Comparison of secondary organic aerosol generated from the oxidation of laboratory precursors by hydroxyl radicals, chlorine atoms, and bromine atoms in an oxidation flow reactor

Abstract

The role of hydroxyl radicals (OH) as a daytime oxidant is well established on a global scale. In specific source regions, such as the marine boundary layer and polluted coastal cities, other daytime oxidants, such as chlorine atoms (Cl) and even bromine atoms (Br), may compete with OH for the oxidation of volatile organic compounds (VOCs) and/or enhance the overall oxidation capacity of the atmosphere. However, the number of studies investigating halogen-initiated secondary organic aerosol (SOA) formation is extremely limited, resulting in large uncertainties in these oxidative aging processes. Here, we characterized the chemical composition and yield of laboratory SOA generated in an oxidation flow reactor (OFR) from the OH and Cl oxidation of n-dodecane (n-C₁₂) and toluene, and the OH, Cl, and Br oxidation of isoprene and α-pinene. In the OFR, precursors were oxidized using integrated OH, Cl, and Br exposures ranging from 3.1 × 10¹⁰ to 2.3 × 10¹², 6.1 × 10⁹ to 1.3× 10¹² and 3.2 × 10¹⁰ to 9.7 × 10¹² molecules cm⁻³ s⁻¹, respectively. Like OH, Cl facilitated multistep SOA oxidative aging over the range of OFR conditions that were studied. In contrast, the extent of Br-initiated SOA oxidative aging was limited. SOA elemental ratios and mass yields obtained in the OFR studies were comparable to those obtained from OH and Cl oxidation of the same precursors in environmental chamber studies. Overall, our results suggest that alkane, aromatic, and terpenoid SOA precursors are characterized by distinct OH- and halogen-initiated SOA yields, and that while Cl may enhance the SOA formation potential in regions influenced by biogenic and anthropogenic emissions, Br may have the opposite effect.

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1 Introduction

The atmosphere is an oxidizing environment. Gas-phase oxidants, including ozone (O₃), hydroxyl radicals (OH), nitrate radicals (NO₃), chlorine atoms (Cl), and bromine atoms (Br), can react with organic and inorganic pollutants to generate a myriad of gas- and condensed-phase oxidation products. The importance of each oxidant in different parts of the atmosphere depends on the local meteorology, emissions, and photochemistry. Globally, OH is the most important oxidant: there are many ways to generate it during the daytime from precursors that are widely distributed throughout the atmosphere, and, unlike O₃, it reacts with most inorganic and organic compounds.¹ With regards to atmospheric aerosols, OH is particularly important in initiating the oxidation of sulfur dioxide (SO₂) to generate sulfuric acid and initiating the oxidation of volatile organic compounds (VOCs) to generate low volatility organic compounds (LVOC) that condense to form secondary organic aerosol (SOA). NO₃ is an important oxidant at nighttime² and in some cases during the daytime,^3,4 particularly in source regions influenced by emissions from unsaturated VOCs emitted from plants and wildfires.⁵

Significant Cl production occurs in regions such as the marine boundary layer,⁶ polluted coastal cities,⁷ and the Arctic atmosphere.^8,9 Most VOCs react with Cl approximately 10 to 100 times faster than their corresponding rate of reaction with OH. Thus, when atmospheric Cl mixing ratios are high enough, Cl may compete with OH in the oxidation of VOCs and/or otherwise enhance the overall oxidation capacity of the atmosphere. Additionally, significant inland Cl production has been observed,^10–12 bleach washing has been shown to initiate significant indoor chlorine chemistry,^13–15 and both Cl and Br have been linked to enhanced secondary aerosol formation in China.¹⁶ Br contributes to springtime polar mercury and O₃ depletion^9,17,18 reacts with dimethyl sulfide, alkenes, and aldehydes at rates that are similar to their reaction rates with OH, and, like Cl, induces significant multiphase chemistry in organic aerosols.^19–21

Large environmental chambers have been used for decades to study complex SOA formation chemistry. Multi-instrument, multi-investigator chamber studies have provided comprehensive data sets that transform existing concepts of SOA formation and aging and SOA models.²² The recent emergence of oxidation flow reactors (OFRs) complements chambers through their lower operation/maintenance costs, portability for in situ oxidative aging of ambient and source emissions, and ability to access photochemical aging timescales of up to several days.^23–28 To date, the vast majority of SOA formation studies in chambers and OFRs have used O₃, OH, and to a lesser extent NO₃, to mimic daytime and nighttime oxidation of hydrocarbons. The handful of studies that have measured yields of SOA obtained from Cl oxidation of VOCs have shown that Cl exposure generates SOA in yields that are comparable to, or exceed, OH oxidation of the same precursors.^29–36 SOA formed from Br oxidation of VOCs has not been studied; models including halogen chemistry assume the same yield of SOA is obtained regardless of whether Cl and Br is the initiating oxidant.¹⁶

To investigate these knowledge gaps, we characterized the chemical composition and yield of laboratory SOA generated in an OFR from the OH, Cl and Br oxidation of a set of anthropogenic and biogenic VOCs. OFRs use residence times that are on the order of minutes and oxidant concentrations that are typically 100–1000 times higher than ambient levels; these factors may make the chemistry and microphysics in the OFR somewhat different from the chemistry and microphysics in the atmosphere.^37,38 Thus, we also compared the chemical composition and mass yields of SOA obtained from OH and Cl oxidation of the same precursors in the OFR with previous chamber studies.

2 Experimental

Experiments were conducted inside a Potential Aerosol Mass (PAM) OFR (Aerodyne Research, Inc.), which is a horizontal 13 L aluminum cylindrical chamber (46 cm long × 22 cm ID) operated in continuous flow mode, with 6.0–6.8 L min⁻¹ flow through the reactor.³⁹ The corresponding calculated mean residence time in the OFR, τ_OFR, ranged from 114 to 130 s. An electroconductive Teflon coating was applied to the OFR to improve chemical compatibility with halogen precursors while maintaining high transmission of gases and particles.⁴⁰ Two low-pressure mercury (Hg) lamps that were isolated from the sample flow using type 214 quartz sleeves were used to photolyze oxidant precursors. As discussed in Section 2.1 and shown in Fig. S1,† different lamps were used for different OFR methods to maximize the overlap between the absorption cross section of the oxidant precursor and the range of achievable oxidant exposure. A fluorescent dimming ballast (IZT-2S28-D, Advance Transformer Co.) was used to regulate current applied to the lamps. The UV irradiance was measured using a photodetector (TOCON-GaP6, sglux GmbH) and was varied by changing the control voltage applied to the ballast between 1.5 and 10 VDC. The corresponding actinic flux ranged from approximately 1 × 10¹⁴ to 3 × 10¹⁵ photons cm⁻² s⁻¹.^39,41

2.1 Oxidant generation

2.1.1 OH production and quantification. OH was generated from the combined photolysis of O₂ and H₂O at λ = 185 nm plus photolysis of O₃ at λ = 254 nm using two low-pressure germicidal Hg lamps (GPH436T5VH/4P, Light Sources, Inc.); this method is hereafter referred to as “OFR185”. Segments of opaque heat shrink tubing were applied to approximately 86% of the illuminated section of the lamps to further reduce the lamp output below what is achievable using the ballast dimming voltage alone.³⁹ Across all OH-OFR experiments, the relative humidity (RH) was controlled with a Nafion membrane humidifier (Perma Pure) and ranged from 30.9 to 43.3%; corresponding H₂O mixing ratios were between 1.01 to 1.52% at OFR temperatures ranging from 26.2 to 28.8 °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 eqn (1) that was developed specifically from OH_exp calibration experiments for the GPH436T5VH/4P lamps:³⁹where OHR_ext (external OH reactivity, s⁻¹) is the product of the SOA precursor mixing ratio and its bimolecular OH rate coefficient (cm³ molecules⁻¹ s⁻¹, hereafter “cm³ s⁻¹”) and [O₃] is the ozone concentration (molecules cm⁻³, hereafter “cm⁻³”) measured at the exit of the OFR. The estimated uncertainty in calculated OH_exp values obtained using eqn (1) was ±50%. Over the range of OFR185 conditions that were used, OH_exp values calculated using eqn (1) ranged from 3.1 × 10¹⁰ to 2.3 × 10¹² molecules cm⁻³ s⁻¹ (hereafter “cm⁻³ s”), or approximately 6 h to 18 d of atmospheric oxidation at [OH] = 1.5 × 10⁶ cm⁻³.⁴²

