# In situ analysis of single secondary organic aerosol from the ozonolysis of α-pinene using laser trapping and Raman spectroscopy

Naoto Tyumyo, Gwendoline Bourdon, Eric Villenave, Sophie Sobanska, Shoji Ishizaka

► To cite this version:

Naoto Tyumyo, Gwendoline Bourdon, Eric Villenave, Sophie Sobanska, Shoji Ishizaka. In situ analysis of single secondary organic aerosol from the ozonolysis of α-pinene using laser trapping and Raman spectroscopy. Chemosphere, 2025, 385, pp.144581. 10.1016/j.chemosphere.2025.144581. hal-05363513

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Distributed under a Creative Commons Attribution 4.0 International License
In situ analysis of single secondary organic aerosol from the ozonolysis
of α-pinene using laser trapping and Raman spectroscopy

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Naoto Tyumyo<sup>a</sup>, Gwendoline Bourdon<sup>b,c</sup>, Eric Villenave<sup>b</sup>, Sophie Sobanska<sup>c</sup> and Shoji Ishizaka<sup>a,*</sup>

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a Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japan

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b Univ. Bordeaux, CNRS, EPOC, UMR 5805, F-33600 Pessac, France

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* Corresponding author:
Shoji Ishizaka – ORCID ID: 0000-0001-9866-5809
Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japan
E-mail: ishizaka@hiroshima-u.ac.jp

Note: Naoto Tyumyo and Gwendoline Bourdon are co-first authors who have contributed equally to this work.

Acknowledgments

This research was supported by the JSPS KAKENHI Grant Numbers JP21H01964. The authors would like to thank the Agence Nationale de la Recherche (ANR) for supporting the SOAPHY project (ANR-21-CE29-0031), the Région Nouvelle Aquitaine for funding the LEVIAERO project (convention n°AAPR2022-2021-17126210) and the INSU LEFE Program of the CNRS for supporting the LEVITEAU project. Gwendoline Bourdon thanks the University of Bordeaux for the international exchange grant.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

* Corresponding author:

28 Shoji Ishizaka – ORCID ID: 0000-0001-9866-5809

29 Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japan

31 E-mail: ishizaka@hiroshima-u.ac.jp
1 A novel approach for analyzing secondary organic aerosol particles is demonstrated

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3 The first laser trapping of single α-pinene SOA particles was successfully achieved

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5 Raman spectroscopy was employed to analyze the chemical composition of α-pinene SOA

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7 Peroxide-containing compounds in α-pinene SOAs were directly detected for the first time

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9 A powerful tool for analyzing SOA formation and aging mechanisms in the atmosphere
<figure><img src="image_2.png" /></figure>

1 Abstract

2 To form secondary organic aerosols (SOAs) from volatile α-pinene via ozone oxidation reactions, the

3 generation of low-vapor pressure compounds capable of acting as condensation nuclei is essential. Both

4 laboratory and field studies have identified low-volatile dimers as key components in SOA formation.

5 Although mechanisms involving stabilized Criegee intermediates and peroxy radicals have been proposed

6 for dimer formation in the gas phase, the detailed processes of SOA formation remain widely debated due

7 to the high diversity of reaction products and the inherent complexity of their formation mechanisms. To

8 clarify certain steps of the SOA formation mechanism from ozone-initiated oxidation of α-pinene, a laser

9 trapping technique coupled with Raman spectroscopy was employed for *in situ* analysis of individual SOA

10 particles. In this study, single micrometer-sized α-pinene SOA particles were successfully trapped in the

11 gas phase. To our knowledge, this represents the first successful application of laser trapping of α-pinene

12 SOA particles directly in the gas phase. An intense Raman peak observed at about 852 cm⁻¹ was attributed

13 to the O–O stretching vibration of peroxides. This finding provides direct evidence for the presence of

14 peroxy functional groups in the dimers, likely formed via stabilized Criegee intermediates. These results

15 demonstrate the capability of laser trapping combined with Raman spectroscopy to detect specific

16 molecular features, such as peroxide bonds, within α-pinene-derived SOAs in the gas phase.

