Characterization of New Particle Formation in Soft X-ray Radiolysis Reactor: AMCs-to-Secondary Inorganic Aerosols

Received: October 30, 2022
Revised: January 23, 2023
Accepted: February 18, 2023

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Shin, J., Cho, K., Ha, Y., Kang, G., Park, J., Park, H., Kim, C. (2023). Characterization of New Particle Formation in Soft X-ray Radiolysis Reactor: AMCs-to-Secondary Inorganic Aerosols. Aerosol Air Qual. Res. 23, 220373. https://doi.org/10.4209/aaqr.220373

HIGHLIGHTS

• A continuous tube reactor was developed to understand soft X-ray radiolysis.
• The length of tube reactor and soft X-ray intensity can be manipulated easily.
• Effects of parameters on the new particle formation and growth were investigated.
• Conversion rates of gaseous precursors to secondary aerosols were calculated.
• Particle formation and growth were strongly correlated with soft X-ray intensity.

Keywords: Soft X-ray, Radiolysis, Gas-to-particle conversion, New particle formation, Flow tube reactor

1 INTRODUCTION

Airborne nanoparticles in cleanrooms are avoided in semiconductor industries because they cause yield loss in the production of semiconductor chips. Ultra or high efficiency air particulate filters have been deployed in cleanrooms to remove particulate contaminants. However, cleanrooms still have different kinds of gaseous contaminants from the manufacturing processes, e.g., etching and solvent cleaning. Airborne molecular contaminants (AMCs) represent a wide range of gaseous chemical contaminants in cleanrooms at part-per-trillion or billion (ppt or ppb, respectively) levels. When AMCs are exposed to short-wavelength ultraviolet (UV) sources during lithography processes, secondary aerosol particles can be formed, which can damage semiconductor chips (Den et al., 2006; Gordon et al., 2005; Lobert et al., 2009; Pic et al., 2010; Weineck et al., 2010). However, ultra-low concentrations of AMCs cause difficulty in the real-time detection of AMCs and their effective removal from cleanrooms.

Kim et al. (2015) developed a real-time detection technique for AMCs in cleanrooms using soft X-ray radiolysis. Soft X-ray radiolysis means the formation of secondary aerosols during the gas-to-particle conversion under soft X-ray irradiation. The AMC detection method using soft X-ray radiolysis was very sensitive in the detection of sulfur dioxide (SO₂), a well-known acidic AMC and could detect SO₂ as low as a few ppt levels. This method can detect in real-time, organic AMCs emitted from adhesives used in cleanrooms for screening materials appropriated in the cleanroom environment (Kim et al., 2016). The results well-match with conventional precise chemical analysis method and thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS). When this method was applied to determine outgassing from PM_2.5 collected in Xi’an, China, it showed a linear correlation between the amount of outgassing and the collected PM_2.5 mass loading (Kim et al., 2019). The secondary aerosol formation assisted by soft X-ray is similar to the new particle formation (NPF) of atmospheric secondary aerosols by photochemical processes using UV light from the sun during the day (Kulmala et al., 2012; Kulmala and Kerminen, 2008; Nieminen et al., 2014; Zhang et al., 2012), or at night (Junninen et al., 2008; Kalivitis et al., 2012) or ion-induced nucleation (IIN) caused by cosmic rays coming from the universe (Arnold, 2006). In addition, NPF and subsequent growth under soft X-ray irradiation may increase the heterogeneous (condensational) growth of pre-existing particles by accelerating the oxidation of gaseous contaminants (Munir et al., 2013). However, the mechanism of soft X-ray radiolysis so far has not been well-studied.

In this study, a continuous flow tube reactor was developed to investigate NPF and subsequent particle growth using soft X-ray radiolysis at different conditions. The flow tube reactor was designed by the use of computational fluid dynamics, and maintains laminar flow under soft X-ray irradiation. In addition, the reactor length can be extended by the addition of modules with soft X-ray emitters for controlling reaction (or soft X-ray exposure) time in the reactor. Conversion rates at different experimental conditions were calculated to understand the effects of each parameter on the NPF assisted by soft X-rays in the reactor based on the measured size distributions of particles.

