Tip-Enhanced Raman Spectroscopy of Atmospherically Relevant

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Tip-enhanced Raman Spectroscopy of Atmospherically relevant Aerosol Nanoparticles Johannes Ofner, Tanja Deckert-Gaudig, Katharina A. Kamilli, Andreas Held, Hans Lohninger, Volker Deckert, and Bernhard Lendl Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02760 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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Analytical Chemistry

Tip-enhanced Raman Spectroscopy of Atmospherically relevant Aerosol Nanoparticles Johannes Ofner*,†, Tanja Deckert-Gaudig‡, Katharina A. Kamilli&, Andreas Held&, Hans Lohninger†, Volker Deckert*,‡,§, and Bernhard Lendl† †

Institute of Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria. Leibniz Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany. & Atmospheric Chemistry, University of Bayreuth, Dr.-Hans-Frisch-Straße 1-3, D-95448 Bayreuth, Germany. § Institute of Physical Chemistry and Abbe Center of Photonics, University of Jena, Helmholtzweg 4, 07743 Jena, Germany. ‡

ABSTRACT: Atmospheric aerosol nanoparticles play a major role in many atmospheric processes and in particular in the global climate system. Understanding their formation by homogeneous or heterogeneous nucleation as well as their photochemical aging and atmospheric transformation is of utmost importance to evaluate their impact on atmospheric phenomena. Single particle analysis like tip-enhanced Raman spectroscopy (TERS) opens access to a deeper understanding of these nanoparticles. Atmospherically relevant nanoparticles, formed above a simulated salt lake inside an aerosol smog-chamber were analyzed using TERS. TERS spectra of eleven nanoparticles were studied in detail. First results of TERS on atmospherically relevant aerosol nanoparticles reveal the presence of inorganic seed particles, a chemical diversity of equally sized particles in the nucleation mode and chemical transformation during photochemical aging. Therefore, single particle analysis by optical near-field spectroscopy such as TERS of atmospheric nanoparticles will significantly contribute to elucidate atmospheric nucleation, photochemical aging and chemical transformation processes by uncovering single particle based properties.

Atmospheric aerosol particles play a crucial role in the global climate system.1 The contribution of secondary organic aerosols (SOA), which are formed from biogenic or anthropogenic precursor gases through gas-to-particle conversion in the atmosphere, is still afflicted with a high uncertainty. As outlined by the International Panel on Climate Change1, the radiative forcing due to organic aerosol-radiation interactions varies from -0.4 to 0.1 Wm-2. While the global SOA production is estimated to be only up to 380 Tg yr-1 (compared to sea spray aerosol with up to 6800 Tg yr-1 or mineral dust with up to 4000 Tg yr-1)1, various physical (e.g., light scattering and absorption), physicochemical (e.g., coating, formation of cloudcondensation and ice nuclei) and chemical properties and transformation processes of SOA (e.g., photochemical aging) can influence the climate system. Large uncertainties in the SOA contribution to radiative forcing result from a large variety of different sources and formation processes. Atmospheric SOA particles, formed by secondary aerosol formation, grow from the nano-meter scale (so-called nucleation mode) through the so-called Aitken mode (2080 nm particle diameters) to the accumulation mode (801000 nm). During the formation process and subsequent photochemical aging, organic precursors and SOA particles permanently undergo chemical transformation (e.g., oxidation) and related changes of the above mentioned (physico-)chemical properties.2,3 A fundamental, comprehensive understanding of processes leading to the formation of

atmospheric nanoparticles and SOA transformation is of utmost importance for evaluating the atmospheric SOA burden and its climate impact3,4. However, the investigation of the chemical composition of atmospheric nanoparticles is still a challenging task, due to their size and the related limited amount of mass per particle. Atmospheric SOA nanoparticles are often analyzed off-line after sampling on filters or by impaction. Various methods have been applied to study the chemical composition of precipitated SOA particles generated from aerosol smogchambere.g.,5,6,7,8 and flow-reactore.g.,9,10,11 experiments or obtained from ambient samplinge.g.,12,13. The current workhorse in online SOA analytics is the so-called aerosol mass spectrometer (AMS).14,15 The bulk chemical evolution of SOA nanoparticles can be followed by metrics such as van Krevelen diagrams (exhibiting the relation between H/C and O/C mass ratios)16, ion factorization (e.g. f43 and f44)14 or the averaged carbon oxidation state17, which can be easily derived from AMS measurements. However, all these methods deliver averaged chemical information of bulk samples of SOA nanoparticles. Atmospheric SOA particles exhibit features, which are based on their individual chemical composition, e.g. size-selective effects4, partitioning of semi-volatile compounds18 or even formation in complex heterogeneous environments19. Single particle analysis can address questions related to single-particle features and the variability of atmospheric aerosols as well as size- and species-selective effects. Several methods have been de-

