Aging of secondary organic aerosol generated from the ozonolysis of α -pinene: effects of ozone, light and temperature

Abstract. A series of experiments was conducted in the CESAM (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) simulation chamber to investigate the evolution of the physical and chemical properties of secondary organic aerosols (SOAs) during different forcings. The present experiments represent a first attempt to comprehensively investigate the influence of oxidative processing, photochemistry, and diurnal temperature cycling upon SOA properties. SOAs generated from the ozonolysis of α-pinene were exposed under dry conditions ( No change in SOA size or chemical composition was observed during O3 and light exposure at constant temperature; in addition, GF and CRI of the SOA remained constant with forcing. On the contrary, illumination of SOAs in the absence of temperature control led to an increase in the real part of the CRI from 1.35 (±0.03) to 1.49 (±0.03), an increase of the GF from 1.04 (±0.02) to 1.14 (±0.02) and an increase of the f44 / f43 ratio from 1.73 (±0.03) to 2.23 (±0.03). The simulation of the experiments using the master chemical mechanism (MCM) and the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) shows that these changes resulted from the evaporation of semi-volatile and less oxidized SOA species induced by the relatively minor increases in temperature (~ 6 °C). These surprising results suggest that α-pinene–O3 SOA properties may be governed more by local temperature fluctuations than by oxidative processing and photochemistry.

mospheric organic aerosol Kanakidou et al., 2005;Turpin and Huntzicker, 1995;Zhang et al., 2007). During their atmospheric lifetime, which ranges from 48 to 72 h (Wagstrom and Pandis, 2009), their physical and chemical properties do not stay constant but rather evolve in response to local atmospheric conditions. For example, studies have shown that SOAs can grow by condensation of low volatility oxidized species (Ellison et al., 1999) and by cloud processing (Cocker et al., 2001;Ervens and Volkamer, 2010;Volkamer et al., 2009). SOAs can also be oxidized by gas-phase oxidants (Kalberer, 2004;Gao et al., 2004), undergo chemical reactions in the particle phase (Kalberer, 2004;Gao et al., 2004) or partially evaporate (Warren et al., 2009). These changes in aerosol properties have also been observed in the field; for example, more oxidized, less volatile and more hygroscopic SOAs are typically observed in remote areas as a result of continuous aging in the atmosphere Ng et al., 2011;Rudich et al., 2007). Since SOA contains a wide variety of organic compounds, which vary in terms of their size, structure, functionality and oxidation state (Kroll and Seinfeld, 2008;Jimenez et al., 2009;de Gouw et al., 2005;Hallquist et al., 2009), the processes associated with SOA aging are very complex.
Experiments performed in simulation chambers have significantly improved our understanding of the SOA aging processes Qi et al., 2012;Yasmeen et al., 2012). In order to provide modellers with accurate parameters for SOA aging in the atmosphere, these experiments must be atmospherically relevant (Kourtchev et al., 2014). While the O : C ratio of laboratory-generated SOAs is similar to that of freshly formed ambient SOAs, it is generally lower than that of aged ambient SOAs (Ng et al., 2010). In addition, while the representation of SOAs in chemical transport models based on parameterization of chamber data showed a good agreement with nighttime SOA concentrations (in a rural site near Rotterdam, the Netherlands), an underprediction of SOA concentrations occurred during the day (Li et al., 2013).
Research has shown that the oxidative aging of SOAs in the atmosphere has a major influence on its properties. For example, studies have shown that highly oxygenated organic particles are likely to have a higher hygroscopicity and cloud condensation nuclei (CCN) activity than freshly emitted particles, due to the increased polarity and solubility of their constituents (Massoli et al., 2010;Jimenez et al., 2009;Chang et al., 2010;Duplissy et al., 2011). Oxidative aging has also been shown to lead to changes in the real part of the complex refractive index (CRI) of SOAs Lambe et al., 2013;Flores et al., 2014), and to an increase in its UV absorption via the formation of additional carbonyl compounds and oligomeric products (Sareen et al., 2013;Nozière and Esteve, 2005;Shapiro et al., 2009;Lambe et al., 2013). To date, laboratory studies have primarily focused on oxidative aging mediated by the heterogeneous reactive uptake of OH radical (Rudich et al., 2007;George and Abbatt, 2010;Smith et al., 2009). On the other hand, oxidative aging by O 3 has received less attention.
Light exposure has also been shown to influence SOA properties; through photodissociation of molecules, such as carbonyls and organic peroxides, either in the gas or the particle phase, it has been shown to induce a decrease in SOA mass concentration (Kroll et al., 2006;Bateman et al., 2011). There are also indications of significant photolytic processing of carbonyl compounds in aerosols during long-range transport (Hawkins and Russell, 2010). In addition, laboratory studies have revealed that photochemical processes alter the chemical composition of SOAs Qi et al., 2012;George and Abbatt, 2010;Donahue et al., 2012) and modify its hydrophilicity George and Abbatt, 2010;, optical properties  and volatility . In these studies, H 2 O 2 was used as a photolytic OH precursor. Since this OH source requires the use of UV light, these experiments did not allow for the individual effects of heterogeneous reactions by OH and direct photolysis on SOA properties to be distinguished.
SOA properties can also be affected by local temperature variations via the evaporation of volatile products. A number of studies have used thermodenuder-based techniques to investigate the volatility of SOAs and to determine the effect of temperature on SOA properties (Asa-Awuku et al., 2008;Huffman et al., 2009;. Volatile tandem differential mobility analyzers (V-TDMAs) have been used to measure the shrinkage of monodisperse particles after heating Salo et al., 2011). In these studies, particles were exposed to elevated temperatures (up to 300 • C) for a short time. It has been shown, however, that SOA exhibits a significantly slower response to changes in temperature than that predicted by models for liquid droplets . Recently, it has been suggested that SOA could be in an amorphous semi-solid or amorphous solid (glassy) state under dry conditions (Renbaum-Wolff et al., 2013;Saukko et al., 2012;Denjean et al., 2014b), which could limit the volatilization kinetics of the aerosol.
This study focuses on the SOA formed from the ozonolysis of α-pinene, which is an important source of SOAs on both regional and global scales (Guenther et al., 1995;). In our companion paper (Denjean et al. 2014b), we explored the evolution of the physical, chemical, optical and hygroscopic properties of α-pinene-O 3 SOAs during the first hours after its formation. However, Wang et al. (2011) have shown that particle lifetime can vary from 10 h to 4 days in the CESAM simulation chamber. CESAM is a powerful tool for the study of SOAs over longer timescales corresponding to their lifetime in the atmosphere (Yasmeen et al., 2012). In the present work, we investigate in the same chamber the effects of (i) additional ozone exposure, (ii) light exposure and (iii) temperature variation on the chemical composition, hygroscopicity and optical properties of α-pinene-O 3 SOA, over timescales reaching 20 h.

