Chemical composition of nanoparticles from α-pinene nucleation and the influence of isoprene and relative humidity at low temperature

New Particle Formation (NPF) from biogenic organic precursors is an important atmospheric process. One of the major species is α-pinene, which upon oxidation, can form a suite of products covering a wide range of volatilities. A fraction of the oxidation products is termed Highly Oxygenated Organic Molecules (HOM). These play a crucial role for nucleation 50 and the formation of Secondary Organic Aerosol (SOA). However, measuring the composition of newly formed particles is challenging due to their very small mass. Here, we present results on the gas and particle phase chemical composition for a system where α-pinene was oxidized by ozone, and for a mixed system of α-pinene and isoprene, respectively. The measurements took place at the CERN Cosmics Leaving Outdoor Droplets (CLOUD) chamber at temperatures between -50 °C and -30 °C and at low and high relative humidity (20 % and 60 to 100 % RH). These conditions were chosen to simulate 55 pure biogenic new particle formation in the upper free troposphere. The particle chemical composition was analyzed by the Thermal Desorption-Differential Mobility Analyzer (TD-DMA) coupled to a nitrate chemical ionization time-of-flight mass spectrometer. This instrument can be used for particle and gas phase measurements using the same ionization and detection scheme. Our measurements revealed the presence of C8-10 monomers and C18-20 dimers as the major compounds in the particles (diameter up to ~ 100 nm). Particularly, for the system with isoprene added, C5 (C5H10O5-7) and C15 compounds (C15H24O5-10) 60 are detected. This observation is consistent with the previously observed formation of such compounds in the gas phase. However, although the C5 and C15 compounds do not easily nucleate, our measurements indicate that they can still contribute to the particle growth at free tropospheric conditions. For the experiments reported here, most likely isoprene might enhance growth at particle sizes larger than 15 nm. Besides the chemical information regarding the HOM formation for the α-pinene (plus isoprene) system, we report on the nucleation rates measured at 1.7 nm and found that the lower J1.7nm values compared 65 with previous studies are very likely due to the higher α-pinene and ozone mixing ratios used in the present study.

