Comparison of oxidation products generated from the reaction of α-pinene with hydroxyl radicals, chlorine atoms, and bromine atoms measured using ammonium adduct chemical ionization mass spectrometry

Abstract

Halogen atoms play important but undercharacterized roles in atmospheric oxidation chemistry. Here, we report laboratory measurements of gas- and condensed-phase products formed from the oxidation of α-pinene by hydroxyl radicals (OH), chlorine atoms (Cl), and bromine atoms (Br) in an oxidation flow reactor (OFR). Products were detected using a Vocus proton-transfer time-of-flight reaction mass spectrometer (PTR-ToF-MS) operated with low-pressure ammonium adduct (NH₄⁺) ionization and a Vaporization Inlet for Aerosols (VIA). We applied Positive Matrix Factorization (PMF) to classify precursor and product ions into early and later-generation oxidation products. While some common products were observed across all oxidants, significant compositional differences were also apparent. Vocus : VIA signal ratios were used to estimate volatility trends, revealing that more highly oxygenated compounds and many halogenated products contributed to SOA formation. Cl and Br oxidation led to the formation of oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA), which retained halogen atoms, with Br-derived products exhibiting the lowest carbon oxidation state and the highest halogen retention. Halogenated oxidation products were less volatile than their non-halogenated counterparts. Photochemical modeling suggests that the fates of organic peroxy radicals (RO₂) were primarily influenced by RO₂ + HO₂ reactions for α-pinene/OH, RO₂ + Cl, RO₂ + HO₂, and potentially RO₂ isomerization/autooxidation reactions for α-pinene/Cl, and RO₂ + Br reactions for α-pinene/Br.

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Environmental significance

Chlorine (Cl) and bromine (Br) atoms are increasingly recognized as important atmospheric oxidants, especially in coastal, marine, and polar environments. This study provides a comprehensive molecular-level comparison of products generated from the oxidation of α-pinene by OH, Cl, and Br. Our analysis reveals distinct chemical pathways and product distributions for each oxidant. By identifying unique halogenated products and quantifying their volatility, this work highlights the need to account for halogen-specific chemistry in models. Our results contribute to an improved understanding of how halogen initiated oxidation of volatile organic compounds influences SOA formation and oxidative aging and support the development of more accurate atmospheric chemical mechanisms.

1 Introduction

Atmospheric oxidation processes play a central role in the transformation of organic and inorganic compounds, with gas-phase oxidants such as ozone (O₃), hydroxyl radicals (OH), nitrate radicals (NO₃), chlorine atoms (Cl), and bromine atoms (Br) initiating these reactions. The relative importance of these oxidants varies with regional meteorology, emissions, and photochemical conditions. Among them, OH is the most globally significant due to its rapid daytime production and its broad reactivity with atmospheric constituents. OH initiates key processes, including the oxidation of sulfur dioxide (SO₂) to sulfuric acid and the transformation of volatile organic compounds (VOCs) into low-volatility products that contribute to secondary organic aerosol (SOA) formation. While NO₃ dominates nighttime oxidation in certain regions, halogen atoms offer unique daytime oxidation pathways, especially in marine, polar, and urban atmospheres.^1,2 Cl is especially relevant in polluted coastal,^3–5 urban regions,^6,7 and other inland sources^8,9 and can react with many VOCs with rate coefficients that are 10 to 100 times greater than OH (Fig. 1a). This reactivity is particularly enhanced for alkanes and siloxanes, where oxidation proceeds exclusively via hydrogen abstraction. Only a small subset of VOCs, such as benzene and naphthalene, exhibit lower reactivity toward Cl than OH. However, Cl does not significantly react with SO₂, limiting its role in sulfate aerosol formation. Br is especially active in the polar boundary layer,¹⁰ where heterogeneous reactions on snow and ice surfaces release reactive bromine species that drive episodic O₃ depletion and oxidant cycling.^11,12 While Br is highly reactive with specific VOC classes such as alkenes and aldehydes, it is generally less reactive with alkanes, aromatics, and alcohols (Fig. 1b). Although Br has a high electron affinity, its H-abstraction reactions are typically endothermic, making them less favorable due to added reaction enthalpy and activation energy. Aldehydes are an exception: the formyl C–H bond is much weaker than other C–H bonds, making H-abstraction by Br nearly thermoneutral.

Fig. 1 Rate coefficients for reactions of (a) Cl and (b) Br versus OH with various classes of VOCs.^13–120 Solid and dashed lines indicate fixed ratios of halogen-to-OH reactivity: 100 : 1, 10 : 1, and 1 : 1 in (a), and 1 : 1, 1 : 10, and 1 : 100 in (b).

The role of Cl and Br in the oxidation of biogenic VOCs (BVOCs) has received far less attention than O₃, OH, and NO₃, particularly for α-pinene – a globally relevant BVOC often used as a model compound due to its well-characterized OH oxidation chemistry. α-Pinene reacts with Cl at near gas-kinetic rates^53,121 and with Br at a rate roughly half that of OH.³⁷ While early studies primarily focused on reaction kinetics, more recent work has begun to examine the chemical composition of SOA formed from Cl-and Br-initiated α-pinene oxidation. Cl-driven oxidation has been shown to generate highly oxygenated molecules – including chlorinated products – with SOA yields that are comparable to or greater than those from OH.^122–125 Far fewer studies have investigated Br-initiated α-pinene oxidation, but existing results indicate very low SOA yields.^125,126 These findings highlight the potential for halogen atoms to significantly alter SOA composition and yield in environments influenced by reactive halogen chemistry, while also underscoring the limited availability of comparative data across oxidants under consistent experimental conditions.

In a companion study, we characterized the chemical composition and yield of laboratory SOA generated from the OH and Cl oxidation of n-dodecane and toluene, and the OH, Cl, and Br oxidation of isoprene and α-pinene.¹²⁵ While OH and Cl produced comparable SOA yields with oxygen-to-carbon (O/C) ratios indicative of multigenerational aging, Br-mediated SOA formation was more limited, yielding lower O/C values. These findings challenge model assumptions that Cl and Br produce SOA with similar efficiency.¹²⁷ To build on that work, this study characterizes the detailed molecular composition of gas- and condensed-phase products obtained from the reaction of α-pinene with OH, Cl, and Br in an oxidation flow reactor (OFR). Using an ammonium-adduct Vocus proton-transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) equipped with a vaporization inlet for aerosols (VIA), we detected a wide range of gas- and condensed-phase oxidation products. We applied PMF to classify these products and compared their composition and volatility across the three oxidants. These measurements provide new insight into the distinct pathways and product distributions associated with halogen- versus OH-mediated oxidation, with implications for the treatment of halogen chemistry in atmospheric models.