CESAM atmospheric simulation chamber
The present experiments were performed in the CESAM (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) atmospheric simulation chamber, which has previously been described in detail by Wang et al. (2011). In brief, CESAM is a stainless-steel chamber with a volume of 4.2 m 3 . Chamber illumination is accomplished using the borosilicate-filtered output of three high-pressure arc xenon lamps (4 kW, XPO 4000 W/HS, OSRAM), which provides a good reproduction of the solar energy distribution at the Earth's surface over the 290-700 nm wavelength region. The inner walls of the chamber are polished in order to provide good reflection inside the chamber and thus enhance the radiation homogeneity. During our experiments, the NO 2 photolysis frequency J NO 2 within the chamber was approximately 3 × 10 −3 s −1 , which corresponds to a solar zenith angle of ∼ 70 • (Carter et al., 2005). The simulation chamber was maintained at room temperature (±1 • C) using a refrigerating liquid (70 % water / 30 % ethylene glycol), which was circulated in the double walls of the chamber. Temperature and relative humidity (RH) are monitored with a transmitter (HMP234, Vaisala) equipped with a thin-film capacitive humidity sensor (HUMICAP ® , Vaisala). During our experiments, the temperature accuracy is ±0.1 • C at 20 • C and the RH accuracy is ±1.9 % (up to 90 % RH).