 
2 MATERIALS AND METHODS

 
2.1 New Particle Formation under Soft X-ray Irradiation (Soft X-ray Radiolysis)

Soft X-ray has been used widely in the field of aerosol science and technology as a charge neutralizer. Different from radioactive materials (e.g., Po-210, Am-241, or Kr-85), soft X-rays do not have a half-life and are easy to turn ON/OFF. This makes soft X-rays an excellent choice to be deployed as a charge neutralizer (Han et al., 2003; Kwon et al., 2007; Shimada et al., 2002). The soft X-ray emitters (SXH-05SP1, Sunje Hitek, Co.) used in this study had 4.99 keV photon energy. The beam angle of soft X-rays emitted from the head was 150°, and the decay time was less than 1 sec at a 100 mm distance from it. Turning ON/OFF the soft X-ray emitter was easily done using a controller, which controls the soft X-ray emitter up to 4 ea., simultaneously. Soft X-rays can produce secondary aerosol particles through soft X-ray radiolysis. Even though particle-free air was introduced into a soft X-ray neutralizer, nanoparticles could be formed from the gaseous contaminants in the air depending on the residence time in the neutralizer (Liu et al., 2020). The possible mechanism of soft X-ray radiolysis is described more in SI.

 
2.2 Design of the Continuous Soft X-ray Flow Tube Reactor

Fig. 1 shows the design of the soft X-ray continuous flow tube reactor. The reactor consists of two cone-shaped ends and cylindrical tube body modules. The tube reactor can be systematically extended on demand for longer residence time in the tube reactor or exposure time to soft X-ray irradiation. The reactor was made of metals such as stainless steel and/or aluminum, to enable the robustness of the structures and prevent the leakage of soft X-rays to the outside. The tube reactor can be decontaminated by heating its surface (Tiszenkel et al., 2019). Every cylindrical tube body module had two holders on both sides for the installation of soft X-ray emitters. The length of inlet tubing (1/4 in. diameter) and tube body diameter (D) were fixed at 5 and 10 cm, respectively. The length (L_body) and diameter for each tube body module were fixed at 19.1 and 10 cm, respectively, which made the bulk residence time of the introduced gaseous precursors in one tube body module to be 60 sec at a laminar flow rate, Q = 1.5 lpm. The soft X-ray window was made of Kapton film to prohibit the contaminants in the room from air smearing into the tube reactor but allow soft X-ray access to the reactor for soft X-ray radiolysis (Kulkarni et al., 2002). The thickness between the Kapton film and the inner surface of the reactor was less than 5 mm to minimize the loss of soft X-ray intensity. For safety, radiation leakage around the soft X-ray reactor was checked using a Geiger counter, and no leakage was found. In between the module parts sealed gaskets were used to prevent gaseous and particulate contaminants coming from outside of the tube reactor, which was checked using a set of vacuum pumps and gauges.

Fig. 1. The design of a continuous soft X-ray flow tube reactor with dimensions (fixed and variable ones) and scheme for the soft X-ray radiolysis in the reactor. A static mixer was installed in the inlet tubing (1/4 in. diameter).

 
2.3 Computational Fluid Dynamics in the Soft X-ray Flow Tube Reactor

The inner cone angle (θ) and corresponding cone length (L_cone) were determined based on the simulation results of computational fluid dynamics (CFD) using COMSOL Multiphysics. The CFD simulations were conducted using realizable k-ε turbulent flow, depending on the simulation conditions for parameters in Table 1. With the fixed dimensions of L_body and D, θ and Q were varied from 15° to 60° and from 0.3 to 1.5 lpm, respectively (Huang et al., 2017; Ihalainen et al., 2019).

Fig. 2 shows the CFD simulation results at different flow rates (Q) and θ. When θ was 15°, the flow in the overall reactor was laminar from the inlet cone through the body at all the simulation conditions. Even though θ = 15° was the best condition for maintaining laminar flows, the corresponding L_cone (36.24 cm, much longer than L_body module) was too long to install the reactor in the laboratory; the total length was ~110 and ~150 cm for 2- and 4-module reactors, respectively. When θ was 30° without a static mixer, backward flows existed within 3 cm from the beginning of the first tube body at Q = 1.5 lpm. However, backward flows disappeared in the first tube body at the same condition by installing a static mixer in the inlet tube. Therefore, the θ = 30° (L_cone = 17.81 cm) with a static mixer was selected for the soft X-ray tube reactor in this study. The total length was reduced to ~74 and ~112 cm for 2- and 4-module reactors, respectively. Q was set to 1.5 lpm to investigate the effects of parameters on the NPF through soft X-ray radiolysis, except for the effect of installed soft X-ray positions (Q = 1.2 lpm). When dry N₂ at 25°C was used as a carrier gas in this study, Reynolds number (Re) in the tube body was approximately 4.1–20.5 for Q = 0.3–1.5 lpm, which was calculated as,

where ρ is the density of the gas, D is the diameter of the tube reactor (10 cm), Q is the volume flow rate in the reactor, and μ is the dynamic viscosity of the gas. The range of Re reveals that the flow in the reactor was laminar (Fomete et al., 2021).

