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# Atmospheric stability of six particulate biogenic secondary organic aerosol markers towards photolysis, hydroxyl radicals and ozone

Pauline Pouyes<sup>a</sup>, Judith Lorin<sup>b</sup>, Pierre-Marie Flaud<sup>a</sup>, Emmanuel Geneste<sup>b</sup>, Hélène Budzinski<sup>b</sup>, Emilie Perraudin<sup>a</sup>, Eric Villenave<sup>a,* ID</sup>

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## HIGHLIGHTS:

* The atmospheric stability of six biogenic secondary organic aerosol markers was investigated.

* Markers were adsorbed onto silica model particles.

* Photolysis rates were measured for 6 carboxylic acids as markers of BSOA.

* Kinetics of heterogeneous reactions of the acids with ozone and hydroxyl radicals were measured.

* Particulate marker concentrations were measured as a function of exposure times by LC-QTOF/MS.

## GRAPHICAL ABSTRACT

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## ARTICLE INFO

Handling Editor: Volker Matthias

**Keywords:**

Biogenic SOA

LC-QTOF MS

Hydroxyl radical

Ozone

Photolysis

Heterogeneous reaction

Lifetime

## ABSTRACT

This study aimed to investigate the kinetics of the heterogeneous reactions of six biogenic secondary organic aerosol (BSOA) markers of atmospheric interest, i.e. the terebic, terpenylic, cis-pinonic, pinic, 3-methyl-1,2,3-butanetricarboxylic (MBTCA) and β-caryophyllinic acids. For this purpose, these compounds were individually adsorbed onto silica model particles and exposed either to solar irradiation, hydroxyl radicals, or ozone. Marker concentrations extracted from particles were subsequently analyzed by liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (LC-QTOF/MS). Pseudo-first-order rate constants were derived from simulations (using either exponential functions or the tangent slope at t₀) of particulate marker concentration decays as a function of exposure time to different oxidants or light. Second-order rate constants were calculated considering the oxidant concentrations under different experimental conditions. The overall atmospheric lifetime

<footnote><sup>a</sup> University of Bordeaux, EPOC UMR 5803 CNRS, B18, Allée Geoffroy Saint Hilaire, CS50023, cedex, 33615, Pessac, France</footnote>

<footnote><sup>b</sup> University of Bordeaux, EPOC UMR 5803 CNRS, A12, 351 cours de la Libération, cedex, 33405, Talence, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: pauline.pouyes@u-bordeaux.fr (P. Pouyes), judith.lorin@u-bordeaux.fr (J. Lorin), pierre-marie.flaud@u-bordeaux.fr (P.-M. Flaud), emmanuel.geneste@u-bordeaux.fr (E. Geneste), helene.budzinski@u-bordeaux.fr (H. Budzinski), emilie.perraudin@u-bordeaux.fr (E. Perraudin), eric.villenave@u-bordeaux.fr (E. Villenave).</footnote>
of each marker was calculated, revealing that β-caryophyllinic acid is the most reactive compound studied, with a lifetime of <14.3 min, followed by cis-pinonic acid (8.6 h), MBTCA (19 h), pinic acid (>2.8 days), terpenylic acid (>4.8 days), and terebic acid (>5.8 years). This work confirms that the atmospheric stability of some BSOA markers is insufficient, to justify their relevance as tracers of particle formation or aging processes.

# 1. Introduction

Atmospheric aerosols are solid or liquid particles suspended in air masses. Their average concentration in the continental boundary layer is estimated to be between 100 and $10000$ particles cm$^{-3}$ (Rose et al., 2021). They have significant impacts on health, causing numerous cardiovascular and respiratory diseases (Pöschl et al., 2005; Oberdörster et al., 2005; Pope III and Dockery, 2006) and increasing premature mortality (Lelieveld et al., 2020; Chowdhury et al., 2022), on climate through direct interactions with solar and terrestrial radiations (Kanakidou et al., 2005; Beres et al., 2020) or indirectly through cloud formation (Tao et al., 2023). Particles may also impact visibility and tourism (Bäumer et al., 2008; Hao et al., 2021; Aniol et al., 2022), ecosystems (Bergin et al., 2001) and cultural heritage (Nava et al., 2010) through dry (Burkhardt and Grantz, 2017) or wet (Farmer et al., 2021) deposition processes.

Organic matter may contributes to 20–60 % of the mass of fine particles in mid-latitude continental areas and up to 90 % in tropical forests (Kanakidou et al., 2005; Zhang et al., 2007; Docherty et al., 2008). The contribution of secondary aerosols to particulate organic matter varies but remains significant, accounting for 20–80 % of the measured particle mass (Song et al., 2018). Secondary organic aerosols (SOAs) represent a large fraction of the total aerosol, and the biogenic SOA (BSOA) fraction is much larger than the anthropogenic one at the global level (Guenther et al., 1995; Huang et al., 2017; Cai et al., 2021). BSOAs are generated in situ in the atmosphere through oxidation processes by various atmospheric oxidants (e.g. hydroxyl (OH) and nitrate ($NO_3$) radicals, ozone ($O_3$), atomic chlorine, etc.) or photodissociation by solar radiation (Atkinson and Arey, 2003; Shen et al., 2013) of volatile organic compounds (VOCs) emitted by biogenic sources like vegetation (Ibanez et al., 2010; Rasulov et al., 2018).

The complexity of particle formation and aging processes, together with the large number of compounds potentially involved in these reactions, makes the study of atmospheric aerosols, particularly SOAs, extremely challenging. It is therefore essential to characterize the chemical composition of secondary aerosols at the molecular level. Rather than attempting to analyze thousands of compounds present in the particulate phase, an approach consists in focusing only on few specific species, identified as “tracers”, that are representative of a larger set of family of molecules and associated processes. For this purpose, the identification and use of source or process tracer molecules has proven to be undeniably appropriate and relevant (Claeys et al., 2007, 2013; Nozière et al., 2015).

The main characteristic of tracers, aside from their specificity, is their high atmospheric stability – due to their molecular structure – against photo- and physicochemical aging processes. Therefore, they retain their structural identity when emitted or generated, and are then transported in the atmosphere until they reach a sink where they will be deposited or transferred to another environmental compartment (Sheppard, 1963). They must be easily identifiable and present in sufficient concentrations to be quantifiable using the analytical tools employed to measure them. The term “tracer” can only be rigorously applied to molecules whose stability over time is sufficiently durable. More, tracers should also have a low vapor pressure, to be mainly associated with the particulate phase, to minimize possible underestimation due to gas phase losses (Lanzafame et al., 2021). Then, they may allow the tracking of a compound’s transport, which is then considered to represent an entire fraction of the aerosol, or the tracking of the trace of a reactive process that is occurring or has occurred. When these

compounds are less stable, meaning they interact with different atmospheric components through various processes (photolysis, reactivity, etc.), they are referred to as molecular « markers », which are often used despite their more limited usefulness. This is for instance the case of levoglucosan, which can be used to identify and follow biomass burning events (Simoneit et al., 1999; Kehrwald et al., 2012). After years of common use as a tracer, its relevance was questioned when its atmospheric lifetime was estimated to be lower than 2.2 days (Hennigan et al., 2010; Hoffmann et al., 2010; Li et al., 2021). Levoglucosan is now only considered as a source marker due to its reactivity with the OH radical and cannot allow to properly follow biomass combustion sources on a couple day basis, but by taking into account its atmospheric degradation pathways (Li et al., 2021).

