Molecular composition of fresh and aged secondary organic aerosol from a mixture of biogenic volatile compounds: a high-resolution mass spectrometry study

Abstract. Field observations over the past decade indicate that a significant fraction of organic aerosol in remote areas may contain highly oxidized molecules. Aerosol processing or further oxidation (aging) of organic aerosol has been suggested to be responsible for their formation through heterogeneous reaction with oxidants and multigenerational oxidation of vapours by OH radicals. In this study we investigated the influence of several aging processes on the molecular composition of secondary organic aerosols (SOA) using direct infusion and liquid chromatography high-resolution mass spectrometry. SOA was formed in simulation chamber experiments from ozonolysis of a mixture of four biogenic volatile organic compounds (BVOC): α-pinene, β-pinene, Δ3-carene and isoprene. The SOA was subsequently aged under three different sets of conditions: in the dark in the presence of residual ozone, with UV irradiation and OH radicals, and using UV light only. Among all studied conditions, only OH radical-initiated aging was found to influence the molecular composition of the aerosol and showed an increase in carbon oxidation state (OSC) and elemental O / C ratios of the SOA components. None of the aging processes produced an observable effect on the oligomers formed from ozonolysis of the BVOC mixture, which were found to be equally abundant in both "fresh" and "aged" SOA. Additional experiments using α-pinene as the sole precursor demonstrated that oligomers are an important group of compounds in SOA produced from both ozonolysis and OH radical-initiated oxidation processes; however, a completely different set of oligomers is formed under these two oxidation regimes. SOA from the OH-initiated oxidation of α-pinene had a significantly higher overall OSC and O / C compared to that from pure ozonolysis experiments confirming that the OH radical reaction is more likely to be responsible for the occurrence of highly oxidized species in ambient biogenic SOA.


Introduction
Biogenic volatile organic compounds (BVOCs) play an important role in atmospheric chemistry and give rise to secondary organic aerosols (SOA) that affect climate and air quality (Kanakidou et al., 2005;Hallquist et al., 2009). Although a substantial fraction (20-90 %) of atmospheric fine particulate matter is comprised of organic compounds , its molecular composition remains poorly understood. The limited knowledge of aerosol composition ultimately restricts our understanding of the most relevant particle sources.
Laboratory chamber experiments have been performed for decades in an attempt to mimic atmospheric SOA formation.

Published by Copernicus Publications on behalf of the European Geosciences Union.
However, it is still unclear how close the aerosol particles generated in laboratory experiments resemble atmospheric SOA with respect to their detailed chemical composition. Field observations over the past decade indicate that a significant fraction of organic aerosol in remote areas may contain highly oxidized molecules Kroll et al., 2011). In contrast, laboratory-generated SOA is oxidized to a much lesser extent, suggesting that the conditions in smog chamber experiments are not optimal for mimicking aging in the atmosphere . One likely reason for this difference in composition is that the reaction times in chamber experiments are significantly shorter than the lifetime of organic aerosol in the real atmosphere (e.g. Shilling et al., 2009). Another explanation for the difference is that typical smog chamber experiments are performed with only one or two SOA precursors and are often limited to one oxidant (e.g. O 3 or OH radicals).
It has been suggested that aerosol processing or further oxidation (aging) of OA could be responsible for formation of highly oxidized OA components through heterogeneous reaction with oxidants and multigenerational oxidation of vapours by OH radicals . Several OH radical initiated aging experiments have been performed with α-pinene, its oxidation products Müller et al., 2012;Denjean et al., 2015;Romonosky et al., 2015) and a mixture with limonene and p-xylene (Emanuelsson et al., 2013;Flores et al., 2014). It was found that hydroxyl radical aging significantly increases the concentration of first-generation BSOA components as determined from both O / C elemental ratios and individual marker compounds. In addition, atmospheric aging was proposed to have a role in the formation of high molecular weight compounds or oligomers (Kalberer et al., 2004) through condensed (Rudich et al., 2007) or aqueous (Renard et al., 2015) phase chemistry.
The influence of aqueous photochemical processing on the average molecular composition of SOA produced from several biogenic and anthropogenic precursors has also been recently investigated (Romonosky et al., 2015). A reduction of the average number of atoms in the SOA molecular formulae was observed after photolysis -however, without a significant effect on the overall O / C and H / C ratios. In another study, condensed-phase photochemistry in the near-ultraviolet (λ > 300 nm) was shown to induce significant changes in SOA particle size and composition (Epstein et al., 2014).
