CAICE 2011. Finally, the similarity of the SFG spectra obtained from the wave flume microlayer and 150 nm-sized aerosol particles above it suggests that the SFG active CH oscillators in the topmost layer of the wave flume and the particle fine mode may be in similar chemical environments.

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ASSOCIATED CONTENT

S Supporting Information *

SFG spectra from multiple laser spots on various MOUDI collection filters, of the wave flume surface microlayer, and of homogeneous and heterogeneous aerosol particles. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.M.G.); kprather@ ucsd.edu (K.A.P.); [email protected] (V.H.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation through a Phase I Center for Chemical Innovation under Grant No. CHE1038028. C.J.E. gratefully acknowledges an NSF Graduate Research Fellowship. F.M.G. gratefully acknowledges support from an Irving M. Klotz professorship in physical chemistry. We also thank Spectra Physics, a Division of Newport Corporation, for equipment loans and donations as well as superb technical support. We thank all those involved with the CAICE intensive campaign, including Dr. Luis Cuadra-Rodriguez, Dr. Timothy Guasco, Dr. 6598

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Defeng Zhao, Douglas Collins, Prof. Timothy Bertram, Michelle Kim, Olivia Ryder, Dr. Grant Deane, Dr. Dale Stokes, Prof. Farooq Azam, and the SIO Hydraulics Laboratory staff. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

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