2.1.2 Cl and Br generation and quantification. Cl was generated via photolysis of chlorine (Cl₂) at λ = 313 or 369 nm (Cl₂ + hν → 2Cl) or photolysis of oxalyl chloride (C₂Cl₂O₂) at λ = 254 or 313 nm (C₂Cl₂O₂ + hν → 2Cl + 2CO). C₂Cl₂O₂ has been used as a Cl precursor in chamber and flow tube studies;^43–45 here, its usage enabled the investigation of the potential role of unwanted photolysis of Cl-SOA at λ = 254 nm, a long-standing concern of OH-OFR studies.⁴⁶ Similarly, Br was generated via photolysis of bromine (Br₂) at λ = 369 or 421 nm (Br₂ + hν → 2Br) or photolysis of oxalyl bromide (C₂Br₂O₂) at λ = 254 nm. C₂Br₂O₂ is less studied than C₂Cl₂O₂, but appears to follow a similar photodissociation mechanism as C₂Cl₂O₂,^47,48 and so we assume its photolysis proceeds via the reaction C₂Br₂O₂ + hν → 2Br + 2CO. These methods are hereafter referred to as “OFR313-iCl₂”, “OFR369-iCl₂”, “OFR254-iC₂Cl₂O₂”, “OFR313-iC₂Cl₂O₂”, “OFR369-iBr₂”, “OFR421-iBr₂”, and “OFR254-iC₂Br₂O₂”, where applicable (“i” = inject; thus, “iCl₂” means Cl₂ was the radical precursor injected into the OFR). Fig. S1† shows absorption cross sections for Cl₂,⁴⁹ Br₂,⁵⁰ C₂Cl₂O₂,⁴⁴ and C₂Br₂O₂,^51,52 as well as emission spectra obtained from the manufacturer for the aforementioned lamp types. Across all Cl-OFR and Br-OFR experiments, the RH ranged from 1.1 to 4.1% at T = 24.2 to 28.0 °C. A compressed gas cylinder containing 0.1% Cl₂ in N₂ (Praxair) was used to supply Cl₂ to the OFR. The Cl₂ mixing ratio entering the OFR (1.9–24.7 ppmv) was calculated from the Cl₂ mixing ratio in the compressed gas mixture and the dilution ratio of 15–200 cm³ min⁻¹ into 8000 cm³ min⁻¹ carrier gas. C₂Cl₂O₂, C₂Br₂O₂, and Br₂ vapor were supplied to the OFR using a sealed permeation tube (EMPTY-HE, VICI) filled with each liquid and placed in a permeation tube oven that was heated to 80–90 °C. A carrier gas flow of 100 cm³ min⁻¹ zero air was used to transfer C₂Cl₂O₂, C₂Br₂O₂, or Br₂ vapor from the oven into the OFR. The C₂Cl₂O₂, C₂Br₂O₂, and Br₂ mixing ratios (C) entering the OFR were calculated using eqn (2):where P was the measured permeation rate (1.74 × 10⁵, 6.32 × 10⁴, 9.68 × 10⁴ ng min⁻¹), MW was the molecular weight (126.93, 215.83, or 159.81 g mol⁻¹), and F was the dilution flow rate (8000 cm³ min⁻¹). Here, the calculated C₂Cl₂O₂, C₂Br₂O₂, and Br₂ mixing ratios were 4.2, 1.8, and 1.9 ppmv, respectively.

Integrated Cl and Br exposures (Cl_exp, Br_exp) were characterized in offline calibration experiments by measuring the decay of O₃ injected into the OFR and measured using an O₃ analyzer (2B Technologies) as a function of lamp voltage. O₃ concentrations were allowed to stabilize before initiating Cl_exp and Br_exp measurements, during which steady-state levels of O₃ were obtained with the lamps turned off (O_3,i). Then, the lamps were turned on, and O₃ concentrations were allowed to stabilize before being measured at illuminated steady-state conditions (O_3,f) following reaction with Cl or Br. The Cl or Br exposure (Cl_exp, Br_exp) at each condition was calculated using eqn (3) and (4):

where and are the bimolecular Cl + O₃ and Br + O₃ reaction rate coefficients. Here, we used = 1.21 × 10⁻¹¹ and = 1.16 × 10⁻¹² cm³ s⁻¹.⁵³

Because [Cl₂] was varied in experiments that used OFR313-iCl₂ and OFR369-iCl₂ to generate Cl, separate calibration experiments were performed to measure Cl_exp as a function of [Cl₂]. To correct for Cl or Br suppression that occurs in the presence of external Cl or Br reactivity (ClR_ext, BrR_ext), which is the product of the O₃ mixing ratio and its bimolecular Cl or Br rate coefficient, [O_3,i] was varied from 242 to 3360 ppbv in Cl-OFR calibration experiments and 369 to 7191 ppbv in Br-OFR calibration experiments. These calibration conditions achieved ClR_ext values ranging from 72 to 1000 s⁻¹ and BrR_ext values ranging from 12 to 226 s⁻¹, which approximately span the range of ClR_ext and BrR_ext values in the OFR conditions listed in Table 1. Example OFR313-iCl₂, OFR254-iC₂Cl₂O₂, OFR369-iBr₂, OFR254-iC₂Br₂O₂ calibration data are shown in Fig. S2–S5.† Calibration results indicate that Cl_exp decreased by a factor of 4 to 12 at each lamp setting over the range of ClR_ext values shown in Fig. S2,† with the largest Cl suppression occurring at lower lamp voltage, as expected. Similarly, Br_exp decreased by a factor of 2 to 60 using OFR369-iBr₂ (Fig. S4†) and by a factor of 2 to 28 using OFR254-iC₂Br₂O₂ (Fig. S5†). Here, we assumed that a specific ClR_ext or BrR_ext value suppressed Cl_exp or Br_exp by the same amount regardless of the source of ClR_ext or BrR_ext (e.g. O₃ in calibrations, or VOCs in SOA studies). This assumption may have introduced uncertainty in some cases, such as OFR conditions where VOCs were short-lived and their Cl or Br oxidation products had significantly different Cl or Br reaction rates. Because ClO_x and BrO_x chemistry in Cl-OFR and Br-OFR calibration experiments was more complex than the analogous HO_x chemistry in OH-OFR calibration experiments, we assumed ±70% uncertainty in Cl_exp and Br_exp values.

Table 1 Summary of OFR experimental conditions. OH was generated via OFR185, Cl was generated via OFR254-iC₂Cl₂O₂, OFR313-iC₂Cl₂O₂, OFR313-iCl₂, or OFR369-iC₂Cl₂O₂, and Br was generated via OFR254-iC₂Br₂O₂, OFR369-iBr₂, or OFR421-iBr₂; for details see Section 2.1

VOC/oxidant	[VOC]0 (ppb)	[C2Cl2O2] (ppm)	[Cl2] (ppm)	[C2Br2O2] (ppm)	[Br2] (ppm)	RH (%)	T (°C)	Oxidant exposure (cm^−3 s)
n-C12/OH	21	—	—	—	—	30.9	26.2	2.2 × 10^11 to 2.1 × 10^12
n-C12/Cl	21	4.2	—	—	—	1.2	26.1	3.3 × 10^10 to 2.3 × 10^11
Toluene/OH	45	—	—	—	—	31.7	26.2	2.6 × 10^11 to 2.3 × 10^12
Toluene/Cl	45	4.2	—	—	—	1.1	26.9	2.7 × 10^10 to 4.5 × 10^11
Toluene/Cl	45	—	4.9–24.7	—	—	1.2	24.2	2.3 × 10^10 to 1.3 × 10^12
Isoprene/OH	48	—	—	—	—	43.3	27.2	3.1 × 10^10 to 9.6 × 10^11
Isoprene/Cl	48	—	1.9–24.4	—	—	4.0	26.5	6.1 × 10^9 to 2.9 × 10^11
Isoprene/Br	144	—	—	1.8	—	4.0	27.8	3.2 × 10^10 to 2.6 × 10^12
α-Pinene/OH	30	—	—	—	—	28.2	28.8	6.7 × 10^10 to 1.2 × 10^12
α-Pinene/Cl	30	4.2	—	—	—	1.4	28.0	3.9 × 10^10 to 1.2 × 10^11
α-Pinene/Br	90	—	—	1.8	—	4.1	25.6	9.8 × 10^10 to 2.9 × 10^12
α-Pinene/Br	90	—	—	—	1.9	3.9	26.0	9.6 × 10^11 to 9.7 × 10^12

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A subset of OFR421-iBr₂ calibration conditions that were applicable to α-pinene/Br experiments (lamp voltages higher than 4 V and/or BrR_ext < 90 s⁻¹) depleted all the O₃ that was injected into the OFR. To constrain Br_exp at these conditions, we compared Br_exp obtained using OFR421-iBr₂ and OFR369-iBr₂ at otherwise identical OFR conditions when Br_exp < 3 × 10¹² cm⁻³ s using both methods. Br_exp obtained using OFR421-iBr₂ was approximately 2.16 times higher than Br_exp obtained using OFR369-iBr₂ (Fig. S6†), which is consistent with a higher Br₂ absorption cross section at λ = 421 nm than at λ = 369 nm.⁵⁰ Br_exp values obtained at the other OFR369-iBr₂ conditions were then multiplied by 2.16 to obtain Br_exp values at OFR421-iBr₂ conditions with equivalent lamp settings and BrR_ext values.

Calculated Cl_exp values in SOA experiments ranged from 6.1 × 10⁹ to 1.3 × 10¹² cm⁻³ s, or approximately 1.2 d to 8.2 months of atmospheric oxidation at [Cl] = 6 × 10⁴ cm⁻³.¹⁸ Similarly, calculated Br_exp values ranged from 3.2 × 10¹⁰ to 9.7 × 10¹² cm⁻³ s, or approximately 1 h to 16 d at [Br] = 7 × 10⁶ cm⁻³.¹⁸ These simple calculations should be interpreted as a rough estimate of the photochemical age in a representative source region with active Cl or Br photochemistry (e.g., Arctic spring), and may vary by orders of magnitude elsewhere.