One of the most prominent biogenic precursors for the formation of particulate material is α-pinene (C10H16). It is 75 known that α-pinene oxidation forms HOM that have the ability to nucleate on their own under atmospheric conditions, without the involvement of other trace gases, e. g., sulfuric acid (Kirkby et al., 2016;Tröstl et al., 2016). Stolzenburg et al. (2018) showed that the rapid growth of organic particles produced by α-pinene dark ozonolysis at +25 °C, +5 °C, and -25 °C is determined by the lower extent of autoxidation at reduced temperatures and the decrease in volatility of all oxidized molecules.
Furthermore, Simon et al. (2020), extended the study of α-pinene gaseous oxidation products to even lower temperatures from 80 +25 °C to -50 °C, showing that the oxygen to carbon ratio (O:C) and the yield for HOM formation decrease as the temperature decreases, whereas the reduction of volatility compensates this effect by increasing the nucleation rates at lower temperatures.
Isoprene (C5H8) is the biogenic vapor with the highest global emission rate. Its estimated emissions are between 500 to 600 Tg per year (Guenther et al., 2006;Sindelarova et al., 2014) and there are many studies that indicate the global importance of isoprene in terms of SOA formation (Surratt et al., 2006;Surratt et al., 2007;Surratt et al., 2010;Paulot et al., 85 2009;Lin et al., 2012;Riva et al., 2016). Kiendler-Scharr et al. (2009) presented observations at 15 ºC of a significant decrease in particle number and volume concentration by the presence of isoprene in an experiment under plant-emitted VOCs conditions. Subsequently, McFiggans et al. (2019) showed that isoprene, carbon monoxide, and methane can each suppress aerosol mass and the yield from monoterpenes in mixtures of atmospheric vapors.
Recently, a study by Heinritzi et al. (2020) revealed that the presence of isoprene in the α-pinene system suppresses 90 new particle formation by altering the peroxy-radical termination reactions and inhibiting the formation of those molecules needed for the first steps of cluster and particle formation (species with 19 to 20 carbon atoms). For these biogenic systems, α-pinene and α-pinene + isoprene, the mechanisms behind the formation of HOM in the gas phase have been studied over a wide temperature range. However, the particle phase has not been characterized to the same extent because of the difficulties in measuring the nanoparticle chemical composition due to their very small mass. Despite that, there have been several efforts 95 for designing and improving techniques to face this problem.
Some particle phase studies exist that report the chemical composition of newly formed nanoparticles. For instance, Kristensen et al. (2017), measuring at 293 K and 258 K, showed an increased contribution of less oxygenated species to αpinene SOA particles formed from ozonolysis at sub-zero temperatures. Ye et al. (2019) measured the particle phase chemical composition from α-pinene oxidation between -50 °C and +25 °C with the FIGAERO . They 100 found that during new particle formation from α-pinene oxidation, gas phase chemistry directly determines the composition of the condensed phase. Highly Oxygenated Organic Molecules are much more abundant in particles formed at higher temperatures, shifting the compounds towards higher O:C and lower volatilities. Additionally, some studies addressing the chemical composition, volatility, and viscosity of organic molecules have provided important insights into their influence on the climate (Huang et al., 2018;Reid et al., 2018;Champion et al., 2019). 105 Here, we present the results from gas and particle phase chemical composition measurements for a system where αpinene was oxidized to simulate pure biogenic new particle formation at free tropospheric conditions in a range from -50 °C to -30 °C. The data are further compared to the mixed system of α-pinene and isoprene in order to better understand the https://doi.org/10.5194/acp-2021-512 Preprint. Discussion started: 7 July 2021 c Author(s) 2021. CC BY 4.0 License. partitioning processes. The particle chemical composition was analyzed by the Thermal Desorption-Differential Mobility Analyzer (TD-DMA) , coupled to a nitrate chemical ionization time-of-flight mass spectrometer. This 110 technique allows a direct comparison between gas and particle phase as both measurements are using the identical chemical ionization source and detector.

The CLOUD chamber at CERN and the experiments
The measurements took place in the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at the European Organization for 115 Nuclear Research (CERN) during the CLOUD14 campaign (Sep -Nov 2019). The CLOUD chamber is a stainless-steel cylinder, with a volume of 26.1 m 3 , which has been built to the highest technical standards of cleanliness (Kirkby et al., 2011;Duplissy et al., 2016). By precisely controlling several parameters such as, gas concentrations, temperature, relative humidity, ultraviolet light intensity and internal mixing, specific atmospheric systems can be recreated in order to study the nucleation and growth processes of aerosols at atmospheric conditions. The biogenic gas concentrations, here α-pinene and isoprene, can 120 be regulated by using individual evaporator supplies, in which dry nitrogen passes through the evaporator containing the precursors in a liquid form, at controlled temperature. In this way, the precursors are evaporated and diluted with clean air to achieve the desired concentration in the chamber. Ozone is introduced via a separate gas line. The chamber is continuously stirred by two magnetically coupled stainless-steel fans placed at the top and at the bottom of the chamber to provide a homogeneously mixed system (Voigtländer et al., 2012). 125 The experiments relevant for this work were done in a through mode with continuous addition of the reactants and performed at -50 °C and -30 °C and at low and high relative humidity to simulate pure biogenic new particle formation at a range of free tropospheric conditions. Isoprene and α-pinene precursor gases were oxidized with O3 and ·OH (produced from O3 photolysis in the presence of H2O and UV light) to induce both dark ozonolysis and photochemistry oxidation reactions.
The α-pinene level was between 1 and 8 ppbv, the isoprene level up to 30 ppbv, and O3 approximately 100 ppbv. The ozonolysis 130 of α-pinene was performed at -50 °C and -30 °C, while the α-pinene + isoprene experiment was performed at -30 °C only. The experimental overview is discussed in more detail in Sect. 3.1.