Experimental details
Prior to each experiment, the chamber was evacuated to a secondary vacuum (typical pressure ∼ 4 × 10 −4 mbar) and kept under vacuum overnight. The chamber was then filled to atmospheric pressure with a mixture of 200 mbar of oxygen (Air Liquide, ALPHAGAZ ™ class 1, purity 99.9 %) and 800 mbar of nitrogen produced from the evaporation of a pressurized liquid nitrogen tank (Messer, purity > 99.995 %, H 2 O < 5 ppm). The background conditions were typically: particles concentration < 0.1 µg m −3 , ozone mixing ratio < 5 ppb, gas-phase organics < 5 ppb and RH < 1 %. To avoid contamination, a slight overpressure of about 5 mbar with respect to the atmospheric pressure was maintained during each experiment by adding nitrogen (Messer, purity > 99.995 %, H 2 O < 5 ppm).
All aging experiments were carried out under the same initial conditions with α-pinene and ozone reacting in the dark, with neither seeds nor OH scavenger. O 3 was generated in an O 2 flow using a commercial dielectric ozone SOA generator (MBT 802N, Messtechnik GmbH, Stahnsdorf, Germany) and introduced to the chamber through an injection port. Quantification of α-pinene was performed by evaporating precisely measured amounts of the terpene into a glass bulb held under vacuum. When the O 3 concentration within the chamber reached ∼ 250 ppb, α-pinene (Aldrich, 98 %) was flushed from the bulb into the chamber in a flow of oxygen to a concentration of ∼ 200 ppb within the chamber. In all experiments, SOAs, formed directly after α-pinene injection, and its precursors were essentially consumed after 4 h of reaction.
In our companion paper, we observed changes in the oxidative degree and optical properties during the formation of SOAs, but these changes ceased after 9 h of reaction (Denjean et al., 2014b). In the present study, therefore, SOAs were allowed to evolve for 14 h prior to simulating atmospheric processing, which was in turn conducted over a period of 6 h. Three different aging regimes were applied after 14 h of α-pinene ozonolysis: (1) 700 ppb of ozone was introduced into the simulation chamber, (2) SOAs were exposed to light for 6 h in the absence of temperature control, during which time the temperature increased by ∼ 6 • C and (3) SOAs were exposed to light for 6 h under controlled temperature conditions (±1 • C). For comparison purposes, control experiments were performed: in these experiments, SOAs were allowed to remain in the chamber for 20 h under dark conditions. The specific experimental conditions associated with each aging regime are shown in Table 1. As discussed in our companion paper (Denjean et al., 2014b), the variation of SOA mass concentration between the experiments was attributed to different concentrations of α-pinene injected to the chamber. Despite this, the chemical, optical and hygroscopic properties of SOA were found to be very similar between the experiments. Therefore, the experiments were considered to be comparable for studying the effect of forcing on SOA properties.
During these experiments, the concentrations of α-pinene, ozone and VOCs were monitored using a Fourier transform infrared spectrometer (FTIR) from Bruker GmbH (Ettlingen, Germany) coupled to a multi-reflection cell with an optical path of 192 m. During the different forcings, the concentrations of gas-phase compounds were found to be below the detection limit. Ozone was also measured with a commercial instrument (Horiba APOA 370, Kyoto, Japan), with a detection limit of 0.2 ppb and a precision of 0.1 ppb.

Size distribution
SOA number size distributions between 14 and 505 nm were monitored using a scanning mobility particle sizer (SMPS; DMA Model 3080, CPC Model 3010; TSI) operated at flow rates of 3/0.3 Lpm (sheath flow/aerosol sample flow). Instrument calibration was conducted using polystyrene latex spheres (PSL) (Duke Scientific). Since the PSL diameters measured during calibration were ∼ 10 % larger than the certified PSL diameters (for 100 nm PSL samples), a correction factor was applied to all measurements. Corrections for particle loss by diffusion in the SMPS tubing and the contribution of multiply charged particles were made using the SMPS software (Aerosol Instrument Manager, version 9, TSI). The number size distribution was used to obtain SOA mass concentrations, assuming homogeneous spherical particles and an effective density of 1.

Chemical composition
SOA chemical composition was analyzed using a highresolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne) (DeCarlo et al., 2006). Instrumental and data treatment details are given in our companion paper (Denjean et al., 2014b), and thus will be described only briefly here. The HR-ToF-AMS was used under standard conditions (vaporizer at 600 • C and electron ionization at 70 eV). The instrument was switched between two modalities: a single-reflectron configuration (V-mode), which offers higher sensitivity but lower resolving power (up to ∼ 2100 at m/z 200), and a double-reflectron configuration (W-mode), which provides a higher resolving power (up to ∼ 4300 at m/z 200) but a lower sensitivity (De Carlo et al., 2006). Default collection efficiencies (CEs) and relative ionization efficiencies (RIEs) were used for quantification of SOA composition. High-resolution analysis was performed using Vmode data, by integrating each C x H y O z ion in the mass range 12-180 m/z; W-mode data were used only to check for possible interferences. The method is based on the RIEs of molecules containing C, H and O atoms. The sum of the ion signal intensities from all fragments was used to estimate the O : C ratio of the SOA, and thus its degree of oxidation. Air interferences were removed by adjusting the fragmenta-tion table (Aiken et al., 2007;Allan et al., 2004). Additional modification of the fragmentation table was made for organic H 2 O + as suggested by Chen et al. (2011). The default fragmentation table derives the contribution from fragmentation of organic compounds (dehydration) to the ion H 2 O + as organic H 2 O + = 0.225 * CO + 2 , considering that the measured excess is due to the interference of water. Since the experiments were run under very dry conditions (RH < 1 %), the ion H 2 O + has been totally assigned to fragmentation of organic compounds which corresponds to a organic H 2 O + to CO + 2 ratio of 0.8-1:1. Similar organic H 2 O + to CO + 2 were obtained in previous studies of α-pinene ozonolysis (Chen et al., 2011;Chhabra et al., 2010). Measurement uncertainties of O : C were estimated to be ±30 %, as determined by Aiken et al (2007).