The influence of aging on oligomer formation is generally inferred from analysis of the elemental O / C and H / C ratios. However, other chemical processes such as carboxylation and carbonylization are also known to affect the elemental ratios. Additionally, the effects of aging on oligomerization have been assessed by monitoring the concentration of 2-5 dimers that could be identified by liquid chromatography mass spectrometry (LC/MS) (Emanuelsson et al., 2013). Other techniques such as high-resolution mass spectrometry (HRMS) often identify hundreds of oligomeric compounds (Tolocka et al., 2004;Kalberer et al., 2004;Reinhardt et al., 2007;Hall and Johnston, 2011;Putman et al., 2012;Kourtchev et al., 2014), which raises the question of whether the small number of dimers that can be quantified with LC/MS reliably represent the entire oligomer content in SOA.
The objectives of this work were to examine the influence of several aging conditions on the molecular composition of biogenic SOA. SOA formed from ozonolysis of a BVOC mixture was exposed to (i) residual ozone in the dark, (ii) OH radicals and UV light, and (iii) UV light only. The BVOC mixture contained the four most abundant compounds (i.e. α and β-pinene, 3 -carene, and isoprene) detected at a remote boreal forest site Hyytiälä, Finland (Hakola et al., 2003;Aaltonen et al., 2011;Bäck et al., 2012;Kourtchev et al., 2014). The aged SOA was characterized using direct infusion and LC HR-MS which allows detection of thousands of individual SOA constituents at once providing their elemental formulae from accurate mass measurements (Nizkorodov et al., 2011).

Methods
All aerosol aging experiments were performed in the CE-SAM chamber in Paris. Supplementary experiments on the ozonolysis and OH-initiated oxidation of α-pinene were conducted in the CRAC chamber in Cork.

CESAM experiments
Aerosol aging experiments were carried out in the CESAM chamber (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber). A detailed description of the chamber is provided elsewhere (Wang et al., 2011). Briefly, the chamber is a 4.2 m 3 stainless steel vessel, operated at 296 ± 2 K using synthetic air at 1 bar atmospheric pressure. The solar irradiation is mimicked using xenon arc lamps (4 kW, XPO 4000 W/HS, OSRAM) fitted with 8 mm Pyrex filters that provide an emission spectrum closely resembling that at the Earth's surface near the equator over the wavelength range 290-700 nm (Wang et al., 2011). For these experiments, the corresponding NO 2 photolysis frequency was J NO 2 = (2.5 ± 0.2) × 10 −3 s −1 . The experimental conditions are outlined in Table 1. The experiments were performed at a relative humidity (RH) of 2-9 %. Neutral seed particles of ammonium sulfate were generated from 3 mM (NH 4 ) 2 SO 4 (Sigma-Aldrich, 99.99 %) solution using an atomiser (TSI ® model 3075) and dried using a diffusion dryer (TSI ® , model 3062) before introduction into the chamber. BVOCs (i.e. α-pinene, β-pinene, 3 -carene and isoprene) were introduced into the chamber by flowing purified air over known amounts of the compounds in a gently heated Pyrex impinger. While the total concentrations of the BVOC mixture used in these chamber experiments exceeded (up to 10 times, depending on the season) those observed at the Finnish site, their molar ratios were kept very close to the reported values (i.e. α-pinene, 0.4; 3 -carene, 0.3; β-pinene, 0.2; and isoprene, 0.1). The total VOC mixture concentration was about 150 ppb for all CESAM experiments. The precursor hydrocarbons concentrations and their decay were measured using in situ FTIR spectroscopy. After injecting the BVOC mixture and allowing it to stabilize for 5-10 min, ozone was introduced into the chamber over a period of 10-15 s from an electric discharge generator. Infrared absorbing species such as SOA precursors and their oxidation products were measured during the reactions using multi-path in situ Fourier transform infrared (FTIR) spectrometry (Bruker GmbH, Ettlingen, Germany) with an optical pathlength of 192 m. The concentration of NO x throughout the experiments was below the detection limit. Particle size distributions (from 19 to 980 nm in diameter) were measured with a TSI ® 3080 scanning mobility particle sizer (SMPS) and a TSI ® 3010 condensation particle counter operating with 0.2 L min −1 sample flow and 2.0 L min −1 sheath flow. For the SOA mass concentration, the density of the organic material was assumed to be 1.0 g cm −3 .
While this choice of density affects the estimated mass of collected SOA, the major conclusions of this work are not sensitive to the actual value of SOA density.
SOA samples were collected in three stages: (a) after reaching the maximum particle concentration (as measured by SMPS) during the dark ozonolysis reaction of the VOC mixture; (b) after 3 h of exposure of the SOA particles to OH radicals and/or UV radiation and (c) after 8-9 h of exposure of the SOA particles to OH radicals and/or UV radiation.
