Extending the Range of Detectable Trace Species with the Fast Polarity Switching of Chemical Ionization Orbitrap Mass Spectrometry.

Chemical ionization (CI) atmospheric pressure interface mass spectrometry is a unique analytical technique for its low detection limits, softness to preserve molecular information, and selectivity for particular classes of species. Here, we present a fast polarity switching approach for highly sensitive online analysis of a wide range of trace species in complex samples using selective CI chemistries and high-resolution mass spectrometry. It is achieved by successfully coupling a multischeme chemical ionization inlet (MION) and an Orbitrap Fourier transform mass spectrometer. The capability to flexibly combine ionization chemistries from both polarities effectively extends the detectability compared to using only one ionization chemistry, as commonly used positive and negative reagent ions tend to be sensitive to different classes of species. We tested the performance of the MION-Orbitrap using reactive gaseous organic species generated by α-pinene ozonolysis in an environmental chamber and a standard mixture of 71 pesticides. Diethylammonium and nitrate are used as reagent ions in positive and negative polarities. We show that with a mass resolving power of 280,000, the MION-Orbitrap can switch and measure both polarities within 1 min, which is sufficiently fast and stable to follow the temporal evolution of reactive organic species and the thermal desorption profile of pesticides. We detected 23 of the 71 pesticides in the mixture using only nitrate as the reagent ion. Facilitated by polarity switching, we also detected 47 pesticides using diethylammonium, improving the total number of detected species to 59. For reactive organic species generated by α-pinene ozonolysis, we show that combining diethylammonium and nitrate addresses the need to measure oxygenated molecules in atmospheric environments with a wide range of oxidation states. These results indicate that the polarity switching MION-Orbitrap can promisingly serve as a versatile tool for the nontargeted chemical analysis of trace species in various applications.

Role of sesquiterpenes in biogenic new particle formation.

Biogenic vapors form new particles in the atmosphere, affecting global climate. The contributions of monoterpenes and isoprene to new particle formation (NPF) have been extensively studied. However, sesquiterpenes have received little attention despite a potentially important role due to their high molecular weight. Via chamber experiments performed under atmospheric conditions, we report biogenic NPF resulting from the oxidation of pure mixtures of ß-caryophyllene, α-pinene, and isoprene, which produces oxygenated compounds over a wide range of volatilities. We find that a class of vapors termed ultralow-volatility organic compounds (ULVOCs) are highly efficient nucleators and quantitatively determine NPF efficiency. When compared with a mixture of isoprene and monoterpene alone, adding only 2% sesquiterpene increases the ULVOC yield and doubles the formation rate. Thus, sesquiterpene emissions need to be included in assessments of global aerosol concentrations in pristine climates where biogenic NPF is expected to be a major source of cloud condensation nuclei.

Pesticide Residue Fast Screening Using Thermal Desorption Multi-Scheme Chemical Ionization Mass Spectrometry (TD-MION MS) with Selective Chemical Ionization.

In this work, the detection characteristics of a large group of common pesticides were investigated using a multi-scheme chemical ionization inlet (MION) with a thermal desorption unit (Karsa Ltd.) connected to an Orbitrap (Velos Pro, Thermo Fisher Scientific) mass spectrometer. Standard pesticide mixtures, fruit extracts, untreated fruit juice, and whole fruit samples were inspected. The pesticide mixtures contained 1 ng of each individual target. Altogether, 115 pesticides were detected, with a set of different reagents (i.e., dibromomethane, acetonylacetone, and water) in different polarity modes. The measurement methodology presented was developed to minimize the common bottlenecks originating from sample pretreatments and nonetheless was able to retrieve 92% of the most common pesticides regularly analyzed with standardized UHPLC-MSMS (ultra-high-performance liquid chromatography with tandem mass spectrometry) procedures. The fraction of detected targets of two standard pesticide mixtures generally quantified by GC-MSMS (gas chromatography with tandem mass spectrometry) methodology was much less, equaling 45 and 34%. The pineapple swabbing experiment led to the detection of fludioxonil and diazinon below their respective maximum residue levels (MRLs), whereas measurements of untreated pineapple juice and other fruit extracts led to retrieval of dimethomorph, dinotefuran, imazalil, azoxystrobin, thiabendazole, fludioxonil, and diazinon, also below their MRL. The potential for mutual detection was investigated by mixing two standard solutions and by spiking an extract of fruit with a pesticide's solution, and subsequently, individual compounds were simultaneously detected. For a selected subgroup of compounds, the bromide (Br-) chemical ionization characteristics were further inspected using quantum chemical computations to illustrate the structural features leading to their sensitive detection. Importantly, pesticides could be detected in actual extract and fruit samples, which demonstrates the potential of our fast screening method.