2.2 Photochemical model

To investigate the fate of O₃ in our Cl_exp and Br_exp calibration experiments, and to build a foundation for characterizing the concentrations of inorganic halogens generated in the OFR, we developed a photochemical box model that was implemented in the KinSim chemical kinetic solver.⁵⁴ The KinSim mechanism shown in Table S1† contains 66 reactions to model HO_x concentrations in OFRs^39,55,56 plus 139 reactions that were added to model concentrations of Cl₂, C₂Cl₂O₂, Br₂, C₂Br₂O₂, Cl, ClO, ClO₂, ClO₃, OClOO, ClOO, Cl₂O, Cl₂O₂, HCl, HOCl, Cl₂O₃, Br, BrO, BrO₂, HBr, HOBr, and BrCl. Inputs to the KinSim model were [O₃] (242 to 7191 ppb), UV flux (3.5 × 10¹³ to 3.5 × 10¹⁵ photons cm⁻² s⁻¹), RH = 1%, T = 25 °C, and τ_OFR = 130 s (modeled as plug flow). Over this range of OFR conditions, the model suggests that >97% of reactive O₃ loss was due to reaction with Cl across all OFR313-iCl₂, OFR369-iCl₂, OFR254-iC_2Cl₂O₂, and OFR313-iC₂Cl₂O₂ calibration conditions, with the remaining O₃ lost to reaction with ClO. Similarly, we estimate that >99% of reactive O₃ loss was due to reaction with Br across all OFR369-iBr₂, OFR421-iBr₂, OFR254-and iC_2Br₂O₂ calibration conditions, with the remaining O₃ lost to reaction with BrO. We estimate that Cl or Br regeneration via photolysis of ClO_x and BrO_x biased calibrated Cl_exp or Br_exp values by <2% or <14% respectively. Because we already applied ±70% uncertainty estimates to Cl_exp and Br_exp values, we did not apply additional correction factors to the calibration data.

2.3 Particle generation

SOA particles were generated via gas-phase OH or Cl oxidation of n-dodecane (n-C₁₂) or toluene, or OH, Cl, or Br oxidation of isoprene or α-pinene, followed by homogeneous nucleation; we hereafter refer to SOA formation initiated by OH, Cl, and Br as “OH-SOA”, “Cl-SOA” or “Br-SOA” respectively. These precursors were chosen to cover a range of surrogate anthropogenic and biogenic compounds that enabled comparison with results from Cl-initiated chamber SOA formation studies.^{29–33,35,36} Particle number concentrations and mobility size distributions were measured with a TSI scanning mobility particle sizer (SMPS), and ensemble aerosol mass spectra were measured with an Aerodyne long high-resolution time-of-flight aerosol mass spectrometer (L-ToF-AMS). Liquid solutions containing the precursor diluted to 10% (v/v) in carbon tetrachloride were injected into the OFR carrier gas flow at liquid flow rates (Q_VOC,l) ranging from 0.94 to 2.8 μL h⁻¹ using a syringe pump. The VOC mixing ratio entering the OFR, r_VOC,g, was calculated using the ideal gas law as applied by Liu et al.⁵⁷ in eqn (5):where ρ (g cm⁻³) and MW (g mol⁻¹) are the liquid density and molecular weight, R (8.314 J mol⁻¹ K⁻¹) is the universal gas constant, T (K) is temperature, P (hPa) is pressure, r_VOC,l is the VOC mixing ratio in solution, and 10⁷ is a lumped pressure, volume, density and time unit conversion factor. Calculated r_VOC,g values for n-C₁₂, toluene, isoprene and α-pinene are listed in Table 1. For isoprene and α-pinene Br-SOA experiments, we used r_VOC,g values that were 3 times higher than in corresponding OH-SOA and Cl-SOA experiments in order to promote homogenous nucleation of Br-SOA.

Over the course of our studies, we learned that conditioning the OFR with humidified carrier gas containing O₃ and OH while transitioning from Cl-SOA to OH-SOA studies generated >100 μg m⁻³ and >10⁷ cm⁻³ of homogenously nucleated particles that gradually subsided over hours to days. As shown in Fig. S7,† L-ToF-AMS spectra of these particles contained chlorinated ion signals (and their primary Cl isotope at m/z +2), at m/z = 35 (Cl⁺), 36 (HCl⁺), 51 (ClO⁺), 67 (ClO₂⁺), 70 (Cl₂⁺), 83 (ClO₃⁺), and 100 (HClO₄⁺), suggesting that these particles contained perchloric acid (HClO₄) and/or perchlorate salts.^58,59 Significant signal at m/z = 44 (CO₂⁺) was also observed and is a known artifact from the interaction of inorganic salts on surfaces inside the AMS.⁶⁰ During these transient conditioning periods, we hypothesize that perchlorate was generated from the reaction of O₃ with aqueous HCl on the walls of the OFR⁶¹ and/or in the gas phase from the reactions:⁶²

HCl + OH → Cl + H₂O (R1)

Cl + O₃ → ClO + O₂ (R2)

ClO + O₃ → ClO₂ + O₂ (R3)

ClO₂ + O₃ → ClO₃ + O₂ (R4)

ClO₃ + OH → HClO₄ (R5)

To minimize the influence of perchlorate on ensuing OH-SOA measurements, the OFR was conditioned with O₃ and OH until ClO_x⁺ signals in the AMS returned to background levels.

2.4 Analysis

2.4.1 L-ToF-AMS. L-ToF-AMS spectra were analyzed using ToF-AMS analysis software,⁶³ which yielded high-resolution mass spectra, hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, and abundances of the default C_xH_y⁺, C_xH_yO⁺, and C_xH_yO_>1⁺ ion groups. We created additional C_xH_yClO_z⁺ (including Cl⁺ and HCl⁺) and C_xH_yBrO_z⁺ (including Br⁺ and HBr⁺) ion groups to distinguish ions containing Cl or Br atoms. Elemental analysis was performed using the methods of Aiken et al.⁶⁴ to enable direct comparison with chamber SOA measurements^30,65,66 that were published prior to more recent methods proposed by Canagaratna et al.⁶⁷

To compare our measurements with chamber Cl-SOA mass spectra obtained with an Aerodyne quadrupole aerosol chemical speciation monitor (Q-ACSM) with unit mass resolution,^32,33,35,36 corresponding O/C ratios for those data were calculated using the equation O/C = 3.82 × f₄₄ + 0.0794,⁶⁴ where f₄₄ was the fraction of Cl-SOA signal at m/z = 44. We used L-ToF-AMS data to confirm the literature O/C-f₄₄ parameterization was accurate for Cl-SOA generated in the OFR in this study. However, H/C ratios of Cl-SOA calculated using the equation H/C = 1.01 + 6.07 × f₄₃–16.01 × f₄₃²,⁶⁸ where f₄₃ was the fraction of Cl-SOA signal at m/z = 43, were systematically ≈20–40% higher than H/C ratios calculated from the L-ToF-AMS spectra (Fig. S8†). Additionally, significant contributions to m/z = 43 from both C₃H₇⁺ and C₂H₃O⁺ ions in n-C₁₂ Cl-SOA spectra complicated application of the H/C-f₄₃ correlation proposed by Ng et al.⁶⁸ Thus, we developed a different parameterization to calculate the H/C ratio of Cl-SOA from the fraction of L-ToF-AMS signal at m/z = 41, which contained one alkyl ion (C₃H₅⁺). We then applied the equation H/C = 5.032 × f₄₁^0.1485 − 1.737 (Fig. S9†) to calculate the H/C ratio of chamber-generated n-C₁₂, isoprene, and α-pinene Cl-SOA obtained with the Q-ACSM. For chamber-generated toluene Cl-SOA data published in Dhulipala et al.,³⁵ because only f₄₃ was available, we calculated f₄₁ : f₄₃ = 0.17 in our toluene Cl-SOA spectra, used this result to estimate f₄₁ for their toluene chamber Cl-SOA spectra, and then calculated H/C ratio using the equation described above.

2.5 SOA yields

SOA mass yields were calculated from the ratio of SOA mass formed to precursor gas reacted. The SOA mass was calculated from the integrated SMPS particle volume and the SOA particle density, ρ_SOA, which was calculated using eqn (6):⁶⁹

We estimated the fraction of precursor gas reacted from the product of the OH_exp, Cl_exp, or Br_exp and the bimolecular rate coefficients of n-C₁₂ + OH/Cl, toluene + OH/Cl, isoprene + OH/Cl/Br, and α-pinene + OH/Cl/Br.^70–76 These calculations suggested that 95–100% of n-C₁₂, 75–100% of toluene, 91–100% of isoprene, and 89–100% of α-pinene reacted across the OFR conditions summarized in Table 1.

SOA yields were corrected for size-dependent particle wall losses in the OFR by applying the particle transmission efficiency measurements of Bhattarai et al.⁷⁷ to the SMPS volume-weighted mobility size distributions. Here, the particle wall loss correction factors ranged from 1.07 to 1.38 for mean volume-weighted particle mobility diameters ranging from 180 nm to 36 nm. To investigate the influence of vapor wall losses on SOA yields, we used the LVOC fate correction model,^78,79 which calculates the timescales of diffusional wall losses, gas-phase oxidative loss, and condensation onto aerosols at a user-specified condensation loss rate, OH/Cl/Br exposure and reaction rate coefficient, and OFR residence time values. We assumed that LVOCs had the same OH, Cl, or Br reaction rate coefficients as their VOC precursors. The condensation sink was calculated using the integrated SMPS number-weighted mobility size distributions and assuming a LVOC diffusion coefficient of 0.07 cm² s⁻¹, mean molecular speed of 2 × 10⁴ cm s⁻¹, and mass accommodation coefficient of unity.^78,80,81 In these experiments, mean fractional LVOC wall losses for each precursor/oxidant combination ranged from 0.003 ± 0.001 (n-C₁₂/Cl) to 0.079 ± 0.076 (α-pinene/Br). Thus, we assumed LVOC wall losses were negligible compared to gas-phase oxidative loss and condensation onto aerosols,^82,83 and did not modify SOA yield values to account for them.