Fig. 2. Computational fluid dynamics (CFD) simulation results for determining the dimensions (inner cone angle and cone length, θ and L_cone) of the continuous soft X-ray flow tube reactor (Red streamlines: backward flows, blue lines: forward flows, and slashed boxes: not converged results).
 

 
2.4 Characterization of the New Particle Formation (NPF) through Soft X-ray Radiolysis

Fig. 3 shows the experimental setup to investigate the effects of various parameters on the NPF using the soft X-ray radiolysis in the flow tube reactor at different conditions as presented in Table 2. The number and volume size distributions of particles (PNSD and PVSD) formed in the continuous soft X-ray tube reactor at different experimental conditions were measured using a set of scanning mobility particle sizer (SMPS), which consists of a nano differential mobility analyzer and an ultrafine condensation particle counter (nano DMA and an UCPC, model 3085 and 3756, TSI, Inc.). The aerosol inlet and sheath flow rates of the DMA were 1.5 and 15 lpm, respectively, which allows the measurement of nanoparticles in the size range of 2.02–63.8 nm. When PNSDs exceeded the upper size limit range of nano DMA depending on the experimental conditions, the nano DMA was replaced by a long DMA (model 3081A, TSI, Inc.) with the same inlet and sheath flow rates. The SMPS with the long DMA measured the particles in the size range of 6.15–216.7 nm. The number and volume concentrations (N_tots and V_tots) for each condition were calculated by integrating PNSDs and PVSDs.

Fig. 3. Experimental setup for investigating the soft X-ray radiolysis in the continuous flow tube reactor at different conditions.

The concentrations of gaseous precursors (SO₂, NH₃, and NO₂) were controlled by mixing 2 ppmV gases with ultra-high-purity (UHP) grade nitrogen (N₂, 99.999%) using mass flow controllers (Alicat Scientific, Inc). The residence time under soft X-ray irradiation and its intensity can strongly influence the NPF via soft X-ray radiolysis (Yun et al., 2009). Even though soft X-ray radiolysis can produce secondary organic aerosols (SOAs), secondary particle formation from the inorganic gaseous precursors (SO₂, NO₂, and NH₃) (secondary inorganic aerosols, SIAs) was mainly investigated in this study. SOA formation through soft X-ray radiolysis will be discussed in subsequent research papers.

The exposure time to soft X-rays was changed by the number (2 or 4) of reactor modules, which were accompanied by soft X-ray emitters (one per module). For example, the exposure time to soft X-ray was 1 minute per a body module at Q = 1.5 lpm. The residence time (or flow rate, Q) of the reactor was adjusted by discharging the excess flow in front of the reactor. At the rear end of the reactor, particle-free air was supplemented to the outflow to make the total flow as 1.5 lpm, which was matched with the sampling flow of SMPS (1.5 lpm) before measuring PNSDs. The measured PNSDs at different Qs were calibrated based on the dilution ratios (1.25–5) by the particle-free air. The gas concentrations for mixtures were adjusted by mixing 2 ppmV SO₂, NO₂, or NH₃ standard gas with ultra-high purity (UHP) grade N₂. The initial SO₂ and NO₂ concentrations were measured using gas analyzers (Serinus 50 and 40, Ecotech, Inc.) to verify the accuracy of the mixing ratio of the injected gas. The NH₃ concentrations were derived from the flow rate mixing ratios with other gases.

The number and position of soft X-ray emitters were changed to investigate the effects of the soft X-ray intensity (Table 3). The serial alignment indicates that soft X-ray emitters were installed in the direction of the gas flow in the reactor, while the “Parallel 2” represents that two emitters were installed per one module; for example, the soft X-ray intensity of Parallel 4 is twice that of Serial 2 in the same position. A maximum of four soft X-ray emitters were deployed in this study. For this parameter, Q was set at 1.2 lpm to investigate the effect of the position of soft X-ray emitters more precisely by further removing the backward flow in the tube body.

