Secondary organic aerosol phase behaviour in chamber photo-oxidation of mixed precursors

Manchester, Manchester M13 9PL, UK State Key Joint Laboratory of Environmental Simulation and Pollution Control, International Joint Laboratory for Regional Pollution Control, Ministry of Education (IJRC), College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China Lancaster Environment Centre, Lancaster University, LA1 4YQ, UK 10 International Joint Laboratory for Regional Pollution Control, 52425 Jülich, Germany, and Beijing 100871, China Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing University of Information Science and Technology, Nanjing 210044, China National Centre for Atmospheric Science, School of Earth and Environmental Sciences, The University 15


Abstract.
The phase behaviour of aerosol particles plays a profound role in atmospheric physicochemical processes, influencing their physical and optical properties and further impacting climate and air quality. However, understanding of aerosol phase behaviour is still incomplete, especially that of multicomponent particles which contain inorganic compounds and secondary organic aerosol (SOA) from mixed volatile organic 25 compound (VOC) precursors. We report measurements conducted in the Manchester Aerosol Chamber (MAC) to investigate the aerosol rebounding tendency, measured as "bounce fraction", as a surrogate of particle phase behaviour during SOA formation from photo-oxidation of biogenic (α-pinene, isoprene) and anthropogenic (o-cresol) VOCs and their binary mixtures on deliquescent ammonium sulphate seed.
Aerosol phase behaviour is RH and chemical composition dependent (key factors determining aerosol 30 water uptake). Liquid (bounce fraction, BF < 0.2) at RH > 80 % and non-liquid behaviour (BF > 0.8) at RH < 30 % were observed, with a liquid-to-nonliquid transition with decreasing RH between 30 % ~ 80 %. This RH-dependent phase behaviour (RHBF=0.2, 0.5, 0.8) increased towards a maximum with increasing organic-inorganic-mass ratio (MRorg/inorg) during SOA formation evolution in all investigated VOC systems. With the use of comparable initial ammonium sulphate seed concentration, the SOA 35 production rate of the VOC systems determines the MRorg/inorg, and consequently the change of the phase behaviour. Although less important than RH and MRorg/inorg, the SOA composition plays a second-order role, with differences in liquid-to-nonliquid transition at moderate MRorg/inorg of ~ 1 observed between biogenic-only (anthropogenic-free) and anthropogenic-containing VOC systems. Considering the combining role of the RH and chemical composition in aerosol phase behaviour, the BF decreased 40 monotonically with increasing hygroscopic growth factor (GF) and the BF was ~ 0 when GF was larger than 1.15. The real atmospheric consequences of our results are that any processes changing ambient RH or MRorg/inorg (aerosol liquid water) will influence their particle phase behaviour. Where abundant anthropogenic VOCs contribute to SOA, compositional changes of SOA may influence phase behaviour at moderate organic mass fraction (~ 50 %) compared with purely biogenic SOA. Further studies are 45 needed on more complex and realistic atmospheric mixtures.

Introduction
Aerosol particles are ubiquitous in the atmosphere and can act as reaction vessels where physicochemical processes occur. As one of the key physical properties of aerosol particles, the aerosol phase behaviour 50 can significantly impact those physicochemical processes (Martin, 2000). Following the pioneering work of Virtanen et al. (2010), there has been considerable recent efforts to resolve the influences of particle phase behaviour from a number of perspectives mainly relating to retardation of diffusion or mobile components, including water. The viscous solid particles have potential impacts on physicochemical processes, such as constraining gas-particle partitioning of semi-volatile organic species (Vaden et al.,55 2011; Shiraiwa et al., 2011;Shiraiwa and Seinfeld, 2012;Zaveri et al., 2014;Renbaum-Wolff et al., 2013), heterogeneous reactions or liquid phase reactions Koop et al., 2011;Kuwata and Martin, 2012;Zhang et al., 2018;Martin, 2000). These processes affect secondary organic and inorganic particulate matters formation in the atmosphere, further impacting their optical properties and air quality.