Optical properties
SOA optical properties were characterized by combining data obtained from an integrating nephelometer (Model M9003, Ecotech), a spectral aethalometer (Model AE31, Magee Scientific) and the SMPS described above. The integrating nephelometer measured the scattering coefficient (σ scatt ) of 525 nm wavelength, as well as the temperature and relative humidity of the incoming airflow, and was calibrated prior to the experiments using filtered air and CO 2 . The nephelometer collected light only from particles at scattering angles between 10 • and 170 • . Measured scattering coefficients were corrected for this angular truncation using Mie calculations, which were performed using the measured SMPS size distribution. The aethalometer measured the SOA ab-Atmos. Chem. Phys., 15, 883-897, 2015 www.atmos-chem-phys.net/15/883/2015/ sorption coefficient at 7 wavelengths (370, 470, 520, 590, 660, 880 and 950 nm) by measuring the increase in attenuation of transmitted light through its quartz fiber filter as a function of particle exposure time. In order to avoid artefacts from the adsorption of ozone and VOCs on the filter, a charcoal denuder was installed upstream of the aethalometer (Weingartner et al., 2003). Measurements of light attenuation were corrected for aerosol scattering effects according to the method described by Collaud Coen et al. (2010) and using the parameters obtained by Denjean et al. (2014aDenjean et al. ( , 2014b for α-pinene-O 3 SOAs. These measurements were ultimately used to calculate the real and imaginary parts of the CRI, which together describe the scattering and absorbing characteristics of SOAs. The CRI retrieval procedure employed has been described and validated in Denjean et al. (2014a). In brief, the CRI of SOAs was retrieved at 525 nm by comparing the measured scattering and absorbing coefficients (σ scat and σ abs ,respectively) with those obtained from Mie scattering calculations (Bohren and Huffman, 1983) performed using the measured number size distribution. The absolute error associated with the real CRI was ±0.03 (Denjean et al., 2014b).

Hygroscopic properties
SOA hygroscopic properties were analyzed with a custombuilt hygroscopic tandem differential mobility analyzer (H-TDMA), which is described in detail in Denjean et al. (2014a). The first differential mobility analyzer (DMA) was used to select particles with a mobility diameter of 200 nm, which were then humidified at a constant RH of 90 ± 1 % (residence time ∼ 15 s). The second DMA, which was coupled to a CPC, measured the humidified size distribution (with respect to the mobility diameter). Both DMAs were calibrated using monodisperse PSL particles (Duke Scientific) with size diameters of 70, 100, 200 and 300 and 500 nm. These size distributions were fitted to log-normal size distributions to obtain the dry geometric mean diameter, D p,m (dry), and the geometric mean diameter of the humidified aerosol, D p,m (90 % RH). These values were used to obtain the size growth factor (GF), which is defined as the ratio of D p,m (90 % RH) to D p,m (dry). The uncertainties in the calculated GF are associated with uncertainties in particle size distributions arising from DMA classification and calibration, as well as with uncertainties in the estimation of D p,m from size distributions, and are estimated to ±0.02. Prior to each experiment, the experimental set-up was validated by comparing measurements of the GF of ammonium sulfate particles to those predicted by Köhler theory (Denjean et al., 2014a).

Modelling SOA formation and aging
In order to assist in the interpretation of experimental results, the α-pinene ozonolysis experiments were simulated using a box model that included the MCM (version 3.1) (Saunders et al., 2003) and the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) (Aumont et al., 2005). The MCM v3.1 oxidation scheme for αpinene contains 329 organic species and 973 reactions. The GECKO-A chemical scheme for α-pinene oxidation, which provides a more detailed description of the gaseous oxidation of organic species and takes into account minor reaction pathways not considered by the MCM, involves 5.7 × 10 4 organic species reacting according to 1.7 × 10 6 reactions (Valorso et al., 2011). Both models simulate gas-particle partitioning in terms of equilibria between the gas phase and an ideal liquid homogeneous condensed phase (Camredon et al., 2007).
Vapour pressures and boiling points for secondary organic species were estimated using the methods developed by Nannoolal et al. (2004Nannoolal et al. ( , 2008, as they have been shown to provide the most reliable estimates for the purpose of SOA formation ( Barley and McFiggans, 2010). Accretion, oxidation and photolysis in the condensed phase were not taken into account in this investigation in the models. In addition, recent studies (Renbaum-Wolff et al., 2013;Saukko et al., 2012;Denjean et al., 2014b) suggest that α-pinene-O 3 SOA viscosity increases with aging. Due to possible kinetic limitations, heterogeneous chemistry and particle-phase reactions can be limited in highly viscous aerosol. The effect of viscosity on rate constants were not taken into account in the current MCM and Gecko-A models.
Time integration of the chemical schemes was solved using the two-step solver (Verwer et al., 1994(Verwer et al., , 1996. The gasparticle partitioning module was solved using the iterative method described in Pankow (2008). Simulations were initialized at a time point corresponding to α-pinene injection. This injection was implemented in the model as a constant flux over the injection time period that reproduced the observed concentration. Dilution within the chamber (arising from the periodical injection of N 2 to compensate for instrumental sampling flows) was described as a measurementconstrained first-order process and applied in the simulations to both gas and aerosols. The temperature used in the simulations was that observed in the CESAM chamber during the experiments. Ozone-wall loss is significant in the CESAM chamber (Wang et al., 2011). The ozone loss rate employed in the simulations, therefore, was adjusted for each experiment in order to reproduce its measured decay. during and after simulated atmospheric processing. In the control experiment (blue symbols), SOA was left to evolve in the chamber under dark conditions. In the O 3 aging experiments (green), SOA was exposed to an excess of ozone (∼ 700 ppb) under constant temperature conditions. In the photochemical aging experiments, SOA was exposed to light for 6 h, either under constant temperature conditions (yellow) or with light-induced heating (red). Here, the initial time t = 0 corresponds to the beginning of simulated aging, which was commenced after SOA was allowed to form and stabilize for 14 h.