2 Experimental

2.1 Oxidation flow reactor setup

Experiments were conducted inside a Potential Aerosol Mass (PAM) OFR (Aerodyne Research), a 13 L horizontal aluminum cylindrical chamber (46 cm long × 22 cm ID) operated in continuous flow mode with a total flow of 6.0–6.8 L min⁻¹, yielding a calculated mean residence time (τ_OFR) of 114–130 s. An electroconductive Teflon coating was applied to the OFR to improve chemical compatibility with halogen precursors while maintaining high gas and particle transmission.^125,128 Two low-pressure mercury (Hg) lamps housed in type 214 quartz sleeves were used to photolyze oxidant precursors, with UV output regulated by a fluorescent dimming ballast (IZT-2S28-D, Advance Transformer Co.). The UV irradiance was controlled by adjusting the ballast control voltage (1.5–10 VDC) and measured using a TOCON-GaP6 photodetector (sglux GmbH). The corresponding actinic flux ranged from approximately 1 × 10¹⁴ to 3 × 10¹⁵ photons cm⁻² s⁻¹.^129,130

2.2 Oxidant, OVOC and SOA generation

OH was generated by photolyzing O₂ and H₂O at λ = 185 nm and O₃ at λ = 254 nm using two low-pressure Hg lamps. The relative humidity (RH) was controlled to 31–43% using a Nafion humidifier (Perma Pure); corresponding H₂O mixing ratios were 1.0–1.5% at OFR temperatures of 26–29 °C. The integrated OH exposure (OH_exp) in the OFR, defined as the product of the mean OH concentration and τ_OFR, was calculated using an empirical estimation equation¹³⁰ and ranged from 6.7 × 10¹⁰ to 1.0 × 10¹² molecules cm⁻³ s (hereafter “cm⁻³ s”) – equivalent to 0.5–8 days of atmospheric oxidation at OH = 1.5 × 10⁶ cm⁻³.¹³¹ The estimated uncertainty in calculated OH_exp values was ±50%. Cl was produced by photolyzing 4.2 ppm oxalyl chloride (C₂Cl₂O₂) at λ = 254 or 313 nm,^125,132,133 and Br was generated by photolyzing 1.8 ppm oxalyl bromide (C₂Br₂O₂) at λ = 254 nm.^125,134,135 These C₂Cl₂O₂ and C₂Br₂O₂ mixing ratios were chosen to ensure >99.5% consumption of α-pinene by reaction with Cl or Br at maximum actinic flux in the OFR. The RH in Cl and Br experiments was maintained at 1.1–4.1%; corresponding H₂O mixing ratios were 0.04–0.12% at OFR temperatures between 24–28 °C. Integrated Cl and Br exposures (Cl_exp, Br_exp) were characterized via offline calibration using O₃ decay measurements, with estimated uncertainties of ±70%.¹²⁵ Estimated Cl_exp values ranged from 4.6 × 10⁸ to 1.3 × 10¹¹ cm⁻³ s, and Br_exp values ranged from 4.5 × 10¹¹ to 3.7 × 10¹² cm⁻³ s, corresponding to 2 h – 25 days ([Cl] = 6 × 10⁴ cm⁻³) and 0.7–6 days ([Br] = 7 × 10⁶ cm⁻³) of equivalent atmospheric exposure.¹³⁶ Gas-phase OH, Cl, or Br oxidation of α-pinene led to the formation of OVOCs and SOA via homogeneous nucleation. α-Pinene (10% (v/v) in carbon tetrachloride) was injected into the OFR carrier gas flow at 0.94–2.8 µL h⁻¹ using a syringe pump, yielding ∼30 ppbv for OH and Cl experiments and ∼90 ppbv for Br experiments to ensure nucleation.¹²⁵

2.2.1 Caution. This study involved preparing polyfluorotetraethylene (PTFE) permeation tubes filled with liquid C₂Cl₂O₂ and C₂Br₂O₂,¹²⁵ which are toxic, corrosive, and release harmful gases upon decomposition. C₂Cl₂O₂ and C₂Br₂O₂ permeation tubes were prepared in a well-ventilated fume hood with appropriate personal protective equipment, including gloves, lab coats, and eye protection. O₃ is a strong oxidant and respiratory irritant. Exhaust flows containing O₃, C₂Cl₂O₂ or C₂Br₂O₂ were vented to a laboratory fume exhaust system to prevent exposure and accumulation.

2.3 Instrumentation

Aerosol number concentrations and size distributions were measured using a scanning mobility particle sizer (SMPS; TSI), and aerosol mass spectra were acquired using an Aerodyne long high-resolution time-of-flight aerosol mass spectrometer (L-ToF-AMS). Gas- and condensed-phase organic compounds were measured with a Vocus 2R PTR-ToF-MS¹³⁷ (hereafter referred to as “Vocus”) following low-pressure ammonium adduct (NH₄⁺) ionization.^138,139 The system alternated between gas-phase and particle-phase sampling using a PTFE solenoid valve manifold (Fig. 2). During gas sampling, the Vocus sampled the OFR through an unheated 0.25 in. o.d. fluorinated ethylene propylene (FEP) line with a PTFE membrane filter to remove particles. For particle sampling, the Vocus sampled the OFR through a charcoal denuder that removed gas-phase organics before evaporation of aerosols in a heated (220 °C) Sulfinert-coated stainless-steel VIA^140–142 at a flow of 1.5 slpm.

Fig. 2 Schematic of (a) gas and (b) particle sampling Vocus configurations used in these experiments. During gas sampling (a), the OFR was sampled through an unheated 0.25 in. o.d. FEP inlet line that was connected to Vocus atmospheric pressure ports A1 and A2. Aerosols were removed using a PTFE membrane filter upstream of three-way valve V1, and sample flow pulled through the VIA was removed through inlet pump port P1. During particle sampling (b), the OFR was sampled through an activated charcoal denuder and the VIA (T = 220 °C), while the unheated FEP inlet line was isolated from the Vocus by switching V1 and closing on/off valve V2. Constant flows were continuously pulled through P1 and either A1 and A2 or pumped to exhaust, while 0.1 L min⁻¹ flow was continuously subsampled into the Vocus PTR reactor.