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18 Keywords: Aerosol, Raman spectroscopy, Laser trapping, α-pinene, ozonolysis, SOA

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20 **1. Introduction**

21 During their atmospheric transport, volatile organic compounds (VOCs) may undergo oxidation reactions
22 by ozone, hydroxyl and nitrate radicals, leading to the formation of secondary organic aerosols (SOAs),
23 which have significant impacts on human health and climate change. Among VOCs, α-pinene, emitted
24 primarily by coniferous trees, is recognized as one of the major biogenic precursors of SOAs worldwide,
25 because of its high emission and reactivity. Following ozone-initiated oxidation processes, the formation
26 of SOAs from volatile α-pinene requires that some of the oxidation products possess sufficiently low vapor
27 pressures, enabling them to condense and contribute to particle growth as condensation nuclei (Frosch et
28 al., 2011; Renbaum-Wolff et al., 2016). The reactivity of α-pinene with major atmospheric oxidants has
29 been extensively studied and an extensive chemical kinetic evaluation is reported in the IUPAC database
30 (Mellouki et al., 2021), enabling most of the main stable components of the SOA to be predicted (Lee et
31 al., 2006; Zhang et al., 2015). In previous studies, SOA particles were collected on filters and then extracted
32 with solvents (Camredon et al., 2010; Christoffersen et al., 1998; Kahnt et al., 2018; Kamens et al., 1999;
33 Lee and Kamens, 2005; Ma et al., 2007a; Ma et al., 2007b; Walhout et al., 2019; Yasmeen et al., 2012;
34 Zhao et al., 2022), or introduced directly into a mass spectrometer (Hoffmann et al., 1998; Kuckelmann et
35 al., 2000) for the analysis of their chemical composition. Over the past decades, low-volatile dimers have
36 been identified as major components of SOA and are considered as important precursors in atmospheric
37 SOA formation. Although mechanisms involving stabilized Criegee intermediates and peroxy radicals have
38 been proposed for dimer formation in the gas phase (Zhao et al., 2022), the exact process of SOA formation
remains controversial due to the diversity of reaction products and the complexity of their formation
mechanisms. Since mass spectrometry has both high sensitivity and high mass resolving power, it is a
powerful tool to study the chemical composition of SOA. However, it cannot distinguish isomers, often
present in complex organic mixtures, and provides only limited structural information through
fragmentation analysis. Therefore, complementary analytical techniques are essential to fully characterize
the organic mixture of α-pinene SOA.

Vibrational spectroscopy techniques such as infrared (IR) and Raman spectroscopies are powerful tools
for identifying the functional groups of organic and inorganic molecules. Using IR spectroscopy and mass
spectrometry, Finlayson-Pitts et al. investigated the chemical composition of SOA formed from the
ozonolysis of α-pinene (Kidd et al., 2014). They reported that chemical species with a molecular weight of
368 are major components of α-pinene SOA and proposed two candidate molecules, shown in Figure 1.
One is the C₁₉H₂₈O₇ diacid (C₁₉ dimer), proposed to be formed from the reaction between cis-pinic acid and
10-hydroxypinonic acid. The other is a C₂₀H₃₂O₆ hydroperoxide (C₂₀ dimer), proposed to result from the
reaction of the stabilized Criegee intermediate with cis-pinonic acid. Although they showed that the IR
spectrum of α-pinene SOA exhibits significant similarities to those of cis-pinonic acid and pinonaldehyde,
it was not possible to clearly distinguish the proposed dimers using IR spectra alone. However, it is
important to note that the C₂₀ dimer contains a peroxy group. The O–O stretching vibration is very strong
and specific in Raman spectra but is undetectable or very weak in IR spectroscopy. Therefore, we employed
Raman spectroscopy to further clarify the chemical composition of α-pinene SOA.
Laser trapping combined with Raman spectroscopy is a valuable analytical technique for investigating the chemical composition of SOAs, as it can selectively identify molecules within the particle phase among the myriad molecules generated during ozone oxidation reactions. Previously, Sullivan et al. had succeeded in laser trapping of SOAs (Gorkowski et al., 2016; Gorkowski et al., 2017; Sullivan et al., 2020). Their experimental approach is to incorporate SOA particles into laser-trapped oil droplets in the gas phase. In contrast, we present here directly laser trapping SOA particles in the gas phase and *in situ* analysis of their chemical composition. In this study, SOAs were generated by ozone oxidation of α-pinene in a reaction chamber mounted on the stage of an optical microscope, and single SOA particles were trapped in the gas phase. To detect the formation of resulting oxidized species, including the O-O stretching vibrations of peroxides in α-pinene SOA, we measured the Raman spectra of the laser-trapped SOA particles.