TD-DMA
The particle chemical composition was analyzed by the Thermal Desorption-Differential Mobility Analyzer (TD-DMA) coupled to a nitrate chemical ionization time-of-flight mass spectrometer. The TD-DMA design and characterization have 135 been described in detail by Wagner et al. (2018). This instrument allows the direct comparison between gas and particle phase chemical composition as both measurements use the same ionization scheme and mass spectrometer (the detection technique will be described in Sect. 2.3).
The TD-DMA uses an online and semi-continuous principle for the detection of the chemical composition of nanoparticles. The particles are sampled from the chamber, charged with an X-ray source, a specific size can be selected and 140 immediately afterwards they are electrostatically collected on a filament. Heating the filament after a defined collection time evaporates the particles into a stream of clean carrier gas (N2). The particle vapor is analyzed by the nitrate CI-APi-TOF mass spectrometer (Kürten et al., 2014). In order to estimate the instrumental background, two heating profiles are recorded: the first heating cycle evaporates all the particulate material collected; a second heating cycle constrains the background due to the heating of the inlet line. All reported particle phase signals are corrected based on this background measurement. 145 For the experiments that are reported in this work, a filament of platinum/rhodium (90:10) was used, and an integral, nonsize -selective mode of operation was chosen in order to maximize the mass of collected particles. Due to the very low experimental temperatures, cold sheath flows and isolated inlet lines were installed in order to avoid drastic temperature changes between the CLOUD chamber and the instrument. Evaporation of particulate material before the analysis should therefore not be substantial. 150

Nitrate CI-APi-TOF mass spectrometer
The gas phase and the evaporated particulate material were measured using a nitrate chemical ionization atmospheric-pressureinterface time-of-flight (CI-APi-TOF) mass spectrometer, which has three major components: an atmospheric pressure ionmolecule reactor where the chemical ionization takes place; an atmospheric pressure interface for transporting the charged ions into the mass classifier; and a time-of-flight mass classifier where the ions are accelerated, separated according to their 155 mass-to-charge ratio and detected with a microchannel plate (Jokinen et al., 2012;Kürten et al., 2014). The nitrate CI-APi-TOF mass spectrometer uses nitrate reagent ions (HNO3)n NO3with n = 0-2, which are created by an ion source using a corona discharge needle (Kürten et al., 2011). With this nitrate chemical ionization technique, sulfuric acid, iodic acid, dimethylamine and HOM can be detected (Kürten et al., 2014;Simon et al., 2016;Kirkby et al., 2016;He et al., 2021). HOM are detected because of the presence of functional groups such as hydroperoxy (-OOH) or hydroxy (-OH), which provide the hydrogen 160 bonds required for clustering with the reagent ions.
Here the nitrate CI-APi-TOF mass spectrometer data for gas and particle phase have been corrected for background signals and the mass-dependent transmission efficiency in the mass classifier . The data analysis and processing were performed using IGOR Pro 7 (WaveMetrics, Inc., USA), Tofware (Version 3.2, Aerodyne Inc., USA) and MATLAB R2019b (MathWorks, Inc., USA). 165