2.4 Data analysis

L-ToF-AMS mass spectra were analyzed using SQUIRREL version 1.63I and PIKA version 1.23I¹⁴³ software, and elemental analysis of high-resolution L-ToF-AMS spectra was performed using the “Improved-Ambient” method.^143,144 High-resolution Vocus mass spectra were analyzed using Tofware version 3.2.5.¹⁴⁵ To aid in the interpretation of Vocus mass spectra, PMF¹⁴⁶ was applied to NH₄⁺ adduct signals with even m/z values between m/z = 150 and 350, exported from Tofware. This range and selection minimized interference from low-mass background and fragment ions in the mass spectra. Ion-specific errors were calculated as 1.28,¹⁴⁷ where I is the high-resolution peak height. PMF was used primarily to group ions with similar temporal behavior trends and to qualitatively apportion ions contributing to multiple factors. Separate PMF analyses were performed for the α-pinene/OH, α-pinene/Cl, and α-pinene/Br systems using the PMF Evaluation Tool (PET) version 3.08C,¹⁴⁸ each yielding a four-factor solution representing precursors and early- and late-generation products. Three-factor PMF tended to combine precursor and product signals, often with higher residuals. Solutions with five or more factors led to the splitting of existing factors without identifying new product groups of products or significantly reducing residuals. To avoid artificially forcing ion signals to zero in specific factors, no rotations were applied.

2.5 Effective saturation concentration (C°) calculations

C° values for C₁₀H_14,16O_x compounds were calculated using the empirical formula: ,¹⁴⁹ where n_C and n_O are the carbon and oxygen numbers of each compound. Vocus:VIA-Vocus ratios of measured C₁₀H_14,16O_x compounds were plotted against their calculated C° values and fit to sigmoid or polynomial regression equations. These equations were then used to infer C° for C₁₀H_13,15ClO_x and C₁₀H_13,15BrO_x from their measured Vocus:VIA-Vocus ratios. This analysis was restricted to C₁₀H_14,16O_x, C₁₀H_13,15ClO_x and C₁₀H_13,15BrO_x signals to minimize the effect of possible thermal decomposition reactions in the VIA on the calculated C° values.

2.6 Peroxy radical fate modeling

To explore the fate of RO₂ generated from OH, Cl, and Br oxidation of α-pinene, we used the KinSim chemical kinetic solver¹⁵⁰ with a simplified mechanism adapted from prior OFR studies.^{125,130,151,152} In the model, RO₂ were generated from the reactions

APINENE + OH → RO₂

APINENE + Cl → RO₂

APINENE + Br → RO₂

While RO₂ composition varies across the OH, Cl, and Br systems, we assumed that rate coefficients for RO₂ remained the same due to the lack of kinetic data. Following the approach introduced by Peng et al.¹⁵³ and used in previous studies by our group^141,154 we applied kinetic data and reaction pathways associated with reactions between Cl/Br and the methylperoxy radical (CH₃O₂) as a surrogate RO₂ species. These RO₂ were assumed to undergo autooxidation via isomerization, react with other RO₂, and/or react with hydroperoxyl radicals (HO₂):

RO₂ + O₂ → RO_2,isom

RO₂ + RO₂ → 2RO + O₂

RO₂ + RO₂ → ROH + R(O) + O₂

RO₂ + RO_2,isom → 2RO + O₂

RO₂ + RO_2,isom → ROH + R(O) + O₂

RO₂+HO₂ → ROOH

RO_2,isom + HO₂ → ROOH

Here, RO, ROH, R(O), and ROOH denote generic alkoxy radical, alcohol, carbonyl, and organic peroxide species, respectively. In the α-pinene/OH system, we assumed RO₂ reacted with OH to generate ROH and O₂,¹⁵⁵ and that RO isomerized and decomposed to generate pinonaldehyde (PINAL) and HO₂:¹⁵⁶

RO₂ + OH → ROH + O₂

RO_2,isom + OH → ROH + O₂

RO + O₂ → PINAL + HO₂

In the α-pinene/Cl system, we assumed RO₂ reacted with Cl to generate RO and ClO,⁴⁴ and that RO₂ reacted with ClO to generate RO and ClO₂ or organic chlorides (ROCl) and O₂.⁵⁹ We also assumed that all RO generated from RO₂ + RO₂ and RO₂ + Cl reactions were chlorinated, and that they subsequently isomerized and decomposed to yield either chloropinonaldehyde (CHLOROPINAL) and HO₂, or pinonaldehyde and Cl,¹²² with branching ratios of 0.33 and 0.67 respectively:¹²⁶

RO₂ + Cl → RO + ClO

RO_2,isom + Cl → RO + ClO

RO₂ + ClO → RO + ClO₂

RO_2,isom + ClO → RO + ClO₂

RO₂ + ClO → ROCl + O₂

RO_2,isom + ClO → ROCl + O₂

RO + O₂ → CHLOROPINAL + HO₂

RO + O₂ → PINAL + Cl

In the α-pinene/Br system, we assumed RO₂ reacted with Br to generate RO and BrO,¹⁵⁷ and that RO₂ reacted with BrO to generate RO₂H and HOBr.^158,159 We also assumed all RO generated from RO₂ + RO₂ and RO₂ + Br were brominated, and that they isomerized and decomposed to produce either bromopinonaldehyde (BROMOPINAL) and HO₂, or pinonaldehyde and Br, with branching ratios of 0.66 and 0.34 respectively:¹²⁶

RO₂ + Br → RO + BrO

RO_2,isom + Br → RO₂

RO₂ + BrO → RO₂H + HOBr

RO_2,isom + BrO → RO₂H + HOBr

RO + O₂→BROMOPINAL + HO₂

RO + O₂ → PINAL + Br

Finally, RO₂ were regenerated following reaction of ROH, R(O), ROOH, RO₂H, ROCl, PINAL, CHLOROPINAL, and BROMOPINAL with OH, Cl, and Br:

ROH + OH/Cl/Br → RO₂

R(O) + OH/Cl/Br → RO₂

ROOH + OH/Cl/Br → RO₂

ROCl + Cl → RO₂

PINAL + OH/Cl/Br → RO₂

CHLOROPINAL + Cl → RO₂

BROMOPINAL + Br → RO₂

RO₂H + Br → RO₂

The kinetic parameters used in these calculations^{16,19,37,44,50,53,59,79,107,156,158,160–165} are listed in Table S2, with the following additional assumptions:

1. A bimolecular rate coefficient of 1.6 × 10⁻¹⁰ cm³ s⁻¹ for RO₂ + Br → RO + BrO.

2. Rate coefficients for ROH + Cl, R(O) + Cl, and ROOH + Cl reactions were 10 times faster than the corresponding ROH + OH, R(O) + OH, and ROOH + OH rate coefficients.

3. Rate coefficients for ROH + Br, R(O) + Br, and ROOH + Br reactions were 100 times slower than the α-pinene + Br rate coefficient.

4. Secondary OH production from halogen-initiated α-pinene oxidation was negligible.

Simulations were run with and without isomerization pathways, assuming first-order isomerization rate coefficients of 0, 0.1, 1, and 4 s⁻¹.^163,165 Results from these simulations were used to calculate the fractional loss of generic RO₂ (denoted F_RO2) as a function of OH_exp, Cl_exp, and Br_exp.