Heterogeneous Ion-Induced Nucleation of Water and Butanol Vapors Studied via Computational Quantum Chemistry beyond Prenucleation and Critical Cluster Sizes.

Water and butanol are used as working fluids in condensation particle counters, and condensation of a single vapor onto an ion can be used as a simple model system for the study of ion-induced nucleation in the atmosphere. Motivated by this, we examine heterogeneous nucleation of water (H2O) and n-butanol (BuOH) vapors onto three positively (Li+, Na+, K+) and three negatively charged (F-, Cl-, Br-) ions using classical nucleation theory and computational quantum chemistry methods. We study phenomena that cannot be captured by Kelvin-Thomson equation for small nucleation ion cores. Our quantum chemistry calculations reveal the molecular mechanism behind ion-induced nucleation for each studied system. Typically, ions become solvated from all sides after several vapor molecules condense onto the ion. However, we show that the clusters of water and large negatively charged ions (Cl- and Br-) thermodynamically prefer the ion being migrated to the cluster surface. Although our methods generally do not show clear sign-preference for ion-water nucleation, we identified positive sign-preference for ion-butanol nucleation caused by the possibility to form stabilizing hydrogen bonds between butanol molecules condensed onto a positively charged ion. These bonds cannot form when butanol condenses onto a negatively charged ion. Therefore, we show that ion charge, its sign, as well as vapor properties have effects on the prenucleation and critical cluster/droplet sizes and also on the molecular mechanism of ion-induced nucleation.

The missing base molecules in atmospheric acid-base nucleation.

Transformation of low-volatility gaseous precursors to new particles affects aerosol number concentration, cloud formation and hence the climate. The clustering of acid and base molecules is a major mechanism driving fast nucleation and initial growth of new particles in the atmosphere. However, the acid-base cluster composition, measured using state-of-the-art mass spectrometers, cannot explain the measured high formation rate of new particles. Here we present strong evidence for the existence of base molecules such as amines in the smallest atmospheric sulfuric acid clusters prior to their detection by mass spectrometers. We demonstrate that forming (H2SO4)1(amine)1 is the rate-limiting step in atmospheric H2SO4-amine nucleation and the uptake of (H2SO4)1(amine)1 is a major pathway for the initial growth of H2SO4 clusters. The proposed mechanism is very consistent with measured new particle formation in urban Beijing, in which dimethylamine is the key base for H2SO4 nucleation while other bases such as ammonia may contribute to the growth of larger clusters. Our findings further underline the fact that strong amines, even at low concentrations and when undetected in the smallest clusters, can be crucial to particle formation in the planetary boundary layer.

Deposition potential of 0.003-10 µm ambient particles in the humidified human respiratory tract: Contribution of new particle formation events in Beijing.

Ultrafine particles (UFPs) usually explosive growth during new particle formation (NPF) events. However, the risk of exposure to UFPs on NPF days has been ignored due to the prevalence of mass-based air quality standards. In this study, the daily deposited doses, i.e., the daily deposited particle number dose (DPNd), mass dose (DPMd), and surface area dose (DPSd), of ambient particles in the human respiratory tract in Beijing were evaluated based on the particle number size distribution (3 nm-10 µm) from June 2018 to May 2019 utilizing a Multiple-Path Particle Dosimetry Model (MPPD) after the hygroscopic growth of particles in the respiratory tract had been accounted for. Our observations showed a high frequency (72.6%) of NPF on excellent air quality days, with daily mean PM2.5 concentrations less than 35 µg m-3. The daily DPNd on excellent air quality days was comparable with that on polluted days, although the DPMd on excellent air quality days was as low as 15.6% of that on polluted days. The DPNd on NPF days was ~1.3 times that on non-NPF days. The DPNd in respiratory tract regions decreased in the order: tracheobronchial (TB) > pulmonary (PUL) > extrathoracic (ET) on NPF days, while it was PUL > TB > ET on non-NPF days. The number of deposited nucleation mode particles, which were deposited mainly in the TB region (45%), was 2 times higher on NPF days than that on non-NPF days. Our results demonstrated that the deposition potential due to UFPs in terms of particle number concentrations is high in Beijing regardless of the aerosol mass concentration. More toxicological studies related to UFPs on NPF days, especially those targeting tracheobronchial and pulmonary impairment, are required in the future.