Changes in SOA size distribution during simulated atmospheric processing
within the chamber) during and after simulated atmospheric processing. As shown in this figure, the SOA mass concentrations in the control experiments decreased by ∼ 15 % over the 6 h duration of the experiment. Since these measurements were performed after the total consumption of O 3 , this mass decrease cannot be attributed to O 3 -induced fragmentation reactions. It is thus probable that the SOA mass decrease observed in the control experiments arose via losses of particle and gaseous compounds to the chamber walls. Similar behaviour was observed when SOA was exposed to ozone and to light under controlled temperature conditions, which suggests that these forcings did not lead to significant fragmentation or functionalization. On the contrary, exposure of SOA to light and increasing temperature led to a 40 % loss in total SOA mass concentration and, as shown in Fig. 2, to a change in the SOA number size distribution: as the temperature increased from 20 • C to 26 • C, the geometric mean diameter of the normalized SOA number size distribution decreased from 286 nm to 249 nm, which also indicates that significant evaporation of SOA particles occurred.

Changes in SOA optical and hygroscopic properties during simulated atmospheric processing
Since the CRI is related to the aerosol chemical composition, density, molecular weight and polarization (Liu and Daum, Figure 2. Temporal evolution of SOA number size distribution (normalized to the total number concentration) (a) during photochemical aging in the absence of temperature control (experiment E120312) and (b) during a control experiment (experiment E160411). The temporal evolution of the median diameter during photochemical aging in the absence of temperature control (experiments E201411, E281111, E051211 and E120312) and during a control experiment (Experiment E160411) is shown in (c).

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, it was thus expected to be influenced by atmospheric processing. The imaginary part of the SOA CRI over the 370-950 nm wavelength range is shown in Fig. 3. Its value was almost zero at all the wavelengths studied, which indicates that SOA, even after simulated atmospheric processing, has a pure scattering effect in the visible to near-UV region. The formation of chromophores has been observed previously from the ozonolysis of biogenic terpenes (Bones , α-pinene-O 3 SOA products did not contain long conjugated structures which absorbed visible radiation, at least under our experimental conditions when conducted in the absence of NO x or seed particles. The influence of atmospheric processing upon the real part of the SOA CRI is shown in Fig. 4. No discernible change in real CRI was observed when the SOA was exposed to O 3 or to light at constant temperature. On the contrary, a significant increase in the real CRI was observed, 1.35 (±0.03) to 1.49 (±0.03), after photolytic forcing with increasing temperature. The hygroscopic properties of SOA were studied by measuring the GF at a constant relative humidity (90 ± 1 %). As shown in Fig. 5, the GF remained constant (∼ 1.04 ± 0.02) both during O 3 exposure and during photochemical aging at constant temperature. These results are consistent with our observations of constant mass concentration and CRI of SOA, and confirm that no significant processing of the SOA occurred under these conditions. In contrast, a significant increase in the GF, from 1.04 (±0.02) to 1.14 (±0.02), was observed with increasing temperature. In the control experiment (blue symbols), SOA was left to evolve in the chamber under dark conditions. In the O 3 aging experiments (green), SOA was exposed to an excess of ozone (∼ 700 ppb) under constant temperature conditions. In the photochemical aging experiments, SOA was exposed to light for 6 h, either under constant temperature conditions (yellow) or with light-induced heating (red). Here, the initial time t=0 corresponds to the beginning of simulated aging, which was commenced after SOA was allowed to form and stabilize for 14 h.