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veloped for single aerosol particle characterization, including atomic force and electron microscopy20,21, vibrational spectroscopy22,23,24,25, elemental composition analysis21,23,24, and mass spectrometry24,26,27. Imaging-based analysis has recently been extended to a combinational approach of several techniques.28 While all these techniques contribute to single particle analysis, almost all of them fail at single-particle analysis of nucleation-mode aerosol due to the limited sample mass and volume. Only scanning probe techniques such as Atomic Force Microscopy (AFM)29 or scanning electron microscopy (e.g. fieldemission-gun SEM)7, which is not bound to the optical limit of diffraction, allow to characterize the morphology of individual nanoparticles but with only limited information of their chemical composition. Surface-enhanced Raman Spectroscopy (SERS) has recently been successfully applied to unravel previously undetectable SOA compounds.30 In SERS the intrinsically weak Raman signal is enhanced by interactions of the sample with rough metal nanoparticles. In this way a more sensitive detection of individual aerosol particles can be achieved. On the other hand, SERS lacks high lateral resolution and the nucleation mode of single particles remains undiscovered. Tipenhanced Raman spectroscopy (TERS), which combines the lateral resolution of a Scanning Probe Microscope (SPM) with the information content and signal enhancement of SERS31,32 should be able to investigate the morphology and the chemical composition of single nucleation-mode nanoparticles, simultaneously. The potential of TERS has been demonstrated in a variety of samples in chemistry and biology33,34,35,36. In the present study TERS is -to the best of our knowledge- the first time used to analyze single precipitated nucleation-mode nanoparticles formed under conditions typical of atmospheric secondary aerosol formation.

METHODOLOGY Simulated Salt Lake. Western Australian salt lakes have been identified as natural sources of atmospheric nanoparticles.37 These environments are characterized by complex multiphase chemistry and multiple contributions to the atmospheric aerosol population. Aerosol multiphase chemistry is a challenging task in atmospheric science, especially in case of nanoparticle formation.38,39 To study the formation of atmospheric nanoparticles in these multiphase environments in detail, and to analyze the chemical signature of the released nanoparticles, aerosol smog-chamber experiments, mimicking salt lake conditions, were carried out.40 Therefore, a simulated salt lake was installed inside an aerosol smog-chamber. The liquid phase of the simulated salt lake was composed of NaCl, Na2SO4 with CaSO4 impurities and Eucalyptus globulus oil (85 % 1,8-cineol and 15 % limonene) to simulate the organic impact of degrading eucalyptus trees. The chemical environment is described in detail by Kamilli et al. (2015).40 The spectral actinic flux inside the smog-chamber mimicks a solar spectrum comparable to the area of the Mediterranean Sea in summer.11 Nanoparticle formation41 and the chemical composition of larger particles (diameter between 0.5 and 20 µm) sampled in Western Australia have been analyzed so far using Raman micro-spectroscopy, electron microscopy, energy dispersive x-ray spectroscopy and ultra-high