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## 2. Experimental

A schematic illustration of the laser trapping and Raman spectroscopy system is shown in Fig. 2. A custom-built reaction chamber (outer diameter: 80 mm; inner diameter: 40 mm; height: 40 mm) was placed on the stage of an inverted optical microscope (Olympus IX73). To prevent optically trapped SOA particles from being blown away by the airflow, a small cylinder (outer diameter: 22 mm; inner diameter: 12 mm; height: 16 mm) was placed on the cover glass at the bottom of the chamber. A few drops of α-pinene were dropped onto the bottom of the chamber, which was then filled with vaporized α-pinene. Ozone was generated by passing pure oxygen through an ozone generator (EcoDesign, Inc., ED-OG-L7) and
77 introduced into the chamber with a flow rate 0.1 L/min (typical O₃ concentration of 90 ppm). Experiments
78 of α-pinene ozonolysis were conducted under dry conditions (i.e. relative humidity RH < 10%) at room
79 temperature (~298 K). A diode-pumped solid-state laser (Laser Century Co., Ltd., GL532T8-1000) emitting
80 a 532-mn laser beam (26-28 mW) was used both as the trapping light source and the excitation source for
81 Raman spectroscopy of optically trapped particles in air. The laser beam was focused into the chamber
82 using an objective lens (Olympus LUCPLFLN60X, ×60, numerical aperture 0.70) to trap single SOA
83 particles in air. Raman scattering light from the trapped SOA particles was collected using the same
84 objective lens. After passing through a single-notch filter (Semrock, NF03-532E-25, optical density > 6) to
85 remove Rayleigh scattering by the particles, the Raman-scattered light was focused onto the entrance slit
86 of a polychromator (SOLAR TII, MS3504i, 300 or 1200 grooves/mm). The Raman spectra of the optically
87 trapped SOA particles were measured using a cooled charge-coupled device (CCD) detector (Andor
88 Technology, DU-401A-BVF). Bright-field images were observed under the optical microscope with a CCD
89 camera (Mintron Enterprise Co., Ltd., MTV-73XX11HN) and recorded as videos on a PC via a USB-
90 connected video capture device.

91 The assignment of the Raman spectra obtained from the SOA particles was performed by comparing
92 the experimental spectra with those of pure reference compounds: α-pinene (>95%, FUJIFILM Wako Pure
93 Chemical Corporation), cis-pinonic acid (98%, Sigma-Aldrich), 3-methylbutane-1,2,3-tricarboxylic acid
94 (MBTCA) (98%; Toronto Research Chemicals, TCR), and α-pinene oxide (97%, Sigma Aldrich). cis-Pinic
95 acid was synthesized according to procedures reported in the literature (Moglioni et al., 2000).
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97 **3. Results and Discussion**

98 **3.1. Laser trapping of single SOA particles in the gas phase**

99 Several minutes after ozone was introduced into the chamber, fine particles were observed in the gas phase. As demonstrated in Movie S1, a SOA particle that drifted into the focal point of the laser beam was 100 successfully trapped in the gas phase. Figure 3 shows an optical image of a single SOA particle trapped by 101 the laser approximately 5 minutes after ozone introduction. The particle shown in Figure 3 is spherical, 102 with a diameter of 4.2 µm. To the best of our knowledge, this represents the first direct laser trapping of 103 single SOA particles produced by the ozonolysis of α-pinene in the gas phase. As shown in Movie S1, it 104 was observed that when SOA particles in the gas phase collide with the laser-trapped SOA, they 105 immediately fuse each other to form a single spherical particle. These observations suggest that the SOA 106 shown in Figure 3 behaves like a liquid at room temperature. 107

108 The viscosity of secondary organic materials is known to vary by more than 12 orders of magnitude 109 depending on humidity and chemical composition. SOAs can be classified as liquids (<10² Pa s), semi- 110 solids (10²–10¹² Pa s) or solids (>10¹² Pa s) (Koop et al., 2011). The viscosity of the SOA particle shown 111 in Figure 3 was roughly estimated, based on the timescale required for an optically trapped SOA particle to 112 coalesce with other SOA droplets, as shown in Movie S1. For the coalescence of two equally sized liquid 113 droplets, the timescale for coalescence, τ, is given by Equation 1 (Abramson et al., 2013; Koch and 114 Friedlander, 1990; Pajunoja et al., 2014):
$$ \tau = \frac{\eta d}{\sigma} \quad (1) $$