Changes in SOA chemical composition during simulated atmospheric processing
The results presented in the previous two sections suggest that the exposure of the aging of SOA to light and increasing temperature resulted in changes in its physical, optical and hygroscopic properties, which are likely to be linked to changes in its chemical composition. Indeed, as shown in Fig. 6a, the increase in the O : C ratio of bulk SOA during its exposure to light and increasing temperature, 0.55 (±0.16) to 0.59 (±0.18), was much larger than that observed for SOA under control conditions. The increase in the O : C ratio was, however, within the measurement uncertainties (±30 %) estimated from Aiken et al. (2007). These uncertainties may be overestimated compared to the experimental variability and even experimental reproducibility observed in this study. In fact, we estimated the experimental uncertainties to be ±0.01 from the standard deviation of the experimental values before the forcing. A new parameterization of the O : C ratio derived from the AMS was recently presented by Canagaratna et al. (2014). Application of this new parameterization to our measurements resulted in a O : C ratio similar to that obtained with our modified fragmentation . Temporal evolution of SOA hygroscopicity, here parameterized using the size growth factor (GF), during and after simulated atmospheric processing. In the control experiment (blue symbols), SOA was left to evolve in the chamber under dark conditions. In the O 3 aging experiments (green), SOA was exposed to an excess of ozone (∼ 700 ppb) under constant temperature conditions. In the photochemical aging experiments, SOA was exposed to light for 6 h, either under constant temperature conditions (yellow) or with light-induced heating (red). Here, the initial time t = 0 corresponds to the beginning of simulated aging, which was commenced after SOA was allowed to form and stabilize for 14 h.
with the Aiken method and can be attributed to an underestimation in the Aiken method of the CO + and H 2 O + ions produced from many oxidized species. The fragments f 44 , defined as the ratio of the m/z 44 (a major fragment of organic acids and hydroperoxides) signal to the total organic aerosol signal, and f 43 , defined as the ratio of m/z 43 (associated with less oxygenated groups, e.g. aldehydes and alcohols) signal to the total organic aerosol signal, have been widely used in laboratory and field studies as indicators of SOA functionality and degree of oxidation (Ng et al., 2010;Alfarra et al., 2013;Poulain et al., 2010;Pfaffenberger et al., 2013). As shown in Fig. 6b, in the experiment performed under illumination but in the absence of temperature control, the f 44 / f 43 ratio of SOA increased from 1.73 (±0.03) to 2.23 (±0.03) as the chamber temperature increased. This result implies a temperature-mediated increase in particle-phase oxidized species. On the contrary, only a small increase in the f 44 / f 43 ratio, from 1.93 (±0.03) to 2.03 (±0.03), was observed during the control experiment, which implies that the SOA composition in this experiment remained relatively constant.

Effect of phase partitioning on SOA properties
During the aging of SOAs by photolysis with increasing temperature, we observed a decrease of the mass concentration ( Fig. 1) which was associated with an increase of the f 44 / f 43 ratio of bulk particles (Fig. 6b). These evolutions may result from possible shifts of some semi-volatile organics to the gas phase as temperature increases. A second possible explanation would be the photochemical reactions that occur in the condensed phase combined with the evaporation of less oxidized compounds. In this case, photolysis could lead to a loss of semi-volatile and less oxidized compounds in the particle phase due to the fragmentation of condensed-phase species Henry and Donahue, 2012).
In order to investigate if the increase in temperature can explain the SOA mass decrease, the SOA formation and evolution were modelled with the detailed chemical schemes GECKO-A and MCM. In these models, the simulated SOA formation started at the initial step of α-pinene ozonolysis. The effects of temperature on phase partitioning were simulated with both models. The effects of photochemical reactions and heterogeneous reactions by O 3 were ignored in these simulations. As shown in Fig. 7 for the initial SOA formation period, the time profiles of α-pinene and ozone concentrations were well reproduced by both models within the measurement uncertainties. Although the temporal profile of the SOA formation was also well reproduced by the models, the simulated mass concentrations of SOAs obtained using the GECKO-A and MCM models (302 and 337 µg m −3 , respectively) were higher than those observed in the experiment (130 µg m −3 ). This overestimation of the simulated SOA mass concentration might be due either to an underestimation of the vapour pressure with the Nannoolal method used in this study or some oxidation processes occurring in the aerosol phase and leading to fragmentation not implemented in the model. In addition, an unavoidable consequence of simulation chamber measurements is the interaction of gases with the chamber wall surfaces. It is plausible that the VOC-wall losses affect the SOA mass concentration by being a sink for semi-volatile products preventing them from condensing on the aerosol phase.
Four experiments, each of which exhibited different temperature increases, were simulated using these models. SOA mass concentration loss was calculated as the difference between the mass concentration before and after each forcing. For the simulations, only the effects of the temperature on phase partitioning were simulated with both models. The effects of photochemical reaction on the gas and particle phase were omitted. A comparison between the observed and modelled loss of SOA mass concentration under these temperature conditions is presented in Fig. 8a. As shown in this figure, both the temporal profile and magnitude of SOA mass loss observed in these experiments were well reproduced by the GECKO-A and MCM models. This finding suggests that the observed heating-induced loss in SOA mass concentration can be explained by the temperature-dependent gasparticle phase partitioning of the semi-volatile components of SOA.
The measured and simulated O : C ratios for these experiments are shown in Fig. 8b. Before the forcing, both models predict an O : C ratio of 0.49, which is within the uncer- Here, the initial time t = 0 corresponds to the beginning of simulated aging, which was commenced after SOA was allowed to form and stabilize for 14 h.