Moreover, the ice nucleation abilities in the upper tropospheric conditions and cloud condensation nuclei 60 activation of aerosol particles are affected by the phase behaviour (Pöschl, 2011;Murray et al., 2010;Murray, 2008;Reid et al., 2018;Shiraiwa et al., 2017;Ignatius et al., 2016;Slade et al., 2017), further impacting cloud formation and regional climate. Better understanding the phase behaviour of atmospheric particles is important for understanding physicochemical processes in the atmosphere, and aerosol-cloud interactions. 65 The particle rebounding property has been widely used to study phase behaviour of aerosol particles (Dahneke, 1971;Stein et al., 1994). In a real atmosphere, the phase behaviour of aerosol particles varied significantly under various environments, depending on the ambient relative humidity (RH) and aerosol chemical composition. For example, background atmospheric particles in the tropical rainforest over central Amazonia, mainly composed of isoprene-derived secondary organic aerosol (SOA), were initially 70 liquid for ambient RH > 80 % and temperature of 23 ~ 27 °C (Bateman et al., 2015b). In contrast, when the measurement site was influenced by anthropogenic air mass from urban pollution and biomass burning, non-liquid PM fraction increased to 60 % at 95 % RH . However, in the boreal forest of northern Finland, atmospheric particles (mainly monoterpene-derived SOA) showed amorphous, solid-like phase state (Virtanen et al., 2010). An enhanced fraction of particulate sulphate can 75 lead to loss of particle bounce and atmospheric particles with high fraction of inorganic compounds showed a liquid-like phase state under moderate and high ambient RH (e.g. urban area , subtropical coastal megacity , south-eastern US rural site (Pajunoja et al., 2016), Northeastern near forest area (Slade et al., 2019)). When the terpene-dominant SOA increased during night time (Slade et al., 2019) or ambient RH dropped under 60 % , the non-liquid PM 80 increased.
Consistent with the main findings in the field studies, the particle bounce of pure SOA from the oxidation of representative biogenic or anthropogenic VOC (e.g. isoprene, α-pinene, toluene) decreased with elevated RH and varied with SOA composition (Bateman et al., 2015a;Saukko et al., 2012). The discrepancy between the Bateman et al. (2015a) and Saukko et al. (2012) was the RH at which aerosol 85 particles fully adhered to the substrate (with < 20 % aerosol particles rebounding; referred as the adhesion RH). In a well-calibrated rebounding impactor system, the rebound or adhering behaviour is highly related to the aerosol phase state, which is determined by material softening. Bateman et al. (2015a) found the complete adhesion RH for the isoprene SOA, α-pinene SOA, 2:1 isoprene/α-pinene mixture and toluene SOA were > 65 %, > 95 %, > 80 % and > 80 %. However, Saukko et al. (2012) observed that SOA from 90 isoprene and α-pinene oxidation, do not fully adhere to the substrate even RH of up to 90 %. This discrepancy might due to the slightly different instrumentation design (pressure drop in Saukko et al. (2012) vs atmospheric pressure in Bateman et al. (2015a)) or SOA composition difference from different oxidation conditions. As observed in the real atmosphere, aerosol particles are usually a mixture with organic and inorganic species (e.g. sulphate and nitrate) (Jimenez et al., 2009). Since the inorganic species 95 have lower glass transition temperature than atmospheric-relevant organics (Pedernera, 2008;Koop et al., 2011), the presence of inorganic species could theoretically lower the glass transition temperature of the estimated SOA compounds mixtures. Furthermore, inorganic species are hydrophilic and the absorbed water molecules can act as a plasticiser, which effectively lower the glass transition temperature and soften the aerosol Martin, 2000). Therefore, ignoring the mixing with inorganic 100 species, or assuming external mixing as in Shiraiwa et al. (2017), could bias our understanding on the phase behaviour of aerosols containing abundant inorganic compounds.