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where η is the viscosity, σ is the surface tension and d is the diameter of the droplet after coalescence. A frame-by-frame analysis of Movie S1 revealed that coalescence was completed within one video frame (33 ms), indicating that τ was less than 33 ms. Assuming that the surface tension of the droplet is equivalent to that of α-pinene (σ = 26 mN/m) (Wagner et al., 2019), and using a droplet diameter is 4.2 µm, the viscosity of the SOA shown in Figure 3 was estimated using Equation 1 to be less than 2 × 10² Pa s. Therefore, the α-pinene SOA observed in this experiment is presumed to be in a semi-solid or liquid state at room temperature.

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Both the chemical composition and relative humidity affect the viscosity of α-pinene SOA. Grayson et al.

reported that the viscosity of SOA under dry conditions decreases with increasing SOA mass concentration

in the gas phase (Grayson et al., 2016). Based on poke-and-flow experiments, they showed that when the

SOA mass concentration is 121 µg m⁻³, the viscosity ranges from 6 × 10⁵ to 5 × 10⁷ Pa s. In contrast, when

the SOA mass concentration reaches 14000 µg m⁻³, the viscosity ranges from 4 × 10⁴ to 1.5 × 10⁶ Pa s.

More recently, using calculations from the GECKO-A model, Shiraiwa et al. reported that the viscosity of

α-pinene SOA decreases from 10⁶ to 10⁴ Pa s as the mass concentration increases from 10² to 10⁴ µg m⁻³

under dry conditions (Galeazzo et al., 2021). In the present experiment, a relatively high ozone

concentration (90 ppm) was introduced into the reaction chamber to form micrometer-sized SOA droplets.

Although the SOA mass concentration in the reaction chamber is not known, it is likely to exceed 10⁴ µg

m⁻³, as the ozone concentration (90 ppm) in this experiment is significantly higher than that used by
134 Grayson et al. (12 ppm) (Grayson et al., 2016). Therefore, the SOA viscosity ($< 2 \times 10^2 \text{ Pa s}$) shown in
135 Figure 3 appears to be a reasonable estimate.

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137 **3.2. Prediction of SOA composition**

138 The ozonolysis of α-pinene has been extensively studied (Gao et al., 2023; Luo et al., 2024). This reaction
139 leads to the formation of cis-pinonic acid (CPA), pinic acid (PA) and 3 methylbutane-1,2,3-tricarboxylic
140 acid (MBTCA) as the most stable species identified in SOA. It also results in the production OH radicals,
141 which can further react with either α-pinene or its oxidation products (i.e. CPA, PA and MBTCA). To
142 predict the most likely compounds formed under our experimental conditions, a model based on reaction
143 kinetics was developed. A detailed description of the methodology and the reactions considered is provided
144 in the Supporting Information (SI). The predicted evolution of the concentrations of α-pinene, O<sub>3</sub>, OH,
145 CPA, PA and MBTCA is shown in Figure S1 in the SI. Based on our experimental conditions (i.e. initial
146 concentrations and reaction time), CPA, PA and MBTCA are expected to be present in the SOA with CPA
147 and PA being the most concentrated species.

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149 **3.3. Raman Spectroscopy of SOA Particles**