3 Results & discussion

3.1 Overview of results obtained from alternating Vocus and VIA-Vocus measurements

Fig. S1 shows example time series of Vocus signals collected during an α-pinene/Cl experiment, alternating between Vocus and VIA-Vocus sampling modes every 15 min. Fig. S1a displays Cl_exp, which was varied in discrete steps from 0 to 1.2 × 10¹¹ cm⁻³ s throughout the experiment, and Fig. S1b–e show representative ammonium adduct ion signals exhibiting distinct trends with Cl_exp. In Fig. S1b, the signal for unreacted α-pinene (NH₄⁺·C₁₀H₁₆) was highest at low Cl_exp and decreased with increasing Cl_exp, as expected. At low Cl_exp, NH₄⁺·C₁₀H₁₆ signal was enhanced during Vocus sampling periods but was negligible during VIA-Vocus sampling periods due to its removal by the charcoal denuder upstream of the VIA. Fig. S1c shows that NH₄⁺·C₁₀H₁₅ClO₂ also peaked during Vocus sampling and reached maximum intensity at intermediate Cl_exp, suggesting it is an early-generation gas-phase oxidation product. Its decline at higher Cl_exp likely reflects further oxidation to more functionalized compounds. In Fig. S1d, both NH₄⁺·C₃H₆O and its chlorinated analog NH₄⁺·C₃H₅ClO plateau at intermediate-to-high Cl_exp, consistent with their formation as fragmentation products from α-pinene and/or its early-generation oxidation products. Finally, Fig. S1e shows NH₄⁺·C₁₀H₁₄O₇ and NH₄⁺·C₁₀H₁₃ClO₇, which peaked at higher Cl_exp and were enriched during VIA-Vocus sampling, indicating their low volatility and likely contribution to SOA. Similar trends were observed in α-pinene/OH and α-pinene/Br experiments. Additional compounds with analogous temporal behavior were identified with PMF and are discussed in the next section.

To examine how Vocus : VIA-Vocus signal ratios relate to compound volatility, Fig. 3 presents three panels showing the ratio of Vocus to VIA-Vocus signals for C₁₀ species detected in α-pinene/OH, α-pinene/Cl, and α-pinene/Br experiments, plotted as a function of oxygen number. Symbols are colored by C°, ranging from approximately 10⁷ µg m⁻³ (purple) to 10⁻⁸ µg m⁻³ (red). In Fig. 3a, α-pinene/OH products show a clear trend: as oxygen number (n_O) increases from 0 to 8, the Vocus : VIA-Vocus ratio decreases from approximately 137 to 0.4, C° decreases. Compounds with n_O ≈ 0–2 exhibit high C° values (10⁴–10⁷ µg m⁻³), consistent with dominant gas-phase behavior, and compounds with n_O = 3–5 and C° ≈ 0.1–10 µg m⁻³ show comparable signals in both configurations, consistent with semivolatile behavior. Highly oxygenated species (n_O ≥ 6) exhibit C° < 10⁻³ µg m⁻³ and low Vocus : VIA-Vocus ratios, indicating low volatility and likely contribution to SOA.

Fig. 3 Ratio of Vocus to VIA-Vocus signal intensities (Vocus : VIA) for C₁₀ ammonium adducts as a function of oxygen number, measured during α-pinene oxidation experiments with (a) OH, (b) Cl, and (c) Br as the primary oxidants. Circle symbols represent non-halogenated species (NH₄⁺·C₁₀H_14,16O_x), triangles represent chlorinated species (NH₄⁺·C₁₀H_13,15ClO_x), and squares represent brominated species (NH₄⁺·C₁₀H_13,15BrO_x). Colors indicate estimated volatility (C°) from 10⁻⁹ to 10⁷ µg m⁻³. C° values were calculated using methods described in Section 2.5.

Fig. 3b compares Vocus : VIA-Vocus ratios for non-halogenated (circles) and chlorinated (triangles) species from α-pinene/Cl experiments. Both groups show decreasing Vocus : VIA-Vocus ratios and C° with higher n_O. Chlorinated compounds generally have similar or slightly lower C° than their non-halogenated counterparts. Cl-containing species with n_O = 1–2 behave predominantly as gases, n_O = 3 are semivolatile, and n_O ≥ 4 mostly partition to the condensed phase. Fig. 3c shows analogous results for brominated products (squares), alongside non-halogenated compounds (circles) from α-pinene/Br experiments. Similar trends are observed: Vocus : VIA-Vocus ratios and C° values decrease with n_O. Br-containing compounds with n_O = 1–2 are mainly in the gas phase, n_O = 3 are semivolatile, and n_O ≥ 4 are low-volatility and likely mostly in the condensed-phase. Overall, Fig. 3 demonstrates a strong correlation between n_O and volatility. Higher n_O corresponds to lower Vocus : VIA-Vocus ratios and C° values, indicating enhanced partitioning to the condensed phase. The presence of Cl or Br further modulates this trend, generally lowering volatility and increasing the likelihood of SOA formation relative to non-halogen species with equal n_O. To extend the volatility estimation framework of Donahue et al.,¹⁴⁹ which relates log(C°) to molecular composition for CHO compounds, we adapted the formulation to include chlorinated (CHOBr) and brominated (CHOBr) species. The modified expression is:

where n_Cl and n_Br are the numbers of Cl and Br atoms, and b_Cl and b_Br represent their respective contributions to log(C°). For compounds with n_O ≥3, Fig. 3b and c suggest that Cl and Br decrease volatility comparably to the addition of ∼2 and ∼4 oxygen atoms, respectively; that is, b_Cl ≈ 2b_O and b_Br ≈ 4b_O. Assuming b_O ≈ 2.3,¹⁴⁹ we arrive at the following revised formulation:

While beyond the scope of this work, a more comprehensive characterization of b_Cl and b_Br using the approach of Li et al.¹⁶⁶ should be explored in future studies.