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resolution mass spectroscopy.28,41 During the smogchamber experiments, particles were sampled onto previously cleaned microscopy glass cover slips (18x18 mm with 0.17 mm thickness; previously cleaned for 2.5 hours using a mixture of HNO3/H2O2 (3:1)) using a Sioutas cascade impactor (SKC, Pennsylvania, USA). AFM overview images (figure 1, above) exhibit a general surface roughness of the glass cover slips of Rq of 0.184 nm, while atmospherically relevant aerosol nanoparticles exhibit a height of at least 1.5 nm. Tip-enhanced Raman Spectroscopy. For the TERS measurements samples of the lowest impactor stage (with a 50% cut-point diameter of 250 nm at 9 Lmin-1 sampling) were selected. While nucleation-mode particles are generally too small for efficient collection by impaction, several individual nanoparticles, which are supposed to have settled by diffusion, were found on the cover slip, close to the central line of impaction. TERS spectra of single atmospheric nanoparticles were acquired using a homemade setup42 equipped with a 532 nm laser and 900 µW laser power at the sample using an acquisition time of 1- 5 s. Agisland coated AFM tips43,44 were used in intermittentcontact mode to image the topography and to acquire TERS spectra. For the measurements a 40x oil immersion objective with a NA 1.35 was used. The laser spot diameter can be calculated around 200 nm. Due to a permanent slight defocusing for TERS spectra acquisition it can be assumed that the actual size was around 500-1000 nm. With this value the irradiance can be estimated to 0.1-2 MW/cm2. Data Processing. AFM images of the atmospheric nanoparticles were analyzed using the software package Gwyddion 2.40. TERS spectra were obtained from eleven nanoparticles by point-wise (in terms of grids) measurements. All obtained Raman spectra were analyzed using the software package Imagelab (Epina GmbH), which was also used to evaluate TERS active spectral regions.

RESULTS AND DISCUSSION AFM topographic images exhibited a waxy nature of the precipitated particles, which is indicated by the dropletlike deposition pattern (figure 1). Based on the AFM topography, the volume was estimated and an averaged volumeequivalent spherical diameter was calculated. Six particles with spherical diameters between 10 and 20 nm were assigned to the aged nucleation mode.45 The remaining five particles (20 to 100 nm) were allocated to the Aitken and accumulation modes.46 Aerosol formation was carried out at high spectral actinic fluxes in an oxidizing atmospheric environment as mentioned in the methodology section. Hence, a further photochemical processing of the nanoparticles by the Raman laser was not expected.


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Analytical Chemistry ly correlate to species concentrations compared to common vibrational bands, the use of the familiar notation (weak, medium, strong) of intensity interpretation was refused. To compare the presence of bands and related functional groups on a semi-quantitative base, the appearance of bands was evaluated by a relative abundance. The presence of a TERS band related to all TERS active spots is represented by a scale ranging from very rare to very often. TERS spectra were separated from all acquired spectra by analyzing enhanced Raman bands in the spectral regions 600 to 900 cm-1 and 1050 to 1800 cm-1. Between 8 and 30 % of the acquired spectra per particle, with 225400 spectra recorded of each, exhibited TERS spectra; all other spectra exhibited an unspecific organic-like characteristic. This might be caused by a loss of feedback of the tip to the surface of the nanoparticles and therefore a loss of the field-enhancement of TERS.

Figure 1. 3-D AFM topography overview of a typical sampling area and AFM topographic images of selected atmospheric nanoparticles of the nucleation, Aitken and accumulation modes and volume-equivalent calculated spherical diameters.

To understand the chemical nature of the nucleation mode, TERS spectra of single nucleation mode particles were analyzed in detail. According to the potential expansion of selection rules in TERS47,48,49, not only Raman bands but also bands corresponding to infrared active vibrational modes were taken into account. TERS spectra exhibit a variety of overlapping bands. Therefore, the assignment of functional groups to TERS bands was assured by two ways: Only functional groups with a complete set of band assignments were taken into account and assigned functional groups were correlated with results obtained from classical analytical methods, as reported by Kamilli et al. (2015).40 Due to the fact that TERS bands do not necessari-

Figure 2. AFM topography image of a 12.5 nm aerosol particle with a measurement grid and TERS active spots (black dots) in the upper left. About 18 out of 33 TERS-active spots exhibited significant contributions of carbonyl vibrations. Spectra of the three most intensive spots are displayed and interpreted in detail.

The spectra of the single nanoparticles reveal that the nucleation mode exhibits an organic-like nature (figure 2). The presence of symmetric and asymmetric vibrations of SO2 and -CO2- containing functional groups indicates that oxidized organic sulphur-containing species like sulphones and sulphates as well as oxidized carbon species like carbonates and carboxylic acids dominate as shown in the selected spectra in figure 2. A possible presence of nitrogen oxides is indicated by bands in the 1250-1565 cm-1 region. Vibrational features that can be assigned to the organic precursor molecules are hardly visible because the characteristic δ(ring) mode of 1,8-cineol at about 660 cm-1 cannot be observed. The detection of δ(CH2) and δ(CH3) modes between 1420-1480 cm-1 point to the presence of aliphatic compounds, most likely from the degradation of the original Eucalyptus globulus oil. These observations denote a ring-opening process with subsequent oxidation of the ether group of the precursor molecule (1,8-cineole)



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under the oxidizing conditions in and above the simulated salt lake. Noteworthy, these TERS-based spectroscopic observations of formed functional groups are in good agreement with conventional Raman spectra of bulk samples and larger particles and related high-resolution mass spectroscopic measurements.40 This chemical relationship indicates a similar formation mechanism of the organic fraction of the larger particles (analyzed by micro spectroscopic methods) and the nucleation-mode particles (analyzed by TERS).