150 The Raman spectrum of the optically trapped SOA shown in Figure 3 was measured and is presented in
151 Figure 4(c). For comparison, the Raman spectra of α-pinene (liquid) and an aqueous solution of cis-pinic
152 acid (at the concentration of 0.48 mol dm⁻³) are shown in Figures 4(a) and (b), respectively. Additional
comparisons between SOA spectra recorded after 5 and 150 min of trapping and those of reference pure compounds are provided in Figure S2 in the SI. As shown in Figure 4(a), many sharp Raman peaks are present in the spectrum of α-pinene. The peak assignments are summarized in Table 1 (An et al., 2016; Qiu et al., 2010; Wilson, 1976). In particular, the sharp peaks observed at 668 cm⁻¹ and 1661 cm⁻¹ are attributed to the breathing mode of six-membered ring and the C=C stretching vibration, respectively (An et al., 2016; Qiu et al., 2010). Notably, these characteristic peaks are absent in the Raman spectrum of the SOA shown in Figure 4(c) (and Figure S2 in the SI), clearly indicating that ozonolysis led to the cleavage of the C=C double bond in α-pinene, and that little to no α-pinene remains in the SOA. Figure 4(b) shows the Raman spectrum of an aqueous solution of cis-pinic acid, a compound known to be formed from α-pinene oxidation by ozone under dry conditions. The peak assignments for cis-pinic acid were primarily made by comparison with those of α-pinene, and are summarized in Table 1. The broad peaks observed around 1650 and 3430 cm⁻¹ in Figure 4(b) arise from Raman scattering by water (Seki et al., 2020), whereas the other peaks originate from cis-pinic acid. As shown in Figure 4(b), cis-pinic acid contains two carboxyl groups. A peak attributed to the C=O stretching vibration is observed at 1700 cm⁻¹, overlapping with the broad H–O–H bending mode of water at 1650 cm⁻¹. Peaks around 906, 1454, and 2965 cm⁻¹ are attributed to the four-membered ring and various C–H vibrations: CH₂ scissoring, CH₃ asymmetric deformation, and C–H stretching, respectively. A similar Raman spectrum is observed for cis-pinonic acid (see Figure S2 in the SI), which is also a main product of α-pinene oxidation. Indeed, the molecular structures of pinic acid and pinonic acid are very similar, differing only at carbon #2 (see Figure 4(b) and S2), which bears a carboxyl
group in pinic acid and a ketone group in pinonic acid. As shown in Figure 4(c) (and in Figure S2), the Raman spectral profile of the SOA is closely similar to that of cis-pinic acid (Figure 4(b)), as well as those of cis-pinonic acid and MBTCA (Figure S2), in agreement with our kinetic predictions. This similarity suggests that the molecules constituting the SOA likely possess chemical structures containing functional groups similar to those found in cis-pinic and cis-pinonic acids. Indeed, the broad Raman peak centered around 1700 cm⁻¹ observed in the SOA spectra can be attributed to the C=O stretching mode, which is characteristic of α-pinene oxidation products. Interestingly, the Raman spectrum of SOA does not change significantly over the reaction time under our experimental conditions (see Figure S2). The SOA Raman spectra are also consistent with the expected vibrational modes based on the chemical structures of the C₁₉ and C₂₀ dimers shown in Figure 1. The peaks observed around 930, 1443, 1670 and 2924 cm⁻¹ can be attributed to the four-membered ring and C–H deformation, CH₂ scissoring and CH₃ asymmetric deformation, C=O stretching vibration, and C–H stretching vibrations, respectively. Considering both the experimental conditions and kinetic predictions, the SOA composition is expected to be a mixture of compounds containing COOH and C=O functional groups, which are capable of readily forming H-bonds and thus promote the formation of dimeric structures. Additionally, it is noteworthy that a peak at 852 cm⁻¹ is observed in the Raman spectrum of SOAs. In the literature, it has been reported that a similar peak around ~850 cm⁻¹ was observed in the Raman spectrum of alkenes following ozonolysis reactions (Cataldo, 2015; Lee and Chan, 2007). For example, Chan *et al.* investigated the ozonolysis of oleic acid particles using an electrodynamic balance coupled with Raman spectroscopy, attributing the peak near 850 cm⁻¹ to
O–O stretching vibration of peroxides and/or ozonides (Lee and Chan, 2007). Cataldo studied the synthesis of secondary ozonides from 1-alkenes using Raman spectroscopy and observed strong peaks between 822 – 830 cm⁻¹, assigned to the O–O stretching vibration of the 1,2,4-trioxolane ring (Cataldo, 2015). The O–O stretching peak observed in our SOA spectrum at 852 cm⁻¹ lies slightly above this range. Furthermore, the Raman spectrum of *tert*-butyl hydroperoxide has been reported to exhibit an O–O stretching peak at 847 cm⁻¹ (Vacque et al., 1997). Taken together, these findings suggest that the peak at around 852 cm⁻¹ in Figure 4(c) likely arises from the O–O stretching vibration of peroxides. Therefore, we conclude that Raman spectroscopy has been successfully used to detect the O–O stretching vibration of the peroxide moiety, presumably associated with the C₂₀ dimer shown in Figure 1 (as an example) or its structural analogue, within the SOA. The presence of a mixture of oxygenated compounds and the formation of dimeric structures within the SOA particles strongly supports the hypothesis of liquid droplet formation, as the corresponding pure compounds are solid under dry atmospheric conditions.