There are few studies on phase behaviour of multicomponent aerosol particles. Saukko et al. (2012) found increasing sulphate fraction mixing with SOA produced by longifolene oxidation can reduce the particle rebounding significantly. Saukko et al. (2015) extended this to study deliquescence hysteresis of 105 ammonium sulphate with condensed SOA from α-pinene and isoprene oxidation as manifested by particle rebound. They found the α-pinene SOA condensing on ammonium sulphate seed particles showed no influences on their deliquescence but significantly attenuated the efflorescence behaviour. In contrast, the isoprene SOA system resulted in a loss of sharp deliquescence and efflorescence behaviour in comparison to pure ammonium sulphate (Saukko et al., 2015). These results are partly consistent with similar studies 110 using a different instrument (Hygroscopicity Tandem Differential Mobility Analyzer) Smith et al., 2012). They found that the α-pinene SOA on ammonium sulphate seed can slightly shift the deliquescence and efflorescence RH by a few percent  while the isoprene SOA can significantly decrease the deliquescence and efflorescence RH depending on the organic fraction (Smith et al., 2012). These contrasting findings above indicate that aerosol phase behaviour could differ 115 between different SOA precursor systems as well as in the presence of inorganic compounds. However, it is still unclear how the phase behaviour (and any associated diffusion limiting behaviour) will be influenced during the formation and evolution of SOA from an increased complexity of mixed precursors in the presence of inorganic seed.
We designed a series of aerosol simulation chamber experiments to study SOA formed from 120 representative biogenic (α-pinene, isoprene) and anthropogenic (o-cresol) VOC photochemistry. The experiments studied single VOC precursors and their binary mixtures under modest-NOx condition on deliquescent ammonium sulphate seed particles. We frame our results around the following hypotheses: 1) Aerosol phase behaviour for SOA mixture is driven by RH and organic-inorganic-mass ratio 2) The difference in SOA composition is less important in determining the phase behaviour than the SOA 125 production rate, which changes the organic-inorganic ratio.
The main objective of this paper is to test the above two hypotheses and discuss their potential atmospheric implications. 130 The aerosol particles for the experiments were produced in the Manchester Aerosol Chamber (MAC).

Reaction chamber and experimental setup
Detailed description of MAC can be found in Shao et al. (2021) and a brief introduction is given as below.
The facility is run as a batch reactor with an 18 m 3 (3 m (H) * 3 m (L) * 2 m (W)) FEP Teflon bag supported by three aluminium frames, in which the upper and the lower frame can move freely with the expansion or collapsing as sampling air is introduced to or extracted from the chamber. The Teflon bag 135 is enclosed inside a housing with temperature and relative humidity controlled by an air conditioning system. Two 6 kW Xenon arc lamps (XBO 6000 W/HSLA OFR, Osram) and a series of halogen bulbs arranged in 7 rows containing 16 bulb each (Solux 50W/4700K, Solux MR16, USA) are mounted inside of the enclosure housing. The combination of 5 rows of halogen bulbs (2 rows spare) and two Xenon arc lamps is chosen to be a good representative to mimic the solar spectrum in the wavelength of 290 ~ 800 140 nm (Alfarra et al., 2012). The calculated photolysis rate of NO2 (jNO2) from O3-NO-NO2 photostationary state was (1.8 ~ 3) × 10 -3 s -1 during the experiment period. The simulated irradiation spectrum intensity in our chamber is around a third of the measured solar spectrum at the mid-day on a clear sky day in June of Manchester (https://www.eurochamp.org/Facilities/SimulationChambers/MAC-MICC.aspx).
To ensure chamber cleanliness and data reproducibility, an automatic fill/flush cycle is conducted pre-145 and post-experiment using 3 m 3 min -1 purified clean air. 5 ~ 6 cycles are routinely conducted between experiments, with total number concentration of aerosol particles usually lower than 10 p cc -1 after cleaning. Air is scrubbed using a series of filters, including Purafil (Purafil Inc., USA) and charcoal removing reactive gaseous compounds and HEPA (Donaldson Filtration, USA) removing particles and a dryer. To remove reactive compounds, the chamber is soaked in high concentrations of O3 (~ 1 ppm) 150 overnight between experiments which is removed during pre-experiment fill/flush cycles on the subsequent day. An additional harsh cleaning procedure is conducted weekly with high O3 (~ 1 ppm) and UV for 4 ~ 5 hours to consume the remaining reactive compounds.