3 Results & discussion

3.1 Sample anthropogenic OH-SOA and Cl-SOA mass spectra

Fig. 1 shows L-ToF-AMS spectra of SOA generated from the OH and Cl oxidation of n-C₁₂ and toluene. To compare results obtained at lower oxidant exposures that were most applicable to urban atmospheres, the spectra shown were obtained at the lowest OH_exp and Cl_exp values at which particle formation was observed: OH_exp = 2.2 × 10¹¹ and Cl_exp = 3.3 × 10¹⁰ cm⁻³ s for n-C₁₂ OH- and Cl-SOA, and OH_exp = 2.6 × 10¹¹ and Cl_exp = 2.7 × 10¹⁰ cm⁻³ s for toluene OH- and Cl-SOA. For both n-C₁₂ OH-SOA and Cl-SOA, the spectra were dominated by C_xH_y⁺ (green) and C_xH_yO₁⁺ (purple) ion groups, which contributed ∼60% and 31% of the total OH- and Cl-SOA signal, respectively (Fig. 1e and f). Some of the most abundant ions within these groups included signals at m/z = 27 (C₂H₃⁺), 29 (CHO⁺), 41 (C₃H₅⁺), 43 (C₂H₃O⁺ + C₃H₇⁺), and 55 (C₃H₅O⁺ + C₄H₇⁺). Additional signals were present at m/z = 44 (CO₂⁺), a marker for organic acids in the AMS,⁸⁴ and at multiple ion clusters above m/z = 60 that contained C_xH_y⁺, C_xH_yO₁⁺, and C_xH_yO_>1⁺ (pink) ions.

Fig. 1 L-ToF-AMS spectra of SOA generated from the (a and e) OH oxidation of n-C₁₂, (b and f) Cl oxidation of n-C₁₂, (c and g) OH oxidation of toluene, and (d and h) Cl oxidation of toluene. OH_exp and Cl_exp values listed in (a)–(d) are in units of cm⁻³ s. The toluene Cl-SOA spectrum presented in (d) was generated using OFR254-iC₂Cl₂O₂. Additional notes regarding (h): toluene Cl-SOA was generated using (1) OFR313/369-iCl₂ or (2) OFR254-iC₂Cl₂O₂.

To characterize the similarity between SOA mass spectra obtained from OH and Cl oxidation of the same precursor, using simple linear regression, we calculated the square of the Pearson correlation coefficient (r²) between the n-C₁₂ OH- and Cl-SOA mass spectra shown in Fig. 1a and b, and between toluene OH- and Cl-SOA mass spectra shown in Fig. 1c and d. OH and Cl oxidation of n-C₁₂ generated SOA with similar AMS spectra (r² = 0.94) because both OH and Cl oxidation of n-C₁₂ proceeded via hydrogen atom abstraction. In contrast, toluene OH-SOA and Cl-SOA spectra were more distinct from each other: while an r² value of 0.87 was obtained between the two spectra, r² decreased to 0.57 when contributions from signals at m/z = 44 and m/z = 28 (CO⁺, set equal to CO₂⁺ by default) were removed from the regression analysis. Notably, toluene Cl-SOA contained enhanced signals at m/z = 77 (C₆H₅⁺), 91 (C₇H₇⁺), and 105 (C₇H₅O⁺) relative to toluene OH-SOA. While the Cl_exp used to generate Fig. 1d was 10 times lower than the OH_exp used to generate Fig. 1c, toluene reacts with Cl ten times faster than OH;⁷⁵ thus, to first order, the extent of OH and Cl oxidation was similar in both cases. The most likely explanation is that Cl oxidation of toluene generated a higher yield of ring-retaining C₆ and C₇ oxidation products because Cl preferentially abstracts H-atoms from the methyl group.³⁰

To compare the similarity of n-C₁₂ and toluene OH-/Cl-SOA spectra across the range of OH_exp or Cl_exp values shown in Fig. 1e–h, Fig. S10† plots r² values between L-ToF-AMS spectra shown in Fig. 1a or c and n-C₁₂ or toluene OH-/Cl-SOA spectra obtained at other OH_exp and Cl_exp. For example, the r² value between Fig. 1a and more-oxidized n-C₁₂ OH-SOA spectra decreased from 0.98 to 0.31 with increasing OH_exp. For n-C₁₂ Cl-SOA, r² decreased from 0.94 to 0.43 with increasing Cl_exp. Likewise, r² between Fig. 1c and more-oxidized toluene OH-SOA spectra decreased from 0.98 to 0.85 with increasing OH_exp, and r² between Fig. 1c and toluene Cl-SOA spectra decreased from 0.87 to 0.71 with increasing Cl_exp.

Fig. 1e, f and g, h plot the fractional contributions of the C_xH_y⁺, C_xH_yO₁⁺, C_xH_yO_>1⁺, and C_xH_yClO_z⁺ ion groups (f_CxHy⁺, f_CxHyO1⁺, f_CxHyO>1⁺, and f_CxHyClOz⁺) (pale orange) present in n-C₁₂ and toluene OH-/Cl-SOA as a function of OH_exp and Cl_exp. Minimum and maximum fractions of each ion group are provided in Table 2, and the corresponding range of OH_exp and Cl_exp values are listed in Table 1. Minor contributions from the C_xH_yClO⁺_z ion group (orange) to the toluene OH-SOA spectra in Fig. 1g were observed, primarily from signals at Cl⁺ and HCl⁺. These signals may be associated with NH₄Cl generated from the reaction of trace NH₃ in the system with residual HCl from Cl-OFR studies. While f_CxHy⁺ and f_CxHyO1⁺ values spanned similar ranges, the maximum f_CxHyO>1⁺ value was lower in n-C₁₂ Cl-SOA (0.34) than in n-C₁₂ OH-SOA (0.48). For both n-C₁₂ OH-SOA and Cl-SOA, f_CxHy⁺ decreased monotonically, f_CxHyO1⁺ increased and then decreased, and f_CxHyO>1⁺ and f_CxHyClOz⁺ increased monotonically as a function of OH_exp and/or Cl_exp. Because n-C₁₂ has no double bonds for direct Cl addition, one possible source of particulate organic chlorides (ROCl) may have been Cl oxidation of unsaturated dihydrofuran intermediates.³³ Another possible ROCl source involves the reaction RO₂ + Cl → RO + ClO followed by the reaction RO₂ + ClO → ROCl + O₂,^85,86 where RO₂ represents organic peroxy radicals derived from Cl oxidation of n-C₁₂ and/or its oxidation products and RO represents alkoxy radicals.

Table 2 Summary of OFR experimental results. The fraction of SOA signals detected in C_xH_y⁺, C_xH_yO⁺, C_xH_yO_≥1⁺, C_xH_yClO_z⁺, and C_xH_yBrO_z⁺ ion groups (f_CxHy⁺, f_CxHyO⁺, f_CxHyO>1⁺, f_CxHyClOz⁺, and f_CxHyBrOz⁺), oxygen-to-carbon (O/C) and hydrogen-to-carbon (H/C) ratios were obtained from high-resolution analysis of L-ToF-AMS spectra over the range of oxidant exposures summarized in Table 1. SOA yields were calculated as described in Section 2.3

VOC/oxidant	f CxHy^+	f CxHyO^+	f CxHyO>1^+	f CxHyClOz^+	f CxHyBrOz^+	O/C	H/C	C SOA (μg m^−3)	Y SOA
n-C12/OH	0.24–0.60	0.30–0.37	0.08–0.48	—	—	0.27–1.05	0.89–1.24	25–132	0.18–0.91
n-C12/Cl	0.30–0.61	0.32–0.37	0.06–0.34	0.009–0.033	—	0.22–0.82	1.20–1.63	160–357	1.1–2.5
Toluene/OH	0.11–0.27	0.19–0.38	0.37–0.66	—	—	0.83–1.51	0.89–1.24	16–93	0.098–0.56
Toluene/Cl	0.18–0.46	0.17–0.25	0.20–0.40	0.09–0.25	—	0.46–1.08	0.86–1.22	24–106	0.15–0.64
Toluene/Cl	0.18–0.44	0.12–0.20	0.24–0.40	0.12–0.31	—	0.53–1.02	0.82–1.07	14–99	0.083–0.58
Isoprene/OH	0.24–0.43	0.37–0.50	0.08–0.38	—	—	0.36–0.91	1.17–1.50	4–53	0.031–0.40
Isoprene/Cl	0.36–0.60	0.30–0.34	0.06–0.17	0.042–0.065	—	0.24–0.55	1.22–1.38	1–29	0.011–0.21
Isoprene/Br	0.46	0.34–0.35	0.05–0.07	—	0.12–0.13	0.29–0.31	1.40–1.44	2–7	0.007–0.018
α-Pinene/OH	0.28–0.46	0.26–0.40	0.13–0.42	—	—	0.38–0.93	1.16–1.48	18–52	0.11–0.31
α-Pinene/Cl	0.29–0.45	0.28–0.34	0.16–0.33	0.058–0.095	—	0.42–0.80	1.18–1.39	37–77	0.22–0.47
α-Pinene/Br	0.49–0.56	0.31–0.32	0.08–0.12	—	0.042–0.063	0.26–0.34	1.36–1.39	0.5–18	0.0006–0.037
α-Pinene/Br	0.55–0.59	0.25–0.26	0.09–0.12	—	0.056–0.063	0.24–0.30	1.33–1.38	1–9	0.003–0.018

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Qualitatively similar changes in toluene OH-SOA and Cl-SOA spectra were observed as a function of OH_exp and Cl_exp. Because toluene is more volatile than n-C₁₂, addition of more oxygen-containing functional groups was required to generate condensable oxidation products, resulting in lower initial f_CxHy⁺ and f_CxHyO1⁺ values, with a monotonic decrease in f_CxHyO1⁺ instead of an increase and then decrease as was observed in n-C₁₂ SOA. Additionally, higher f_CxHyO>1⁺ values were measured in toluene OH-/Cl-SOA, and higher f_CxHyClOz⁺ values were observed in toluene Cl-SOA than in n-C₁₂ SOA. Notably, for Cl-SOA generated from toluene (and, to a lesser extent, n-C₁₂), f_CxHyClOz⁺ followed the same trend as f_CxHyO>1⁺. This suggests that this ion group was also associated with later-generation oxidation products. Because direct Cl addition to the aromatic ring is a minor pathway (e.g. Cai et al.³⁰), ROCl in toluene Cl-SOA may have been generated from RO₂ + ClO reactions, as was hypothesized earlier for ROCl observed in n-C₁₂ Cl-SOA. Overall, these observations were consistent with multigenerational oxidative aging of n-C₁₂ and toluene OH- and Cl-SOA, where early-generation oxidation products that contributed to the less-oxidized C_xH_y⁺ and C_xH_yO₁⁺ ion groups were converted to later-generation oxidation products that contributed to the C_xH_yO_>1⁺ ion groups.