Among the most commonly identified or used tracers in studies related to biogenic SOA formation and fate (both on field and lab studies), carboxylic acids (mono- or polyfunctionalized) as primary or secondary oxidation products of terpenes in the atmosphere have been extensively measured (in the particle phase) and studied (Nozière et al., 2015). Hence, the main objective of this work was to assess the stability of 6 markers of BSOAs resulting from the oxidation of α- and β-pinene (i. e. terebic acid, terpenylic acid, cis-pinonic acid, pinic acid, and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA)) and β-caryophyllene (β-caryophyllinic acid) by considering their heterogeneous reactivity (i. e. at the gas/particle interface) under solar irradiance, exposure to OH radicals and ozone.

Even if most of these carboxylic acids are either semi- or low-volatile, gas/particle partitioning studies show that they are mainly distributed in the particulate phase (Ylisirniö et al., 2020; Lanzafame et al., 2021). If many previous studies already investigated the reactivity of biogenic SOA markers, almost all were performed in the aqueous phase because of the polar and acidic nature of such molecules (Lignell et al., 2013; Aljawhary et al., 2016; Enami and Sakamoto, 2016; Witkowski and Gierczak, 2017; Witkowski et al., 2019a, 2019b; Amorim et al., 2020, 2021; Witkowski et al., 2023; Blaziak et al., 2024; Sun et al., 2024), highlighting the importance of such oxidative processes occurring in the condensed phase (cloud water accounting for 15 % of the air masses).

The heterogeneous reactivity of cis-pinonic acid was first investigated by Lai et al. (2015). The average lifetime of the compound was estimated between 2 and 3 days. They emphasized that the residence time of aerosols in the atmosphere is longer than the lifetime of the compound in the environment, further questioning the use of cis-pinonic acid as a tracer to quantitatively follow the formation and fate of biogenic SOAs. Very recently, Wang et al. (2025) investigated the heterogeneous reactivity of SOAs formed during α-pinene oxidation in the presence of OH radicals. They focused their study on typical tracers of biogenic SOA formation, such as pinic, cis-pinonic, and hydroxypinonic acids. With an exposure equivalent to 12 h of atmospheric aging, the lifetimes of these compounds were less than half a day, regardless of their experimental conditions.

The comparison of ambient air measurements at different specific sites with the output of international chemistry-transport models (CTMs) - used to model air quality or climate change - showed that the secondary fraction of organic aerosols is often underestimated by these models. In order to improve its description and limit potential biases, the introduction of tracers and markers have been included in 3D - air quality models recently (Zhang et al., 2021; Lanzafame et al., 2022). While fundamental knowledge of the kinetics and reaction mechanisms involved in heterogeneous processes remains essential to understand the atmospheric fate of tracers, the main and essential objective of this work
was to determine a single parameter that could be used directly in statistical approaches or 3D CTMs: their atmospheric lifetimes, towards the main oxidative processes.

This work on heterogenous reactions of atmospheric potential tracers plans to reinforce the idea that it is necessary to consider all possible reaction processes in the atmosphere to minimize bias in the determination of source and process tracer concentrations leading to the formation of biogenic secondary organic aerosols.

## 2. Material and methodologies

Given the heterogeneity and the variability of the chemical composition of ambient atmospheric particles (Zhang et al., 2007; Jimenez et al., 2009), it is often useful in the laboratory to use model particles under controlled conditions. This approach allows a reproducible representation to be achieved while simplifying the studied matrix and isolating complex processes so that they can be addressed and understood in greater depth. Organic coated- or pure silica particles, selected in this work, have been widely used as model particles to study the heterogeneous reactivity of many organic compounds of atmospheric interest like carboxylic acids (Rubasinghege et al., 2013), polycyclic aromatic hydrocarbons (Wang et al., 1999; Perraudin et al., 2007; Miet et al., 2009) or pesticides (Socorro et al., 2015; Han et al., 2021; Samia et al., 2024).

### 2.1. Chemicals

Among the oxidation products of mono- and sesquiterpenes of interest, six carboxylic acids were selected in this study: terebic acid, terpenylic acid, cis-pinonic acid, pinic acid, 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA)) and β-caryophyllinic acid. This selection was

guided by several criteria: (i) the presence of these compounds derived from distinct precursors of secondary particles, (ii) their identification as first- or second-generation oxidation products, indicative of varying degrees of SOA aging, and (iii) the fact that some of these compounds were extensively studied, allowing for a meaningful comparison with existing literature, while others have far fewer references. They are all presented in Table 1.

### 2.2. Silica particles

Silica particles were supplied by Interchim (IT-70-5). They have an average particle diameter of 5 µm, an average pore diameter of 70 Å, and a specific surface area of 500 m² g⁻¹. These particles were extensively analyzed and described in our previous works, e.g. by Perraudin et al. (2005) and the homogeneity of the particle size distribution and the hydrophilicity of the particles were verified and assessed using optical microscopy and infrared spectroscopy techniques, respectively.

### 2.3. Preparation and storage

The particles were successively cleaned by ultrasonication in dichloromethane and acetonitrile. Residual solvent was evaporated by placing particles under a fume hood (48 h). None of the six compounds was found in the clean particles. Six batches of particles were prepared by coating the acids individually by liquid-solid deposition using a rotary evaporator. The target particle concentration was about 500 µg g⁻¹. Pure crystals (or a solution in the case of β-caryophyllinic acid) were weighed on a microbalance, transferred to a flask with 20 mL of acetonitrile, and ultrasonicated for 15 min. Clean silica particles were then weighed and added to the flask containing the coating solution, and sonicated again for 5 min. The solvent was evaporated with a rotary

<caption>**Table 1**</caption>

List of the different markers selected in this study.