OH radicals for the aging experiments were produced by photolysis of H 2 O 2 (60 % w/v, Fisher Scientific). H 2 O 2 (produced by evaporation of 60 % w/v solution, Fisher Scientific) was continuously injected into the smog chamber approximately 1 h after reaching the maximum SOA concentra-tion (mean maximum concentration 122 ± 21 µg m −3 , n = 6) produced from the dark ozonolysis reaction.
For the two OH-induced aging experiments, the OH concentration was estimated using the Master Chemical Mechanism (Jenkin et al., 1997(Jenkin et al., , 2015Saunders et al., 2003), MCM v3.3, via website: http://mcm.leeds.ac.uk/MCM. The MCM contains detailed chemical mechanisms for isoprene, α-pinene and β-pinene, but not for 3 -carene. As a result, a module describing the chemistry of 3 -carene was constructed using the well-characterized reactions of the structurally similar compound α-pinene as a basis. Following the work of Atkinson et al. (1986Atkinson et al. ( , 1990, the rate constants used for the reaction of ozone and OH with 3 -carene were 0.44× k(O 3 + α-pinene) and 0.55 × k(OH + α-pinene) respectively. The concentration of H 2 O 2 , determined from FTIR measurements, was constrained in the model, as well as the temperature, the relative humidity and the dilution flow induced by sampling. The initial concentration of VOCs and ozone used in the MCM were measured at the start of the ozonolysis reaction. A value of J (H 2 O 2 ) = (3.2 ± 0.4) × 10 −6 s −1 was used, based on the measured light intensity distribution in the CESAM chamber (Wang et al., 2011). Using these inputs, the overall OH exposure was determined to be 1.7 × 10 7 and 1.3 × 10 7 molecule cm −3 h in the two experiments, which is comparable to similar studies in the literature (Eddingsaas et al., 2012;Qi et al., 2012;Henry and Donahue, 2012).

CRAC experiments
Supplementary experiments on the ozonolysis and OHinitiated oxidation of α-pinene were performed at the Centre for Research into Atmospheric Chemistry (CRAC) simulation chamber in Cork (Thüner et al., 2004, Kourtchev et al., 2014. The chamber is a cylinder made of fluorine-ethenepropene (FEP) Teflon ® foil with a volume of 3.91 m 3 . It was operated at 296 ± 2 K using purified air at 0.1-1 mbar above atmospheric pressure. The experiments were performed at 55 ± 2 % relative humidity produced from bubbling purified air through heated water. The humidity and temperature were measured using a dew point meter (DRYCAP ® DM70 Vaisala). Between experiments the chamber was cleaned by introducing about 1 ppm of ozone into the chamber and flushing with purified air at a flow rate of 0.15 m 3 min −1 . Aerosol seed particles produced from atomising (NH 4 ) 2 SO 4 were neutralized by a Krypton-85 (Kr-85) charge neutralizer before introduction to the chamber with a concentration of approximately 5 µg m −3 . α-pinene was introduced into the chamber in the similar manner as described above for the CESAM experiments. OH radicals were generated from the photolysis of hydrogen peroxide (H 2 O 2 ), which was added to the chamber by bubbling purified air into a slightly heated (∼ 40-50 • C) solution of 50 % H 2 O 2 . After the introduction of all reactants, the mixture was allowed to mix for 10 min before turning on 12 Philips TL12 (40 W) lamps with λ max = 310 nm to initiate photolysis of the OH radical precursor. Based on the measured distribution of light intensity in the chamber (Healy et al., 2012), the steadystate concentration of OH radicals was estimated to be ca. 4×10 6 molecule cm −3 . For the ozonolysis-only experiments, ∼ 2 ppm of ozone was introduced into the chamber over a period of 1-2 min from an electric discharge generator. Cyclohexane (∼ 40 ppm, Sigma, > 99 % purity) was used as an OH scavenger in the ozonolysis-only experiments. The concentration of NO x was monitored using a standard NO x analyzer (Thermo Model 42i) was found to increase gradually from 0 to around 25 ppb during the course of the reactions, due to NO x offgassing from the Teflon walls (Carter et al., 2005;Rohrer et al., 2005).