3.2 Sample biogenic OH-SOA, Cl-SOA, and Br-SOA mass spectra

Fig. 2a–f shows L-ToF-AMS spectra of SOA generated from the OH, Cl, and Br oxidation of isoprene and α-pinene. The isoprene OH-SOA, Cl-SOA and Br-SOA mass spectra were obtained at OH_exp, Cl_exp, and Br_exp values of 3.1 × 10¹⁰, 6.1 × 10⁹, and 3.2 × 10¹⁰ cm⁻³ s, and the α-pinene OH-SOA, Cl-SOA, and Br-SOA spectra were obtained at OH_exp, Cl_exp, and Br_exp values of 6.8 × 10¹⁰, 3.9 × 10¹⁰, and 5.9 × 10¹¹ cm⁻³ s. All SOA spectra were dominated by signals at m/z = 29 (CHO⁺), 39 (C₃H₃⁺), 43 (C₂H₃O⁺), 44 (CO₂⁺), and 55 (C₃H₅O⁺). Thus, at the lower OH, Cl, and Br exposures used to generate Fig. 2 spectra, the main differences were associated with C_xH_yClO_z⁺ and C_xH_yBrO_z⁺ ions present in Cl-SOA and Br-SOA.

Fig. 2 L-ToF-AMS spectra of SOA generated from the (a and g) OH oxidation of isoprene, (b and h) Cl oxidation of isoprene, (c and i) Br oxidation of isoprene, (d and j) OH oxidation of α-pinene, (e and k) Cl oxidation of α-pinene, and (f and l) Br oxidation of α-pinene. OH_exp, Cl_exp, and Br_exp values listed in (a)–(f) are in units of cm⁻³ s. The α-pinene Br-SOA spectrum presented in (f) was generated using OFR254-iC₂Br₂O₂. Additional figure notes regarding (l): α-pinene Br-SOA was generated using (1) OFR369/421-iBr₂ or (2) OFR254-iC₂Br₂O₂.

As was done in Section 3.1, to characterize the similarity of SOA mass spectra obtained from OH, Cl, and Br oxidation of the same precursor, we calculated r² values between the isoprene OH-/Cl-/Br-SOA spectra shown in Fig. 2a–c, and between the α-pinene OH-/Cl-/Br-SOA shown in Fig. 2d–f. The r² values between isoprene OH-/Cl-SOA and OH-/Br-SOA were 0.83 and 0.93 respectively. Likewise, r² values between α-pinene OH-/Cl-SOA and OH-/Br-SOA were 0.86 and 0.92. In each of these cases, removing contributions from m/z = 43 or m/z = 44 from the regression analysis resulted in a minimal change in r². To compare the similarity of isoprene and α-pinene OH-SOA, Cl-SOA and Br-SOA spectra across the full range of experimental conditions that were used, Fig. S11† plots r² values between the L-ToF-AMS spectra shown in Fig. 2a and d and corresponding isoprene or α-pinene OH-/Cl-/Br-SOA spectra obtained at other OH_exp, Cl_exp, and Br_exp. The r² value between Fig. 2a and more-oxidized isoprene OH-SOA spectra decreased from 0.99 to 0.32 with increasing OH_exp. For isoprene Cl-SOA, r² increased from 0.83 to 0.87 before decreasing to 0.63 at higher Cl_exp, whereas r² = 0.91–0.92 as a function of Br_exp for isoprene Br-SOA. The r² value between Fig. 2d and more-oxidized α-pinene OH-SOA spectra decreased from 0.96 to 0.46 with increasing OH_exp. For α-pinene Cl-SOA, r² decreased from 0.86 to 0.57, and for α-pinene Br-SOA, r² ranged from 0.92–0.93 (OFR254-iC₂Br₂O₂) or 0.83–0.86 (OFR369/421-iBr₂) as a function of Br_exp.

Fig. 2g–l plot f_CxHy⁺, f_CxHyO1⁺, f_CxHyO>1⁺, f_CxHyClOz⁺, and f_CxHyBrOz⁺ (blue) present in isoprene and α-pinene OH-/Cl-/Br-SOA as a function of oxidant exposure. Here, as was observed in Fig. 1g, minor contributions from the C_xH_yClO_z⁺ ion group (orange) to the α-pinene OH-SOA spectra in Fig. 2g were mostly Cl⁺ and HCl⁺, and may be associated with NH₄Cl generated from incidental NH₃ + HCl reactions. For isoprene and α-pinene OH-/Cl-SOA, as with toluene OH-/Cl-SOA, f_CxHy⁺ and f_CxHyO1⁺ decreased, and f_CxHyO>1⁺ and f_CxHyClOz⁺ increased with increasing OH_exp or Cl_exp (Table 2). Despite a similar decrease in f_CxHy⁺ of isoprene OH-SOA and Cl-SOA, the decrease in f_CxHyO1⁺ and increase in f_CxHyO>1⁺ were smaller in isoprene Cl-SOA than in isoprene OH-SOA. This was probably because a higher oxidation state was achieved for isoprene OH-SOA than for isoprene Cl-SOA: the maximum OH_exp (9.6 × 10¹¹ cm⁻³ s) was 6.5 times higher than the maximum Cl_exp (1.5 × 10¹¹ cm⁻³ s) shown in Fig. 2h, but the isoprene + Cl reaction rate is only 4.3 times faster than the isoprene + OH reaction rate. Due to Cl-induced fragmentation at high Cl_exp, the yield and size of isoprene SOA particles generated at Cl_exp = 2.9 × 10¹¹ cm⁻³ s were too small for efficient transmission through the L-ToF-AMS inlet.

On the other hand, changes in isoprene and α-pinene Br-SOA composition were minor by comparison. For isoprene Br-SOA, f_CxHy⁺ was approximately constant, f_CxHyO⁺ decreased by less than 0.01, and f_CxHyO>1⁺ and f_CxHyBrOz⁺ increased slightly. For α-pinene Br-SOA, f_CxHy⁺ decreased slightly, while f_CxHyO1⁺, f_CxHyO>1⁺, and f_CxHyBrOz⁺ increased slightly. Following direct Br addition to α-pinene, additional organic bromide (ROBr) formation may have occurred via RO₂ + Br and/or RO₂ + BrO reactions.^87,88 At a specific Br_exp, α-pinene Br-SOA had lower f_CxHyO⁺ and higher f_CxHyO1⁺ when generated via OFR254-iC₂Br₂O₂ (lines) compared to OFR369/421-iBr₂ (symbols). We do not think that α-pinene Br-SOA photolysis at λ = 254 nm was important when using OFR254-iC₂Br₂O₂ (Section 3.5), but other potential reasons for these Br-SOA compositional differences are unclear. Overall, based on these results, we hypothesize that multigenerational oxidative aging of Br-SOA was less extensive than in OH-SOA and Cl-SOA because Br is a more selective oxidant. While OH and Cl are likely reactive towards the majority of early-generation isoprene and α-pinene oxidation products, Br is only known to react efficiently with alkenes and aldehydes, with reactivity towards alcohols, ketones, and peroxides that is orders of magnitude slower than OH or Cl (e.g. Manion et al.⁸⁹). This hypothesis and its implications will be explored further in Sections 3.4 and 3.5.

3.3 Cl-SOA and Br-SOA mass spectral markers

Fig. 3 shows L-ToF-AMS spectra of C_xH_yClO⁺_z ions present in the n-C₁₂, toluene, isoprene and α-pinene Cl-SOA spectra plotted in Fig. 1b, d and 2b, e. The y-axis in Fig. 3a (n-C₁₂ Cl-SOA) was multiplied by a factor of 10 to put it on the same scale as Fig. 3b–d. The f_CxHyClOz⁺ was lowest for n-C₁₂ Cl-SOA (0.0083) because Cl oxidation proceeded practically exclusively via H-abstraction, and was highest for isoprene Cl-SOA (0.114) because Cl oxidation proceeded mostly via addition to double bonds. Intermediate f_CxHyOzCl⁺ values of 0.077 and 0.063 were observed for toluene and α-pinene SOA, respectively. Overall, signals at m/z = 35 (Cl⁺) and m/z = 36 (HCl⁺) and their isotopes contributed 66 to 81% of the C_xH_yClO_z⁺ signal across all Cl-SOA types. Other ions that were detected in multiple Cl-SOA spectra included m/z = 49 (CH₂Cl⁺), 61–63 (C₂H₂₋₄Cl⁺), 76 (C₂HClO⁺) and 77 (C₂H₂ClO⁺), albeit usually at much lower levels than Cl⁺ and HCl⁺.