Formation rates
The particle number size distribution between ~ 1 nm and 1 μm is measured using a suite of particle counters namely a particle size magnifier, PSM (Vanhanen et al., 2011), a condensational particle counter (CPC 3776, TSI), a nano scanning mobility particle sizer (nano-SMPS 3982, TSI), and a home-built long scanning mobility particle sizer (long-SMPS). The PSM measures the size distribution between ~1 and 3 nm as well as the total particle number concentration above a defined cutoff, 1.7 nm in 170 https://doi.org/10.5194/acp-2021-512 Preprint. Discussion started: 7 July 2021 c Author(s) 2021. CC BY 4.0 License. this study. The CPC on the other hand is used to measure the total particle number concentration above 2.5 nm. The nano-SMPS and long-SMPS together cover the particle number size distribution between 6 nm and 1 μm. The same set-up has been used in previous CLOUD experiments, see for example Lehtipalo et al. (2018) and Heinritzi et al. (2020).
The particle formation rate (Jdp), which is defined as the flux of particles of a certain size as a function of time, is calculated using the method proposed by Dada et al. (2020), see equation (9) therein. For this study, the formation of particles 175 with a diameter ≥ 1.7 nm is calculated (J1.7) using the derivative of the total concentration of particles measured with the PSM while accounting for size-dependent losses to the chamber wall, by coagulation or via dilution. The error on J1.7 is 30% based on run-to-run repeatability (Dada et al., 2020).

Experimental overview 180
An overview of the experiments performed at -30 °C and -50 °C at low and high relative humidity is shown in Fig. 1. The mixing ratio of ozone was stable at ~ 100 ppbv for all of the experiments reported in this work (not shown). In order to represent pure biogenic new particle formation events, no other trace gases were added to the chamber and the levels of SO2, NOx, and other trace gases were monitored to remain always below the detection limits of the respective measurement devices. By using the TD-DMA, particles were collected in every NPF system (without resolving the particle size), the shaded area in nucleated particles. The effect of isoprene in terms of total HOM concentration in the gas phase and on the measured new particle formation rates will be discussed in more detail in Sect. 3.4.2.
The third panel of Fig. 1 shows the α-pinene and isoprene mixing ratios. For all of the systems, α-pinene was between 205 1 and 8 ppbv, while isoprene was only present during experiment αIP-30,20 up to 30 ppbv. The precursor gases were measured by using a proton transfer reaction time-of-flight (PTR-TOF) mass spectrometer (Graus et al., 2010;Breitenlechner et al., 2017), which is capable of measuring VOCs.
The bottom panel of Fig. 1 shows the total HOM concentration in the gas phase. Here, the total HOM is defined as the sum of C5, C10, C15 and C20 carbon classes; these classes consider compounds with C2 -C5, C6 -C10, C11 -C15 and C16 -C20, 210 respectively and considered as a HOM such compounds with five or more oxygen atoms as suggested in Bianchi et al. (2019).
The total HOM was measured with a calibrated nitrate CI-APi-TOF mass spectrometer . Additionally, a temperature dependent sampling loss correction factor is applied. From the evolution of these traces, it can be observed that, C5 and C15 carbon classes have higher concentrations (approximately by a factor of 2.5) in experiment αIP-30,20 compared with α-30,20, which can be explained by the presence of isoprene. However, possible fragmentation in the α-pinene ozonolysis 215 systems also can lead to some C5 and C15 compounds produced without the presence of isoprene. Figure 2 shows the carbon distribution as an overview of the compounds detected in gas and particle phase for a system where only α-pinene was oxidized (α-30,20). C8-10 monomers (Fig. 2a) and C18-20 ( Fig. 2b) dimers are observed in the gas as well as in the particle phase. For instance, some of the signals with the highest intensity correspond to C10H16O3-9, and C20H32O5-13, 220 especially C10H16O6 and C10H16O7 have an important presence in both phases. Overall, most of the compounds that are present in the gas phase are detected as well in the particle phase, although their relative contribution to the total signal can differ between the phases. Figure 3 shows mass defect plots of gas and particle phase and the intensity difference between them for the experiments at -225 30 °C. Figure 3a and Fig. 3d display the gas and particle of α-pinene at -30 °C and 20 % RH (α-30,20). While the gas and particle of α-pinene + isoprene at -30 °C and 20 % RH 20) are shown in Fig. 3b and Fig. 3e, respectively. As both phases were measured with the same instrument, they can be directly inter-compared.