<table><thead><tr><th>Name</th><th>Precursor</th><th>Formula</th><th>Molecular weight (g.mol<sup>-1</sup>)</th><th>Purity (%)</th><th>Supplier</th></tr></thead><tbody><tr><td>Terebic acid</td><td>α-pinene<br>β-pinene</td><td></td><td>158</td><td>&gt;98</td><td>TCI</td></tr><tr><td>Terpenylic acid</td><td></td><td></td><td>172</td><td>95</td><td>Akos</td></tr><tr><td>cis-Pinonic acid</td><td></td><td></td><td>184</td><td>98</td><td>Sigma-Aldrich</td></tr><tr><td>Pinic acid</td><td></td><td></td><td>186</td><td>98</td><td>Sigma-Aldrich</td></tr><tr><td>MBTCA</td><td></td><td></td><td>204</td><td>98</td><td>TRC</td></tr><tr><td>β-caryophyllinic acid</td><td>β-caryophyllene</td><td></td><td>254</td><td>&gt;95</td><td>(a)</td></tr></tbody></table>

<footnote>(a) supplied from the T. Hoffmann group at the Johannes Gutenberg-University of Mainz (Germany).</footnote>
evaporator (T = 45 °C, 100 rpm, P = 160 Torr). Residual solvent was evaporated under a fume hood (48 h). All particles were stored in amber vials, wrapped in aluminium foil, and protected from light. Coated particles were extracted and analyzed to measure the concentration levels of the six markers investigated in this work. Experimental mass concentration values are shown in Fig. S1 for coated particles with individual acids. To investigate a possible mixing effect, a batch of particles was prepared by mixing an aliquot of each individually coated batch. The final marker concentrations in the mixture were then 132 µg g⁻¹ (terebic acid), 91 µg g⁻¹ (terpenylic acid), 73 µg g⁻¹ (cis-pinonic acid), 74 µg g⁻¹ (pinic acid), 111 µg g⁻¹ (MBTCA) and 82 µg g⁻¹ (β-caryophyllinic acid). Note that the marker surface coverage was assessed to be lower than 1 % of a monomolecular layer.

## 2.4. Analytical procedure

Marker-coated particles (5 mg for one single compound or 30 mg for the mix), previously mixed with a vortex, were deposited onto a 47 mm diameter quartz fiber filter, and then weighed before and after the reaction to monitor potential particle losses, during filter handling as well as gas flow start and stop. After each reaction time, filters were placed into a 15 mL polypropylene tube (Falcon™) where a solution of sodium decyl sulfate (Acros organics, HPLC grade, ≥98.5 %) was added as an internal standard (80 µL, 100 µg g⁻¹). Particles were extracted two times in 3 mL of a mixed acetonitrile/water solution (70/30) under ultrasonication for 15 min. Each extract was then centrifuged and the supernatant was transferred in an amber glass vial, twice. A syringe standard solution (80 µL, 100 µg g⁻¹) containing sebacic acid (Acros Organics, 98 %) was added in a known quantity to determine a potential loss of internal standards along the analytical protocol. Finally, 100 µL of extracts were taken and introduced into injection vials, then completed with 300 µL of injection solvent (H₂O/ACN, 90/10).

Authentic standards of terebic acid, terpenylic acid, cis-pinonic acid, pinic acid, MBTCA, and β-caryophyllinic acid were used for calibration as external standards for quantifying targeted compounds. For each series, a blank and a spiked sample were prepared by introducing the internal standard solution into both samples and external standards only into the spike. In almost all samples, the recovery yields of the internal standard were ranging from 80 to 120 % (Fig. S2). Target SOA tracer compounds were not detected in the protocol and injection blank samples.

## 2.5. LC-QTOF/MS analysis

Sample analyses were performed using liquid chromatography coupled to quadrupole time-of-flight mass spectrometry set-up (LC-QTOF/MS, Agilent 1290 LC coupled to 6540 QTOF/MS). This coupling type is effective for analyzing markers in organic aerosols (Amarandei et al., 2023). In this study, the analytical protocol was based on a previous work reported by Surratt et al. (2008) and optimized in our group (Duporté et al., 2016; this work).

The separation was achieved using a silica-grafted column (Acquity UPLC HSS T3, 100 × 2.1 mm, 1.8 µm film thickness, 30 °C). The mobile phase consisted of purified water (A) and acetonitrile (B) with 0.1 % formic acid. The initial concentration of acetonitrile was 10 %, then increased to 40 % in 22 min, increased to 100 % in 8 min, kept at 100 % for 2 min, then decreased to 10 % in 2 min, and held at 10 % for 4 min. The mass spectrometry was operated with an electrospray ionization source (Jet Stream) in the negative mode. The analyzer was used in the full-SCAN mode in the 50–1000 Da range. All other optimized LC-QTOF/MS parameters are presented in Table S1.

## 2.6. Experimental set-ups

### 2.6.1. Photolysis

A solar simulator (SUNTEST CPS, Atlas Material Testing Solutions)

was used to conduct irradiation experiments on particulate markers. It is equipped with a 1500 W Xenon arc lamp (type NXE1500). Such photo-simulators are often used to investigate atmospheric removal processes, such as photoreaction experiments of particulate organic mixtures (Zhao et al., 2015; Hems et al., 2020).

Samples were exposed to wavelengths ranging from 290 to 800 nm and irradiated at 765 W m⁻². The light emission spectrum was similar to the solar radiation (see Fig. S3a and S3b). Heating inside the chamber was limited by the presence of reflective surfaces and a ventilation system (120 m³ h⁻¹). Coated particles were placed on filters in glass boxes to prevent any potential loss due to the ventilation. The exposure time to light varied from 2 min to 24 h, depending on the marker photolysis rates.

Due to small local temperature increase, specific experiments were dedicated to study potential marker desorption. In that case, the boxes were systematically placed in the darkness, covered with aluminium foil.

### 2.6.2. OH reactivity

The setup was initially developed to study gas-phase reaction kinetics of volatile organic compounds with OH (Loison et al., 1998), and adapted to study the heterogeneous reactivity of particulate organics (like PAHs) with OH and NOₓ radicals (Estève et al., 2003, 2004, 2006). Briefly, it consists of a low-pressure laminar fast-flow reactor coupled to a Laser-Induced Fluorescence (LIF) of OH radical detection.

Hydroxyl radicals are initially formed by the reaction of nitrogen dioxide with a hydrogen atom. The energy provided by a microwave discharge (105 W, 2450 MHz) allows to dissociates the dihydrogen present as an impurity in a Helium cylinder (He, Messer, purity 99.999 %) (R1). When in contact with nitrogen dioxide (4.82 % NO₂ in helium, Linde) in excess, downstream of the discharge, hydrogen atoms react instantaneously to form hydroxyl radicals and nitrogen monoxide (R2). Mixed gas flows (1 L min⁻¹, 1.5 Torr) are controlled by mass flow controllers (MFC).

$$H_2 + e^- \rightarrow 2H \quad (R1)$$

$$H + NO_2 \rightarrow OH + NO \quad (R2)$$

Hydroxyl radicals are further introduced via a mobile quartz injector (50 cm x Ø 10 mm, coated with halogenated wax) into a 40 cm long glass reactor containing the filter on which the particles are deposited.