Seed particles, VOC, NOx and water vapour are injected into chamber before illumination. Deliquescent ammonium sulphate (AS) seed (Puratronic, 99.999 % purity) are nebulized and introduced into a drum to 155 mix before flushing into chamber. Liquid VOC precursors (α-pinene, isoprene, o-cresol; Sigma Aldrich, GC grade ≥ 99.99 % purity) are injected using a syringe into a heated glass bulb in which the VOCs are instantaneously vaporized. The vaporized VOCs are flushed into the chamber with a flow of ~ 0.5 bar high purity nitrogen (ECD grade, 99.997 %). Here, the experiments will investigate single and binary mixtures under modest-NOx condition (VOC/NOx of 3 ~ 10). NOx (mostly as NO2 in this study) is 160 introduced through a cylinder with a flow of ~ 0.3 bar high purity nitrogen (ECD grade, 99.997 %) and the O3-NO-NO2 photostationary state will establish immediately after illumination. The detailed initial conditions for chamber setup are shown in Table 1 and the experimental design will be described in details in Sec. 2.3.

Instrumentation
A series of instruments are equipped for gas-phase and particle-phase measurements in MAC. NO, NO2 and NOx are recorded by NO-NO2-NOx analyser (Model 42i, Thermo, USA) and O3 is measured by O3 170 analyser (Model 49C, Thermo, USA). RH and T inside of MAC are recorded by an Edgetech dewpoint hygrometer (DM-C1-DS2-MH-13, USA) and two Sensirion SHT75 sensors (Farnell 413-0698, USA).
The particle bounce behaviour (bounce fraction, BF) was measured by a three-arm particle rebound apparatus with RH adjustment system. A brief instrumental description is provided below and more details can be found in Bateman et al. (2014) and  along with a schematic diagram in 180 Figure S1 of the latter. Three single-stage impactors operated in parallel in the system combined with a condensation particle counter (CPC, Model: 3772, TSI, USA). The first impactor is not equipped with a plate (step 1 in Figure S1 in ), so particles can pass through the first impactor directly, measuring the total particle population (N1). The second impactor is equipped with a smooth plate (step 2 in Figure S1 in ), which provides a solid surface and allows particles rebounding from 185 the impactor. The particle population measured after the second impactor represents the sum of particles that do not strike the impactor and that strike but rebound from the impactor (N2). The third impactor is equipped with a grease-coated plate (step 3 in Figure S1 in ). The coated grease is quite sticky and all particles striking on the plate will be stuck. Therefore, the particle number population after grease-coated plate provides a measure of the particles that don't strike the impactor (N3 was used to select a monodisperse aerosol particles from chamber, with mobility diameter of 100 ~ 200 nm following the growth of the aerosol particles. The selected particle sizes are larger than the 50 % transmission diameter of the used impactor (84.9 ± 5.4 nm) (Bateman et al., 2014) to ensure a reliable bounce fraction measurement.