Fig. 3 C_xH_yClO_z⁺ ions present in L-ToF-AMS spectra of (a) n-C₁₂, (b) toluene, (c) isoprene, and (d) α-pinene Cl-SOA displayed in Fig. 1b, d and 2b, e. The y-axis scale in (a) is multipled by 10 to put it on the same scale as (b)–(d).

To compare these results with chamber Cl-SOA studies that reported f_HCl⁺ values, Table 3 lists minimum and maximum f_HCl⁺ values measured in OFR- and chamber-generated Cl-SOA at their corresponding Cl_exp values. In OFR-generated SOA, as expected, f_HCl⁺ increased as a function of Cl_exp in a similar manner to the C_xH_yClO_z⁺ ion signals shown in Fig. 1f, h and 2h, k. The most significant absolute change in f_HCl⁺ was observed for OFR-generated toluene Cl-SOA, where f_HCl⁺ increased from 0.050 to 0.16 as Cl_exp increased from 1.5 × 10¹⁰ to 1.3 × 10¹² cm⁻³ s. By comparison, f_HCl⁺ measured in chamber-generated toluene Cl-SOA ranged from 0.051 to 0.069.³⁵ For the other Cl-SOA types, f_HCl⁺ increased from 0.0059 to 0.020 (n-C₁₂), 0.046 to 0.071 (isoprene), and 0.041 to 0.065 (α-pinene), respectively. Chamber-generated Cl-SOA had f_HCl⁺ values ranging from 0.008 to 0.014 (n-C₁₂), 0.034 to 0.067 (isoprene), and 0.013 to 0.056 (α-pinene).^32,33,36

Table 3 Comparison of OFR (this work) and chamber Cl-SOA (all other references listed in table) experimental conditions and results; “N/S” = not specified. O/C and H/C ratios of chamber Cl-SOA were calculated as described in Sect. 2.3. In chamber Cl-SOA studies, the fraction of AMS or ACSM signals contributed by HCl⁺ (f_HCl⁺) was calculated from signals at m/z = 36 + 38

VOC	Clexp (cm^−3 s)	O/C	H/C	f HCl^+	C SOA (μg m^−3)	Y SOA	Reference
n-C12	3.3 × 10^10 to 2.3 × 10^11	0.22–0.82	1.63–1.20	0.006–0.020	160–357	1.1–2.5	This work
n-C12	9.8 × 10^10 to 1.3 × 10^11	0.32–0.88	1.50–1.26	0.006–0.014	99–149	1.10–1.65	Wang and Hildebrandt Ruiz^33
Toluene	2.3 × 10^10 to 1.3 × 10^12	0.53–1.02	1.07–0.82	0.067–0.159	14–99	0.083–0.58	This work
Toluene	3.3 × 10^10 to 2.3 × 10^11	0.46–1.08	1.22–0.86	0.050–0.127	24–106	0.15–0.64	This work
Toluene	N/S	0.69–0.81	0.94–0.89	0.050–0.069	53–136	0.33–0.67	Dhulipala et al.^35
Toluene	N/S	0.65	1.31	0.071	3–12	0.030–0.079	Cai et al.^30
Isoprene	6.1 × 10^9 to 2.9 × 10^11	0.24–0.55	1.38–1.22	0.042–0.065	1–29	0.011–0.21	This work
Isoprene	N/S	0.45–0.72	1.32–1.11	0.033–0.067	9–80	0.08–0.29	Wang and Hildebrandt Ruiz^32
α-Pinene	3.9 × 10^10 to 1.2 × 10^11	0.42–0.80	1.39–1.18	0.040–0.065	37–77	0.22–0.47	This work
α-Pinene	1.9 × 10^11	0.46–0.65	1.25	0.013–0.056	14–247	0.44–0.96	Masoud and Hildebrandt Ruiz^36
α-Pinene	N/S	N/S	N/S	N/S	0.0013–176	3.7 × 10^−5 to 0.62	Ofner et al.^31
α-Pinene	N/S	N/S	N/S	N/S	8–33	0.079–0.22	Cai and Griffin^29

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L-ToF-AMS spectra of Br-SOA contained a series of C_xH_yBrO_z⁺ ions (Fig. 4), with f_CxHyBrOz⁺ = 0.115 and 0.039 for isoprene and α-pinene Br-SOA, respectively. Because Br oxidation proceeded via double-bond addition, f_CxHyBrOz⁺ was higher in isoprene Br-SOA. Signals at m/z = 79 (Br⁺) and 80 (HBr⁺) and their isotopes contributed 58 and 68% of the C_xH_yBrO_z⁺ signal for isoprene and α-pinene Br-SOA; f_HBr⁺ ranged from 0.035 to 0.042 and from 0.016 to 0.022 for isoprene and α-pinene Br-SOA (Table 3). Analogous to the usage of HCl⁺ as a marker ion for ROCl in the AMS,³² we hypothesize that H⁷⁹Br⁺ and/or H⁸¹Br⁺ may be used as simple markers for ROBr in the absence of inorganic halides such as NH₄Br. Depending on the source region, H⁸¹Br⁺ may be easier to resolve than H⁷⁹Br⁺ due to sulfate (SO₃⁺) interference at m/z = 79. Other signals that were detected in both isoprene and α-pinene Br-SOA spectra included m/z = 93 and 95 (CH₂Br⁺), 106 and 108 (C₂H₃Br⁺), 121 and 123 (C₂H₂BrO⁺) and 133 and 135 through 138 (C₃H₅₋₆BrO⁺). Larger C_xH_yBrO_z⁺-containing ions up to m/z = 245 and 247 were identified in α-pinene Br-SOA (Fig. S12–S13;† both m/z values shown to indicate similar concentrations of C₁₀H₁₄BrO₂⁺ and its ⁸¹Br-containing isotope).

Fig. 4 C_xH_yBrO_z⁺ ions present in L-ToF-AMS spectra of (a) isoprene and (b) α-pinene Br-SOA displayed in Fig. 2c and f. The y-axis scale in (b) is multipled by 2.5 to put it on the same scale as (a).

3.4 H/C and O/C ratios of OH-SOA, Cl-SOA and Br-SOA

Van Krevelen diagrams that show H/C ratio as a function of O/C ratio have been used to provide information about the nature of SOA formation and oxidative aging.⁹⁰ Typically, with oxidative aging the O/C ratio increases and H/C ratio of SOA decreases as oxygen-containing functional groups are added to a carbon backbone. Here, we use Van Krevelen diagrams to compare the elemental ratios of OH-SOA, Cl-SOA, and Br-SOA discussed in the previous sections. Where applicable, data from chamber SOA studies are included for comparison.

Fig. 5a–d show that chamber- and OFR-generated OH-SOA generally have similar Van Krevelen plots within the limited range of overlap of O/C and H/C values. Chamber-generated n-C₁₂ OH-SOA had O/C and H/C values ranging from 0.18 to 0.30 and 1.83 to 1.67, respectively,⁶⁶ and OFR-generated n-C₁₂ OH-SOA had O/C and H/C values ranging from 0.27 to 1.05 and 1.59 to 1.08. Thus, SOA elemental ratios agree within approximately 5–10% for the most-oxidized chamber-generated n-C₁₂ OH-SOA and the least-oxidized OFR-generated n-C₁₂ OH-SOA. Similar trends were observed with the other OH-SOA types studied in this work (Fig. 5b–d). Whereas chamber studies have limited ability to generate highly oxidized OH-SOA, here, it was possible to compare H/C and O/C ratios of chamber- and OFR-generated Cl-SOA over a wide range of SOA oxidation state. Chamber-generated n-C₁₂ Cl-SOA had O/C and H/C ratios ranging from 0.30 to 0.88 and 1.50 to 1.26 respectively³³ compared to O/C and H/C ratios ranging from 0.22 to 0.82 and 1.63 to 1.20 for OFR-generated n-C₁₂ Cl-SOA. At O/C = 0.36, n-C₁₂ Cl-SOA H/C ratios agreed within ∼1% for chamber and OFR-generated Cl-SOA; likewise, at O/C = 0.82, the H/C ratios agreed within ∼5%. These results suggest that Cl-SOA elemental composition is the same regardless of whether it is generated at lower oxidant concentrations over longer exposure times in chambers, or higher oxidant concentrations over shorter exposures times in OFRs, similar to OH-SOA.^91,92

Fig. 5 Van Krevelen diagrams showing H/C ratio as a function of O/C ratio for SOA generated from (a) OH and Cl oxidation of n-C₁₂, (b) OH and Cl oxidation of toluene, (c) OH, Cl, and Br oxidation of isoprene, and (d) OH, Cl, and Br oxidation of α-pinene. Additional figure notes for superscripts: ¹Cl generated using OFR254/313-iC₂Cl₂O₂; ²Cl generated using OFR313/369-iCl₂; ³Br generated using OFR254-iC₂Br₂O₂; ⁴Br generated using OFR369/421-iBr₂; ⁵Yee et al.;⁶⁶⁶Chhabra et al.;⁶⁵⁷Wang and Hildebrandt Ruiz;³³⁸Dhulipala et al.;³⁵⁹Cai et al.;³⁰¹⁰Wang and Hildebrandt Ruiz;³²¹¹Masoud and Hildebrandt Ruiz.³⁶