Influence of isoprene on α-pinene system at -30 °C and 20 % RH
The intensity difference is calculated based on the normalized signal (each single signal divided by the total signal for each system and phase). Essentially, the normalized signal can be understood as a measure of the fraction or contribution 230 of every compound in the entire system. By looking at the intensity difference in the gas phase (Fig. 3c), it can be observed that some C5 and C15 contribute significantly more in the system with isoprene added (αIP-30,20) that are not as pronounced in the system where only α-pinene was oxidized (α-30,20). This observation can be attributed to the presence of isoprene in the system. As described by Heinritzi et al. (2020), C15 dimers are formed in the gas phase when C10 RO2· radicals from α-https://doi.org/10.5194/acp-2021-512 Preprint. Discussion started: 7 July 2021 c Author(s) 2021. CC BY 4.0 License. pinene ozonolysis undergo terminating reactions with C5 RO2· radicals from the isoprene oxidation with ·OH. Additionally, 235 C19 and C20 dimers contribute more in the system where only α-pinene was oxidized (α-30,20).

Fig. 3f
shows the intensity difference in the particle phase. This indicates that there is an enhancement of C4-5 and C13-16 compounds in the system with isoprene 20). There is especially a group of C15 compounds C15H24O5-10 (see Fig.   S1 in the supplement) with significant signal in the particle phase. Since the particles collected reached sizes up to ~ 100 nm, and it has been shown in previous studies (Heinritzi et al., 2020) that C15 dimers do contribute to the growth, this observation 240 confirms the existence of these species in the condensed material. Certainly, isoprene can suppress new particle formation (it will be discussed in Section 3.4.2). However, isoprene can still contribute to the growth of particles by C5 or by C15 compounds.
Thus, these species can be an important fingerprint to identify SOA from a mixture of biogenic vapors containing isoprene.
For the experiments presented in this study, we report in Table 1 the particle growth rates (GR) determined from the nSEMS size distributions. The growth rates in 3.2-8 nm and 5-15 nm were calculated using the 50% appearance time method 245 described in Stolzenburg et al. (2018). From the calculated values in Table 1, we observe that GR3.2-8 nm for the α-pinene + isoprene system 20) at the first concentration stage is around 18 nm h -1 compared to ~ 77 nm h -1 for the α-pinene only system (α-30,20). This is a factor of ~ 4 difference. While GR5-15 nm represents a factor of 2 to 3 difference between αIP-30,20 compared to α-30,20. From these values, one would conclude that isoprene does not contribute to the growth in the size range reported here. Nevertheless, by looking at the aerosol mass concentration (see Fig. S3 in the supplement), the mass reached 250 during the experiment αIP-30,20 is identical in the presence and absence of isoprene at -30 ºC and 20 % RH. Reaching the same mass with a lower number of particles for the experiment with isoprene (αIP-30,20) compared to α-30,20, means that the growth rates at larger sizes (> 15 nm) are higher in the presence of isoprene. This is consistent with the fact that the particle size reached in the presence of isoprene is higher. Most likely, isoprene might enhance growth at higher sizes (> 15 nm) in this study. 255 3.2.2 Influence of relative humidity on α-pinene system at -50 °C Figure 4 shows mass defect plots for the pure α-pinene experiments at -50 °C at low and high relative humidity: the gas and particle phase of α-pinene at -50 °C, 20 % RH ( Fig. 4a and Fig. 4d); and the gas and particle phase of α-pinene at -50 °C, 60 to 100 % RH ( Fig. 4b and 4e). In both gas and particle phase at high and low RH, we detected C8-10 monomers and C18-20 dimers. C10H16O4-7 and C20H32O5-11 are the most prominent signals (see Fig. S2 in the supplement). 