## 4. Conclusions

In this study, single α-pinene SOA particles were successfully trapped in the gas phase using a laser trapping technique. The trapped SOAs were found to be spherical and exhibited liquid-like behavior at room temperature. Viscosity is a key physical property of SOAs as it governs the diffusion rate and chemical heterogeneity within individual particles. Consequently, the viscosity of α-pinene SOAs has been measured using various experimental methods. However, a discrepancy remains between experimentally
210 determined and theoretically calculated viscosity values, which is still a matter of debate (Galeazzo et al.,

211 2021). Although a preliminary estimate, the viscosity of the optically levitated α-pinene SOA in the gas

212 phase was approximated to be less than 2 × 10² Pa s, based on the fusion time of the droplets. It is well

213 known that SOA viscosity depends on both chemical composition and relative humidity. Therefore, further

214 experimental validation is needed, and more detailed viscosity measurements are planned for future work.

215 Raman spectroscopic analysis revealed that SOAs consist primarily of oxygenated compounds, notably

216 *cis*-pinic acid and *cis*-pinonic acid, in agreement with our kinetic predictions based on the experimental

217 conditions. An intense Raman peak observed at approximately 852 cm⁻¹ was attributed to the O–O

218 stretching vibration of peroxides. This observation does not preclude the presence of compounds without

219 peroxy group, such as the C₁₉ dimer depicted in Figure 1, in the SOA. Instead, it indicates that at least some

220 peroxide-containing compounds, presumably the C₂₀ dimer or structural analogues, are present in the SOA.

221 Such dimeric structures can be easily formed in a mixture of oxygenated species derived from α-pinene

222 ozonolysis, due to the structural similarity of the molecules involved. The formation of weak intermolecular

223 interactions in this complex mixture supports the idea that SOAs exist in the form of liquid droplets, as the

224 individual pure compounds are solid under dry atmospheric conditions. Recently, Zhao et al. reported that

225 both acyl organic peroxy radicals and stabilized Criegee intermediates play key roles in dimer formation in

226 the particle phase during α-pinene ozonolysis. These dimers, which contain peroxide functional groups, are

227 formed via stabilized Criegee intermediates. Therefore, the presence of the O–O bond observed in the

228 Raman spectrum of the SOA supports the hypothesis that dimer was formed through reactions involving
229 stabilized Criegee intermediates. The O–O stretching vibration is a highly intense and specific feature in

230 Raman spectra, but it is undetectable or very weak in IR spectroscopy. In this study, we successfully

231 achieved, for the first time, the direct detection of peroxide-containing compounds in α-pinene SOAs, using

232 laser trapping coupled with Raman spectroscopy, compounds that could not previously be detected via IR

233 spectroscopy. This demonstrates that the laser trapping technique coupled with Raman spectroscopy is a

234 powerful and highly selective method for identifying molecular components within the particle phase

235 among the multitude of species generated during ozonolysis reactions. As such, we are confident that this

236 combined method represents a highly effective analytical approach for unraveling the formation and aging

237 mechanisms of SOAs in the atmosphere.