Figure 3. Demonstration of the variations of the presence of carbonyl stretch vibrations for the six individual nucleationmode particles in selected TERS spectra. The measured particles exhibit an inhomogeneous presence of different carbonylcontaining functional groups.

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Further, organic of atmospheric nucleation50. 51 carboxylates play an important role, which is confirmed by the presence of bands in the spectral regions of νs(CO2-) and νas(CO2-) at 1335-1440 and 1540-1695 cm-1. Additionally, a contribution of organic nitrates and nitrocompounds is visible (org. nitrates: νas(NO3) at 1610-1660 cm-1 and νs(NO3) at 1250-1300 cm-1; org. nitrocompounds: νas(NO2) at 1505-1565 cm-1 and νs(NO2) at 1360-1385 cm-1). Highly oxidized species52 are further visible by the detection of marker bands of ν(C=O) vibrations of carboxylic acids, esters and carbonates above 1700 cm-1. Possible contributions of inorganic sulfates (CaSO4) were found on all particles with varying band intensities, indicating a possible contribution of inorganic sulfates to the composition of the nucleation-mode nanoparticles. Single particle spectroscopy of the nucleation mode confirms general findings from bulk analytical analysis performed using Raman micro-spectroscopy and ultra-highresolution mass spectrometry.40 However, the main spectral features could directly be allocated to single nanoparticles beside the overall analysis of e.g. a bulk filter or impactor sample. All observed Raman vibrations of the analyzed nucleation-mode nanoparticles support a common formation process. Slight variations of sulfur-, nitrogen-, carbon- and oxygen- containing species indicate a unique processing of each single nanoparticle. Two spectral regions have been analyzed in detail. The sulfur-rich and acidic environment of the simulated salt lakes allows the formation of inorganic sulfates. A characteristic inorganic sulfate band at 1018 cm-1 (possibly CaSO4) was found on several nanoparticles with varying intensities and was present in 2-33 % of all acquired spectra per particle. The abundance of carbonyl groups of esters, carbonates and carboxylic acids varied between the six analyzed nanoparticles of the nucleation mode (Figure 3). Carbonyls were determined in 4-60 % of the acquired TERS spectra per particle. On two particles no carbonyltypical stretch vibration could be detected. These variations in the carbonyl content indicate an inhomogeneous organic composition of the nucleation mode. The variable organic composition of individual nucleation-mode nanoparticles as observed in the acquired TERS spectra suggests that different oxidation pathways contribute to SOA formation simultaneously. However, a more detailed analysis of oxidation states of single nucleation-mode particles is necessary to verify the variable composition of nucleation-mode atmospheric SOA particles.

Six particles, which exhibit comparable spherical diameters between 10.9 and 16.1 nm allocated to the aged nucleation mode45 were analyzed in detail (Table 1). All particles exhibit Raman bands in the spectral region of the ν(CS) or ν(C-Cl) stretch vibrations. Due to fact that νs(SO2) and νas(SO2) of sulfates and sulfones are significantly represented in the TERS spectra, the assignment of the ν(C-S) to this spectral region is more likely. In general, all particles appear to be dominated by sulfur-containing species, which is consistent with sulfuric acid being the key driver


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Table 1. Assignment of TERS bands to vibrational modes for six nucleation mode particles: observed occurrence (implying relative intensities and frequencies) in all TERS spectra: vr: very rare, r: rare, m: medium, o: often, vo: very often, np: band is not present in the related spectra.