260 The relative humidity change from 20 % to 60 -100 % does not have a significant influence on the gas phase composition at temperatures of -50 °C (Fig. 4c), meaning that most of the gaseous compounds detected contribute practically equal to the total signal when the humidity changes over the reported range. In contrast, there are changes in the particle phase signal. Although, the intensity difference (Fig. 4f) does not show a clear humidity effect on the particle chemical composition; however, this comparison is based on the normalized signal (contribution of every compound to the total intensity). When 265 looking only at the total intensity in the particle phase, we do observe an increase by a factor of ~ 3 in the total signal for the system at high RH (α-50,60-100) compared with the system at low 20). This observation can be likely attributed to https://doi.org/10.5194/acp-2021-512 Preprint. Discussion started: 7 July 2021 c Author(s) 2021. CC BY 4.0 License. the change on the mass distribution (see Fig. S3 in the supplement), which indicates that at similar α-pinene and ozone mixing ratio, and the same temperature, the mass concentration increases possibly due to the effect of the relative humidity in the system. Besides, a possible impact of relative humidity on particle viscosity can influence particle mass formed, studies by 270 Grayson et al. (2016) and Galeazzo et al. (2021) have reported lower viscosity with higher SOA mass concentration along with RH-dependence of viscosity for organic particles.
Our findings are consistent with previous experiments. Saathoff et al. (2009) observed that humidity has a significant influence on α-pinene SOA yields for lower temperatures. Cocker III et al. (2001) reported that the yield of SOA at higher RH for α-pinene ozonolysis (relative to the system at dry conditions) increases possibly due to the uptake of water. One explanation 275 for this observation could be that the rate constant value of the α-pinene ozonolysis can be affected by the RH (Zhang et al., 2018). Nevertheless, our semi-continuous particle phase measurements do not allow to draw any conclusions on the magnitude of the rate constants. Continuous particle phase measurements under different RH conditions are required in order to better understand the RH effect on the SOA formation.
In general, for the experiments presented in this work, most of the compounds that are present in the gas phase are 280 detected as well in the particle phase, although the relative contributions to the total signal can vary depending on the phase.
The more oxygenated material in the gas phase, specifically for C20 dimers with nO> 13 is not observed in the particle phase. This is probably because of their very low concentrations and the difficulty to distinguish between real particle signal and background. We conjecture that especially at low temperatures this issue might be related to the fact that at lower temperatures, the autooxidation process to form HOM is slower, therefore, the oxygen content and O:C decrease (Stolzenburg et al., 2018;285 Ye et al., 2019;Simon et al., 2020). The low contribution of these compounds in the gas phase might be reflected in the particle phase. Additionally, the heating cycle that evaporates all the particulate material collected on the filament can potentially result in the thermal decomposition of some of the larger molecular weight compounds. Therefore, it is possible that a break-up of some molecules occurs. significant differences between the experiments at -30 °C (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)20) or between the experiments at -50 °C (α-50,20 compared with α-50,60-100), which indicates that temperature is the main parameter affecting the volatility distribution for the experiments reported here.

Volatility distribution of particle phase compounds 290
According to the volatility regimes proposed by Donahue et al. (2012) and Schervish and Donahue (2020), the particle phase detected compounds correspond mainly to Low Volatility Organic Compounds (LVOC) and Extremely Low Volatility 305 Compounds (ELVOC) and Ultralow Volatility Organic Compounds (ULVOC). With this parametrization we are able to approximate the saturation mass concentration for the particle phase compounds measured using the TD-DMA in the CLOUD chamber. For this parametrization we assume that the elemental composition is one of the main parameters to take into account.

Nucleation rates as a function of the total HOM
Previous CLOUD studies have reported nucleation rates (J1.7nm) as a function of the total HOM concentration from α-pinene 310 oxidation for different temperatures and gas mixtures (Kirkby et al., 2016;Heinritzi et al., 2020;Simon et al., 2020). For the experiments discussed in the present study the new particle formation rates have not been reported yet. For this reason, Table   1 gives an overview of the experimental conditions for the experiments α- 30,20,20,; it further includes the HOM total concentration and derived J1.7nm from the PSM data (see method description in Section 2.4).