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tainty of the experimental value (0.55 ± 0.16) obtained with the AMS. During the aging experiments, the O : C ratio simulated with the two models increased with increasing temperature; for example, experiment E200411, which showed the highest temperature increase (+6.5 • C; red in Fig. 8c), also showed the highest increase in simulated O : C ratio (from 0.49 to 0.51); on the contrary, experiment E060512 Figure 7. Comparison of temporal profiles of measured α-pinene (red circles), ozone (blue circles) and SOA mass concentration (green circles) with those modelled using GECKO-A (dashed lines) and the master chemical mechanism (MCM) (solid lines). The experimental data are taken from experiment E160410.
exhibited constant values for both temperature and simulated O : C ratio (yellow in Fig. 8b and c). These results suggest that the changes in SOA physical and chemical properties observed in these experiments resulted from the heatinginduced evaporation of semi-volatile and less oxidized SOA species, which in turn modified the optical and hygroscopic properties of the condensed phase.

Discussion
Our results have shown that exposure of SOAs to increasing temperature enhances its hydrophilicity, degree of oxidation and scattering properties. On the contrary, the few previous studies investigating the changes in physical and chemical properties of α-pinene-O 3 SOAs as a function of temperature have largely found little effect:  observed a reduction in the total mass concentration but no change in the mass spectra of α-pinene-O 3 SOAs after heating at 170 • C, and Kim and Paulson (2013) showed no significant change over temperatures ranging from 23 to 86 • C in the real CRI of SOAs. Using AMS data, Huffman et al. (2009) observed an increase in SOA oxygen content during SOA evaporation. This trend is in good agreement with the increase in the CO + 2 fragment (m/z 44) with SOA evaporation observed by Kostenidou et al. (2009). Finally, Warren et al. (2009) showed that the GF of SOAs decreased with decreasing temperature from 27 • C to 5 • C. These disagreements with the present study may be attributable to two differences in experimental conditions. First, in the present study, the long timescale of the experiments (20 h) allows for stability in the chemical composition of the semi-volatile component of SOAs before processing. On the contrary, in the previous studies, SOA volatility was measured only several minutes after its formation. Since the chemical composi- Here, the experimental data (filled circles) are compared with results obtained using the MCM (solid lines) and the GECKO-A (dashed lines) models. The blue points refer to the control experiment (E160411), the yellow points refer to an experiment in which SOA was exposed to light under controlled temperature conditions (E060512), and the red and brown points refer to experiments in which SOA was exposed to light and increasing temperature (E200411; E120312). Here, the initial time t = 0 corresponds to the beginning of simulated aging, which was commenced after SOA was allowed to form and stabilize for 14 h. tion of SOAs has been observed to vary significantly during its formation (Shilling et al., 2008;Chhabra et al., 2010;Denjean et al., 2014b), it is likely that the volatility of the condensed species also varies during this time period. Second, in most of the previous studies, temperature variation was performed using a thermodenuder. The timescale for aerosol evaporation within thermodenuders (∼ 16 s) is significantly lower than in simulation chambers (several hours) and, owing to mass transfer limitations, may not be long enough for SOA evaporation to occur .
Surprisingly, our results suggest that exposure of α-pinene SOAs to light and O 3 does not significantly change its chemical, hygroscopic and optical properties. These observations suggest that, under our experimental conditions, neither significant photolysis nor ozonolysis of the particle-phase products occurred.
Several previous studies have reported a decline in αpinene-O 3 SOA mass after irradiation under exposure to low OH concentrations Henry and Donahue, 2012). These authors used UV lights (∼ 360 nm) to initiate photochemistry; on the contrary, a more realistic reproduction of the solar energy distribution at the Earth's surface was used in our study (Wang et al., 2011). In a test of the effect of light source upon SOA aging, Donahue et al. (2012) observed a monotonic decrease in SOA mass during aging with 360 nm UV lights, but an increase in SOA mass during aging with sunlight or quasi-solar lamps. These observations can be explained by the fact that some oxygenated organics in the SOA condensed phase, such as carbonyls or peroxides, undergo photodissociation by photolysis at the specific wavelength of 360 nm. Together, these results highlight the strong influence of the light source on the photodissociation of SOAs, and underscore the utility of experiments performed under realistic illumination conditions.
In addition, the molecular structure of α-pinene, which contains only one double bond and thus one active site for ozone reaction, limits the gas-phase oxidation by ozone of reaction products of α-pinene. Since no scavenger of OH radicals was added, α-pinene-O 3 SOA is likely a mixture of OH and O 3 oxidation products of α-pinene. Therefore, some of the products may still contain unreacted C=C bonds, which can then react with O 3 . Our results indicate that the relative contribution of the products of these reactions may not be sufficient to significantly affect the chemical properties of SOAs during the ozone-induced forcing.
Recent literature has highlighted that α-pinene-O 3 SOA is not composed of a homogeneous chemical mixture. For example, when Maurin et al. (2015) exposed α-pinene-O 3 SOA to high concentrations of O 3 , they observed a constant particle-phase concentration of verbenone, an unsaturated oxygenated product. Since this molecule would have been expected to be consumed by heterogeneous reaction with O 3 , and since heterogeneous particle-phase reactions are predicted to occur mainly at the surface of the particles (Shiraiwa et al., 2013), these authors postulated that reaction Atmos. Chem. Phys., 15, 883-897, 2015 www.atmos-chem-phys.net/15/883/2015/ of O 3 with verbenone was kinetically limited by the diffusion of buried verbenone to the particle surface. This hypothesis is also supported by our observation that α-pinene-O 3 SOA exhibits a core-shell structure, with less oxidized species at its surface (Denjean et al., 2014b). The effect of the forcing would depend not only on the chemical composition of the bulk particle, but also on the chemical surface and the phase of the particle.