 
2.5 Calculation of the Conversion Rates of SO₂ into Particles Using Soft X-ray Radiolysis

The parameter effects on the soft X-ray radiolysis were also evaluated using conversion rates (CRs) of gaseous SO₂ into H₂SO₄ particles using the V_tots calculated from the measured PVSDs. CRs of SO₂ into H₂SO₄ particles at different experimental conditions are calculated using the formula:

where V_p,H2SO4 (D_p) is the volume concentration (nm³ cm^–3) at particle diameter, D_p (nm), V_{p,total,H2SO4} is the total volume concentration (μg m^–3) obtained by integrating the volume concentration of each particle diameter from the minimum (lower detection limit, LDL) to the maximum (higher detection limit, HDL) diameter, ρ_p,H2SO4 is the bulk density of H₂SO₄ particles (1.83 g cm^–3), and M_H2SO4 is the molecular mass of H₂SO₄ particles.

 
3 RESULTS AND DISCUSSION

 
3.1 Effects of SO₂ Concentration and Exposure Time to Soft X-ray on SIA Formation

Fig. 4 shows the temporal PNSDs and PVSDs as results of soft X-ray radiolysis in the 2- and 4-module reactors. One emitter was installed in each body module. In the 2-module reactor, most particles were quickly formed at the beginning of the formation, and the size was generally smaller than 20 nm as shown in the PNSD (Fig. 4(a)). However, particles were continuously grown in the reactor up to 60 nm in the 4-module reactor and higher concentrations around 10 nm was observed 10 mins after turning soft X-ray ON (Fig. 4(b)). In the PVSDs, higher volume concentration was shown between 10 and 30 nm in the 2-module reactor (Fig. 4(c)), while the mode of the PVSD was established at ~20 nm and was wider after ~7 min in the 4-module one (Fig. 4(d)). The bulk residence time in the 2- and 4-module reactors were 2 min 40 sec and 4 min 40 sec, respectively. After its bulk residence time, of 4 min 40 sec, the particle formation was almost done within the bulk residence time for the shorter reactor but was still ongoing for the longer one.

Fig. 4. Temporal number and volume size distributions of particles (PNSDs and PVSDs) formed through the soft X-ray radiolysis in the tube reactors (SO₂ = 100 ppb, Q = 1.5 lpm). (a) and (b) are PNSDs for 2- and 4-module reactors, and (c) and (d) are PVSDs for 2- and 4-module reactors.

Fig. 5 shows the PNSDs and PVSDs as well as CRs at different exposure times to soft X-rays. As shown in Figs. 5(a) and 5(c), the number concentrations of the mode diameter particles for the 4-module reactor were generally two times lower than those for the 2-module reactor. However, most particles were formed below 10 nm for the 2-module reactor with two soft X-ray emitters, even though the inlet gas concentrations were double the conditions for the 4-module reactor (Fig. 5(a)). Otherwise, more than half of the particles formed in the longer reactor were larger than 10 nm (Fig. 5(c)). When the number of modules increased, the exposure time to soft X-ray also increased, promoting particle growth. This was found in the PVSDs for the two reactors, as shown in Figs. 5(b) and 5(d). Despite higher inlet gas concentrations for the 2-module reactor, the mode sizes for the shorter reactor were smaller than those for the longer ones. In addition, the volume concentrations at the mode diameters for the 2-module reactor were much lower than those for the 4-module reactor (> 10 times), owing to the longer exposure time to soft X-ray irradiation. In terms of V_tot as shown in Fig. 5(e), the 4-module reactor converted more SO₂ gas molecules into H₂SO₄ particles than the 2-module reactor; for example, 4.44 × 10⁹ nm³ cm^–3 particles were formed in the 4-module reactor at 100 ppb SO₂, while the 2-module one produced 6.05 × 10⁸ nm³ cm^–3 particles at the same SO₂ concentration (~7.33 times higher in the 4-module reactor). As the number of modules (emitters) increased, it extended chemical reaction time between radicals and gaseous precursors for NPF. In addition, the increased residence time also enhanced physical reactions among various sized particles for the particle growth, such as collision and coagulation as well as the condensation of gas molecules on the particle surfaces. This caused the enhanced V_tot for the 4-module reactor than the 2-module one, even at the same inlet SO₂ concentration. When CRs were calculated from the V_tots (Fig. 5(f)), ~1.87 of 100 ppb introduced SO₂ was converted into particles in the 4-module reactor (~0.0187), but ~0.254 ppb SO₂ was consumed to form particles in the 2-module one (~0.00254). It shows that the exposure time to soft X-ray was one of the major factors to enhance the NPF by drastically using soft X-ray radiolysis (Lammel, 1991; Li et al., 2022). However, the difference in CRs between the two reactors decreased as the inlet SO₂ concentration increased, due to the insufficient yield of OH radicals via soft X-ray irradiation relative to the concentration of SO₂ (Boy et al., 2005). This was also shown in the correlation equations between inlet SO₂ concentration and CR for the two reactors with negative exponents (−0.6829 and −0.4813).