New Particle formation on pure α-pinene experiments 315
Figure 6 displays pure biogenic J1.7nm vs total HOM concentration at different temperatures for pure α-pinene (Simon et al., 2020), in which can be seen that the total HOM concentration and their nucleation rates have a strong dependence on the temperature. As the temperature decreases, the nucleation rates increase strongly for a given HOM concentration. In other terms, the total HOM concentration needed to reach the same nucleation rate can be up to 2 orders of magnitude higher for +25 °C compared to -50 °C. As described by Simon et al. (2020) this can be attributed to the reduction in volatility with 320 decreasing temperature. In other words, at low temperatures, molecules with less oxygen content can lead to the same nucleation rate as more highly oxygenated molecules at higher temperatures. Additionally, Fig. 6 includes the data points at -30 °C and -50 °C from pure α-pinene experiments reported in this study (α-30,20, α-50,20 and α-50,60-100). However, it can be observed that they do not follow the trend at their corresponding temperature.
For the pure α-pinene systems (α-30,20, α-50,20 and α-50,60-100) and complementary pure α-pinene experiments at 325 +5 °C and at -10 °C, we have calculated the HOM yield as described in Simon et al. (2020) and found that the resulting values are higher than previously reported (see Fig. S5 in the supplement). In order to investigate a possible reason for this finding, we have chosen two representative experiments at -10 °C and 80 to 90 % RH with different levels of α-pinene and ozone. Fig.   7 shows the mass defect plots for the gas phase chemical composition of the oxidation products. In one experiment (Fig. 7a) α-pinene and the ozone mixing ratio were between 0.2 to 0.8 ppbv and 40 to 50 ppbv, respectively, while for the second 330 experiment (Fig. 7b) the mixing ratios were 2 to 3 ppbv and 100 ppbv, respectively. From Fig. 7c it can be concluded that the formation of HOM with low oxygen content is favored when the α-pinene and ozone mixing ratio are higher (relative to the https://doi.org/10.5194/acp-2021-512 Preprint. Discussion started: 7 July 2021 c Author(s) 2021. CC BY 4.0 License. system at low levels of precursor gases). An explanation for this is that the high concentration of RO2 enhances the terminating reactions before the autoxidation can lead to high oxygen content for the products. As the compounds with low oxygen content tend to have higher saturation vapor pressures, they do not contribute efficiently to new particle formation. For this reason, a 335 given total HOM concentration is not unambiguously tied to a new particle formation rate (even at constant temperature). The magnitude of the precursor gas mixing ratio (more specifically the full volatility distribution of the products and not just the simple measure of total HOM) also needs to be taken into account (see Fig.S6 in the supplement). In summary, the lower J1.7nm values compared with previous studies are very likely due to the higher α-pinene and ozone mixing ratios used in the present study. There are several compounds with low oxygen content that contribute to the total HOM concentration in the gas phase 340 while these do not contribute to the formation of new particles.

The influence of isoprene on new particle formation
In order to make the present study comparable with other studies that reported a suppression effect of isoprene on biogenic new particle formation, the values of the isoprene-to-monoterpene carbon ratio (R) are also provided in Table 1, here and in previous studies R is essentially the ratio between isoprene and α-pinene; for experiment αIP-30,20, R equals to 14.4 and 6.1 345 (for two steady-state periods in αIP-30,20). Fig. 8 shows pure biogenic nucleation rates at 1.7 nm against total HOM concentration at different temperatures for the α-pinene and α-pinene + isoprene systems (Kirkby et al., 2016;Heinritzi et al., 2020;Simon et al., 2020). How rapidly particles are formed in a pure biogenic system depends strongly on the temperature and on the ion conditions. In general, we observe increasing nucleation rates at lower temperatures and at GCR conditions. The presence of isoprene lowers the 350 nucleation rate (relative to the pure α-pinene system at similar conditions); this is known as isoprene suppression of new particle formation. In this regard, there is a suppression on the new particle formation caused by adding isoprene on an αpinene system at -30 ºC and 20 % RH. However, it has been reported that the suppression effect is stronger when α-pinene is lower (and R is higher, see Fig. S7 in the supplement). For instance, for a plant chamber experiment that R = 19.5 resulted in no significant new particle formation (Kiendler-Scharr et al., 2009). Additionally, in the Michigan forest with R = 26.4, NPF 355 events did not occur frequently (Kanawade et al., 2011). In spite of that, one has to consider that the suppression effect at a given value of R likely decreases as temperature decreases and so does the saturation vapor pressure of the oxidation products.