Aerosol sample collection
Total aerosol mass was collected on prebaked (at 650 • C) quartz fibre filters (47 mm diameter, Tissuquartz 2500 QAT-UP, Pall Life Sciences) using a stainless steel 47 mm inline filter holder (Pall Life Sciences). The gas phase species were removed using a custom-built 1 m long denuder packed with activated charcoal (NORIT ® ROW 0.8 SUPRA, Sigma-Aldrich). The denuder was purged using clean air for at least 12 h prior to experiments. The sampling was performed at flow rate of 18-30 L min −1 for 1-2 h depending on the SOA concentration in the chamber. To maintain constant pressure in the CESAM chamber, synthetic air was added during aerosol sample collection. A series of chamber blanks were collected by drawing clean air containing aerosol seed that was exposed to ozone, H 2 O 2 and UV irradiation from the smog chamber. Aerosol samples were immediately placed into prewashed glass vials and stored in the freezer until analysis.

Aerosol analysis
Depending on the aerosol loading of the filter samples, which varied between 50 and 150 µg per filter, a part of the quartz fibre filter (5-20 cm 2 ) was extracted three times with 5 mL of methanol (Optima TM grade, Fisher Scientific) under ultrasonic agitation in slurry ice for 30 min. The extracts were combined, filtered through a Teflon filter (0.2 µm, ISO-DiscTM Supelco), and reduced by volume to approximately 50-200 µL under a gentle stream of nitrogen. The sample was split into two parts for direct infusion and LC/MS analyzes. The concentration of SOA extracts for direct infusion analysis was adjusted to the same level of approximately 0.25 µg organic carbon µL −1 . The LC/MS portion was further evaporated to 20 µL and diluted by 0.1 % aqueous solution of formic acid to 100 µL. Bateman et al. (2008) observed facile formation of hemiacetals in reaction of SOA carbonyls with methanol and slow formation of esters in reaction of SOA carboxylic acids with methanol. Although the detailed evaluation of the differences between the two solvents was not performed in this study, a preliminary comparison of the mass spectra for both methanol and acetonitrile extracts of several boreal forest samples did not show any major differences that could be attributed to methylesters of carboxylic acids.
The Orbitrap MS was calibrated using an Ultramark 1621 solution (Sigma-Aldrich, UK). The mass accuracy of the instrument was routinely checked before the analysis and was below 1 ppm. The instrument mass resolution was 100 000 at m/z 400. A mixture of camphor sulfonic acid (20 ng µL −1 ), glutaric acid (30 ng µL −1 ), and cis-pinonic acid (30 ng µL −1 ) in methanol and Ultramark 1621 solution were used to optimize the ion transmission settings.
The direct infusion nanoESI parameters were as follows: the ionization voltage and back pressure were set at −1.4 kV and 0.8 psi, respectively. The inlet temperature was 200 • C. The sample flow rate was approximately 200-300 nL min −1 . The negative ionization mass spectra were collected in three replicates at ranges m/z 100-650 and m/z 150-900 and processed using Xcalibur 2.1 software (Thermo Scientific).
Examples of the spectra obtained for both analytical and smog chamber replicates are shown in Figs. S1 and S2 (in the Supplement). The average percentage of common peaks between analytical replicates was 78 % (range of 70-87 %), which agrees with literature reports for similar data analysis (Sleighter et al., 2012). The percentage of common peaks between smog chamber replicates varied between 82 and 90 %. A largest fraction of non-common ions in the replicates had RI < 0.05 %.
LC-MS ESI parameters were as follows: spray voltage On the basis of prescan information from the full scan MS, a parallel datadependent collision-induced dissociation (CID) multistage mass spectrometry (MS n ) (n = 1, 2, 3, and 4) was performed on the most intense precursor ion in three scans at a resolution of 30 000. For the majority of the identified organic species, authentic standards were not available; therefore, cis-pinonic acid, ketopinic acid and terebic acid were used as surrogates to quantify most of the acids in the samples (Kristensen et al., 2014). The identification was performed by comparing MS fragmentation patterns and chromatographic elution times reported in the literature (Yasmeen et al., 2010(Yasmeen et al., , 2012Kristensen et al., 2014). The dimers were quantified using cispinonic acid as surrogate. Six-point calibration curves were constructed over two concentration ranges 0.2-50 and 50-200 ng µL −1 .

High MS resolution data analysis
The direct infusion data analysis was performed using procedures described in detail by Kourtchev et al. (2013). Briefly, for each sample analysis, 60-90 mass spectral scans were averaged into one mass spectrum. Molecular assignments were made using Xcalibur 2.1 software applying the following constraints 12 C ≤ 100, 13 C ≤ 1, 1 H ≤ 200, 16 O ≤ 50, 14 N ≤ 5, 32 S ≤ 2, 34 S ≤ 1. The data filtering was performed using a Mathematica 8.0 (Wolfram Research Inc., UK) code developed in-house that employed several conservative rules and constraints used in previous studies (Kourtchev et al., 2013). In this study, only ions that appeared in all three replicates were kept for evaluation. Figure 1 shows direct infusion negative ionization (-) na-noESI HR mass spectra for "fresh" SOA collected after 1 h of dark ozonolysis reaction of the BVOC mixture and "aged" SOA collected after 8-9 h of aging under different atmospheric conditions (i.e. dark exposure of SOA in the presence of residual O 3 , exposure to OH radicals and UV light, exposure to UV light only). It should be noted that all mass spectra are blank corrected and show only ions containing carbon, hydrogen and oxygen that appeared in three analytical replicates of two repeated chamber experiments. ESI is a soft ionization technique which usually shows very little or no fragmentation. Thus, in the negative ionization mode it is expected that the detected ions correspond to the deprotonated molecules.