Laser-induced fluorescence is a powerful and efficient technique for detecting OH radicals in real time (Stevens et al., 1994). For this purpose, a coupling between a Nd/YAG laser (Quantel, Brillant b) and a dye laser (Quantel, TDL IV) was used. The first laser beam excites the dye which emits in return a light at 564 nm. A KDP crystal then doubles the light frequency, generating a beam at 282 nm, used to excite OH radicals generated in the reactor. The fluorescence signal of OH radicals is detected at 308 nm, and monitored using a photomultiplier connected to a digital scope.

Briefly, the OH calibration was carried out by monitoring the temporal evolution of the OH fluorescence signal while gradually decreasing the concentration of NO₂. When nitrogen dioxide is no longer in excess in the system and the stoichiometric conditions between H and NO₂ are no longer met, a gradual decrease in the OH signal is observed, allowing to determine the breakpoint (see the Fig. S4). Knowing NO₂ concentrations below this point allows to assess (from the stoichiometry) the concentrations of hydroxyl radicals produced at different positions in the setup (by moving the injector from the LIF measurement point). This has been done in this work under different conditions. A series of tests was conducted to determine all possible OH signal losses as a function of the mobile injector position, (i) in the absence/presence of a filter, (ii) in the presence of non-coated/coated silica particles deposited on a filter.

For all experiments, the concentration of hydroxyl radicals in the system was always approximately 1.3 × 10¹² molecule.cm⁻³. The exposure time of the particles to OH varied from 10 s to 45 min,
depending on the reactivity of the markers. During experiments, the reactor was systematically covered with a black sheet to minimize direct potential photochemical phenomena. The stability of the OH fluorescence signal was verified along experiments, the renewal rate of hydroxyl radicals in the system being fast enough to consider their concentration as stationary throughout experiments, allowing pseudo-first-order conditions to be achieved. All experiments were performed at room temperature (i.e. 295 K) and total pressure was held at (1.500 ± 0.005) torr.

### 2.6.3. Ozone reactivity

The ozonolysis setup used in this work was first developed in the lab to study heterogeneous reactions of particulate-aromatics with ozone (Perraudin et al., 2007). Briefly, it consists of a quasi-static reactor at atmospheric pressure, coupled to an ozone generator and a UV absorption spectrophotometer. Ozone is produced in a quartz cell (60 cm x Ø 24 mm) during the photolysis of a mixture of N₂ (Messer, purity >99.999 %) and O₂ (Messer, purity >99.999 %) through the irradiation from a low-pressure mercury vapor lamp (NIQ 120/80, Heraeus SAS). Gas flows are regulated by MFC (total = 3 L min⁻¹). A filter, where the marker-coated particles were deposited, is introduced into the reactor (10 cm x Ø 24 mm) at different time exposures (varying between 1 and 60 min). In these experiments, ozone concentration typically ranged between 6.3 × 10¹⁴ and 8.5 × 10¹⁴ molecule.cm⁻³. In the case of β-caryophyllinic acid, a lower ozone concentration of 6.0 × 10¹² molecule.cm⁻³ was used, because of its high reactivity with O₃. The residence time of ozone in the reactor was approximately 1 s. As the ozone concentration was maintained in a stationary state and controlled along all experiments, pseudo-first-order kinetic conditions were achieved. During experiments, the reactor was covered with a black sheet to avoid any potential photolysis. All experiments were performed at room temperature (i.e. 295 K).

### 2.7. Data analysis

For each marker, kinetics were measured from the fit of normalized particulate concentration values as a function of exposure time to light or oxidants. Experimental data points were least-square fitted using simple first-order exponential function (Eq (1))

$$C_t = C_0 \exp^{-kt} \qquad (\text{Eq 1})$$

where C_t and C_0 represent the particulate marker concentrations at times t and t₀, k the exponential coefficient derived from the fit and t the exposure time.

In the case of photolysis experiments, k is a first order photolysis constant (hereafter named J, in min⁻¹). For ozone reactions, k is the pseudo-first order reaction rate constant (in min⁻¹). In order to propose second order rate constants (i.e., k¹¹ in cm³.molecule⁻¹.s⁻¹), k was divided by the ozone concentration used during exposure experiments. Finally, for OH reactions, two cases could be distinguished. For cis-pinonic and pinic acids, k¹¹ were determined as described above for ozone. For terebic acid, terpenylic acid and MBTCA, the observed decays were more complex to fit (highlighting secondary processes, see Section 3.2). Therefore, pseudo-first order reaction rate constants were determined from the traditional methodology using the slope of the initial decay tangent, to prevent from any further distortion, but allowing only upper limit rate constant determinations.

Additional experiments were conducted using only one marker on particles for photolysis, OH and ozone oxidations. For all individually coated particles, the results were in excellent agreement with those obtained using the mixture (as illustrated in Fig. S5 for cis-pinonic acid photolysis). This demonstrates that (i) the effect of the total mass of particles deposited on the filter was negligible under our experimental conditions and (ii) the presence of other markers when using the particle mixture did not alter the reactivity (either the photolysis or the

oxidation processes, as light intensity or atmospheric oxidants were maintained constant and in large excess along exposures, evidencing the first order conditions). Hence, all data points were undifferentiated and gathered to fit experimental decays.

## 3. Results and discussion

Before studying the different reactive processes, experiments were systematically performed in the same conditions as those used for the (photo-) oxidation reactions, but without light, OH or O₃, to check any potential desorption of markers during experiments. Desorption was either shown to be negligible, i.e. within the uncertainties on the marker concentration values, or included in the uncertainties of the rate constant determinations, allowing to conclude that marker loss observed in the presence of light, OH or O₃, was mainly due to these reactions only.

### 3.1. Photolysis

Photolysis of the mixture of 6 BSOA marker-coated particles was studied, leading to the photolysis rates presented in Table 2. They were derived from the exponential fit of the experimental data presented in Fig. 1 and S6. The first-order rate constants (J<sub>marker</sub>) varied from 1.8 × 10⁻³ s⁻¹ for β-caryophyllinic acid to 1.0 × 10⁻⁴ s⁻¹ for terpenylic acid, with uncertainties varying between 6 and 17 % for all compounds except for terpenylic acid (78 %) which presented more scattered experimental data points associated to a low photolysis decay.

As it can be seen, cis-pinonic acid, β-caryophyllinic acid and MBTCA were photolyzed to a certain extent under our experimental conditions, whereas pinic, terebic and terpenylic acids potential losses could not be well differentiated from desorption processes. The decays of pinic and terpenylic acids could yet be fitted by first order exponential functions. Therefore, for these two markers, photolysis rates were only proposed as upper limits (in Table 2).