3.2 Molecular characterization of α-pinene/OH, α-pinene/Cl, and α-pinene/Br oxidation products

3.2.1 α-Pinene/OH. Fig. 4 presents results from PMF analysis of α-pinene/OH oxidation products detected with the Vocus. Four factors were identified, each characterized by its evolution with OH_exp (Fig. 4a, c, e and g) and corresponding Kendrick Mass Defect (KMD) plots (Fig. 4b, d, f and h), where symbol size is proportional to signal intensity. Factor 1 (“VOC/OVOC1”) consists primarily of gas-phase species detected in Vocus mode (Fig. 4a). Its signal peaks at the lowest OH_exp and decreases rapidly, approaching zero by approximately 4 × 10¹¹ cm⁻³ s. The dominant signals include α-pinene (NH₄⁺·C₁₀H₁₆), NH₄⁺·C₁₀H₁₄O and NH₄⁺·C₁₀H₁₆O (Fig. 4b). These signals likely represent unreacted precursor and either α-pinene impurities that the Vocus is highly sensitive to, or early-generation oxidation products that PMF could not fully separate from α-pinene due to their fast formation and consumption. Factor 2 (“OVOC2”) also comprises gas-phase compounds, peaking at OH_exp ≈ 1 × 10¹¹ cm⁻³ s before decreasing (Fig. 4c.). Major signals include NH₄⁺·C₁₀H₁₆O₂ – possibly contributed from pinonaldhyde, 2-hydroxy-3-pinanone, and/or α-pinene hydroperoxide^156,167 – and NH₄⁺·C₉H₁₄O₄, likely representing pinic acid¹⁶⁷ (Fig. 4d). Other species include NH₄⁺·C₁₀H₁₄O₂, NH₄⁺·C₁₀H₁₆O, and various C₈–C₁₀ compounds. Factor 3 (“OVOC3”) peaks at OH_exp ≈ 4 × 10¹¹ cm⁻³ s and then gradually declines (Fig. 4e). Like the previous factors, it primarily consists of gas-phase species. Although NH₄⁺·C₃H₆O was excluded from the PMF analysis because it was below m/z 150 (Sect. 2.4), its temporal trend matches this factor. Major constituents include NH₄⁺·C₅H₈O₄ and NH₄⁺·C₅H₈O₅, with additional contributions from NH₄⁺·C₄H₆O_5–6, NH₄⁺·C_6,7H₈O_4–7, NH₄⁺·C₇H₁₀O_4–6, and NH₄⁺·C₈H₁₂O_2–4 (Fig. 4f). These species generally have lower n_C and higher n_O than those in Factors 1 and 2. Factor 4 (“SOA”) includes 70 signals dominated by low-volatility, condensed-phase compounds and peaks at OH_exp = 2.3 × 10¹¹ cm⁻³ s (Fig. 4g), suggesting formation via continued oxidation of Factor 2 compounds. The KMD plot shows a complex mixture of highly oxygenated C₄ to C₁₀ compounds (Fig. 4h).

Fig. 4 Results from PMF analysis of Vocus and VIA-Vocus measurements of the α-pinene + OH system. (a, c, e and g) Signals of four distinct factors as a function of OH exposure (OH_exp): initial α-pinene precursor (Factor 1), gas-phase oxidation products (Factors 2 and 3), and condensed-phase oxidation products (Factor 4). (b, d, f and h) Corresponding Kendrick Mass Defect (KMD) plots showing the chemical composition of each factor. Symbol size is proportional to signal intensity. For clarity, only species with relative abundances ≥ 0.040 (Factor 1), 0.0080 (Factor 2), 0.0044 (Factor 3), and 0.002 (Factor 4) are shown.

The Factor 4 elemental composition is further examined in Fig. 5, which overlays the H/C and O/C ratios of individual factor compounds on a Van Krevelen diagram, allowing qualitative assessment of their contributions to ensemble O/C and H/C ratios derived from L-ToF-AMS measurements of α-pinene OH-SOA. Several prominent homologous series of C₅–C₁₀ compounds are evident. To contextualize observed oxidation products in terms of known α-pinene/OH chemistry, we compared measured molecular formulas to those predicted by the Master Chemical Mechanism (MCM), a near-explicit gas-phase chemical model that describes the degradation of VOCs like α-pinene through detailed reaction pathways.¹⁵⁶ Here, diamond symbols denote molecular formulas represented in the MCM, including the largest signal, NH₄⁺·C₅H₈O₄, which may originate from 3-hydroperoxy-4-oxo-pentanal (“C511OOH” in the MCM) and/or 3-hydroxy-4-oxopentanoic acid (“H3C2C4CO2H”). Similarly, NH₄⁺·C₆H₈O₄ may arise from 6-hydroxyhexane-2,3,5-trione (“C614CO”) and/or 4-hydroxy-2,5-dioxi-hexanal (“H3C25C5CHO”). Structures and formulas for MCM-predicted and detected compounds are shown in Fig. S2. Triangle symbols represent known α-pinene products that are not included in the MCM, such as terpenylic and 2-hydroxyterpenylic acids (NH₄⁺·C₈H₁₂O_4–5), diaterpenylic acid acetate (NH₄⁺·C₁₀H₁₆O₆),¹⁶⁸ and 3-methyl-1,2,3-butanetricarboxylic acid (NH₄⁺·C₈H₁₂O₆),^169,170 along with others.^171–174 Circle symbols denote 39 molecular formulas not previously reported in α-pinene/OH oxidation studies. Some may result from thermal degradation in the VIA, especially those with low H/C ratios (0.5 ≤ H/C < 1). Others may represent later-generation oxidation products formed via functionalization or fragmentation of known precursors. For example, NH₄⁺·C₆H_6,8O_5–6 may result from alcohol addition to C₆H_6,8O₄-type compounds like “CO235C5CHO”, “C614CO” and/or “H3C25C5CHO”, while NH₄⁺·C₁₀H₁₂O_5–7 may result from addition of two carbonyl groups to known C₁₀H₁₆O_3–5 products,¹⁷¹ along with other possible pathways. Additional signals such as NH₄⁺·C₄H_4,6O₅ likely stem from OH-initiated fragmentation of earlier-generation α-pinene/OH oxidation products.

Fig. 5 Van Krevelen diagram showing H/C and O/C ratios of SOA generated from the OH oxidation of α-pinene. L-ToF-AMS data (gray squares) show bulk SOA elemental composition, while VIA-Vocus data (diamonds/triangles/circles) identify individual molecular formulas of Factor 4 components (Fig. 4). Colored lines show homologous series of oxidation products. Additional figure notes: ¹molecular formulas of oxidation products included in the Master Chemical Mechanism (MCM);^{156 2}molecular formulas of previously-reported oxidation products that are not included in the MCM,^{168–174 3}molecular formulas not previously reported in α-pinene/OH studies.