Fig. 5. Measured number and volume size distributions of particles (PNSDs and PVSDs) formed in the 2- and 4-module reactors at different SO₂ concentrations: (a) PNSDs and (b) PVSDs for the 2-module reactor, and (c) PNSDs and (d) PVSDs for the 4-module reactor. (e) Total volume concentrations and (f) conversion rates (CRs) are also derived from the PVSDs to compare the new particle formation and subsequent growth in the two size reactors.

 
3.2. Effects of Residence Time in the Soft X-ray Flow Tube Reactor on SIA Formation

Fig. 6 shows the PNSDs and PVSDs at different inlet flow rates (Qs). The particles are mainly formed in the nano DMA range (2.5–40 nm) at Q = 1.2 and 1.5 lpm (Fig. 6(a)). As the flow rate decreased (Q = 0.3–0.9 lpm), the residence time in the reactor increased, thus accelerating particle growth. Particularly, the mode number concentration at Q = 0.9 lpm was lower than those at Q = 1.2 and 1.5 lpm because the bi-modal division appeared as a transition period in the growth into large particles (SI). In addition, the PNSDs and PVSDs were truncated at the high detection limit of the nano DMA at Q < 0.9 lpm (Fig. 6(b)). The PNSDs and PVSDs could be measured completely by replacing the nano DMA with the long DMA as shown in Figs. 6(c) and 6(d). The volume concentration at each mode increased exponentially from 6.38 × 10⁹ to 5.89 × 10¹⁰ nm³ cm^–3 as Q decreased from 1.5 to 0.3 lpm (Fig. 6(d)). When the CRs were calculated from the V_tots based on the PVSDs in Figs. 6(b) and 6(d) (Fig. 6(e)), the CRs for the long DMA measurements were higher between Q = 0.3 and 0.6 lpm. Then, the CRs for the two DMA measurements intersected each other at Q = 0.9 lpm, and they were reversed at Q = 1.2 and 1.5 lpm due to the truncation of particle number and volume size distributions for the nano DMA. Therefore, the overall CR at the residence time conditions in this study was obtained by choosing the higher CRs at each Q as shown in Fig. 6(f). The CR showed a linear correlation with the bulk residence time (t) in the 4-module reactor. It reflects the linear effect of residence time on NPF and particle growth in the reactor (Li et al., 2015; Young et al., 2008).

Fig. 6. Measured number and volume size distributions of particles (PNSDs and PVSDs) formed in the 4-module reactor at different inlet flow rates (SO₂ = 100 ppb): (a) PNSDs and (b) PVSDs measured by the scanning mobility particle sizer (SMPS) with a nano differential mobility analyzer (DMA), and (c) PNSDs and (d) PVSDs measured by the SMPS with a long DMA. (e) Total volume concentrations and conversion rates (CRs) were also derived from the PVSDs according to the flow rates (Q, lpm) to compare the new particle formation and subsequent growth measured by the two SMPS setups. (f) higher CR measured by the SMPS with nano or long DMA was calculated according to the corresponding residence time (s).