Desorption experiments were carried out at three different exposure times: 2, 8, and 12 h. It can be seen that, except for terebic acid, no compound desorbed significantly. Note that the terebic acid desorption observed here is consistent with a very recent study reported by Florou et al. (2024), showing that this compound must be considered as a semi-volatile compound in the atmosphere.

For terpenylic acid (Fig. S6), most of the experimental points were centred around 95 %, but this compound exhibited much larger analytical variability than the other acids tested in this study. Terebic acid was not significantly photolyzed (Fig. S6), regardless of the exposure time, its relative concentration being centred around 80 %, a value indistinguishable from that obtained during the desorption step only.

To our knowledge, this is the first study on the photolysis of carboxylic acids adsorbed on silica particles. Therefore, no reported study allows us to easily compare our results.

Lignell et al. (2013) studied the cis-pinonic acid direct photolysis in the ultraviolet tropospheric window (280–400 nm) and showed that a Norrish type I reaction (α-C-C bond cleavage excited at singlet or triplet states) would yield minor products. The Norrish type II (intramolecular abstraction of a γ-hydrogen atom) of the compound would lead to the formation of possible isomers like limononic acid. This was also confirmed by Amorim et al. (2020), reporting the efficiency of the direct

<table><caption>**Table 2**
Photolysis constants J (min⁻¹) obtained for the photolysis reactions of 5 BSOA markers.</caption><thead><tr><td>BSOA markers</td><td>J (min<sup>-1</sup>)</td></tr></thead><tbody><tr><td>Terpenylic acid</td><td>&lt; (1.0 ± 0.8) x 10<sup>-4</sup></td></tr><tr><td>Cis-pinonic acid</td><td>(1.3 ± 0.1) x 10<sup>-3</sup></td></tr><tr><td>Pinic acid</td><td>&lt; (1.7 ± 0.3) x 10<sup>-4</sup></td></tr><tr><td>MBTCA</td><td>(6.2 ± 0.7) x 10<sup>-4</sup></td></tr><tr><td>β-caryophyllinic acid</td><td>(1.8 ± 0.2) x 10<sup>-3</sup></td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Decay of the normalized *cis*-pinonic acid (top-left), pinic acid (top-right), MBTCA (bottom-left) and *β*-caryophyllinic acid (bottom-right) concentrations (N = 10) as a function of light exposure (at 765 W m⁻²). Desorption experiments (i.e. without light exposure) are indicated by black crosses (N = 5). Error bars represent the standard deviations only.</figcaption></figure>

photolysis of *cis*-pinonic acid. Aljawhary et al. (2016) showed that the lifetime of *cis*-pinonic acid during its photolysis, related to its reactivity with OH, was 3.8 h in their experiments (performed in the aqueous phase), corresponding to an atmospheric lifetime of approximately 27 h. Meanwhile, Enami and Sakamoto (2016) studied the reactivity at the

air/water interface of *cis*-pinonic acid. They showed that its photolysis was very rapid and leads to the formation of many products, 70 % of which being re-emitted into the gas phase.

Concerning pinic acid, Amorim et al. (2020) showed that, contrary to the *cis*-pinonic acid, the pinic acid did not undergo direct photolysis in

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Decays of the normalized *cis*-pinonic acid (left, N = 11) and pinic acid (right, N = 11) concentrations as a function of OH exposure. Desorption experiments in the presence of He/NO₂ are indicated by black crosses (N = 3). Error bars represent the standard deviations from triplicates.</figcaption></figure>
their system, which is rather consistent with this study, carried out with coated particles (Fig. 1).

## 3.2. OH reaction

Kinetics of the reactions of 6 BSOA markers with OH radicals have been investigated at different exposure times: 0, 3, 7, 12, 22, 35, and 45 min. For the most reactive markers with OH, i.e. cis-pinonic and pinic acids, additional experiments were conducted at shorter times (i.e. 30 s and 1, 2, 3, and 5 min). Decay of the variations of their normalized concentrations within time are presented in Fig. 2 and S7.

Second-order rate constants ($k^{II}$) for the OH reactions of cis-pinonic acid and pinic acid were respectively equal to (1.6 ± 0.4) x 10⁻¹⁴ cm³·molecule⁻¹·s⁻¹ and (1.2 ± 0.2) x 10⁻¹⁴ cm³·molecule⁻¹·s⁻¹ at 293 K, and reactions were completed after 7 and 12 min, respectively. These values were determined for an OH concentration of 1.3 × 10¹² molecule·cm⁻³, at 65 % humidity.

Desorption and reactivity experiments with NO₂, used here in excess as the source of OH (Fig. 2 and S7) were conducted for a 45 min exposure time and results show that there was no very significant difference from initial concentrations obtained at t₀, within uncertainties, indicating their negligible impact on the fate of each marker.

Lai et al. (2015) reported a study on the heterogeneous kinetics of cis-pinonic acid with OH radicals under different environmental conditions. In this study, instead of using particles, a dry film of cis-pinonic acid was prepared then placed on a disk to investigate the kinetics. The second order rate constant $k^{II}$ was determined to be equal to (6.2 ± 1.1) x 10⁻¹² cm³·molecule⁻¹·s⁻¹, at 298 K and 40 % humidity. This value is much larger than the one measured in our study. Such a difference may be explained by the different experimental conditions (dry film against coated particles), the methodologies used to measure the kinetics (relative rate in Lai's work against absolute rate measurements here) or the source and the calibration of the OH initial concentration (absolute in our work compare to a relative OH calibration using salicylic acid).

To our knowledge, no other study has reported on the heterogeneous reactivity of any of the other 5 acids with OH, which would allow us to compare our results. It is nevertheless interesting to look at the few works reporting studies on the gas- or aqueous-phase reactions of these compounds. When studying the formation of MBTCA via the gas-phase oxidation of cis-pinonic acid, Müller et al. (2012) estimated that the rate constants for the reaction of OH with cis-pinonic acid was ranging from 0.1 × 10⁻¹² to 4.1 × 10⁻¹² cm³·molecule⁻¹·s⁻¹, depending on the site of hydrogen abstraction by OH radicals. As quoted by the authors, such estimation based on the sum of partial site-rate coefficients is subject to very large uncertainties. In the work of Amorim et al. (2020), performed in the aqueous phase, the rate constants $k^{II}$ for the reactions of pinic and cis-pinonic acids with OH were measured equal to 2.8 × 10⁻¹² to 4.8 × 10⁻¹² cm³·molecule⁻¹·s⁻¹ and to 3.5 × 10⁻¹² to 6.1 × 10⁻¹² cm³·molecule⁻¹·s⁻¹ respectively, depending on the pH values. If the rate constants measured at the gas/particle interface in this work are much lower (by a factor around 300) than those determined in the aqueous phase, it can be noted that cis-pinonic acid reacts approximately 30 % faster than pinic acid with OH in both cases. These results should be compared with those recently published by Blaziak et al. (2024), which demonstrated the significant variability in the lifetime of pinic acid, fluctuating in aqueous phase between several hours and seconds, depending on the experimental conditions. As described in the work of Amorim et al. (2021), the H-abstraction by a OH radical can occur at different reactive sites of the pinic acid molecule. This leads to the formation of peroxy radicals (RO₂), which then react with a hydroperoxy radical (HO₂), resulting in the formation of several isomers of hydroperoxides (ROOH). Depending on the reaction pathway, a wide variety of products is generated, which will ultimately lead to the formation of norpinic acid and MBTCA.