Isomer-Resolved Mobility-Mass Analysis of α-Pinene Ozonolysis Products.

Highly oxygenated organic molecules (HOMs) are important sources of atmospheric aerosols. Resolving the molecular-level formation mechanisms of these HOMs from freshly emitted hydrocarbons improves the understanding of aerosol properties and their influence on the climate. In this study, we measure the electrical mobility and mass-to-charge ratio of α-pinene oxidation products using a secondary electrospray-differential mobility analyzer-mass spectrometer (SESI-DMA-MS). The mass-mobility spectrum of the oxidation products is measured with seven different reagent ions generated by the electrospray. We analyzed the mobility-mass spectra of the oxidation products C9-10H14-18O2-6. Our results show that acetate and chloride yield the highest charging efficiencies. Analysis of the mobility spectra suggests that the clusters have 1-5 isomeric structures (i.e., ion-molecule cluster structures with distinct mobilities), and the number is affected by the reagent ion. Most of the isomers are likely cluster isomers originating from binding of the reagent ion to different sites of the molecule. By comparing the number of observed isomers and measured mobilities and collision cross sections between standard pinanediol and pinonic acid to the values observed for C10H18O2 and C10H16O3 produced from oxidation of α-pinene, we confirm that pinanediol and pinonic acid are the only isomers for these elemental compositions in our experimental conditions. Our study shows that the SESI-DMA-MS produces new information from the first steps of oxidation of α-pinene.

Atmospheric pressure thermal desorption chemical ionization mass spectrometry for ultra-sensitive explosive detection.

Illegal explosives are a threat to aviation, transport sector, critical infrastructure and generally to public safety. Their detection requires extremely sensitive instruments with efficient workflows that allow large throughput of items. In this study, we built a trace explosives detection instrument that requires minimal sample treatment and reaches ultra-low picogram level detection limits for many common explosives. The instrument is based on thermal desorption of filters, which allows analysis of liquid and solid phase samples, and subsequent selective atmospheric pressure chemical ionization and detection with a mass spectrometer. We performed experiments to scope the optimal ionization chemistry for the system and selected Br- as the reagent ion, and measured the limit of detection for 14 different explosives that were generally in the picogram range. Finally, we demonstrate the usability of the system by sampling air to a filter from a storage room known to contain explosives, from which we detect four different explosives.

Electrical Mobility as an Indicator for Flexibly Deducing the Kinetics of Nanoparticle Evaporation.

Condensation and evaporation of vapor species on nanoparticle surfaces drive the aerosol evolution in various industrial/atmospheric systems, but probing these transient processes is challenging due to related time and length scales. Herein, we present a novel methodology for deducing nanoparticle evaporation kinetics using electrical mobility as a natural size indicator. Monodispersed nanoparticles are fed to a differential mobility analyzer which serves simultaneously as an evaporation flowtube and an instrument for measuring the electrical mobility, realizing measurements of evaporation processes with time scales comparable to the instrument response time. A theoretical framework is derived for deducing the evaporation kinetics from instrument responses through analyzing the nanoparticle trajectory and size-mobility relationship, which considers the coupled mass and heat transfer effect and is applicable to the whole Knudsen number range. The methodology is demonstrated against evaporation but can potentially be extended to condensation and other industrial/atmospheric processes involving rapid size change of nanoparticles.

Influence of Aerosol Chemical Composition on Condensation Sink Efficiency and New Particle Formation in Beijing.