Vibrational modes

Group frequencies & Raman / IR intensities (cm-1)

Spherical particle diameter (nm) and number of TERS active spots 10.9 ±1.7 (44)

11.8 ±1.5 (24)

12.5 ±2.0 (33)

14.2 ±2.9 (120)

15.7 ±4.2 (52)

16.1 ±1.3 (38)

580-720 (s/w)/505-760 (s/s)

m

m

m

o

o

o

915-1000 (m-s/m-w)

r

o

r

r

np

np

1018 (s/-)

o

m-o

np

m

m

o

νs(SO2) of org. salts

1075-1140 (s/m)

m

m

r

np

o

o

νs(SO2) of sulfones

1120-1220 (s/vs)

o

o

m

m

o

m

νs(SO2) of org. sulfates

1185-1200 (s/vs)

o

o

m

o

m

1220-1315 (s-m/vs)

o

m-o

o

M

o

o

νs(NO2) of org. nitrates

1250-1300 (s/s)

o

o

m

np

o

r

νas(SO2) of sulfones

1270-1390 (v/vs)

o

m

m-o

m

o

o

νs(NO2) of org. nitro comp.

1360-1385 (s/vs)

o

o

m

o

m

1370-1415 (s-m/s)

o

o

m

vo

o

m

δs(CH)

1370-1390 (w-m/m-s)

o

o

r

m

o

m

νs(CO2-) of org. carbox. acid salts

1335-1440 (m-s/m-s)

o

o

m

o

o

o

δas(CH)

1440-1465 (m/m)

o

o

m-o

m

m

r

νas(NO2) of org. nitro comp.

1505-1565 (m-s/s)

m

o

o

np

o

m

νas(CO2-) of org. carbox. acid salts

1540-1695 (w/s)

o

o

o

m

o

o

νas(NO2) of org. nitrates

1610-1660 (s/s)

o

o

o

vo

o

ν(C=O) of carboxylic acids

1700-1740 (w-m/vs)

r

o

o

m

o

vr

ν(C=O) of esters & carbonates

1720-1760 (w-m/vs)

np

r

m

m-o

o

np

ν(C-S) / ν (C-Cl)

ν (C-N) inorganic sulfate (CaSO4)

νas(SO2) of org. salts

νas(SO2) of org. sulfates


np

np

np


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Table 2. Assignment of TERS bands to vibrational modes of Aitken-(20-80 nm) and accumulation- (80-100 nm) mode nanoparticles with their observed occurrence (implying relative intensities and frequencies) in all TERS spectra: vr: very rare, r: rare, m: medium, o: often, vo: very often, np: band is not present in the related spectra.

Vibrational modes

Group frequencies & Raman intensities (cm-1)

Spherical particle diameter (nm) and number of TERS active spots 20.9 ±4.0 nm (47)

34.2 ±1.5 nm (27)

48.3 ±4.0 nm (41)

50.0 ±2.4 nm (21)

97.6 ±2.1 nm (23)

580-720 (s/w)/505-760 (s/s)

o

r

m

o

o

inorganic sulfate (CaSO4)

1018 (s/-)

m

r-m

m

m

r

ν(C=O) of carboxylic acids

1700-1740 (w-m/vs)

r

r-m

m-o

o

o

ν(C=O) of esters & carbonates

1720-1760 (w-m/vs)

np

np

m-o

o

o

ν(C-S) / ν (C-Cl)

indicated by an enhanced presence of carbonyl stretch vibrations (Table 2 and Figure 4), the dominance of the inorganic sulfate decreases (Table 2). This suggests that the inorganic sulfate of larger particles is covered by condensing organic compounds, and therefore hardly detectable by TERS, due to the techniques extreme surface sensitivity.53 Thus, organic compounds are suspected to play a key role in particle growth, which causes an increasing contribution to TERS spectra from the nucleation mode to the Aitken and accumulation modes. In particular, sulfurcontaining oxidized organic compounds contribute to SOA formation, as indicated by carbon-sulfur stretch vibrations (ν(C-S), Table 2).

CONCLUSION

Figure 4. AFM topographic images of typical analyzed Aitken and accumulation mode particles with selected TERS spectra. The accumulation mode particle exhibits the typical composed structure. The TERS spectra indicate the evolution of vibrational features related to carboxylic acids, esters and carbonates with increasing particle size.