Conclusions
In this study, we showed the capability of the Thermal Desorption-Differential Mobility Analyzer (TD-DMA) coupled to a chemical ionization time-of-flight mass spectrometer for measuring HOMs in newly formed nano aerosol particles. Together 360 with the nitrate CI-APi-TOF mass spectrometer, this set up is capable of measuring gas and particle phase, allowing a direct comparison as both measurements use the identical chemical ionization and detector. For the pure biogenic NPF experiments performed at -50 °C and -30 °C in the CLOUD chamber at CERN, we detected in the particle phase (diameter up to ~ 100 nm) compounds such as C10H16O3-9, and C20H32O5-13. Especially for the system with isoprene added, C5 (C5H10O5-7) and C15 compounds (C15H24O5-10) can be an important fingerprint to identify secondary organic 365 aerosol from this biogenic source. Based on the elemental composition, we calculated the saturation mass concentration, and according to the volatility regimes, the particle phase compounds correspond mainly to Low Volatility Organic Compounds (LVOC) and Extremely Low Volatility Compounds (ELVOC) and Ultralow Volatility Organic Compounds (ULVOC).
We also showed that at -30 °C and an isoprene-to-monoterpene carbon ratio R = 14.4 and 6.1, there is a reduction of the nucleation rate (compared to the pure α-pinene system at similar conditions). In this way, isoprene suppresses NPF at -30 370 °C. Nevertheless, this suppression effect can be stronger at higher temperatures and at high R.
Lastly, the lower J1.7 values compared with previous studies are very likely due to the higher α-pinene and ozone mixing ratios used in the present study. There are several compounds with low oxygen content that contribute to the total HOM concentration in the gas phase while these do not contribute to the formation of new particles. For this reason, a given total HOM concentration is not unambiguously tied to a new particle formation rate (even at constant temperature). The magnitude 375 of the precursor gas mixing ratio, and thus the full volatility distribution, also needs to be taken into account. 575 Phys., 9, 1551Phys., 9, -1577Phys., 9, , 10.5194/acp-9-1551Phys., 9, -2009Phys., 9, , 2009 Schervish, M., and Donahue, N. M.: Peroxy radical chemistry and the volatility basis set, Atmos. Chem. Phys., 20, 1183-1199, 10.5194/acp-20-1183

655
°C and 20 % RH (α-50,20) and α-pinene at -50 °C and 60-100 % RH (α-50,60-100). The color scale represents the log 10 of the normalized particle concentration in cm -3 . Second panel: Particle number concentration in cm -3 measured by the PSM with a cutoff diameter of 1.7 nm and CPC 2.5 nm. Third panel: Mixing ratio in ppbv for the biogenic precursor gases, isoprene and α-pinene. Fourth panel: Evolution of total HOM concentration in molec. cm -3 , measured in the gas phase by the Nitrate CI-APi-TOF mass spectrometer. HOM total is defined as the sum of C5, C10, C15 and C20 carbon classes which are shown as well. Ozone level is not 660 shown, though remains stable over the whole period ~ 100 ppbv. The shaded areas refer to the time where the particles were collected using the TD-DMA.