Direct infusion results
Irrespective of the applied atmospheric oxidation conditions, the mass spectra from all SOA samples contained distinct groups of monomers, dimers and trimers in the mass range 100-650. Similar to laboratory studies with biogenic SOA from previous studies (e.g. Tolocka et al., 2004;Hall and Johnston, 2011;Putman et al., 2012;Kundu et al., 2012), the ion intensities in the oligomeric region (m/z > 280) were very high (up to 25 % relative intensities, see Fig. 1) and exhibited a bell shape distribution. This is in contrast to the HRMS analysis of ambient organic aerosol from various sampling locations (e.g. Wozniak et al., 2008;Kourtchev et al., 2013Kourtchev et al., , 2014, where mass spectra generally have a unimodal distribution with relatively low ion intensities in the high mass range. None of the aging reactions studied here caused any visible influence on the ion intensity distributions except for an ion at m/z 357.15509. Moreover, several of the oligomers were not affected by the 8-9 h OH radical/UV light and UV light-only exposures. This clearly shows that even prolonged exposure to OH radicals and UV light does not cause decomposition of oligomers formed under the employed ozonolysis conditions, thus indicating their stability once they are formed in the atmosphere. This is in contrast to results reported for aqueous photochemical processing of SOA from various biogenic (α-pinene, β-pinene, D-limonene, αhumulene) and anthropogenic (1,3,5-trimethylbenzene and guaiacol) precursors (Romonosky et al., 2015). In this latter study, a reduction of the average number of atoms in the SOA molecular formulae was observed after photolysis suggesting that the presence of water plays an important role in the aging processes of SOA. It has also been shown that relative humidity can affect the viscosity of SOA particles and potentially affect heterogeneous oxidation kinetics (e.g. Zhou et al., 2013).
In all experiments, the mass spectra were dominated by the ions at m/z 185.08167 and m/z 357.15509 in the monomeric and dimeric regions respectively. As confirmed by LC/MS analysis (discussed below), the ion at m/z 185.08167 corresponds to at least three different oxidation products with the same molecular formula (C 9 H 14 O 4 ) which are formed from α-, β-pinene, and 3 -carene, the major compounds in the studied BVOC mixture (Table S1). They include cis-pinic acid, homoterpenylic acid, and cis-caric acid. A dimer at m/z 357.15509 was previously identified as pinyl-diaterpinyl ester MW 358 in the SOA from ozonolysis of α-pinene (Müller et al., 2008(Müller et al., , 2009Camredon et al., 2010;Yasmeen et al., 2010;Gao et al., 2010;Kristensen et al., 2013). Since pinyl-diaterpinyl ester MW 358 was not observed in the SOA from the OH radical initiated oxidation of α-pinene, it was suggested that high molecular weight dimers are formed through gas phase reaction of the stabilized Criegee Intermediate formed from ozonolysis of the monoterpene (Kristensen et al., 2014).
In addition, the data were visualized using carbon oxidation state (OS C ) plots. The OS C was introduced in aerosol science by Kroll et al. (2011) to describe the composition of a complex mixture of organics undergoing dynamic oxidation processes. It is strongly linked to aerosol volatility and thus is a useful parameter to classify SOA. While highly oxidized organics in organic aerosol have OS C ≥ +1, the reduced molecules have OS C ≤ 0. The maximum in chemical complexity is located at OS C = 0 (Kroll et al., 2011). Carbon oxidation state was calculated for each molecular formula identified in the mass spectra using the following equation: where OS i is the oxidation state associated with element i, and n i / n C is the molar ratio of element i to carbon (Kroll et al., 2011). Figure 2 shows two overlaid OS C plots for the "fresh" and "aged" SOA from the "dark aging" experiments. Consistent with previous studies, the majority of molecules in the SOA had OS C between −1 and +1 with up to 30 (n C ) carbon atoms (Kroll et al., 2011, and references therein). It has been suggested that semivolatile and low-volatility oxidized organic aerosol (SV-OOA and LV-OOA) produced by multistep oxidation reactions have OS C between −1 and +1 with 13 or less carbon atoms (Kroll et al., 2011). It should be noted that all SOA samples contained a cluster of molecules with OS C between −1 and −1.5 with n C less than 10 which could possibly be associated with OH radical oxidation products of isoprene, which was present in the BVOC mixture. The dark ozonolysis experiments were performed without an OH scavenger and thus it is likely that OH radicals produced from the ozonolysis reactions could further react with isoprene resulting in the molecules with very low OS C state. The large cluster of molecules with 15 or more carbon atoms is likely to be associated with dimers and trimers of the BVOC oxidation products.