Larger particles of the Aitken and accumulation modes (cf. Figure 4) may have grown from the nucleation mode by condensation and/or coagulation. Some of these particles are composed of several nucleation mode particles (Figure 4, 50.0 and 97.6 nm particles), and exhibit TERS spectra similar to individual nucleation mode particles. Further, the calculated spherical diameter (dsph) differs from the visually expected diameter. However, while the oxidation state increases with increasing diameter, as

By analyzing TERS spectra obtained from eleven nanoparticles in the nucleation, Aitken and accumulation modes, several features of secondary aerosol formation under atmospherically relevant conditions are uncovered. In general, the formation of SOA nanoparticles with their chemical fingerprint related to the sulfur-rich and acidic environment of the simulated salt lake could be verified using TERS on a single nanoparticle basis. Spectral interpretation and assignment of TERS spectra of single nanoparticles is consistent with previous studies.40 Therefore, TERS confirms the basic composition of the formed SOA, and additionally deepens the understanding of SOA nanoparticle formation by single particle analysis. The specific surface sensitivity54 of TERS will allow to deepen the understanding of chemical reactivity of atmospheric nanoparticles with gas-phase components and therefore assist the interpretation of atmospheric behavior of these particles. Moreover, the comparison of single nanoparticle spectra reveals the chemical variability within the nucleation mode. Bulk analytical methods do not allow studying these variations of the nucleation mode. Inorganic sulfate is mainly present in the nucleation-mode nanoparticles but hardly detectable within the TERS spectra of larger parti-


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cles. This can be explained by the fact that TERS is a surface sensitive technique, which does not reveal the inorganic core of larger particles totally covered by organic condensing material. Consequently tip-enhanced Raman spectroscopy of atmospheric nanoparticles provides access to a deeper understanding of the chemical composition and diversity of aerosol resulting from atmospheric nucleation. Single particle analysis allows identifying the relevant chemical mechanisms of secondary aerosol formation, evaluating the chemical variability of aerosol populations as well as characterizing the chemical signatures of photochemical aging and atmospheric transformation processes. However, for atmospheric secondary aerosol formation, a large, representative ensemble of atmospheric nanoparticles has to be analyzed using a combination of nano-spectroscopic techniques such as TERS, AFMInfrared spectroscopy, and SNOM (scanning-nearfieldoptical-microscopy)-Raman spectroscopy. Moreover, such a complex, multi-dimensional data set requires a general statistical approach to evaluate the obtained spectra and allocate spectral features to size- and time-dependent properties in the atmosphere. The first demonstration of TERS on atmospherically relevant nanoparticles presented in this study bears great potential, and suggests near-field optical spectroscopy being a possible target technology for elucidating nanoparticle formation and transformation processes.

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AUTHOR INFORMATION (6)

Corresponding Author

* Johannes Ofner: TU Wien, Institute of Chemical Technologies and Analytics, Getreidemarkt 9, johan1060 Vienna, Austria; [email protected]; +43 (0)1 58801 – 15177. * Volker Deckert: University of Jena, Institute of Physical Chemistry and Abbe Center of Photonics, Helmholtzweg 4, 07743 Jena, Germany: [email protected]; +49 (0)3641 – 9 48347. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Samples were prepared in Bayreuth by Johannes Ofner, Katharina A. Kamilli and Andreas Held. TERS experiments were carried out in Jena by Tanja Deckert-Gaudig, Johannes Ofner and Volker Deckert. Evaluation of the datasets was assisted by Hans Lohninger.

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ACKNOWLEDGMENT Funding by the German Research Foundation (DFG) grant HE 5214/5–1 and within the DFG research group HaloProc (FOR 763) is gratefully acknowledged. Further the authors like to thank the Austrian Research Promotion Agency within the K-project imPACts (FFG 843546) for additional funding.

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Figure 1. 3-D AFM topography overview of a typical sampling area and AFM topographic images of selected atmospheric nanoparticles of the nucleation, Aitken and accumulation modes and volume-equivalent calculated spherical diameters. 104x203mm (150 x 150 DPI)



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Figure 3. Demonstration of the variations of the presence of carbonyl stretch vibrations for the six individual nucleation-mode particles in selected TERS spectra. The measured parti-cles exhibit an inhomogeneous presence of different carbonyl-containing functional groups. 114x181mm (150 x 150 DPI)


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Figure 4. AFM topographic images of typical analyzed Aitken and accumulation mode particles with selected TERS spectra. The accumulation mode particle exhibits the typical composed structure. The TERS spectra indicate the evolution of vibrational features related to carboxylic acids, esters and carbonates with increasing particle size. 175x173mm (150 x 150 DPI)


Tip-Enhanced Raman Spectroscopy of Atmospherically Relevant

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