Relatively high concentrations of preexisting particles, acting as a condensation sink (CS) of gaseous precursors, have been thought to suppress the occurrence of new particle formation (NPF) in urban environments, yet NPF still occurs frequently. Here, we aim to understand the factors promoting and inhibiting NPF events in urban Beijing by combining one-year-long measurements of particle number size distributions and PM2.5 chemical composition. Our results show that indeed the CS is an important factor controlling the occurrence of NPF events, with its chemical composition affecting the efficiency of the background particles in removing gaseous H2SO4 (effectiveness of the CS) driving NPF. During our observation period, the CS was found to be more effective for ammonium nitrate-rich (NH4NO3-rich) fine particles. On non-NPF event days, particles acting as CS contained a larger fraction of NH4NO3 compared to NPF event days under comparable CS levels. In particular, in the CS range from 0.02 to 0.03 s-1, the nitrate fraction was 17% on NPF event days and 26% on non-NPF event days. Overall, our results highlight the importance of considering the chemical composition of preexisting particles when estimating the CS and their role in inhibiting NPF events, especially in urban environments.

Insights into vertical differences of particle number size distributions in winter in Beijing, China.

Particle number size distribution (PNSD) is of importance for understanding the mechanisms of particle growth, haze formation and climate impacts. However, the measurements of PNSD aloft in megacities are very limited. Here we report the first simultaneous winter measurements of size-resolved particle number concentrations along with collocated gaseous species and aerosol composition at ground level and 260 m in Beijing. Our study showed that the vertical differences of particle number concentrations between ground level and aloft varied significantly as a function of particle size throughout the study. Further analysis illustrated the impacts of boundary dynamics and meteorological conditions on the vertical differences of PNSD. In particular, the temperature and relative humidity inversions were one of the most important factors by decoupling the boundary layer into different sources and processes. Positive matrix factorization analysis identified six sources of PNSD at both ground level and city aloft. The local source emissions dominantly contributed to Aitken-mode particles, and showed the largest vertical gradients in the city. Comparatively, the regional particles were highly correlated between ground level and city aloft, and the vertical differences were relatively stable throughout the day. Our results point towards a complex vertical evolution of PNSD due to the changes in boundary layer dynamics, meteorological conditions, sources, and processes in megacities.

The impact of ammonium on the distillation of organic carbon in PM2.5.

Thermal desorption coupled with different detectors is an important analysis method for ambient carbonaceous aerosols. However, it is unclear how the compounds coexisting in both the gas and particle phases affect carbonaceous aerosol concentrations and measurements during thermal desorption. We observed matrix effects leading to a redistribution of different OC fractions (OC1 to OC4) during the thermal desorption process. These factors led to the formation of OC with low volatility (OC4), mainly from high-volatility OC (OC1 and OC2). Laboratory studies further indicated that ammonium promotes such matrix effects by transforming OC in the particle phase. Therefore, in addition to providing insights into the chemical evolution of OC during haze events, we argue that thermal-desorption-based OC measurements should be used with caution, which is an important step towards a more accurate measurement of OC in the ambient atmosphere.

Molecular Composition of Oxygenated Organic Molecules and Their Contributions to Organic Aerosol in Beijing.

The understanding at a molecular level of ambient secondary organic aerosol (SOA) formation is hampered by poorly constrained formation mechanisms and insufficient analytical methods. Especially in developing countries, SOA related haze is a great concern due to its significant effects on climate and human health. We present simultaneous measurements of gas-phase volatile organic compounds (VOCs), oxygenated organic molecules (OOMs), and particle-phase SOA in Beijing. We show that condensation of the measured OOMs explains 26-39% of the organic aerosol mass growth, with the contribution of OOMs to SOA enhanced during severe haze episodes. Our novel results provide a quantitative molecular connection from anthropogenic emissions to condensable organic oxidation product vapors, their concentration in particle-phase SOA, and ultimately to haze formation.

Heterogeneous Nucleation of Butanol on NaCl: A Computational Study of Temperature, Humidity, Seed Charge, and Seed Size Effects.

Using a combination of quantum chemistry and cluster size distribution dynamics, we study the heterogeneous nucleation of n-butanol and water onto sodium chloride (NaCl)10 seeds at different butanol saturation ratios and relative humidities. We also investigate how the heterogeneous nucleation of butanol is affected by the seed size through comparing (NaCl)5, (NaCl)10, and (NaCl)25 seeds and by seed electrical charge through comparing (Na10Cl9)+, (NaCl)10, and (Na9Cl10)- seeds. Butanol is a common working fluid for condensation particle counters used in atmospheric aerosol studies, and NaCl seeds are frequently used for calibration purposes and as model systems, for example, sea spray aerosol. In general, our simulations reproduce the experimentally observed trends for the NaCl-BuOH-H2O system, such as the increase of nucleation rate with relative humidity and with temperature (at constant supersaturation of butanol). Our results also provide molecular-level insights into the vapor-seed interactions driving the first steps of the heterogeneous nucleation process. The main purpose of this work is to show that theoretical studies can provide molecular understanding of initial steps of heterogeneous nucleation and that it is possible to find cost-effective yet accurate-enough combinations of methods for configurational sampling and energy evaluation to successfully model heterogeneous nucleation of multicomponent systems. In the future, we anticipate that such simulations can also be extended to chemically more complex seeds.