Fig. 5 also provides insight into differences between OH-, Cl-, and Br-SOA oxidative aging pathways. For systems where H-atom abstraction was the dominant reaction pathway (e.g., n-C₁₂), the OH- and Cl-SOA Van Krevelen plots were essentially identical within the range of overlapping H/C and O/C ratios. On the other hand, the H/C ratio of toluene Cl-SOA was consistently ∼25–30% lower than the H/C ratio of toluene OH-SOA at a specific O/C ratio. For example, at O/C ≈ 0.8, H/C = 1.24 for toluene OH-SOA and 1.01 for toluene Cl-SOA. Because the Van Krevelen slopes (Δ(H/C)/Δ(O/C)) ranged from −0.50 to −0.55 for toluene OH-/Cl-SOA, the higher H/C ratio observed for toluene OH-SOA was probably a consequence of OH addition to the aromatic ring for (at least) the first toluene + OH reaction step, given that the H/C ratio of toluene is 1.14. This pathway would initially increase the H/C ratio. Thereafter, addition of similar oxygen-containing functional groups to toluene OH-SOA and Cl-SOA over multiple oxidation steps resulted in similar reductions in H/C as a function of O/C. Similar trends were observed for isoprene and α-pinene OH-/Cl-SOA, with H/C offsets of approximately 0.15 (isoprene) and 0.07 (α-pinene) at a specific O/C ratio. Corresponding Van Krevelen slopes ranged from −0.55 to −0.60 for each OH- and Cl-SOA type. As with OH oxidation of toluene via direct addition, OH addition to double bond(s) present in isoprene and α-pinene would have also initially increased the H/C ratio relative to Cl-SOA generated from the same precursor. Here, the lower increase in H/C of isoprene OH-SOA relative to toluene OH-SOA suggests multiple OH additions to the toluene backbone occurred via ring-opening reactions. Subsequent addition of similar oxygen-containing functional groups by both OH and Cl oxidation would have then generated similar Van Krevelen slopes.

Whereas the O/C and H/C ratios of OH-SOA and Cl-SOA changed significantly as a function of OH_exp or Cl_exp, changes in the elemental ratios of isoprene and α-pinene Br-SOA were relatively minor. The O/C and H/C ratios of isoprene Br-SOA ranged from 0.24 to 0.31 and 1.40 to 1.41, and O/C and H/C ratios of α-pinene Br-SOA ranged from 0.24 to 0.32 and 1.33 to 1.39. Along with Fig. 2f and l, Fig. 5 provides additional evidence that multistep oxidative aging of Br-SOA is less extensive than in OH-SOA and Cl-SOA.

3.5 OH-SOA, Cl-SOA, and Br-SOA mass yields

Fig. 6a–d shows mass yields of SOA as a function of OH_exp and Cl_exp for n-C₁₂, toluene, isoprene, and α-pinene. SOA yields obtained from Br oxidation of isoprene and α-pinene are also shown in Fig. 6c and d. Fig. S14† shows the same data that is plotted in Fig. 6, plus the corresponding SOA yield values without applying particle wall loss correction (pWLC) factors (Section 2.3.2). Results obtained from Cl oxidation of the same precursors in environmental chamber studies is provided in Table 3. Some of the environmental chamber α-pinene Cl-SOA yields, and all the n-C₁₂ Cl-SOA yields, were measured in the presence of added NO_x, whereas no NO_x was added in OFR experiments. However, yields of n-C₁₂ OH-SOA do not display a systematic NO_x dependence,⁹³ and it is not yet clear to what extent NO_x affects α-pinene Cl-SOA yields.^29,31,36 Thus, to first order we assume NO_x has less influence than other experimental variables such as oxidant type or Cl_exp on yields of n-C₁₂ or α-pinene Cl-SOA.

Fig. 6 Mass yields of SOA generated from (a) OH and Cl oxidation of n-C₁₂, (b) OH and Cl oxidation of toluene, (c) OH, Cl, and Br oxidation of isoprene, and (d) OH, Cl, and Br oxidation of α-pinene as a function of OH_exp, Cl_exp, or Br_exp. Different y-axis scales are used in each subpanel. Error bars indicate ±1σ uncertainty in binned SOA yield values, ±50% uncertainty in OH exposure values, and ±70% uncertainty in Cl and Br exposure values. Additional figure notes: ¹Cl generated using OFR254/313-iC₂Cl₂O₂; ²Cl generated using OFR313/369-iCl₂; ³Br generated using OFR254-iC₂Br₂O₂; ⁴Br generated using OFR369/421-iBr₂.

Fig. 6a shows that yields of n-C₁₂ Cl-SOA and OH-SOA initially increased at lower Cl_exp and OH_exp following functionalization reactions that produced condensable LVOCs. At higher Cl_exp and OH_exp, yields decreased due to fragmentation reactions that generated higher-volatility oxidation products. Here, the observation that Cl-SOA yields exhibit similar trends as OH-SOA yields with increasing oxidation therefore builds on results obtained in previous OH-OFR laboratory and field studies.^{23–28,78,94,95} For n-C₁₂ Cl-SOA, the yield increased from 1.1 to 2.5 (0.92 to 1.9 without pWLC; Fig. S14†) as Cl_exp increased from 3.3 × 10¹⁰ to 8.2 × 10¹⁰ cm⁻³ s, then decreased to 1.7 (1.3 without pWLC) at Cl_exp = 2.3 × 10¹¹ cm⁻³ s. For n-C₁₂ OH-SOA, the yield increased from 0.18 to 0.91 (0.14 to 0.74 without pWLC) as OH_exp increased from 2.3 × 10¹¹ to 5.4 × 10¹¹ cm⁻³ s, then decreased to 0.42 (0.31 without pWLC) at OH_exp = 2.1 × 10¹² cm⁻³ s. Thus, over the range of conditions shown in Fig. 6, the maximum n-C₁₂ Cl-SOA yield was approximately 2.7 times higher than the maximum n-C₁₂ OH-SOA yield. In chamber studies, n-C₁₂ Cl-SOA yields ranged from 1.10 to 1.65 (ref. 33) (Cl_exp = 9.8 × 10¹⁰ to 1.3 × 10¹¹ cm⁻³ s) and n-C₁₂ OH-SOA yields ranged from 0.15 to 0.28 (OH_exp = 2.2 × 10¹¹ to 4.3 × 10¹¹ cm⁻³ s).⁹³

Fig. 6b shows yields of toluene Cl-SOA and OH-SOA as a function of OH_exp and Cl_exp. Toluene Cl-SOA yields obtained via OFR254-iC₂Cl₂O₂ and OFR313/369-iCl₂ are represented by different symbols. At the lowest and highest Cl_exp values that were used (Table 1), Cl-SOA yields were 0.083 and 0.090, respectively. Maximum toluene Cl-SOA yields were 0.58 ± 0.13 at Cl_exp = 2.5 × 10¹¹ cm⁻³ s via OFR313/369-iCl₂ and 0.64 ± 0.28 at Cl_exp = 2.2 × 10¹¹ cm⁻³via OFR254-iC₂Cl₂O₂. Toluene Cl-SOA yield values obtained in chambers ranged from 0.030 to 0.079 (ref. 30) and 0.33 to 0.67 (ref. 35) (Table 3). For toluene OH-SOA, the yield increased from 0.24 to 0.56 as OH_exp increased from 2.6 × 10¹¹ to 5.8 × 10¹¹ cm⁻³ s, then decreased to 0.079 at OH_exp = 2.3 × 10¹² cm⁻³ s. At the lowest OH_exp used here, our toluene OH-SOA yield value of 0.24 agrees within 14% of the toluene OH-SOA mass yield of 0.21 obtained at OH_exp = 2.3 × 10¹¹ cm⁻³ by Hildebrandt Ruiz et al.⁹⁶ Overall, over the range of OFR conditions shown in Fig. 6b, Cl and OH oxidation of toluene generated maximum Cl-SOA and OH-SOA yield values that were within 14% of each other.

Fig. 6c shows yields of isoprene Cl-SOA, Br-SOA, and OH-SOA as a function of OH_exp, Cl_exp, and Br_exp. At the lowest and highest Cl_exp values shown in Fig. 6c, Cl-SOA yields were 0.027 and 0.011, respectively. The maximum isoprene Cl-SOA yield was 0.21 at Cl_exp = 3.0 × 10¹⁰ cm⁻³ s. By comparison, chamber isoprene Cl-SOA yield values ranged from 0.08 to 0.29.³² The maximum isoprene Br-SOA yield was only 0.018 at Br_exp = 3.8 × 10¹¹ cm⁻³ s, even when using a significantly higher isoprene mixing ratio than what was used in OH and Cl experiments (Table 1). The maximum isoprene OH-SOA yield measured here was 0.40 at OH_exp = 6.8 × 10¹⁰ cm⁻³ s, which was a higher yield than expected based on recent isoprene OH-SOA yield values measured in the absence of NO_x in both OFRs (0.032 at OH_exp = 7.8 × 10¹¹ cm⁻³ (ref. 92)) and chambers (≤0.15 at OH_exp ≈ 5.1 × 10¹⁰ cm⁻³ s (ref. 97)). Our results agree with those obtained by Lambe et al.⁹² at comparable OH_exp because we observed an isoprene OH-SOA yield of 0.031 at OH_exp = 9.6 × 10¹¹ cm⁻³ s. Thus, the lower OH_exp achieved in this study was a contributing factor to the higher OFR isoprene OH-SOA yield, presumably due to less fragmentation of the SOA. While OH_exp = 6.8 × 10¹⁰ cm⁻³ s is within ≈ 30% of the OH_exp we estimate was used by Liu et al.,⁹⁷ photochemical box modeling calculations suggest that mixing ratios of hydroperoxyl (HO₂) radicals were approximately 6 times higher in the OFR (≈3 ppb vs. 0.55 ppb). Because isoprene OH-SOA yields are sensitive to the rate of reaction between HO₂ and RO₂, we hypothesize that our results may have been obtained under conditions that favored HO₂ + RO₂ reactions to a greater extent, thereby leading to ≈2 times higher SOA yield values. Overall, over the range of OFR conditions shown in Fig. 6, Cl and Br oxidation of isoprene generated SOA with maximum yields that are 52% and 5% of those obtained via OH oxidation.