BF can be used as a proxy of aerosol phase behaviour, with limited capability representing semi-solid or solid particles over 100 Pa s in viscosity (Bateman et al., 2015a;Reid et al., 2018). Nevertheless, it provides insights into the transition process between liquid and solid/semi-solid phase, referred to as liquid-to-nonliquid transition below. Calibrations were performed before and after the campaign using monodisperse (350 nm) ammonium nitrate and ammonium sulphate particles following the standard procedure in Jayne et al. (2000) and 210 Jimenez et al. (2003). An averaged ionization efficiency of ammonium nitrate was 9.38 × 10 -8 ions molecule -1 from the two calibrations. According to the ion balance of ammonium nitrate and ammonium sulphate in the calibrations, the specific relative ionization efficiencies (RIE) for NH4 + and SO4 2are determined as 3.57 ± 0.02 and 1.28 ± 0.01, respectively. The RIE of all organic compounds used the default value of 1.4 (Alfarra et al., 2004). In this study, the AMS mass concentrations were corrected by 215 comparing to the real-time DMPS unit mass multiplied by concurrent calculated density from AMS species. The mass ratio of AMS/(DMPS*density) is 0.4 ~1.0 in all investigated VOC systems.  (Coggon et al., 2019;Koss et al., 2018) and indirectly from the oxidation of toluene (e.g. motor vehicles, solvent use (Fishbein, 1985)). Therefore, anthropogenic source is one of the main contributors to the o- gives a good contrast with the low-yield isoprene and high-yield α-pinene. The overall experiment design 235 enables us to explore SOA formation in initially iso-reactive single, binary and ternary VOC mixtures oxidation, details can be found in Voliotis et al. (to be submitted to this issue). This manuscript focuses on aerosol phase behaviour of seeded SOA from iso-reactive single and binary VOCs photochemistry.

Rationale behind the choice of precursor
Unfortunately, the bounce impactor was unavailable for the ternary mixture experiment.  Figure 1 shows the rebound curves of the 100 ~ 200 nm multicomponent aerosol particles formed in various VOC systems for the period when organic mass fraction in NR-PM1 is larger than 0.05. For all investigated VOC systems, the aerosol particles exhibited BF > 0.8 at RH < 30 % and BF < 0.  (Tang and Munkelwitz, 1993;Kreidenweis and Asa-Awuku, 2014). Owing to limitations of 250 the technique in differentiating particle viscosity at high values (Bateman et al., 2014), the non-liquid phase could represent semi-solid or solid phase. In addition, the rebound curves varied along with SOA formation and subsequent evolution as the photochemistry continues in all the investigated VOC systems (as shown in Figure 1). To illustrate the influences of chemical composition, the overview of rebound curves as a function of organic-inorganic-mass ratio (MRorg/inorg) in all VOC systems is shown in Figure   255 S1, indicating the potential important role of MRorg/inorg (and maybe OA composition) in determining the phase behaviour as RH.

BF dependence on RH and organic-inorganic-mass ratio
To clearer describe the phase behaviour during SOA formation evolution, the RHBF=0.2, RHBF=0.5, and RHBF=0.8 are determined representing the RH at which the BF is close to 0.2 (± 0.15), 0.5 (± 0.15), and 0.8 (± 0.15). It is worth noting that the determination of RHBF=0.2, 0.5, 0.8 carry a maximum error of ± 5 % 260 owing to the measurement resolution and resultant number of data points. By tracking the variation of RH BF=0.2, 0.5, 0.8 during SOA formation evolution, we can understand the tendency of phase behaviour change and insight into the liquid-to-nonliquid transition at the variation of RH at which BF changes from  Figure 2, the RHBF=0.2, 0.5, 0.8 increased towards a maximum as an increase of the MRorg/inorg in all investigated VOC systems, indicating an increase of rebound 270 tendency during SOA evolution on deliquescent sulphate seed. Figure 2 a-c showed a co-increasing trend between RHBF=0.2, 0.5, 0.8 and MRorg/inorg as MRorg/inorg < 2, thereafter RHBF=0.2, 0.5, 0.8 remained constant at 70 ~ 75 %, 65 ~ 70 % and 50 ~ 65 %, respectively, in all VOC systems. Herein, the reason for the decrease of RHBF=0.5 from 60 % to 50 % at MRorg/inorg ~ 1 in the o-cresol system was not known and the same behaviour was not observed for RHBF=0.2, 0.8.