Role of iodine oxoacids in atmospheric aerosol nucleation.

Iodic acid (HIO3) is known to form aerosol particles in coastal marine regions, but predicted nucleation and growth rates are lacking. Using the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we find that the nucleation rates of HIO3 particles are rapid, even exceeding sulfuric acid-ammonia rates under similar conditions. We also find that ion-induced nucleation involves IO3 - and the sequential addition of HIO3 and that it proceeds at the kinetic limit below +10°C. In contrast, neutral nucleation involves the repeated sequential addition of iodous acid (HIO2) followed by HIO3, showing that HIO2 plays a key stabilizing role. Freshly formed particles are composed almost entirely of HIO3, which drives rapid particle growth at the kinetic limit. Our measurements indicate that iodine oxoacid particle formation can compete with sulfuric acid in pristine regions of the atmosphere.

Is reducing new particle formation a plausible solution to mitigate particulate air pollution in Beijing and other Chinese megacities?

Atmospheric gas-to-particle conversion is a crucial or even dominant contributor to haze formation in Chinese megacities in terms of aerosol number, surface area and mass. Based on our comprehensive observations in Beijing during 15 January 2018-31 March 2019, we are able to show that 80-90% of the aerosol mass (PM2.5) was formed via atmospheric reactions during the haze days and over 65% of the number concentration of haze particles resulted from new particle formation (NPF). Furthermore, the haze formation was faster when the subsequent growth of newly formed particles was enhanced. Our findings suggest that in practice almost all present-day haze episodes originate from NPF, mainly since the direct emission of primary particles in Beijing has considerably decreased during recent years. We also show that reducing the subsequent growth rate of freshly formed particles by a factor of 3-5 would delay the buildup of haze episodes by 1-3 days. Actually, this delay would decrease the length of each haze episode, so that the number of annual haze days could be approximately halved. Such improvement in air quality can be achieved with targeted reduction of gas-phase precursors for NPF, mainly dimethyl amine and ammonia, and further reductions of SO2 emissions. Furthermore, reduction of anthropogenic organic and inorganic precursor emissions would slow down the growth rate of newly-formed particles and consequently reduce the haze formation.

Particle growth with photochemical age from new particle formation to haze in the winter of Beijing, China.

Secondary aerosol formation in the aging process of primary emission is the main reason for haze pollution in eastern China. Pollution evolution with photochemical age was studied for the first time at a comprehensive field observation station during winter in Beijing. The photochemical age was used as an estimate of the timescale attributed to the aging process and was estimated from the ratio of toluene to benzene in this study. A low photochemical age indicates a fresh emission. The photochemical age of air masses during new particle formation (NPF) days was lower than that on haze days. In general, the strongest NPF events, along with a peak of the formation rate of 1.5 nm (J1.5) and 3 nm particles (J3), were observed when the photochemical age was between 12 and 24 h while rarely took place with photochemical ages less than 12 h. When photochemical age was larger than 48 h, haze occurred and NPF was suppressed. The sources and sinks of nanoparticles had distinct relation with the photochemical age. Our results show that the condensation sink (CS) showed a valley with photochemical ages ranging from 12 to 24 h, while H2SO4 concentration showed no obvious trend with the photochemical age. The high concentrations of precursor vapours within an air mass lead to persistent nucleation with photochemical age ranging from 12 to 48 h in winter. Coincidently, the fast increase of PM2.5 mass was also observed during this range of photochemical age. Noteworthy, CS increased with the photochemical age on NPF days only, which is the likely reason for the observation that the PM2.5 mass increased faster with photochemical age on NPF days compared with other days. The evolution of particles with the photochemical age provides new insights into understanding how particles originating from NPF transform to haze pollution.