Reactions of OH with terebic, terpenylic acids and MBTCA were almost - or all - completed after 45 min, meaning that they are reacting

more slowly than cis-pinonic and pinic acids (for the same initial OH concentration value). Unlike the previous cases, the observed kinetics could not be fitted using a simple exponential function, highlighting a more complex process involving either OH or other radicals, or a new formation of the acids along the exposure time. This is particularly the case of the terpenylic acid, generating in fine as a secondary source, both terebic and 2-hydroxyterpenyl acids, when reacting with OH (Kahnt et al., 2014). Similarly, MBTCA is generated through the OH-oxidation of cis-pinonic acid and such a secondary formation may participate to the shape of the MBTCA decay within OH exposure time when studying the reactivity of coated particle mixtures.

Considering that the first part of the decays determines more specifically the main kinetic information of the reaction of OH with each acid, as it corresponds to the fastest processes, the pseudo-first-order rate constants of terebic acid, terpenylic acid, and MBTCA were determined from the slopes of the tangents (red, blue, and orange dotted lines in Fig. S7) at t₀ and are reported in Table 3. Even though there is logically a larger uncertainty on these kinetic data (compared to the cases of pinonic and pinic acids), these constants correspond to the maximum reaction rate of each marker with OH and allow to calculate lower limits of their corresponding lifetime.

Florou et al. (2024) reported very recently a study on the gas-phase oxidation of terebic acid. First, they observed that the concentration of particulate terebic acid was reduced significantly after its exposure to OH. They suggested that this was due to the importance of the fragmentation pathways during the gas-phase oxidation of terebic acid: as terebic acid was rapidly evaporating from the particulate phase in order to replace the part that reacted in the gas-phase (as a feedback in the equilibrium). Their experiments suggested that particulate terebic acid could have a lifetime of hours in the ambient atmosphere due to its gas-phase reaction with OH and because of its semi-volatile nature, and hence that it is a rather poor quantitative tracer of monoterpane SOA formation.

MBTCA was first identified as a tracer in 2007 by Szmigielski et al. and its reactivity has been little studied since. Aljawahry et al. (2016) confirmed that MBTCA is a very relevant tracer for determining aerosol aging, as it is derived from the oxidation of two compounds (C₁₀H₁₄O₅ and C₁₀H₁₄O₆) formed during the OH-initiated photolysis of pinonic acid. Under the conditions of their study, the lifetime of MBTCA in the aqueous phase was 41 h, which would correspond to an atmospheric lifetime approximately 7 times higher, i.e. around 12 days. A recent work by Blaziak et al. (2024) shown that its atmospheric lifetime — when exposed to the OH radical — was consistently less than 90 min, regardless of the experimental conditions and the aqueous fraction in which it was present (fog, cloud, rain). However, they observed that this reaction occurred at a slower rate compared to other terpenic organic acids, such as pinic acid and norpinonic acid, which is very consistent with our observations. This difference is attributed to the absence of a methyl group and the additional protection conferred to the chemical structure by the carboxyl groups. They also showed that for higher acidity, i.e. low acid conversion, their reactivity is low, in agreement with Amorim et al. (2020). More, under basic conditions (pH = 8), Amorim et al. (2021) showed that the yield of MBTCA formation following the photo-oxidation of pinic acid was 2 times higher than under acidic conditions (pH = 2), varying from 5.0 % to 2.6 %,

<table><caption>**Table 3**
Rate constants ($k^{II}$) for OH-initiated reactions of 5 biogenic SOA markers.</caption><thead><tr><th>BSOA markers</th><th>k<sup>II</sup> (cm<sup>3</sup>.molecule<sup>-1</sup>.s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Terebic acid</td><td>&lt; (5.5 ± 0.2) x 10<sup>-15</sup></td></tr><tr><td>Terpenylic acid</td><td>&lt; (1.8 ± 0.1) x 10<sup>-14</sup></td></tr><tr><td>Cis-pinonic acid</td><td>(1.6 ± 0.4) x 10<sup>-14</sup></td></tr><tr><td>Pinic acid</td><td>(1.2 ± 0.2) x 10<sup>-14</sup></td></tr><tr><td>MBTCA</td><td>&lt; (7.6 ± 0.1) x 10<sup>-15</sup></td></tr></tbody></table>
respectively.

Under our experimental conditions, the OH concentration generated in our system turned out to be too high to determine β-caryophyllinic acid reactivity. The reaction rate was so fast that its reaction constant could not be determined, even though new experiments were conducted with exposure times ranging from 10 s to 1 min, and considering that no controlled dilution of OH (with enough accuracy) was possible in lower concentration range, using our experimental set-up. Unfortunately, even with the shortest exposure time, the β-caryophyllinic acid could no more be detected on model particles. This very rapid reaction may be explained by the probable addition of OH radicals to the C=C bond of the particulate β-caryophyllinic acid. Note that the desorption (in the presence of only He/NO₂) of particulate β-caryophyllinic acid under our experimental conditions was no more than 15 % (within uncertainties) after 45 min.

If there is no study reported on the β-caryophyllinic acid reactivity, few studies have been reported on the β-caryophyllonic acid, a mono acid which present large structural similarities (but a methyl instead of a OH) with it. The reaction of β-caryophyllonic acid with OH was studied in the aqueous phase by Witkowski et al. (2019a). Its atmospheric lifetime was estimated to be between 24 min and 1.5 h, depending on the pH values. The addition of OH to the terminal C=C bond of the β-caryophyllonic acid leads to the formation of keto-β-caryophyllonic acid as well as the associated hydroxy-hydroperoxide complex related to this compound. Other products, oxygenated derivatives of β-caryophyllonic acid, were generated and exhibited a much higher O/C ratio than that of the parent compound, highlighting the aging of aerosols (Witkowski et al., 2019a; Sun et al., 2024).