3.2.2 α-Pinene/Cl. Fig. 6 presents the PMF analysis of α-pinene/Cl oxidation products measured with the Vocus. As with the α-pinene/OH system, four distinct factors were identified, each characterized by unique mass spectral signatures and trends with Cl_exp. Factor 1 closely resembles Factor 1 from the α-pinene/OH system. It consists mainly of unreacted α-pinene plus NH₄⁺·C₁₀H₁₄O and NH₄⁺·C₁₀H₁₆O, and its signal decreases steadily with increasing Cl_exp. Factor 2 contains gas-phase species and peaks at Cl_exp ≈ 5.3 × 10¹⁰ cm⁻³ (Fig. 6c). The largest contributor is NH₄⁺·C₁₀H₁₄O, followed by chlorinated compounds including NH₄⁺·C₁₀H₁₅ClO, NH₄⁺·C₁₀H₁₇ClO, and NH₄⁺·C₁₀H₁₅ClO₂, along with additional chlorinated and non-chlorinated adducts. Factor 3 also contains gas-phase compounds, but its signal increases continuously with Cl_exp. It includes NH₄⁺·C₅H₆Cl₂O₂, NH₄⁺·C₆H₉ClO₂, and NH₄⁺·C₇H₉ClO₃, along with 33 other C₃–C₉ chlorinated and non-chlorinated species. NH₄⁺·C₃H₆O and its NH₄⁺·C₃H₅ClO chlorinated analog followed a similar trend, but plateaued at lower Cl_exp than Factor 3; additionally, the yield of NH₄⁺·C₃H₆O was lower than in the α-pinene/OH system. Factor 4 is composed primarily of low-volatility, condensed-phase products and peaks at Cl_exp ≈ 8.2 × 10¹⁰ cm⁻³ s (Fig. 6g), suggesting it results from continued oxidation of compounds in Factor 2. Like the α-pinene OH-SOA factor, this Cl-SOA factor is chemically complex, containing 45 chlorinated and 87 non-chlorinated C₄ to C₁₀ ammonium adducts.

Fig. 6 Results from PMF analysis of Vocus and VIA-Vocus measurements of the α-pinene + Cl system. (a, c, e and g) Signals of four distinct factors as a function of Cl exposure (Cl_exp): initial α-pinene precursor (Factor 1), gas-phase oxidation products (Factors 2 and 3), and condensed-phase oxidation products (Factor 4). (b, d, f and h) Corresponding KMD plots showing the chemical composition of each factor. Symbol size is proportional to signal intensity. For clarity, only species with relative abundances ≥ 0.043 (Factor 1), 0.010 (Factor 2), 0.0059 (Factor 3), and 0.0015 (Factor 4) are shown.

Factor 4 components are examined in detail in Fig. S3a and b, which plot their H/C and O/C ratios on Van Krevelen diagrams, alongside ensemble AMS-derived O/C and H/C ratios for α-pinene Cl-SOA. Given the greater complexity of α-pinene Cl-SOA compared to α-pinene OH-SOA, formulas for non-chlorinated (NH₄⁺·C_xH_yO_z) and chlorinated (NH₄⁺·C_xH_yCl_wO_z) compounds are shown separately. Triangle symbols represent species previously reported by Masoud and Hildebrandt Ruiz,¹²⁴ including 37 of the 87 non-chlorinated compounds and 10 of the 45 chlorinated compounds. The only other study characterizing α-pinene/Cl oxidation products used a method selective for highly oxygenated organic molecules (HOM)¹²³ which NH₄⁺ CIMS is less sensitive to. Circle symbols represent compounds not previously reported in α-pinene/Cl oxidation studies. As with the α-pinene/OH system, species with low H/C ratios (0.5 ≤ H/C < 1) may result from thermal decomposition in the VIA. Non-chlorinated species detected in Cl-SOA include many also observed in α-pinene OH-SOA, though with different relative abundances. The most intense were NH₄⁺·C₇H₈O₄ and NH₄⁺·C₆H₈O₄. Species present in OH-SOA but largely absent in Cl-SOA include: C₄H₆O₅, C₇H₁₂O₅, C₈H₁₀O_6–7, C₈H₁₂O₆, C₈H₁₄O₂, C₁₀H₁₆O_5–6, and C₁₀H₁₈O₅. Conversely, non-chlorinated species enriched in Cl-SOA include C₈H_10,12O₂, C₉H₁₀O_1–3, C₉H_12,14O_2–3, C₁₀H₁₂O_2–4, and C₁₀H₁₄O_1–3. These are likely thermal or fragmentation artifacts, given their volatilty and absence in OH-SOA. Some may also result from HCl eliminations not present in the α-pinene/OH system. Chlorinated adducts in the Cl-SOA include NH₄⁺·C₆H₇ClO₃ (most abundant), NH₄⁺·C₅H₇ClO_2–3, NH₄⁺·C₇H₉ClO_3–4, and NH₄⁺·C₁₀H₁₃ClO₅. In α-pinene OH-SOA, NH₄⁺·C₆H₈O₄ was hypothesized to originate from “C614CO” and/or “H3C25C5CHO” based on MCM predictions. The presence of the corresponding chlorinated analog NH₄⁺·C₆H₇ClO₃ suggests Cl-initiated oxidation of similar precursors. Analogous relationships likely exist between NH₄⁺·C₅H₈O₄ and NH₄⁺·C₅H₇ClO₃, and between NH₄⁺·C₇H₁₀O_4–5 and NH₄⁺·C₇H₉ClO_3–4.

3.2.3 α-Pinene/Br. Fig. 7 presents the PMF analysis of α-pinene/Br oxidation products measured with the Vocus. Factor 1 closely resembles the first factor in the OH and Cl systems – it is dominated by unreacted α-pinene, NH₄⁺·C₁₀H₁₄O, and NH₄⁺·C₁₀H₁₆O, and its signal decreases steadily with Br_exp. Factor 2 contains gas-phase species, peaking at Br_exp ≈ 2 × 10¹² cm⁻³ s before decreasing (Fig. 7c). The largest signal is NH₄⁺·C₁₀H₁₄O, followed by NH₄⁺·C₉H₁₀O, NH₄⁺·C₁₀H₁₃BrO, NH₄⁺·C₁₀H_14,16O₂, and NH₄⁺·C₁₀H₁₅BrO_1,2. Factor 3 includes predominantly C₃–C₆ brominated gas-phase species whose signals increase with Br_exp, suggesting progressive fragmentation or oxidation of larger molecules. The dominant signal is NH₄⁺·C₃H₅BrO, along with lesser signals from NH₄⁺·C₉H₁₀O and C₄–C₆ species such as NH₄⁺·C₄H_5,7BrO₂, NH₄⁺·C₅H_5,7BrO₂, NH₄⁺·C₅H₇BrO₃, and NH₄⁺·C₆H_7,9BrO₂. NH₄⁺·C₃H₆O plateaued at lower Br_exp than Factor 3 and at lower yield than in the α-pinene/OH system, similar to its behavior in the α-pinene/Cl system.

Fig. 7 Results from PMF analysis of Vocus and VIA-Vocus measurements of the α-pinene + Br system. (a, c, e and g) Signals of four distinct factors as a function of Br exposure (Br_exp): initial α-pinene precursor (Factor 1), gas-phase oxidation products (Factors 2 and 3), and condensed-phase oxidation products (Factor 4). (b, d, f and h) Corresponding KMD plots showing the chemical composition of each factor. Symbol size is proportional to signal intensity. For clarity, only species with relative abundances ≥0.031 (Factor 1), 0.014 (Factor 2), 0.012 (Factor 3), and 0.0026 (Factor 4) are shown.