 
3.3. Effects of Multi-component Reactions on SIA Formation

Fig. 7 shows the PNSDs and PVSDs for gas mixtures (SO₂-NH₃-NO₂) at different mixing conditions. The N_tots and V_tots for each condition calculated from the measured PNSDs and PVSDs were depicted in Table 4. First, mixing NH₃ or NO₂ with SO₂ did not significantly change N_tot, which remained between 1.88 to 2.22 × 10⁶ # cm^–3. On the other hand, V_tot drastically changed by adding NH₃ or NO₂ with SO₂. For example, V_tot was almost half of that at the SO₂ only condition, when the concentration of the mixed NH₃ or NO₂ was 200 ppb (Figs. 7(b) and 7(d)). In Figs. 7(a) and 7(c), PNSDs were shifted to smaller sizes and the number concentrations of 2–4 nm particles increased significantly as the concentration of mixed NH₃ or NO₂ increased. There was a simultaneous decrease in the volume concentration of the mode particle size as shown in Figs. 7(b) and 7(d).

Fig. 7. Measured number and volume size distributions of particles (PNSDs and PVSDs) formed in the 4-module reactor at different gas mixture conditions (SO₂:NH₃:NO₂) (SO₂ = 100 ppb): (a) PNSDs and (b) PVSDs for different SO₂-NH₃ mixtures, (c) PNSDs and (d) PVSDs for different SO₂-NO₂ mixtures, and (e) PNSDs and (f) PVSDs for different SO₂-NH₃-NO₂ mixtures.

For NH₃ addition, many researchers reported that NH₃ molecules contribute to the nucleation and ammonium sulfate or ammonium nitrate formation by stabilizing pH (Duan et al., 2021; Lehtipalo et al., 2018; Temelso et al., 2018; Zhu et al., 2015). However, so far, the suppressive effects on subsequent cluster growth to larger particles after nucleation is not well known. Unlike other studies, this study conducted multi-component experiments excluding the humidity effect. In other words, the extreme dry conditions for the experiments using dry compressed gases produced much lower water contents of particles than general atmospheric PM_2.5 particles (Zhang et al., 2021). It is the most unlikely environment for small clusters to uptake NH₃ together with water vapors. With low NO₂ concentration, NO₂ and hydroxyl radical (OH⁻) produce NO₂⁻ and OH. NO₂⁻ enhances IIN, and OH facilitates H₂SO₄ homogeneous nucleation by reacting with SO₂ and forming H₂SO₄ vapors. In contrast, the high NO₂ concentration condition shows different particle formation phenomena from the low NO₂ one. Excess NO₂ reacts with OH to produce HNO₃, which can suppress the reaction between OH and SO₂ to form H₂SO₄, because of the faster reaction rate constant of “OH + NO₂” (550 times higher than that of “OH + SO₂”). Therefore, IIN and homogeneous nucleation are reduced simultaneously (Kim et al., 2002; Li et al., 2017). Moreover, the insufficient OH production at dry conditions (~4%RH) may enhance the suppression effect at high NO_x conditions on nucleation as well as on the subsequent particle growth. The high NO₂ concentration (e.g., 200 ppb) showed lower particle volume concentrations and smaller particle sizes than the lower NO₂ concentrations (e.g., 10 ppb) (Fig. 7(d)). When NH₃ and NO₂ were simultaneously mixed with SO₂, the suppressive effects of the two added gaseous precursors were also observed as shown in Figs. 7(e) and 7(f). Even though NH₃ and NO₂ were mixed first with SO₂, the reduction of particle growth was enhanced again as the concentrations of NH₃ and/or NO₂ increased. Interestingly, the reduction amounts of PVSDs by adding more NH₃ or NO₂ were similar between the conditions, SO₂:NH₃:NO₂ = 100:100:10 and SO₂:NH₃:NO₂ = 100:10:100, which were decreased from the PVSDs SO₂:NH₃:NO₂ = 100:10:10 (Table 4). Further studies are recommended for SO₂-NH₃-NO₂ reaction with % RH control.

 
3.4. Effects of Soft X-ray Intensity and Position on SIA Formation

Fig. 8 shows PNSDs and PVSDs at various configurations of soft X-ray emitters (number and position) installed in the 4-module reactor. By increasing the number of soft X-ray emitters from 2 to 4, the enhanced soft X-ray intensity promotes the NPF and subsequent particle growth (Figs. 8(a) and 8(c)). The four soft X-ray emitters accelerated the particle growth up to ~60 nm, while most particles were smaller than 40 nm for the two emitter conditions. At the same time, the number concentrations of particles larger than 10 nm were also boosted, which shifted the PVSDs to larger sizes with higher volume concentrations (Figs. 8(b) and 8(d)). In addition, the PVSDs was changed further depending on the installation position of soft X-ray emitters, even though the number of emitters was still the same. Compared to the PNSDs in Fig. 8(a), those at 4 (1)–(3) and 4 (3)–(4) conditions in Fig. 8(c) had higher number concentrations. The conditions including position (3) facilitated NPF as well as the subsequent particle growth more than other conditions. This was observed from the PVSDs in Fig. 8(d) at the soft X-ray configurations including the position (3). The volume concentrations at the mode diameters of the PVSDs for the configurations including position (3) were generally higher than 1.2 × 10¹⁰ (4 emitters) and 5 × 10⁹ nm³ cm^–3 (2 emitters).