275
As expected, the change in MRorg/inorg during SOA formation and evolution differed in various VOC systems (as shown in Figure 3), depending on their SOA production rate (as shown in Figure S2), noting the comparable initial sulphate seed concentrations. The order of the SOA production rate (from low to high) in all the investigated VOC systems was, o-cresol/isoprene < 50 % reactivity o-cresol < o-cresol < α-pinene/isoprene < α-pinene/o-cresol < 50 % reactivity α-pinene < α-pinene (note that insufficient mass 280 was generated from isoprene system). It is worth noting that the SOA production rate in α-pinene/isoprene, α-pinene/o-cresol and 50 % reactivity α-pinene systems are very similar, and their order is determined by the minor difference of the slopes in Figure S2. Clearly, a higher SOA production rate increased the MRorg/inorg faster and consequently the phase behaviour change. As shown in Figure 4, RHBF=0.2,0.5,0.8 showed an increasing trend towards a maximum across the various VOC systems, highly related to how 285 fast the SOA were formed. With an increasing SOA formation rate from the lowest o-cresol/isoprene to highest α-pinene system, a shorter time was taken for RHBF=0.2, 0.5, 0.8 to reach the maximum. Take RHBF=0.5 as an example, it can be seen from Figure 1 that the RHBF=0.5 at the beginning of the photochemistry was lower than 40 % for all investigated VOC systems. Figure 4 b showed that, for the o-cresol/isoprene system with the lowest SOA formation rate, it took ~ 3.5 h for RHBF=0.5 to increase to 50 % and ~ 5 h to 290 approach 60 %. For comparison, for α-pinene/isoprene (the moderate) and α-pinene (the highest) systems, it takes only ~ 1.5 h and 0.5 h for the RHBF=0.5 increasing to 50 % and 70 %, respectively. Similar results can also be found for the cases in RHBF=0.2, 0.8.
It can be seen that in all of the investigated VOC systems, when the SOA mass fraction was > 0.05, the phase dependence on MRorg/inorg was qualitatively similar irrespective of the single/binary 295 biogenic/anthropogenic SOA precursors. That is, the RHBF=0.2, 0.5, 0.8 increased towards a maximum value with an increase of the MRorg/inorg during SOA formation evolution. In regard to the SOA from biogenic or anthropogenic VOCs oxidation showed amorphous solid property rather than the expected liquid phase as deliquescent inorganic particles (Bateman et al., 2015a;Virtanen et al., 2010;Saukko et al., 2015;Saukko et al., 2012), such as the RHBF=0.2, 0.5, 0.8 of SOA formed from α-pinene and toluene photo-300 oxidation were 80 ~ 90 %, 80 ~ 85 %, and ~ 65 %, respectively (Bateman et al., 2015a). It is expected that the increasing aerosol rebounding tendency can happen with increasing SOA mass condensing on the deliquescent sulphate seed. This speculation is supported by our results that an increase of RHBF=0.2, 0.5, 0.8 toward the maximum with more SOA condensation. Additionally, the maximum RHBF=0.2, 0.5, 0.8 at high MRorg/inorg of ~ 8 in our study were 10 ~ 15 % lower than SOA mentioned above, indicating the presence 305 of small mass fraction of inorganic compounds (~ 10 %) makes multicomponent aerosol particles bounce less than the SOA. Moreover, the time taken for RHBF=0.2, 0.5, 0.8 to reach the maximum depended on how rapidly the SOA was formed, shorter time for the faster SOA production rate of the investigated VOC systems. This general behaviour was independent of the yield of the VOC and whether the precursor was biogenic or anthropogenic. In addition, the individual VOC systems behaved in the same way as the VOC 310 mixtures.