Fig. 6d shows yields of α-pinene Cl-SOA, Br-SOA and OH-SOA. As was done in isoprene Br-SOA studies, a higher α-pinene mixing ratio was used to generate enough mass of low-volatility oxidation products to promote homogenous nucleation of Br-SOA (Table 1). The maximum α-pinene Cl-SOA yield was 0.47 at Cl_exp = 5.8 × 10¹⁰ cm⁻³ s. Maximum α-pinene Br-SOA yields were 0.037 at Br_exp = 1.7 × 10¹² cm⁻³ s (OFR254-iC₂Br₂O₂) and 0.018 at Br_exp = (2.8–3.8) × 10¹² cm⁻³ s (OFR421-iBr₂). As summarized in Table 3, our α-pinene Cl-SOA yield values were within the range of chamber α-pinene Cl-SOA yield values between 0.079 to 0.22,²⁹ 0.11 to 0.62,³¹ and 0.44 to 0.96.³⁶ For α-pinene OH-SOA, the maximum yield was 0.31 at OH_exp = 1.8 × 10¹¹ cm⁻³ s, which is generally consistent with chamber α-pinene OH-SOA yield values obtained at comparable OH_exp.⁹⁸ Over the range of conditions shown in Fig. 6d, Cl and Br oxidation of α-pinene generates SOA at yields that are approximately 150% and 6–12% of those obtained via OH oxidation.

SOA photolysis at λ = 254 nm is a concern under certain OH-OFR conditions,⁴⁶ but direct experimental evaluation is difficult due to the lack of operable OH sources at longer photolysis wavelengths. Here, our measurements enable an investigation into the potential role of Cl-SOA and Br-SOA photolysis at λ = 254 nm through a comparison of the yields of toluene Cl-SOA generated via OFR254-iC₂Cl₂O₂versus OFR313/369-iCl₂, and of α-pinene Br-SOA generated via OFR254-iC₂Br₂O₂versus OFR369/421-iBr₂. If SOA photolysis at λ = 254 nm was significant, yields of toluene Cl-SOA and α-pinene Br-SOA obtained using OFR254-iC₂Cl₂O₂ and OFR254-iC₂Br₂O₂ would have been lower than those obtained via OFR313/369-iCl₂ and OFR369/421-iBr₂. This was not the case. For toluene Cl-SOA, in the three regions of approximate Cl_exp overlap – (2.3–2.7) × 10¹⁰, (2.2–2.5) × 10¹¹, and (4.5–4.7) × 10¹¹ cm⁻³ s – toluene Cl-SOA yields obtained using OFR254-iC₂Cl₂O₂ were either higher than, or in agreement with, yields obtained using OFR313/369-iCl₂ within measurement uncertainties: 0.25 ± 0.08 versus 0.083 ± 0.038, 0.64 ± 0.28 versus 0.58 ± 0.13, and 0.15 ± 0.07 versus 0.32 ± 0.16, respectively. Similarly, α-pinene Br-SOA yields, obtained at Br_exp ranging from 9.5 × 10¹¹ to 2.9 × 10¹² cm⁻³ s, were between 0.024 and 0.032 using OFR254-iC₂Br₂O₂, compared to 0.0028 to 0.017 when using OFR369/421-iBr₂. Thus, because halocarbons are typically photolabile, our results suggest that photolysis of Cl-SOA or Br-SOA was too slow to compete with multigenerational Cl- or Br-induced oxidative aging in the OFR. Because OH-SOA was also generated under conditions using λ = 254 nm radiation, and because OH-induced oxidative aging of SOA occurs to a similar or greater extent than Cl or Br, our results support previous modeling studies suggesting that OH-SOA measurements obtained using OFR185 are not significantly impacted by SOA photolysis at λ = 254 nm.⁴⁶

4 Conclusions

In this study we characterized mass spectra, elemental ratios, and yields of SOA generated from the OH and Cl oxidation of representative anthropogenic precursors (n-C₁₂ and toluene) and the OH, Cl and Br oxidation of representative biogenic (isoprene and α-pinene) precursors. Overall, r²-values between L-ToF-AMS spectra of Cl-SOA (and, where applicable, Br-SOA) and OH-SOA generated from the same precursor ranged from 0.57 to 0.94 at low oxidant exposures (Fig. 1a–d and 2a–f). The highest r²-value was observed between n-C₁₂ OH- and Cl-SOA spectra, which was expected because OH- and Cl-induced oxidative aging occurred primarily via hydrogen atom abstraction. Van Krevelen diagrams of n-C₁₂ OH- and Cl-SOA also had the highest degree of similarity (Fig. 5a). In cases where OH, Cl, and/or Br addition to unsaturated precursors (toluene, isoprene, α-pinene) was possible, the r² values between Cl-/Br-SOA and OH-SOA were lower, and the H/C ratios of Cl-/Br-SOA were systematically lower than H/C ratios of OH-SOA (Fig. 5b–d). Additionally, the presence of C_xH_yClO⁺_z and C_xH_yBrO⁺_z ions in Cl-SOA and Br-SOA were clear indicators of halogen-initiated oxidative aging that may be used to investigate Cl- and Br-induced oxidative aging signatures in ambient AMS datasets. Across the full range of oxidant exposures that were studied, fundamental differences between Br- and OH-/Cl-initiated oxidative aging pathways were evident; namely, that multistep SOA oxidative aging is significant when initiated by OH and Cl, but not by Br.

Our SOA yield measurements indicate that specific precursors generated SOA at yields that were strongly dependent on the oxidant and exposure time. Across all OFR conditions that were used, OH oxidation of n-C₁₂, toluene, isoprene and α-pinene resulted in maximum OH-SOA yields ranging from 0.31 to 0.91; Cl oxidation of the same precursors, at the same precursor mixing ratios, generated Cl-SOA at maximum yields ranging from 0.21 to 2.5. Br oxidation of isoprene and α-pinene generated Br-SOA at maximum yields ranging from 0.018 to 0.037, suggesting that multigenerational oxidative aging may be required to achieve the range of yield values obtained across the OH-SOA and Cl-SOA systems examined here.

Notably, even though n-C₁₂ OH-SOA and Cl-SOA had the highest degree of similarity in chemical composition, maximum n-C₁₂ Cl-SOA and OH-SOA yields were the most different: the yield of n-C₁₂ Cl-SOA was 2.7 times higher than the corresponding maximum n-C₁₂ OH-SOA yield. This may be due to Cl preferentially reacting with terminal carbons on the n-C₁₂ backbone,⁹⁹ which could provide a longer effective carbon chain length for peroxy radicals to undergo intramolecular hydrogen shift reactions leading to SOA formation via autooxidation.¹⁰⁰ Because OH reacts with both terminal and non-terminal carbons with similar probability, this may have resulted in more n-C₁₂ OH-SOA fragmentation relative to n-C₁₂ Cl-SOA.⁹⁹ On the other hand, while toluene OH-SOA and Cl-SOA had the lowest degree of chemical similarity, maximum toluene OH-SOA and Cl-SOA mass yields were the closest in value. Overall, our results suggest that alkane, aromatic, and terpenoid SOA precursors are characterized by distinct OH- and halogen-initiated SOA yields, and that while Cl may enhance the SOA formation potential in regions influenced by biogenic and anthropogenic emissions, Br may have the opposite effect. Characterizing the molecular composition of specific oxidation products that contribute to OH-, Cl- and Br-SOA formation, and expanding the photochemical model introduced in Section 2.1.3 to include VOC + Cl/Br oxidation chemistry, may provide further insight into the trends observed here and will be the focus of future work.

Author contributions

AL conceived and planned the experiments. AL and AA carried out the OFR experiments and performed data analysis. DW and CM provided unpublished data from previous chamber Cl-SOA experiments, and MM calculated Cl exposure values in those chamber Cl-SOA experiments. AL, AA, NB, LHR, and WHB 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.

Acknowledgements

This work was supported by the Atmospheric Chemistry Program of the National Science Foundation: grants AGS-1934352 to Aerodyne Research, Inc.; AGS-1934369 to the University of Texas at Austin; and AGS-1934345 to Pennsylvania State University. AL thanks Lindsay Yee (University of California at Berkeley) for providing published H/C and O/C ratios for n-C₁₂ OH-SOA, and Benjamin Nault, Leah Williams, Donna Sueper, (Aerodyne), Pedro Campuzano-Jost (University of Colorado at Boulder), and Sergey Nizkorodov (University of California at Irvine) for helpful discussions.

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Footnotes

† Electronic supplementary information (ESI) available: mercury lamp emission spectra, Cl and Br exposure calibration data, supplemental high-resolution aerosol mass spectra, KinSim mechanism. See https://doi.org/10.1039/d2ea00018k

‡ Present address: Paul Scherrerr Institute, Villagen, Switzerland.

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