Atmos. Chem. Phys., 15, 5683-5695, 2015 www.atmos-chem-phys.net/15/5683/2015/ Figure 3. Carbon oxidation state of SOA from OH-initiated aging of aerosol from ozonolysis of BVOC mixture. Blue diamonds correspond to molecules in "fresh SOA", red squares to "aged SOA" for 3 h and green triangles to aged SOA for 9 h. Figure 3 shows a carbon oxidation plot for the OH radical aging experiments performed in the presence of UV light. In contrast to dark aging experiments, a very small shift in the oxidation state throughout the entire mass range could be observed with aging time. This shift is not visible in the mass spectrum shown in Fig. 1c because the affected ions had very low intensities (mostly < 5 % relative intensity). The observed increase in the oxidation state of the SOA components is consistent with the results obtained for the OH radical aging of SOA produced by dark ozonolysis reaction of α-pinene . Using an aerosol mass spectrometer (AMS), an increase of the O / C ratio, which is another metric to describe the oxidation state of the SOA components, was observed after addition of OH radicals .
To identify a possible reason for the small shift in OS C to the higher values during the aging reaction observed in the present study, we performed separate O 3 and OH radical initiated oxidation experiments with α-pinene (the major component of the BVOC mixture) in the CRAC chamber. In a previous study (Kourtchev et al., 2014) we compared molecular composition of SOA from ozonolysis of a single VOC (α-pinene) with that from the VOC mixture (αpinene, β-pinene, 3 -carene and isoprene) and showed that the molecular composition of α-pinene SOA was very similar to that from the BVOC mixture. It must be noted that the later experiments were performed at the different smog chamber (CRAC) and high RH (∼ 55 %). Since an OH radical scavenger was used in these ozonolysis experiments, we assume that all OH radicals were efficiently removed from the system. SOA from both ozonolysis and OH radical reaction with α-pinene contained distinguishable groups of monomers, dimers and trimers (Fig. 4). However, a clear shift to higher masses is observed in the mass spectra from the OH radical experiments. The presence of a large number of oligomers in the SOA from the OH-initiated reaction indicates that this oxidation regime also results in a significant degree of oligomerization. In-source fragmentation tests with a fragmentation voltage of up to 70 V showed no significant change in the number of observed oligomers in the mass spectra, thus ruling out the possibility that these oligomers are noncovalent compounds formed in the ESI source. When the fragmentation voltage was increased to 80 V, a decrease in the intensities for both monomers and dimers was observed, confirming that detected dimers are covalent species.
The SOA from the OH reaction clearly shows higher OS C (Fig. 5, red squares) compared to that from the dark ozonolysis experiments (blue diamonds in Fig. 5). Moreover, it contained a very large number of LV-OOA species, which are often referred to as aged SOA (Kroll et al., 2011). As indicated in the Van Krevelen diagram (Fig. 6), where H / C ratio is plotted as a function of the O / C ratio for each mass and corresponding formula identified in the sample (Nizkorodov et al., 2011), the SOA from the OH-initiated reaction had substantially higher O / C ratios than those for SOA obtained from ozonolysis. On the other hand, H / C ratios were very similar in SOA from both reactions. This may be explained by addition of carboxylic or carbonylic groups to the backbone of the hydrocarbon structure, which generally occurs without substantial loss of hydrogen (Zhao et al., 2015).
In the UV-only aging experiments (without addition of H 2 O 2 ) no visible effect on the mass spectral features of the oligomers (Fig. 1d) and OS C (not shown here) could be observed. Considering that during the UV-only experiments the RH was less than 9 %, the production of OH radicals from the photolysis of unreacted ozone should be minimal (Rohrer et al., 2005), confirming that the increase in the OS C in the OH radical initiated aging experiments (discussed above) is not due to UV-initiated decomposition of the SOA products, but to OH radical reactions. This is in line with a recent study where no significant change in the O / C ratio was determined by a time-of-flight aerosol mass spectrometer (ToF-  AMS) when the SOA produced from dark ozonolysis of αpinene was exposed to light representing the solar energy distribution at Earth's surface (Denjean et al., 2015). On the other hand, this is in contrast to findings reported by Wong et al. (2015) who observed photolytic aging of α-pinene SOA in the presence of ammonium sulfate that exhibited a dependence on relative humidity.