Factor 4 represents condensed-phase products and peaks at Br_exp ≈ 2.4 × 10¹² cm⁻³ s (Fig. 7g), suggesting formation via further oxidation of Factor 2 species. This factor includes 57 brominated and 77 non-brominated C₃ to C₁₀ ammonium adducts. Their H/C and O/C ratios are shown in Van Krevelen space in Fig. S4a and b alongside α-pinene Br-SOA elemental ratios measured by L-ToF-AMS. As with Cl-SOA, brominated and non-brominated formulas are plotted separately. To our knowledge, this is the first study to report the molecular composition of α-pinene/Br oxidation products. As with the OH and Cl systems, species with 0.5 ≤ H/C < 1 are likely artifacts of thermal decomposition in the VIA. Many of the nonbrominated compounds shown in Fig. S4a overlap with those found in OH- and Cl-SOA, though in different proportions. The most abundant are NH₄⁺·C₁₀H₁₄O₄, NH₄⁺·C₆H_6,8O₄, and NH₄⁺·C₇H₈O_3,4. Unique to the Br system are NH₄⁺·C₆H₁₂O₃ and NH₄⁺·C₆H₁₀O₄, which were not significant in OH-/Cl-SOA. Fig. S4b shows that the most intense brominated signal is NH₄⁺·C₃H₅BrO – likely bromoacetone – which is too volatile to be a true condensed-phase species. Its Vocus : VIA-Vocus ratio was nearly 100, qualitatively consistent with other volatile C₁₀ species (Fig. 3) and suggesting possible charcoal denuder breakthrough from the gas phase. After NH₄⁺·C₃H₅BrO, the next largest brominated adducts are NH₄⁺·C₁₀H_13,15BrO_3,4. While the Br-SOA composition differs markedly from OH- and Cl-SOA, some chemical parallels remain. For instance, α-pinene/OH pathways in the MCM leading to“PINALOH” (C₁₀H₁₆O₃) and “C106OH” (C₁₀H₁₆O₄) can be adapted to α-pinene/Br chemistry. Br-initiated oxidation of bromopinonaldehyde (C₁₀H₁₅BrO₂) may produce a C₁₀H₁₄BrO₄ peroxy radical, followed by RO₂ + RO₂ forming a C₁₀H₁₄BrO₃ alkoxy radical and subsequent C₁₀H₁₃BrO₃ (carbonyl) and C₁₀H₁₅BrO₃ (alcohol) products. Further isomerization and O₂ addition yield a C₁₀H₁₅BrO₅ peroxy radical, which could react with RO₂ again to produce C₁₀H_13,15BrO₄ carbonyl and alcohol products.

3.3 Comparison of α-pinene/OH, α-pinene/Cl, and α-pinene/Br OVOC and SOA factors

Comparison of Fig. 4d, 6d and 7d reveals notable differences in the composition of the “OVOC2” factors derived from α-pinene oxidation by OH, Cl and Br. In the Cl and Br systems, we hypothesize that NH₄⁺·C₁₀H₁₅ClO, NH₄⁺·C₁₀H₁₇ClO and NH₄⁺·C₁₀H₁₅BrO are first-generation products formed via halogen addition to the α-pinene endocyclic double bond, followed by RO₂ + RO₂ reactions. This is analogous to OH-initiated oxidation in the MCM, which generates C₁₀H₁₆O₂ (“APINBCO”) and C₁₀H₁₈O₂ (“APINBOH”) through similar pathways.¹⁵⁶ All three OVOC2 factors contain NH₄⁺·C₁₀H₁₄O, though its relative abundance is lower in the OH system. While pinonaldehyde (C₁₀H₁₆O₂) is known to undergo dehydration to form C₁₀H₁₄O in H₃O⁺ PTR-MS instruments,^175–177 separate measurements of a pinonaldehyde standard using NH⁺₄ Vocus show that this dehydration product is much less prominent. Specifically, NH⁺₄-adduct signals for C₁₀H₁₄O were more than 250 times weaker than those for C₁₀H₁₆O₂ (NH₄⁺·C₁₀H₁₄O: NH₄⁺·C₁₀H₁₆O₂ = 0.0038; Fig. S5). In contrast, H₃O⁺ Vocus measurements of the same standard showed a much higher relative abundance of C₁₀H₁₄O (H⁺·C₁₀H₁₄O: H⁺·C₁₀H₁₆O₂ = 0.47), consistent with known dehydration artifacts during proton-transfer ionization. This suggests that NH₄⁺·C₁₀H₁₄O represents something else.

Given the greater complexity of the OVOC3 and SOA factors, Fig. 8 summarizes their composition, grouped by carbon and oxygen number, with OVOC2 factors included for reference. Solid bars denote non-halogenated species, vertically striped bars represent chlorinated species, and diagonally striped bars denote brominated species. Several trends are evident:

Fig. 8 Carbon number distributions for OVOC2, OVOC3, and SOA factors resulting from oxidation of α-pinene by (a–c) OH, (d–f) Cl, and (g–i) Br. Fill patterns distinguish non-halogenated (C_xH_yO_z, solid), chlorinated (C_xH_yClO_z, vertical stripes), and brominated (C_xH_yBrO_z, diagonal stripes) compounds, while the colors indicate the number of oxygen atoms. Pie charts in the halogen experiments quantify the total contribution of halogenated products to each fraction.

1. Carbon oxidation state increases in the order Br < Cl < OH across all systems.

2. Within each system, OVOC3 and SOA factors are more oxidized than the OVOC2 factor.

3. Carbon number distributions are broader and more complex for OH and Cl systems than for Br, suggesting more extensive fragmentation and multigenerational oxidative aging.

4. The fraction of halogenated species, shown in the pie chart insets of Fig. 8d–i, increased in the order SOA < OVOC2 < OVOC3 for Cl and OVOC2 < SOA < OVOC3 for Br. These trends suggest that Cl and Br atoms are more likely retained in gas-phase products than in lower-volatility species that contribute to SOA.

5. Br-SOA contained a higher fraction of halogenated signals than Cl-SOA, which is qualitatively consistent with previous findings from α-pinene ozonolysis SOA exposed to Cl and Br.¹²⁶These trends are complicated by potential HCl and HBr elimination via chemical processes in the OFR and/or thermal degradation in the VIA, making quantitative interpretation uncertain. Nonetheless, the enrichment of halogenated signals in the OVOC3 factors combined with lower levels of halogenated signals in the SOA factors supports the idea that Cl and Br are preferentially retained in more volatile fragmentation products.

