Ultrafine aerosol particles (<100 nm) might cause severe effects on human health and impact the global climate through the aerosol indirect effect . In the atmosphere, aerosol particles have primary and secondary sources. Secondary particles form when gaseous precursors oxidize to low-volatility compounds, which at high enough abundances can form molecular clusters. Clusters grow by condensation into the nucleation- and Aitken-mode size range and eventually reach sizes at which they can act as cloud condensation nuclei (CCN). A detailed understanding of the mechanisms leading to aerosol formation and growth, i.e., new particle formation (NPF; ), requires the careful measurement of particles in the cluster–particle transition and nucleation-mode size regime. Quantitative measurements of aerosol particles in the size range 1–10 nm remain a major challenge for the understanding of the mechanisms responsible for NPF.

Condensation particle counters (CPCs), which optically detect nanometer-sized particles after condensational growth, are state-of-the art instruments for particle measurements in the sub-10 nm size regime. Since the introduction of Aitken's “dust counter” towards the end of the 19th century , particle counters capable of detecting smaller and smaller aerosol particles have been developed . Thus, CPCs are used in many different fields, including atmospheric studies , the characterization of combustion processes , monitoring occupational health hazards and clean-room monitoring systems .

CPCs are based on two fundamental processes: heterogeneous nucleation and subsequent condensational growth. The seed particles are mixed with a condensable vapor, followed by a rapid temperature change, leading to vapor supersaturation, particle activation by heterogeneous nucleation and subsequent condensational growth. The seed particles grow to sizes large enough that they can be detected optically. This process can be achieved by different working fluids such as n-butanol, water or diethylene glycol (DEG), which are commonly used in commercial instruments. Previous research has shown that the molecular weight and the surface tension of the working fluid have an impact on particle activation . Higher molecular weights and higher surface tensions allow for the activation of smaller particles.

The detection performance of CPCs is commonly quantified by the 50 % cutoff diameter (dp,50). The total detection efficiency of a CPC, ηtot(dp), is given by the following relation : 1ηtot(dp)=ηs(dp)⋅ηa(dp)⋅ηd(dp). Here, ηs(dp) is the sampling efficiency accounting for the particle losses inside the instrument. ηa(dp) corresponds to the activation efficiency of the seed particles by heterogeneous nucleation, and ηd(dp) is the detection efficiency of the particles in the optical system.

Heterogeneous nucleation processes are influenced by the physicochemical properties of the seed particles. If the seed particle is entirely soluble in the condensing liquid, Köhler theory can describe the activation behavior. If the seed is insoluble, heterogeneous nucleation theory is applicable. Other relevant seed particle properties for their activation are size, shape, wettability and charging state . Previous studies have shown that the charging state of the seed particles might also have an influence on their respective detection efficiencies . Chemical processes on the molecular level mediate particle activation, including interactions related to the chemical composition of the seed particles and the molecules of the working fluids . Previous research has identified potential molecular characteristics that act as docking stations to the seed particle. These docking stations are polar groups, like –OH . The distribution of polar groups, in general, defines the chemical polarity of a vapor molecule. A table summarizing the relevant physical properties of the working fluids can be found in the Supplement (Table S1). Furthermore, it has been shown that the activation probability is influenced by the supersaturation occurring in the condenser of the CPC and the working fluid used . As a result the combination of CPCs based on different working fluids might even provide information concerning the chemical composition of the seed particles by showing differing detection efficiencies . Additionally, previous studies have shown that the detection efficiency of CPCs for sub-2 nm particles can be improved by tuning the instrument settings .

Here we present the results of studying the effect of the working fluid on the activation efficiencies with respect to various seed particle compositions. The broad array of investigated particle detectors was composed of n-butanol-based CPCs (TSI 3772, TSI 3776 and Airmodus A20), water-based CPCs (TSI 3788 and V-WCPC 3789) and a DEG-based booster stage (TSI 3777). The experiments have been performed under reproducible laboratory conditions and provide further insights into particle activation in CPCs.