Seed-Adsorbate Interactions as the Key of Heterogeneous Butanol and Diethylene Glycol Nucleation on Ammonium Bisulfate and Tetramethylammonium Bromide.

Condensation particle counter (CPC) instruments are commonly used to detect atmospheric nanoparticles. They operate on the basis of condensing an organic working fluid on the nanoparticle seeds to grow the particles to a detectable size, and at the size of few nanometers, their efficiency depends on how well the working fluid interacts with the seeds under the measurement conditions. This study models the first steps of heterogeneous nucleation of two working fluids commonly used in CPCs (diethylene glycol (DEG) and n-butanol) onto two positively charged seeds, ammonium bisulfate and tetramethylammonium bromide. The nucleation process is modeled on a molecular level using a combination of systematic configurational sampling and density functional theory (DFT). We take into account the conformational flexibility of DEG and n-butanol and determine the key factors that can improve the efficiency of nanoparticle measurements by CPCs. The results show that hydrogen bonding between the seed and the working fluid molecules is central to the adsorption of the first DEG/n-butanol molecules onto the seeds. However, intermolecular hydrogen bonding between the adsorbed molecules can also enhance the nucleation process for the weakly adsorbing vapor molecules. Accordingly, the heterogeneous nucleation probability is higher for working fluid-nanoparticle combinations with a higher potential for hydrogen bonding; in this case, DEG and ammonium bisulfate. Moreover, conformational analysis and methodology evaluations indicate that the consideration of adsorbate conformers and step-wise addition of the vapor molecules to the seeds is not essential for qualitative modeling of heterogeneous nucleation systems, at least for systems where the adsorbate and seed chemical properties are clearly different. This is the first molecular-level modeling study reporting detailed chemical reasons for experimentally observed seed and working fluid preferences in CPCs and reproducing the experimental observations. Our presented approach can be likely used for predicting preferences in similar nucleating systems.

Unprecedented Ambient Sulfur Trioxide (SO3) Detection: Possible Formation Mechanism and Atmospheric Implications.

Sulfur trioxide (SO3) is a crucial compound for atmospheric sulfuric acid (H2SO4) formation, acid rain formation, and other atmospheric physicochemical processes. During the daytime, SO3 is mainly produced from the photo-oxidation of SO2 by OH radicals. However, the sources of SO3 during the early morning and night, when OH radicals are scarce, are not fully understood. We report results from two field measurements in urban Beijing during winter and summer 2019, using a nitrate-CI-APi-LTOF (chemical ionization-atmospheric pressure interface-long-time-of-flight) mass spectrometer to detect atmospheric SO3 and H2SO4. Our results show the level of SO3 was higher during the winter than during the summer, with high SO3 levels observed especially during the early morning (∼05:00 to ∼08:30) and night (∼18:00 to ∼05:00 the next day). On the basis of analysis of SO2, NO x , black carbon, traffic flow, and atmospheric ions, we suggest SO3 could be formed from the catalytic oxidation of SO2 on the surface of traffic-related black carbon. This previously unidentified SO3 source results in significant H2SO4 formation in the early morning and thus promotes sub-2.5 nm particle formation. These findings will help in understanding urban SO3 and formulating policies to mitigate secondary particle formation in Chinese megacities.

Seasonal Characteristics of New Particle Formation and Growth in Urban Beijing.

Understanding the atmospheric new particle formation (NPF) process within the global range is important for revealing the budget of atmospheric aerosols and their impacts. We investigated the seasonal characteristics of NPF in the urban environment of Beijing. Aerosol size distributions down to ∼1 nm and H2SO4 concentration were measured during 2018-2019. The observed formation rate of 1.5 nm particles (J1.5) is significantly higher than those in the clean environment, e.g., Hyytiälä, whereas the growth rate is not significantly different. Both J1.5 and NPF frequency in urban Beijing show a clear seasonal variation with maxima in winter and minima in summer, while the observed growth rates are generally within the same range around the year. We show that ambient temperature is a governing factor driving the seasonal variation of J1.5. In contrast, the condensation sink and the daily maximum H2SO4 concentration show no significant seasonal variation during the NPF periods. In all four seasons, condensation of H2SO4 and (H2SO4)n(amine)n clusters contributes significantly to the growth rates in the sub-3 nm size range, whereas it is less important for the observed growth rates of particles above 3 nm. Therefore, other species are always needed for the growth of larger particles.