Differences in OVOC and SOA composition among the α-pinene/OH, α-pinene/Cl, and α-pinene/Br systems are linked to differences in the fate of RO₂ formed from OH, Cl and Br oxidation. Fig. 9 shows F_RO2 in the absence of isomerization/autooxidation. In the OH system, RO₂ loss is dominated by reactions with HO₂ and OH: F_RO2+HO2 decreased from 0.96 to 0.73 and F_RO2+OH increased from 0.02 to 0.27 with rising OH_exp (Fig. 9a). In contrast, RO₂ produced from Cl and Br oxidation show more complex RO₂ fates. In the α-pinene/Cl system (Fig. 9b), F_RO2+RO2 decreases from 0.50 to 0.11, F_RO2+HO2 decreases from 0.36 to 0.22, F_RO2+Cl increases from 0.08 to 0.37, and F_RO2+ClO rises from 0.06 to 0.30. In the α-pinene/Br system (Fig. 9c), F_RO2+RO2 drops from 0.23 to 0.002, F_RO2+HO2 decreases from 0.23 to 0.05, F_RO2+Br increases from 0.30 to 0.90, and F_RO2+BrO decreases from 0.24 to 0.05. To assess the role of RO₂ isomerization/autooxidation, we ran separate simulations assuming isomerization rate coefficients (k_isom) of 0.1, 1, and 4 s⁻¹ (Fig. S6–S8). These values span the range of experimentally observed rates,¹⁶³ with k_isom = 4 s⁻¹ corresponding to the isomerization rate coefficient of the “APINCO2” peroxy radical^156,165 derived from H-abstraction on α-pinene's terminal methyl group. In the α-pinene/OH system, RO₂ loss remains dominated by RO₂ + HO₂ reactions at k_isom = 0.1 s⁻¹. Isomerization becomes competitive with RO₂ + HO₂ at k_isom = 1 s⁻¹ and becomes dominant at k_isom = 4 s⁻¹. In the α-pinene/Cl system, isomerization is competitive with RO₂ + Cl at k_isom = 0.1 s⁻¹ and becomes dominant at k_isom ≥ 1 s⁻¹. Finally, in the α-pinene/Br system, RO₂ + Br remains dominant at k_isom = 0.1 s⁻¹ and under most k_isom = 1 s⁻¹ conditions. However, isomerization plays an important role at k_isom = 1 s⁻¹ and becomes dominant at k_isom = 4 s⁻¹. In summary, while exact contributions are uncertain, this analysis suggests that RO₂ + HO₂ reactions dominate in the α-pinene/OH system, multiple pathways (including RO₂ + Cl and isomerization) are important in the α-pinene/Cl system, and RO₂ + Br reactions dominate in the α-pinene/Br system, with increasing isomerization at higher at higher k_isom values. These mechanistic differences contribute to the observed variability in product distributions and SOA formation across the three oxidant systems.

Fig. 9 Modeled fates of the organic peroxy radical (RO₂) as a function of oxidant exposure during the (a) OH, (b) Cl, and (c) Br oxidation of α-pinene in the absence of isomerization/autooxidation reactions. Reactions and kinetic rate coefficients used in these calculations are provided in Table S2.

4 Conclusions

In this study, we presented a comprehensive laboratory investigation into the gas and condensed-phase products generated from the oxidation of α-pinene by OH, Cl, and Br. By employing a Vocus equipped with a VIA and NH₄⁺ reagent ion chemistry, we characterized a wide array of oxidation products, including multifunctional organic compounds and previously challenging-to-measure low-volatility species, using a single reagent ion. PMF analysis revealed consistent formation of four distinct factors corresponding to early-generation VOCs/OVOCs, later-generation oxidized products, and low-volatility species contributing to SOA.

Our results demonstrate distinct chemical pathways and product distributions for each oxidant. While the initial oxidation of α-pinene by all three oxidants yielded C₁₀H_14,16O and C₁₀H₁₆O₂ products, among others, subsequent reactions led to significantly different compound classes. OH-initiated oxidation produced a complex mixture of OVOC and SOA components that were consistent with established α-pinene/OH chemistry. In contrast, α-pinene reactions with Cl and Br atoms led to the formation of a substantial number of halogenated organic compounds. Key early-generation gas-phase products identified included C₁₀H₁₅ClO, and C₁₀H₁₅ClO₂ from the Cl reaction, and C₁₀H₁₅BrO and C₁₀H₁₅BrO₂ from the Br reaction. The product mass spectra obtained for each oxidation system provide unique fingerprints that can aid in identifying the chemical history and sources of ambient organic aerosol. Cl- and Br-initiated oxidation produced condensed-phase products that were generally less oxidized than those from OH oxidation, with Br-SOA being the least oxidized overall. However, Br-SOA retained a higher fraction of halogenated products than Cl-SOA. Photochemical modeling demonstrated oxidant-specific differences in RO₂ fate, with RO₂ + HO₂ reactions dominating under OH oxidation, a more complex interplay of reactions under Cl oxidation, and RO₂ + Br reactions dominating under Br oxidation. These differences help explain the observed variation in SOA composition and oxidative aging between systems.

Altogether, these results underscore that oxidation by halogen atoms leads to distinct chemical pathways and product distributions compared to OH oxidation. This highlights the need for updated chemical mechanisms and SOA yield treatments that explicitly account for Cl- and Br-mediated oxidation chemistry, especially in regions where halogen atoms are abundant. The presence of these species may alter the physicochemical properties of atmospheric aerosols, including their hygroscopicity, optical properties, and potential toxicity, particularly in marine and coastal environments, with important implications for air quality, climate, and human health. This work highlights the power and utility of NH₄⁺ CIMS as a tool for elucidating complex atmospheric oxidation mechanisms, from initial gas-phase reactions to the formation and chemical evolution of aerosols. Future studies should focus on extending this methodology to other precursors and exploring the evolution of these distinct SOA types under a broader range of atmospheric conditions to better constrain their impact in regional and global models.

Author contributions

AL and JK conceived, planned, and carried out the experiments. AL performed data analysis. AL, MC, AA, MA and ND contributed to the interpretation of the results. AL took the lead in writing the manuscript. All authors provided feedback on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data for this article, including source files used to generate Fig. 1 and 3–9, are available at Open Science Framework at [https://osf.io/6fc3q/overview].

Supplementary information (SI): additional experimental time series, molecular formula assignments and proposed structures, Van Krevelen analyses of α-pinene Cl-SOA and Br-SOA composition, data obtained from Vocus measurements of pinonaldehyde, and detailed chemical mechanisms and kinetic parameters used in the OFR modeling. See DOI: https://doi.org/10.1039/d5ea00091b.

Acknowledgements

This work was supported by the Atmospheric Chemistry Program of the United States National Science Foundation: grant AGS-1934352 to Aerodyne Research. AL thanks Lea Hildebrandt Ruiz (University of Texas at Austin) and Harald Stark (Aerodyne Research) for helpful discussions.

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Footnote

† ‘Present address:’ Osmo Labs, PBC, Somerville, Massachusetts, United States.

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