238

239 **Supplementary Information**

240 Supplementary data to this article can be found online at https://xxxx.

241 Movie S1: Laser trapping of single α-pinene SOA particles in the gas phase.

242

243 **Data availability**

244 Data will be made available on request.

245
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<table><caption>Table 1 Assignments of Raman peaks of pure α-pinene and cis-pinic acid aqueous solution.</caption><thead><tr><th colspan="2">α-pinene</th><th colspan="2">cis-pinic acid (aq)</th><th rowspan="2">Ref.</th></tr><tr><th>Raman shift/cm<sup>-1</sup></th><th>Assignment<sup>b</sup></th><th>Raman shift/cm<sup>-1</sup></th><th>Assignment<sup>b</sup></th></tr></thead><tbody><tr><td>621(w.)<sup>a</sup></td><td>six-membered ring deformation; four-membered ring breath</td><td>635(w.)</td><td>four-membered ring breath</td><td>(Qiu et al., 2010)</td></tr><tr><td>668(vs.)</td><td>six-membered ring breath; C(6)-H in plane wagging</td><td>694(w.)</td><td>C(6)-H in plane wagging</td><td>(Qiu et al., 2010)</td></tr><tr><td>773(m.)</td><td>six-membered ring breath</td><td></td><td></td><td>(Qiu et al., 2010)</td></tr><tr><td>823(w.)</td><td>six and four membered ring deformation; C(4, 6, 8, 9)-H out-of-plane wagging</td><td>810(w.)</td><td>C(4, 6, 8, 9)-H out-of-plane wagging</td><td>(Qiu et al., 2010)</td></tr><tr><td>844(m.)</td><td>six and four membered ring deformation; C(6)-H wagging</td><td></td><td></td><td>(Qiu et al., 2010)</td></tr><tr><td>888(w.)</td><td>six-membered ring breath; C(4, 6)-H in-plane wagging</td><td>866(w.)</td><td>C(4, 6)-H in-plane wagging</td><td>(Qiu et al., 2010)</td></tr><tr><td>908(m.)</td><td>six and four membered ring deformation; C(1, 3, 4, 5, 6, 8, 9, 10)-H deformation</td><td>906(m)</td><td>four membered ring deformation; C(1, 4, 5, 6, 8, 9)-H deformation</td><td>(Qiu et al., 2010)</td></tr><tr><td>931(m.)</td><td>six-membered ring deformation; CH<sub>3</sub> asymmetric deformation</td><td>933(m.)</td><td>CH<sub>3</sub> asymmetric deformation</td><td>(An et al., 2016)</td></tr><tr><td>953(m.)</td><td>C(1, 3, 4, 5, 6, 8, 9, 10)-H deformation</td><td>949(m.)</td><td>C(1, 4, 5, 6, 8, 9)-H deformation</td><td>(Qiu et al., 2010)</td></tr><tr><td>998(w.)</td><td>six membered ring deformation; four-membered ring breath; C(8, 9)-H out-of-plane wagging</td><td>994(m.)</td><td>four-membered ring breath; C(8, 9)-H out-of-plane wagging</td><td>(Qiu et al., 2010)</td></tr><tr><td>1015(w.)</td><td>C(1, 3, 4, 5, 6, 8, 9, 10)-H out-of-plane wagging</td><td>1015(w.)</td><td>C(1, 4, 5, 6, 8, 9)-H out-of-plane wagging</td><td>(Qiu et al., 2010)</td></tr><tr><td>1044(m.)</td><td>C(10)-H twisting</td><td></td><td></td><td>(Qiu et al., 2010)</td></tr><tr><td>1087(m.)</td><td>six-membered ring deformation; C(1, 5, 6, 10)-H wagging</td><td>1100(w.)</td><td>C(1, 5, 6)-H wagging</td><td>(Qiu et al., 2010)</td></tr><tr><td>1127(m.)</td><td>C(2)-C(10), C(7)-C(9) stretch; C(1, 3, 4, 5, 6, 8)-H deformation</td><td>1146(m.)</td><td>C(7)-C(9) stretch; C(1, 4, 5, 6, 8)-H deformation</td><td>(Qiu et al., 2010)</td></tr><tr><td>1167(m.)</td><td>C(2)-C(10) stretch; C(1, 3, 4, 5, 6)-H wagging</td><td>1163(m.)</td><td>C(1, 4, 5, 6)-H wagging</td><td>(Qiu et al., 2010)</td></tr><tr><td>1182(m.)</td><td>C(1, 3, 4, 5, 6, 8, 9)-H wagging; four-membered ring deformation</td><td></td><td></td><td>(Qiu et al., 2010)</td></tr><tr><td>1205(w.)</td><td>C(4)-H twist; C(6)-H wagging; six and four membered ring deformation</td><td></td><td></td><td>(Qiu et al., 2010)</td></tr><tr><td>1222(m.)</td><td>C(1, 4, 6)-H wagging; six and four membered ring</td><td>1219(m.)</td><td>C(1, 4, 6)-H wagging; four membered ring deformation</td><td>(Qiu et al., 2010)</td></tr></tbody></table>