As shown in Figure 3, it should be noted that, to avoid high noise-signal-ratio for low organic mass loading measured by HR-ToF-AMS, the data points with organic mass fraction larger than 0.05 (assuming uniform chemical composition) were selected for consideration. It was observed that particulate nitrate (Max. 5 % ~ 16 % of NR-PM1) was formed during photochemistry in all investigated VOC systems. 315 Nitrate can be formed either from oxidation of NO2 followed by neutralisation by NH3 or organic oxidation product (organic nitrate). The organic nitrate fraction in total nitrate signal can be estimated following the NO + /NO2 + ratio method proposed by Farmer et al. (2010), given the differentiation of NO + /NO2 + ratio for pure NH4NO3 (2.6, from calibrations) and organic nitrate of 10 ~ 15 (Bruns et al., 2010;Fry et al., 2009;Kiendler-Scharr et al., 2016;Reyes-Villegas et al., 2018). As shown in Figure S3, 320 organic nitrate fraction in total nitrate signal was lower than 20 % in almost all investigated VOC systems except for the last two hour of o-cresol system (up to ~ 26 %). This NO + /NO2 + ratio method can indicate organic nitrate statistically only if the organic nitrate fraction is larger than 15 % (Bruns et al., 2010).
Thus the observed particulate nitrate is mainly inorganic nitrate with small contribution of organic nitrate (< 20 %) for all VOC systems in this study. The small contribution of organic nitrate mass have little 325 influences on the onset of organic mass fraction > 0.05. Moreover, Li et al. (2017) found that the pure NH4NO3 particles adhered on the impactor even the RH has been reduced to ~ 5 %. It is worth noting that the variable particulate nitrate across all VOC systems in this study might have some influence on the phase behaviour of the multicomponent aerosol particles during SOA formation evolution and the influence of changing inorganic component ratios on the BF in multicomponent mixtures should be the 330 focus of this work.

BF dependence on OA composition
In addition to the control of the BF by MRorg/inorg, there is an indication that the SOA composition across the various VOC systems may also impact aerosol phase behaviour. As indicated in Figure 2 b-c, the increase of the RHBF=0.5, 0.8 as a function of MRorg/inorg is less rapid in the α-pinene/isoprene, 50 % reactivity 335 α-pinene and α-pinene system (referred as BVOC systems, anthropogenic-free) than the anthropogenic-  Shiraiwa et al., 2017). In contrast, Saukko et al. (2012) showed the O:C ratio only influenced rebounding behaviour of SOA formed from n-heptadecane in a Potential Aerosol 360 Mass (PAM) reactor, with no influence in the α-pinene, longifolene, and naphthalene systems. There was some evidence showing that aerosol morphology (single well-mixed phase or phase separation) was closely related to O:C ratio of SOA and organic-inorganic-mass ratio (Bertram et al., 2011;Krieger et al., 2012;You et al., 2014;Freedman, 2017;Song et al., 2012;Smith et al., 2013). It is unknown whether the morphology plays a role in the phase behaviour discrepancy between BVOC and AVOC-containing 365 systems at moderate MRorg/inorg considering their O:C difference, which is of interest and needs further investigations.

Mixing role of chemical composition and RH on BF
As we know that the chemical composition and RH are key factors influencing aerosol water uptake at given size (Kreidenweis and Asa-Awuku, 2014). To discuss the mixing role of chemical composition and 370 RH on BF, the GF at the RH of BF measurement was calculated from HTDMA measurement using κ-Kӧhler equation, and the relation between the BF and the GF was plotted in Figure 5. It can be seen that the BF showed a monotonic decrease from ~ 1 to ~ 0 with the increasing GF from ~ 1 to ~ 1.15 in all VOC systems. As the GF is larger than 1.15, the aerosol particles kept in liquid phase (BF ~ 0). This evidence indicated the key role of increasing aerosol water uptake on the phase transition from the non-375 liquid to the liquid. The BF-GF relation of the varying multicomponent aerosol particles including SOA and inorganic compounds is comparable with the previous study (Bateman et al., 2015a). They measured the BF of the SOA from α-pinene, toluene and isoprene and found that the BF is nearly 0 when the GF is larger than > 1.15 (Bateman et al., 2015a).