LC/MS results
Due to competitive ionization of analytes in the ESI direct infusion analysis of the aerosol samples that are known to have a very complex matrix, the ion intensities do not directly reflect the concentration of the molecules in the sample. Therefore, the effect of aging processes on the SOA composition was additionally investigated using LC coupled with HRMS. All data described in this section (except discussion of Figs. S3 and S4) correspond to experiments performed at CESAM chamber. A list of tentatively identified prod-ucts formed during dark ozonolysis of the BVOC mixture is shown in Table S1. The majority of the identified compounds were attributed to α-and β-pinene oxidation products which can be explained by the fact that both of these precursors contributed to about 60 % of the total BVOC mixture concentration (ppbv) used in this study. On the other hand, none of the chromatographic peaks were associated with isoprene oxidation products which could be due to the relatively low (about 10 %) contribution of isoprene to the total mixture. Moreover, isoprene is known to produce very low aerosol yields (less than 0.01) when reacted with ozone . Most of the tentatively identified compounds have been previously observed in aerosol samples from laboratory (e.g. Yu et al., 1999;Szmigielski et al., 2007;Glasius et al., 2000;Müller et al., 2008Müller et al., , 2009Camredon et al., 2010;Gao et al., 2010;Yasmeen et al., 2010;Kourtchev et al., 2014) and field studies (e.g. Gómez-González et al., 2012;Kristensen et al., 2013Kristensen et al., , 2014Kourtchev et al., 2013). Therefore, to avoid unnecessary repetition, the sources and processes leading to the formation of these compounds are not discussed here. Although the LC/MS allowed separation and identification of a considerably smaller number of molecules compared to direct infusion analysis, some of them are very useful markers for aerosol source characterization and are good indicators of the processes involved in the formation of the SOA. These include well-known first-generation oxidation products of α-pinene such as cis-pinic acid, terpenylic acid and pinonic acid, as well as later generation oxidation products of monoterpenes, i.e. 1,2,3-butanetricarboxylic acid (MBTCA). It has been suggested that MBTCA is a product of the hydroxyl radical initiated oxidation of pinonic acid in the gas phase (Szmigielski et al., 2007;Müller et al., 2012;Yasmeen et al., 2012). Therefore, both pinonic acid and MBTCA could be used to monitor the evolution of OH radical initiated aging of the SOA.
Compared to the direct infusion analysis, only a very small number of dimers were separated using the LC/MS method in all samples, irrespective of the aging conditions. These dimers include isomers with m/z 337. 20172, m/z 343.13983, m/z 357.15509, m/z 367.17599, m/z 387.20221 and m/z 369.19141, consistent with previous studies that applied LC/MS for the analysis of SOA (e.g. Kristensen et al., 2013Kristensen et al., , 2014. Four of these dimers -pinyl-diaterpenyl ester MW 358 (m/z 357.15509), pinyl-diaterebyl dimer MW 344 (m/z 343.13983), pinonylpinyl dimer MW 368 (m/z 367.17599) and MW 388 dimer ester (m/z 387.20221) -have been identified previously in SOA from ozonolysis of α-pinene (e.g. Yasmeen et al., 2010;Kristensen et al., 2013Kristensen et al., , 2014, while none of them were observed in the SOA from the OH radical initiated oxidation of α-pinene (Kristensen et al., 2014). It must be noted that these dimers were also observed during the direct infusion analysis. Interestingly, the tentative structures of the three dimers reported in the literature (Yasmeen et al., 2010;Kristensen et al., 2013Kristensen et al., , 2014 contained pinic acid, which is a less impor-Atmos. Chem. Phys., 15, 5683-5695, 2015 www.atmos-chem-phys.net/15/5683/2015/ tant product in the OH radical initiated oxidation than in the ozonolysis of α-pinene (Kristensen et al., 2014). The chromatographic peak areas for all four dimers, determined by LC/MS, showed relatively good linear fit with R 2 > 0.90 suggesting that these compounds are possibly formed through similar processes. It should be noted that the observed small number of dimers in the LC/MS chromatogram from αpinene ozonolysis, and their absence in the SOA generated from the OH radical initiated oxidation of α-pinene, in previous studies and the current work, does not necessarily mean that oligomers are not formed in the latter reaction. The lack of chromatographically resolved dimers in the OH radical initiated SOA could be explained by the selectivity of the utilized LC columns. The integration of chromatographic "humps" eluting at the end of the chromatograms from both ozonolysis and OH radical initiated reaction of αpinene (Figs. S3 and S4) reveals the presence of a large number of dimers, trimers and tetramers that were not resolved by the LC column. The overall oligomer distribution of these