<table><thead><tr><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td></td><td>deformation</td><td></td><td></td><td></td></tr><tr><td>1266(m.)</td><td>C(7)-(C1, 5) stretch; C(1, 5)-H wagging; C(4)-H twist</td><td>1241(m.)</td><td>C(7)-(C1, 5) stretch; C(1, 5)-H wagging; C(4)-H twist</td><td>(Qiu et al., 2010)</td></tr><tr><td>1309(w.)</td><td>C(1,3,4,5)-H wagging; C(4)-C(5) stretch</td><td></td><td></td><td>(Qiu et al., 2010)</td></tr><tr><td>1330(m.)</td><td>C(1,3,4,5)-H wagging; six-membered ring C-C stretch; four membered ring breath</td><td></td><td></td><td>(Qiu et al., 2010)</td></tr><tr><td>1376(m.)</td><td>CH<sub>3</sub> symmetric deformation</td><td>1372(w.)</td><td>CH<sub>3</sub> symmetric deformation</td><td>(An et al., 2016)</td></tr><tr><td>1437(vs.)</td><td>C(4)-H scissoring</td><td>1417(w.)</td><td>C(4)-H scissoring</td><td>(Qiu et al., 2010)</td></tr><tr><td>1448(vs.)</td><td>C(8, 9)-H asymmetric deformation</td><td>1454(s.)</td><td>C(8, 9)-H asymmetric deformation</td><td>(Qiu et al., 2010)</td></tr><tr><td>1470(s.)</td><td>C(6)-H scissoring; C(8, 9)-H asymmetric deformation</td><td>1472(m.)</td><td>C(6)-H scissoring; C(8, 9)-H asymmetric deformation</td><td>(Qiu et al., 2010)</td></tr><tr><td>1661(vs.)</td><td>C(2)=C(3) stretching</td><td></td><td></td><td>(Qiu et al., 2010)</td></tr><tr><td></td><td></td><td>(1650)<sup>b</sup></td><td>H-O-H bending of water</td><td>(Seki et al., 2020)</td></tr><tr><td></td><td></td><td>1700(m.)</td><td>C=O stretching</td><td>(Walhout et al., 2019)</td></tr><tr><td>2836(vs.)</td><td>CH<sub>2</sub> symmetric stretching</td><td></td><td></td><td>(Wilson, 1976)</td></tr><tr><td>2878 vs.)</td><td>CH<sub>3</sub> symmetric stretching</td><td>2880(s.)</td><td>CH<sub>3</sub> symmetric stretching</td><td>(Wilson, 1976)</td></tr><tr><td>2924 vs.)</td><td>CH<sub>2</sub> asymmetric stretching</td><td>2933 vs.)</td><td>CH<sub>2</sub> asymmetric stretching</td><td>(Wilson, 1976)</td></tr><tr><td>2953 vs.)</td><td>CH<sub>3</sub> asymmetric stretching</td><td>2965 vs.)</td><td>CH<sub>3</sub> asymmetric stretching</td><td>(Wilson, 1976)</td></tr><tr><td>2990 vs.)</td><td>CH<sub>3</sub> asymmetric stretching</td><td></td><td></td><td>(Wilson, 1976)</td></tr><tr><td>3029(s.)</td><td>=CH stretching</td><td></td><td></td><td>(Wilson, 1976)</td></tr><tr><td></td><td></td><td>(3430)<sup>b</sup></td><td>OH stretching of water</td><td>(Seki et al., 2020)</td></tr></tbody></table>

a Intensity: very strong (vs.), strong (s.), medium (m.), weak (w.)

b Numbering of carbon atoms in α-pinene and cis-pinic acid are shown in Figure 4.

c Raman scattering from water used as solvent

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**Declaration of interests**

[x] The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[ ] The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Molecular structures of dimers with a molecular weight of 368, proposed to be major components of SOA from α-pinene ozonolysis.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Schematic illustration of the Laser trapping and Raman spectroscopy system.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** An optical image of optically levitated α-pinene SOA in air.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** (a) Raman spectra of (a) α-pinene, (b) *cis*-pinic acid aqueous solution, and (c) SOA (shown in Fig. 3), respectively. Numbering of carbon atoms in α-pinene and *cis*-pinic acid are shown in (a) and (b), respectively.</figcaption></figure>
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**Supplementary Material**

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**Video**
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<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>