Conclusions and implications
Our experiments support the validity of our two proposed hypotheses: 1) Aerosol phase behaviour for SOA mixture is determined by RH and organic-inorganic-mass ratio (MRorg/inorg). 2) The difference in SOA composition is less important in determining the phase behaviour than the rate of SOA production, which changes the MRorg/inorg. 385 First, aerosol phase behaviour is clearly RH-dependent as already widely known. Multicomponent aerosol particles were always found to exhibit liquid-like behaviour (BF < 0.2) above 80 % RH, and nonliquidlike behaviour (BF > 0.8) at below 30 % RH. The bounce measurements always indicated continuously increasing nonliquid-like behaviour as RH decreased from 80 % to 30 %. These RH-dependent rebound curves strongly depended on the increase in MRorg/inorg during SOA formation in all investigated VOC 390 systems. The identified RHBF=0.2, 0.5, 0.8 increased towards a maximum with the increase of the MRorg/inorg, and the increase rate of RHBF=0.2, 0.5, 0.8 is determined by the SOA production rate on sulphate seed across the VOC systems. This general behaviour was independent of the yield of the SOA precursors and whether the precursor was biogenic or anthropogenic. In some ways, this is an obvious result that follows from the rate of SOA mass increase in the system under investigation. However, this production rate will 395 be dependent on the reactivity and yield of VOCs in the mixture, their concentrations and interactions influencing SOA mass formation. This set of complex dependencies will control the changes in particle MRorg/inorg in mixtures and hence in phase behaviour.
Although less important than the RH and MRorg/inorg, the SOA composition plays a secondary role affecting the phase behaviour. It was observed that the RHBF=0.2 as a function of MRorg/inorg was the same 400 for BVOC (AVOC-free) and AVOC-containing systems, however the decreased RH to achieve liquid-to- This discrepancy cannot be explained by the atomic O:C ratio of SOA during SOA formation evolution.
It should be the focus of future work to investigate whether SOA composition has a more pronounced effect outside the dynamic O:C range of the mixtures in our experiments. Additionally, by combining the chemical composition and RH, we calculated the hygroscopic growth factor (GF) and found its key role in impacting phase behaviour. The multicomponent aerosol particles were liquid in all VOC systems 410 when the GF is higher than 1.15 at room temperature and transmitted from liquid to non-liquid when the GF decreased to ~ 1.
Multicomponent aerosol phase behaviour depends on RH and organic-inorganic-mass ratio in a particle and in the atmosphere as well as our chamber. Any interactions influencing these two factors will therefore influence the phase behaviour. For example, a fast formation of SOA or significant ambient RH 415 change in real atmosphere could change phase behaviour of particles, and consequently influencing atmospheric physicochemical processes. There is an additional indication that an increased anthropogenic VOC contribution to the mixture could give different phase behaviour at certain moderate organic mass fraction (~ 50%), hence there will be some second-order differences depending on the relative contributions of anthropogenic and biogenic VOC. Further studies should be carried out on more complex 420 and realistic atmospheric mixtures.

Data availability
The observational dataset of this study will be available on the open dataset of EUROCHAMP-2020 programme shortly (https://data.eurochamp.org/data-access/chamber-experiments/). Figure 1. Time series of the Bounce fraction (BF) as a function of RH measured by three-arm particle rebound apparatus with RH adjustment system of the 100-200 nm secondary organic aerosol (SOA) formed from various iso-reactive single/binary biogenic/anthropogenic VOCs (volatile organic compounds) photochemistry under relative low-NOx condition on deliquescent ammonium sulphate seed (as the SOA mass fraction in NR-PM1 is larger than 0.05). Figure 2. The measured (a) RHBF=0.2, (b) RHBF=0.5 and (c) RHBF=0.8 as a function of organic-inorganic ratio (MRorg/inorg) and coloured by atomic O:C ratio in various VOC systems photochemistry on deliquescent ammonium sulphate seed. Black box points out the RHBF=0.2, 0.5, 0.8 at moderate MRorg/inorg of ~1 for biogenic VOC systems (α-pinene/isoprene, 50% reactivity α-pinene).