Fig. 8. Measured number and volume size distributions of particles (PNSDs and PVSDs) formed in the 4-module reactor at different configurations of soft X-ray emitters (number and position) (SO₂ = 100 ppb, Q = 1.2 lpm): (a) PNSDs and (b) PVSDs for the lower concentration group, and (c) PNSDs and (d) PVSDs for the higher concentration group.

When the N_tot and V_tot are compared among the soft X-ray configuration conditions as shown in Fig. 9, N_tots and V_tots of secondary particles produced under the soft X-ray irradiation by four emitters were higher than those by two emitters. Particularly, the V_tots at four emitter conditions were more than two times greater than those at two emitter conditions. In this case, the soft X-ray intensity had higher effects on the particle growth than the soft X-ray exposure time (Huart et al., 2020), even though the total number of soft X-ray emitters was the same as 4. In terms of the soft X-ray position, the effects of the position on NPF and subsequent growth by soft X-ray radiolysis were in the order of (3) > (4) > (2) > (1). When the growth rates at different soft X-ray configurations were calculated, the 4 (3)–(4) condition showed the highest growth rates for all the size ranges (3–25, 25–50, and 3–50 nm) (Table S1). This also supports that position (3) had the highest effects on particle growth. The backward flows at the beginning of the flow reactor might influence the velocity field and the concentrations of SO₂ in the continuous tube reactor, thus the soft X-ray position (3) had a higher potential to produce the highest V_tots of secondary inorganic aerosols in the reactor (Huang et al., 2017).

Fig. 9. The total (a) number (N_tot) and (b) volume (V_tot) concentrations of H₂SO₄ nanoparticles formed in the 4-module reactor at different configurations (soft X-ray number and position) of soft X-ray emitters (SO₂ = 100 ppb, Q = 1.2 lpm).

 
4 CONCLUSION

In this study, a continuous flow tube reactor was developed to investigate NPF and subsequent particle growth through soft X-ray radiolysis. The flow tube reactor was designed to optimize the reactor length and the number of soft X-ray emitters using tube body modules. PNSDs and PVSDs formed in the reactor were measured and compared according to the parameters of NPF and particle growth, such as precursor gas concentrations, soft X-ray exposure time, residence time, mixing status of gas precursors, and soft X-ray intensity.

The longer exposure time to soft X-ray promoted particle growth in the reactor, increasing CRs for the 4-module reactor one order higher than those of the 2-module at the same SO₂ concentrations. When the residence time increased by reducing the inlet flow rates, particles were also grown, and the corresponding CR (SO₂ to H₂SO₄) was linearly correlated to the residence time for the 4-module reactor. For the multi-component experiments, NH₃ stabilized NPF in the tube reactor but suppressed the particle growth as its concentration increased at the fixed SO₂ concentration. This might be due to insufficient OH radicals due to the low %RH of gas mixtures. NO₂ also retarded the particle growth as observed in the atmospheric photochemical processes. The particle growth was suppressed more by NH₃ mixing in the reactor at dry RH conditions between NH₃ and NO₂. The soft X-ray intensity was one of the important parameters to boost particle growth. However, the enhancement effect depended on the soft X-ray installation position together, and the tendency was not matched with the expectation. The small backward flows in the inlet cone might affect NPF and subsequent particle growth in the reactor.

Further study will be conducted to modify the continuous soft X-ray tube reactor for removing backward flows in the reactor. The results will help understand the underlying mechanism of NPF and particle growth using soft X-ray radiolysis.

ACKNOWLEDGEMENTS

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science and ICT, Korea (NRF-2019R1F1A1058854 and NRF-2022R1C1C1008367). In part also, by the Fine Particle Research Initiative in East Asia Considering National Differences Project through the National Research Foundation of Korea funded by the Ministry of Science and ICT, Korea (NRF-2020M3G1A1114548).