Conclusions
We have demonstrated that the size, chemical composition, hygroscopicity and optical properties of α-pinene-O 3 SOAs change dramatically in response to relatively minor increases in temperature (∼ 6 • C). These changes, in turn, have implications for the role that SOA plays in climate. For example, the volatilization-induced increase in the real part of the SOA CRI is likely to enhance the direct radiative effect of SOA by increasing its ability to scatter radiation. The direct radiative effect of SOA can be parameterized using the mass extinction efficiency k ext at 525 nm, which is defined as the ratio of the measured σ scat at 525 nm to the SOA mass concentration. In our companion paper (Denjean et al. 2014b), we have estimated a value of k ext = 1.7 ± 0.5 m 2 g −1 for unprocessed SOAs. In the present experiments, we estimate a value of k ext = 2.8 ± 0.5 m 2 g −1 for SOAs exposed to light and temperature increase, which implies that this aging regime led to a ∼ 40 % enhancement in this parameter. We attribute this enhancement to both the decrease in SOA particle size and the increase in the scattering component of the CRI upon heating. As shown in this work, exposure of SOAs to increasing temperature also leads to an increase in SOA hygroscopicity: the SOA GF increased from 1.04 to 1.14 as a result of this forcing. In order to estimate the influence of these changes in hygroscopicity on the direct radiative effect of SOAs, we made the simplified assumption that SOA exhibits a homogeneous mixing state with water. We used Mie scattering calculations for homogeneous spheres to determine σ scat at 90 % RH both from the measured GF and the CRI. The CRI calculations were based on volume-weighted CRI of values of α-pinene-O 3 SOA and water. We estimate that k ext (90 % RH) = 2.8 ± 0.7 m 2 g −1 after SOA volatilization, which is significantly higher than the value of k ext (90 % RH) = 2.3 ± 0.7 m 2 g −1 calculated in Denjean et al. (2014b) for unprocessed SOAs. Since we have shown that volatilization of condensed-phase species takes place over a range of temperatures consistent with diurnal fluctuations, we suggest that these changes in k ext are representative of the diurnal evolution of SOAs during its lifetime in the atmosphere.
Our results also suggest that α-pinene-O 3 SOA is quite insensitive to light-and ozone-induced aging under our experimental conditions. Insensitivity to ozone-induced aging is most likely a result of the molecular structure of α-pinene which limit the gas-phase oxidation by ozone of reaction products of α-pinene. In addition, several recent studies have reported that α-pinene-O 3 SOA undergoes a transition from a more glassy state to a more liquid state with increasing RH (Saukko et al., 2012;Renbaum-Wolff et al., 2013;Denjean et al., 2014b). On the basis of viscosity data and the Stokes-Einstein equation, Renbaum-Wolff et al. (2013) estimated that the change in α-pinene-O 3 SOA viscosity associated with its transition from a solid to a semi-solid state increases the bulk diffusion coefficient of particles by 6 orders of magnitude (from < 10 −17 cm 2 s −1 to 10 −11 cm 2 s −1 ). Since an increase in bulk diffusion coefficient would be expected to be accompanied by an increase in particle reactivity, we suggest that future studies examine the effect of ozone and light exposure on α-pinene-O 3 SOA properties under humidified conditions (i.e. RH > 30 %). Beyond its effect on the physical state of the particles, the relative humidity may influence the chemical composition of SOAs. Recently, Kristensen et al. (2014) observed that α-pinene-O 3 SOAs increased concentrations of high molecular weight compounds, such as dimer esters, at higher RH (> 50 %) relative to lower RH (< 30 %). An increase in high molecular weight compounds with increase RH could lead to a change in hygroscopic and optical properties of α-pinene-O 3 SOAs as well as its sensitivity to aging processes. In addition, further experimental studies on SOAs which exhibit different viscosity are required in order to evaluate the atmospheric implication of the oxidative processing and photochemistry.