unresolved LC "humps" resembles that of the respective direct infusion mass spectra. Figure 7 shows concentrations of selected first-and latergeneration oxidation products, as well as dimers in the SOA produced under different aging conditions. Irrespective of the aging conditions, and even during the dark ozonolysis experiments (without addition of OH radicals), all samples contained MBTCA, the OH radical initiated oxidation product of α-pinene. It has been estimated that the ozonolysis of αpinene results in the formation of OH radicals with a yield between 0.8 and 1.0 (Atkinson et al., 1997;Presto and Donahue, 2004). Therefore, without the use of an OH radical scavenger, a substantial fraction of α-pinene and other terpenes present in the mixture can be oxidized by OH radicals (Henry and Donahue, 2011). During the dark aging experiments (with SOA exposure to residual ozone), the concentrations of the marker compounds remained unchanged even after 9 h of SOA exposure, indicating that all OH radicals produced from the BVOC ozonolysis were immediately consumed in the first hour of the reaction (Fig. 7a). The concentrations of the first-generation products and the dimers did not change during the dark aging experiments either. In contrast, the concentration of MBTCA increased when OH radicals were introduced into the system (Fig. 7b) followed by a decrease in the concentration of pinonic acid and pinic acid, confirming that pinonic acid is further oxidized into MBTCA (Müller et al., 2008).
In the UV-only aging experiments the concentrations of all marker compounds in the SOA remained unaffected suggesting that the observed changes in the photolysis experiments with OH radicals (discussed above) are due to the OH radical initiated chemistry rather than photolytic degradation of the first-generation products. Irrespective of the tested conditions, the contribution of the dimers to the SOA mass showed no statistical difference (Table S2) indicating that none of the Figure 7. Normalized concentrations (µg µg −1 SOA) of terpenylic acid (dark blue), MBTCA (red), pinic acid (light green), pinonic acid (violet), and a dimer MW 358 (light blue) at three aging conditions: (a) dark aging with exposure to residual ozone, (b) OH radical initiated aging and (c) UV-only aging experiments. Pinic, pinonic and terpenylic acids represent organic species associated with fresh SOA, whereas MBTCA is associated with aged SOA. The peak areas for all four dimers, determined by LC/MS, showed relatively good linear fit with R 2 > 0.90 suggesting that these compounds are formed through similar processes. To avoid overloading the figure, only one, the most commonly observed dimer with MW 358 in both ambient and laboratory-generated OA, is shown in the figure.
studied conditions have an effect on dimer decomposition or formation.

Conclusions
In this study the influence of several aging processes on the molecular composition of organic aerosols has been investigated using direct infusion and liquid chromatography HRMS. SOA formed from ozonolysis of a BVOC mixture was aged for 8-9 h in the dark with residual ozone, under UV irradiation, and by OH radicals formed from the continuous photolysis of H 2 O 2 . Dark aging and UV aging in the presence of ozone and UV irradiation closely resembling the solar energy distribution at Earth's surface did not significantly affect the molecular composition of studied SOA. In contrast, OH radical initiated aging showed an increase in OS C and elemental O / C ratios of the BSOA components from the studied BVOC mixture. None of the aging processes produced an observable effect on the oligomers formed from the ozonolysis of the BVOC mixture and they were found to be equally abundant in both "fresh" and "aged SOA". Additional separate dark ozonolysis and OH-initiated experiments with αpinene (a major component of the studied mixture) showed that oligomers are an important group of compounds in both oxidation schemes. However, oligomers in the OH-initiated SOA were shifted towards higher masses and were not readily resolved by the LC techniques applied here, indicating that their importance could have been underestimated in previous similar studies. SOA from the OH-initiated oxidation of α-pinene had a significantly higher OS C and O / C compared to that from pure ozonolysis experiments, confirming that the OH radical reaction is more likely to be responsible for the occurrence of highly oxidized species in ambient biogenic SOA. Considering that the timescale (8-9 h) of the aging reaction in our experiments was still substantially lower than the lifetime of organic aerosol in the real atmosphere (up to 12 days) (Robinson et al., 2007;Rudich et al., 2007), the observed OH aging effects could be substantially stronger when SOA is aged over a longer period in the ambient atmosphere.
The Supplement related to this article is available online at doi:10.5194/acp-15-5683-2015-supplement.