### 3.3. Ozonolysis

Marker-coated silica particles were exposed to ozone at different exposure times between 1 and 60 min. As expected, due to their chemical structures (absence of a C=C double bond leading ozonide formation impossible), terebic acid, terpenylic acid, cis-pinonic acid, pinic acid, and MBTCA did not react with ozone (Fig. S8), whatever its concentration under our experimental conditions. This is also totally consistent with a previous study in the aqueous phase by Nepotchatykh and Ariya (2002), who reported a reaction rate constant for the pinic acid with ozone of $4.15 \times 10^{-23} \text{ cm}^3 \text{.molecule}^{-1} \text{.s}^{-1}$ at room temperature, concluding that ozonolysis is not a significant removal pathway for carboxylic acids.

For an ozone concentration of $7.8 \times 10^{14} \text{ molecule} \cdot \text{cm}^{-3}$, only the β-caryophyllinic acid reacted with ozone, but the observed reaction was too fast to measure properly its rate constant. After ozone concentration dilution (down to $6.0 \times 10^{12} \text{ molecule} \cdot \text{cm}^{-3}$), a second order rate constant ($k^{II}$) for the heterogeneous reaction (β-caryophyllinic acid + O₃) at the gas/particle interface (Fig. 3) was determined to be $(1.1 \pm 0.1) \times 10^{-15} \text{ cm}^3 \text{.molecule}^{-1} \text{.s}^{-1}$, at 295 K. Desorption experiments were carried out in a pure nitrogen flow rate, at three different exposure times: 10, 30, and 60 min. Again, no very significant difference was observed from initial concentrations obtained at t₀, within uncertainties due to measurement variability, indicating the negligible impact of desorption of β-caryophyllinic acid on its loss by OH reaction.

This is to our knowledge the first kinetic measurement of the reaction of ozone with β-caryophyllinic acid, whatever the phase. Van Eijck et al. (2013) reported an atmospheric chamber study on the particulate oxidation products from the ozonolysis of β-caryophyllene reaction. Five products were identified: the β-caryophyllinic acid, the β-caryophyllonic acid, the 3,3-dimethyl-2-(3-oxobutyl)cyclobutanecarboxylic acid, the β-nocaryophyllonic acid, and the 2-(2-carboxyethyl)-3,3-dimethylcyclobutanecarboxylic acid. If no kinetics data were proposed in this work, it can be noted that β-caryophyllinic acid exhibited the lowest product yield (defined in their study as the total amount of acid formed minus the amount of acid loss by further reactions) in the reaction chamber experiments, which is consistent with its possible further fast reactions

<figure><img src="image_6.png" /><figcaption>Fig. 3. Decay of the normalized concentration of β-caryophyllinic acid adsorbed on silica particles (N = 13) and exposed to ozone at different reaction times. Desorption experiments (in a pure nitrogen flow; N = 6) are indicated by black crosses. Error bars represent the standard deviations.</figcaption></figure>

with ozone or OH in their system.

As in the previous case of OH, it is interesting to compare the β-caryophyllinic acid reactivity with that of a close compound, the β-caryophyllonic acid, for which the ozone reactivity has been studied. In the work of Witkowski et al. (2019b), the rate constant for the ozonolysis of β-caryophyllonic acid ranged from $8.0 \times 10^{-16}$ to $9.9 \times 10^{-16} \text{ cm}^3 \text{.molecule}^{-1} \text{.s}^{-1}$, for an ozone concentration of $8.5 \times 10^{11} \text{ molecule} \cdot \text{cm}^{-3}$. As the chemical structure of this compound presents large similarities with that of β-caryophyllinic acid, it can be concluded that our results are very consistent with this study.

### 3.4. Atmospheric implications

Atmospheric lifetimes for polar semi-volatile organic compounds like the carboxylic acids studied in this work, selected as potential tracers of SOA formation and fate, should include the contribution of their fate through gaseous phase processes (for their volatile fraction), aqueous phase reactions (depending on relative humidity and on the pH of the condensed phase of the aerosol) and heterogeneous processes at the gas/particle interface. According to Witkowski et al. (2023), the cis-pinonic acid atmospheric lifetime depends on the water content of aerosols as well as the environment where the acid is formed and reacts, since the concentrations of oxidants vary between rural and urban environments. They estimated that the lifetime of cis-pinonic acid may fluctuate between several minutes and a few weeks, highlighting the difficulty in proposing robust atmospheric lifetimes and considering the choice of a tracer to represent different sources and oxidation processes in different environments. Therefore, the lack of information about the atmospheric reactivity of most of these compounds leads us to consider the lifetimes determined in this study only as upper limit values.

The atmospheric lifetimes associated to the heterogeneous reactions of the six particulate markers, i.e. the terebic, terpenylic, cis-pinonic, pinic, 3-methyl-1,2,3-butanetricarboxylic and β-caryophyllinic acids were derived from both experimental reaction rate constants measured at the gas/particle interface in this work and typical atmospheric oxidant concentrations. The total lifetime of each marker was then derived from the following equation (Eq (2)) and is reported in Table 4:

$$ \tau_{(tot)} = \frac{1}{\frac{1}{\tau_{(h)}} + \frac{1}{\tau_{(OH)}} + \frac{1}{\tau_{(O3)}}} \qquad (\text{Eq 2}) $$
<caption>**Table 4**</caption>

Atmospheric lifetimes (in min, hours or days) related to the heterogeneous processes of the 6 studied biogenic SOA markers. Values for the total lifetimes of terebic, terpenylic and pinic acids represent lower limits whereas that of β-caryophyllinic acid is an upper limit.

<table><thead><tr><th rowspan="2">Markers</th><th colspan="3">Lifetimes related to heterogeneous processes</th><th rowspan="2">TOTAL τ</th></tr><tr><th>O<sub>3</sub></th><th>hν</th><th>OH</th></tr></thead><tbody><tr><td>Terebic acid</td><td>-</td><td>-</td><td>>2107 d</td><td>>2107 d</td></tr><tr><td>Terpenylic acid</td><td>-</td><td>>4.8 d</td><td>>637 d</td><td>>4.8 d</td></tr><tr><td>Cis-pinonic acid</td><td>-</td><td>8.6 h</td><td>722 d</td><td>8.6 h</td></tr><tr><td>Pinic acid</td><td>-</td><td>>2.9 d</td><td>935 d</td><td>>2.8 d</td></tr><tr><td>MBTCA</td><td>-</td><td>19.0 h</td><td>>1518 d</td><td>19.0 h</td></tr><tr><td>β-caryophyllinic acid</td><td>12–18 min</td><td>7.0 h</td><td>see text</td><td><14.3 min</td></tr></tbody></table>

As photolysis is a first-order process, the related atmospheric lifetimes of each marker, measured thanks to the solar simulator, were simply calculated considering the equation (Eq (3)):

$$ \tau_{(h\nu)} = \frac{1}{J_{\text{marker}}} \qquad (\text{Eq 3}) $$

where J is the photolysis rate constant (in time⁻¹).