A schematic of the experimental setup is displayed in Fig. . The counting efficiencies of the CPCs were measured using four different types of seed particles generated from sodium chloride (NaCl, pro analysi, Merck KGaA, Darmstadt, Germany), silver (Ag, wool for elemental analysis, CAS no.: 7440-22-4, Merck KGaA, Darmstadt, Germany), ammonium sulfate ((NH4)2SO4, pro analysi, Merck KGaA, Darmstadt, Germany) and β-caryophyllene (C15H24, BCY, ≥80 %, CAS no.: 87-44-5, Sigma Aldrich, St. Louis, USA). To avoid a possible influence of the relative humidity of the carrier gas on the detection efficiency , synthetic air (ALPHAGAZ 1 Air, 99.999 % (5.0), H2O<3.0 ppm mol-1, Air Liquide) was used as a carrier gas and the aerosol was generated in dry conditions in all measurements. Sodium chloride, silver and ammonium sulfate particles were produced in a tube furnace manufactured by Carbolite Gero GmbH & Co. KG, Germany. The particle material is put onto a crucible and inserted into the cylindrical furnace. The material evaporates in the heated furnace, and rapid cooling downstream of the heated section leads to particle formation. Table S2 contains the approximate furnace temperatures for different seed particle materials.

Subsequent to the aerosol generator, a dilution flow of synthetic air is joined with the aerosol flow. A soft X-ray charger (TSI 3088 Advanced Aerosol Neutralizer) is used to achieve a steady-state charge distribution for the analyzed aerosol flow. In the next step, the aerosol enters a custom-made Vienna-type differential mobility analyzer (DMA, as presented in , and referred to as nano-DMA; see the Supplement), where particles are selected according to their electrical mobility. The electrical mobility diameters of the selected particles range between 1 and 25 nm. According to , the flow rate ratio δ defines the limiting resolution in the absence of diffusion when the DMA flows are balanced and is given by the sum of the aerosol inlet flow Qa and the sample outlet flow Qs divided by the sum of the sheath flow rate Qsh and the excess flow rate Qe: 2δ=Qa+QsQsh+Qe. The limiting resolution throughout our experiments is therefore approximated by 0.15. It is important to note that in the 1–2 nm size range diffusional broadening can degrade the DMA resolution . The selected aerosol might be polydisperse and the selected particle diameter must be seen as an average diameter with an envelope (see Fig. S1). In our study this envelope is given by ±0.5 nm based on a maximum uncertainty approximation by taking into account the results of . The diffusional broadening of the transfer function of the nano-DMA between 1 and 5 nm was also presented by . In their study the authors calculated a geometric standard deviation of approximately 1.07 for a size-classified output aerosol composed of particles with a mean diameter of 1 nm. By applying positive voltage to the nano-DMA, particles with negative charge were selected. Negatively charged particles of the same composition are assumed to achieve higher activation efficiencies at small sizes independently of the CPCs used and are thus preferred for applications in size distribution measurements down to particles as small as 1nm. Previous studies on the counting efficiency of CPCs involving sub-2 nm particles were conducted using high-resolution DMAs . In order to assess the suitability of the nano-DMA used for seed particles with diameters smaller than 2 nm, previous measurements of negatively charged Ag and NaCl seeds using a tuned TSI 3776, the TSI 3777 and a high-resolution DMA (UDMA) were used. Parts of the results and the experimental setup used were published by . The results of the comparison are displayed in the Supplement.

A flow unit, including a total particle filter and a silica gel dryer, maintains a sheath airflow rate of 19.5 L min-1 (see Fig. a) or 33.0 L min-1 (see Fig. b), respectively, in the DMA. The silica gel was exchanged frequently in order to maintain a dry sheath air cycle (closed-loop system). Downstream of the DMA the aerosol is evenly distributed among a CPC and a Faraday cup electrometer (TSI 3068B Aerosol Electrometer) using a T junction. The flow rates were adjusted and monitored using a TSI 4140 mass flowmeter. For the measurements with the DEG-based booster stage, a different flow setup was used due to the higher inlet flow rate of the instrument (see Fig. b). Accordingly, the tube furnace was operated with 2.5 L min-1 of synthetic air and the aerosol inlet flow of the nano-DMA was increased to 5.0 L min-1. Consequently, the setup used for the generation of seed particles based on NaCl, Ag and (NH4)2SO4 bears a close resemblance to the setup described in , wherein the authors also present mass spectra of furnace-generated clusters. The study by can be used to infer the chemical composition of some of the seed particles used in this study.

Particles consisting of oxidized β-caryophyllene (C15H24Ox, BCYOx) were produced in a flow tube . The seeds were produced by evaporating BCY into a clean airstream and subsequently mixing it with ozone, allowing the ozonolysis reaction to take place inside the flow tube. Ozone was generated using a custom-made ultraviolet (UV) lamp with adjustable intensity. An intensity–ozone calibration was performed prior to the experiments with an ozone monitor (ThermoFisher Scientific, model i49), suggesting that the ozone concentrations were in the range of 100–500 ppb. The mode diameter of the resulting size distribution can be shifted by varying the length of the reaction path of the flow tube using an adjustable piston and thus the reaction time of the organic compounds. The diameter of the flow tube used was 0.05 m, and the length of the reaction path was varied between 0.1 and 0.4 m. The temperature of the BCY evaporator was kept at 283.15 K.