Photolysis appears in this study the fastest degradation process (except for β-caryophyllinic acid) for the marker-coated particles. Note that regarding the gas-phase, Eger et al. (2021) reported a complete study on the atmospheric implications of the photolysis of another carboxylic acid, i.e. the pyruvic acid, showing the importance of photolysis as one of its major sinks in the boundary layer, with a mean lifetime of (2 ± 0.5) h.

The lifetimes related to the OH-initiated oxidation of markers were calculated from the pseudo-first-order rate constants measured in this work, assuming a global average concentration of hydroxyl radicals of 1.0 × 10⁶ molecule·cm⁻³ in the atmosphere (Lawrence et al., 2001; Bloss et al., 2005). These values, on the order of several hundred days, indicate that despite certain uncertainties, the stability of these compounds through heterogeneous reactions with hydroxyl radicals has been demonstrated. Although it has been many times demonstrated in the literature that heterogeneous reactions of particulate organics with OH may be slow (e.g. Estève et al., 2004; Miet et al., 2009; Mattei et al., 2019), such observation is very far from the results reported by Lai et al. (2015) (as discussed in section 3.2), where the average lifetime of cis-pinonic acid associated to its heterogeneous reaction with OH radicals was determined to be (2.3 ± 0.3) days, or by Wang et al. (2025), who determined that the lifetimes of cis-pinonic and pinic acids ranged between 1 h and 11 h, depending on the relative humidity in their system.

Due to experimental constrains, it was not possible to investigate the reactivity of β-caryophyllinic acid with OH (see section 3.2). However, assuming that β-caryophyllinic and β-caryophyllonic acids present similar reactivities with OH, a fast reaction rate constant in the range (10⁻¹² – 10⁻¹⁰) cm³·molecule⁻¹·s⁻¹ would lead to an associated lifetime of 12 d–3 h, much longer anyway compared to its loss through ozonolysis.

β-caryophyllinic acid was the only marker reacting with ozone. Its related lifetime was determined considering two different O₃ concentrations: the longer lifetime was calculated using an annual average atmospheric ozone concentration of 36.5 ppb (i.e. 9.0 × 10¹¹ molecule·cm⁻³) (Guion et al., 2023; Ferretti et al., 2024; Real et al., 2024), while the shorter was calculated based on a O₃ concentration of 55.1 ppb (i.e. 1.3 × 10¹² molecule·cm⁻³), representative of levels reached during a heatwave episode (Guion et al., 2023). For the other acids, the heterogeneous reaction rate constants resulting from this work allowed for an estimation of a global lifetime for each marker (or only lower limits in the case of terebic, terpenylic and pinic acids). Among the six markers studied in this work, the β-caryophyllinic acid exhibited the shortest lifetime, followed by cis-pinonic acid, MBTCA, pinic acid, terpenylic acid, and terebic acid. This is consistent with the observations of Van

Eijck et al. (2013), who reported that one of the major carboxylic acids as oxidation products from β-caryophyllene in ambient air samples (in Hyytiälä during the HUMPPA campaign) was the β-nocaryophyllonic acid, with concentrations generally higher than those of β-caryophyllinic acid, suggesting that β-nocaryophyllonic acid could be more suitable as a tracer compound for β-caryophyllene secondary organic aerosol formation.

As a result of this work, it can be noted that the usual assumption, based on the argument that chemical compounds presenting structural similarities (like carboxylic acids here) may display similar reaction rate constant values, implying that organic aerosol compounds can be lumped to one single or only few compounds in three-dimensional chemical transport models to reduce model complexity is not verified.

It was beyond the scope of this study to present more than a detailed investigation of the stability of the present markers towards some of the main atmospheric degradation processes, as almost no mechanistic information was available in the literature on heterogeneous reactions of BSOA markers. In a next step, it will be interesting to assess the importance of NO₃ heterogeneous reactivity, and for instance to check whether chemical mechanisms rely on gaseous phase-like reactions involving radical intermediates or if the reactions take place primarily on the surfaces. Additional work (including theoretical approaches) will be necessary to carefully describe step-by step oxidation products and mechanisms involved in the heterogeneous reactions of carboxylic acids in the atmosphere. Finally, the importance of the choice and properties (size, specific area, hygroscopycity) of selected model particles will be assessed using different models like well-characterized carbonaceous particles, to be ultimately confronted to real BSOA particles collected on the field.

In summary, this work is the first study providing new kinematic parameters describing the stability of some widely used biogenic SOA tracers, by investigating the heterogeneous photolysis and reactivity of six carboxylic acids with OH and O₃. In contrast to terebic, terpenylic and pinic acids, which display a sufficient stability, that of β-caryophyllinic acid, cis-pinonic acid and MBTCA, especially against photolysis or ozonolysis processes, the stability of these compounds is not sufficient to consider them as good atmospheric tracers of biogenic secondary organic aerosols.

It remains many uncertainties in studying the stability of biogenic SOA tracers or markers. As seen above, many studies present contrasted results, demonstrating the need of further investigation of the reactivity of such compounds in laboratory. So far, the comparison between measurements of gaseous and particulate species in ambient air at different specific sites and the concentrations calculated by chemistry-transport models used to simulate air quality or climate change has shown, for example, that the secondary fraction of organic aerosols is often underestimated by the models (Lane et al., 2008; Barsanti et al., 2013; Lanzafame et al., 2022). The parameterization of the lifetimes determined in this work could help to improve aerosol source attribution methods by integrating these new information into models.

### CRediT authorship contribution statement

**Pauline Pouyes:** Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. **Judith Lorin:** Methodology, Investigation, Formal analysis, Conceptualization. **Pierre-Marie Flaud:** Methodology, Investigation, Formal analysis, Conceptualization. **Emmanuel Geneste:** Methodology, Investigation. **Hélène Budzinski:** Methodology, Investigation. **Emilie Perraudin:** Writing – review & editing, Supervision, Project administration, Methodology, Investigation, Formal analysis, Conceptualization. **Eric Villenave:** Writing – review & editing, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
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.

Acknowledgements

The authors wish to thank the University of Bordeaux for its financial support of the PhD grant of Pauline Pouyes. Dr. Mahmoud Idir and Dr. Véronique Daële (from ICARE, CNRS Orléans, France) and Prof. Dr. Thorsten Hoffmann (Johannes Gutenberg University, Mainz, Germany) are gratefully acknowledged for the loan of the Nd-YAG laser during the study and for providing the β-caryophyllinic acid standard, respectively. Karyn Le Menach (from EPOC UMR 5805 CNRS - Univ. Bordeaux, France) is gratefully acknowledged for her help in analytical developments.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2025.144453.

Data availability

The authors do not have permission to share data.

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