The CPCs used and the booster stage are presented in Table . All particle counters are based on the laminar-flow principle and have 50 % cutoff diameters below 10 nm according to the manufacturers. Here, it should be noted that the TSI 3789 was operated in the 2 nm mode for all experiments. The 2 nm mode is the default setting by the manufacturer for low-cutoff measurements based on calibrations with sucrose. The n-butanol- and DEG-based instruments implement a typical architecture consisting of a heated saturator and a cooled condenser. In the case of the TSI 3772 and the Airmodus A20 the entire aerosol flow reaches the heated saturator. The TSI 3776 and TSI 3777 are additionally equipped with a capillary regulating the aerosol flow . This capillary–sheath layout helps to keep the aerosol flow centered and enhances the detection efficiency by achieving a higher and sharper supersaturation profile within the condenser and reducing diffusional losses . In the case of the water-based TSI 3788 a cooler conditioner is followed by a heated growth tube . Here, the aerosol flow is first cooled and then enters a heated region of supersaturated vapor. The temperatures of the sections have to be switched due to the higher diffusion coefficient of water compared to heat transfer. Lastly, the TSI 3789 (V-WCPC 3789) is based on a three-step principle: a cool conditioner, a warm initiator and a cooler moderator . The moderator is necessary for removing water vapor and heat while maintaining supersaturated conditions . In contrast to the TSI 3788, the TSI 3789 is not based on a capillary–sheath layout.

By changing the internal settings of three selected CPCs, the 50 % cutoff diameters were pushed to smaller sizes using BCYOx particles. In the case of the TSI 3777 and the TSI 3789, the boosting was achieved by changing the temperature settings. In the case of the tuned TSI 3776 (denoted as TSI 3776T), the internal flows were also adjusted. The valve regulating the sheath airflow is adjusted such that the sample flow rate of the CPC increases to 2.5 L min-1 .

The detection efficiency η is determined by comparing the particle number concentration measured by a CPC, NCPC, to the particle number concentration of the Faraday cup electrometer (FCE), NFCE: 3η=NCPCNFCE. After the tube furnace had reached a stable state, the number concentrations were simultaneously measured for 45 s. An uncertainty analysis of measurement data was performed according to the rules of Gaussian error propagation.

The effect of the chemical composition of seed particles on the detection efficiency in commonly used CPCs based on different working fluids was investigated. These characterizations included organic seed particles that were generated in a controlled way. We present the first characterization measurements of the newly developed TSI 3789 using seed particles with diameters between 1 and 25 nm. Furthermore, the onset of detection in CPCs was successfully and correctly predicted based on simulated supersaturation profiles.

Chemical similarities between the seed particle material and the working fluids used have been found to have an impact on the detection efficiency of CPCs. It has been shown that shifts in the detection efficiencies also occur for CPCs with 50 % cutoff diameters larger than 5 nm. Additionally, it was confirmed that the detection efficiency can be improved by changing the temperature and flow rate settings of the instruments. Remarkably, the TSI 3776 also did not show any shifts in the detection efficiency for NaCl, (NH4)2SO4 and BCYOx seeds when operated using the tuned settings. This behavior is linked to the higher saturation ratio after tuning the CPC. The saturation ratio might be high enough that differences in the detection efficiencies of different seed particles no longer occur.

Consequently, we conclude that the 50 % cutoff diameter as the sole parameter is not sufficient to characterize the detection efficiency of a CPC. Shifts of the detection efficiency and curve shapes are influenced by the aforementioned interactions. We recommend that CPC characterizations be performed using various polar and less polar seed particles, including Ag seeds, in order to correctly present the detection efficiency of a CPC. The chemical composition of the measured aerosol particles should be considered when instruments are calibrated. The authors recommend following the calibration standard based on Ag seeds, introduced by , when a variety of seed particle materials is not at hand. In the future, measurements of the activation efficiency with Ag could then be used to infer the achieved supersaturation in the CPC. Subsequent comparisons to further SANC measurements with other seed materials (e.g., NaCl; ) then allow researchers to infer the corresponding counting efficiencies. Future research needs to focus on the extension and verification of the presented results for sub-2 nm particles with known composition. Hence, the presented study suggests a novel and improved approach to determining the counting efficiencies of CPCs when a calibration with a variety of seed particles is not available.