Laboratory Study on the Effects of Pressure on Growth of Sulfuric Acid Aerosol Generated by Photo-Oxidation of SO 2

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1 Research Paper Earozoru Kenkyu, 28 4, Laboratory Study on the Effects of Pressure on Growth of Sulfuric Acid Aerosol Generated by Photo-Oxidation of SO 2 Toshiyuki FUJIMOTO 1 *, Shinya YAMANAKA 1 and Yoshikazu KUGA 1 Received 5 March 2013 Accepted 17 July 2013 Abstract In this study an experimental setup was developed that enabled us to investigate effects of pressures on generation and growth of sulfuric acid aerosol by photo-oxidation of gaseous SO 2. Changes in particle size distributions of generated aerosol at reduced and standard atmospheric pressures were measured. Particle size distributions of generated sulfuric acid aerosols were measured at the initial SO 2 concentration of 1 and 5 ppm and mean residence time of 2.31, 4.62 and 9.25 s, and the effects of pressure on the growth of sulfuric acid aerosols were investigated. The growth of sulfuric acid aerosol by coagulation as well as condensation was enhanced at reduced pressure due to the increase in particle diffusion coefficient in both transition and continuous regimes. Keywords : SO 2 Gas-to-Particle Conversion, Sulfuric Acid Aerosol, Secondary Aerosol, Photo Oxidation of SO 2, Particle Size Distribution. 1. Introduction Aerosol particles are ubiquitous in the Earth s atmosphere, and they influence life on our planet in many ways (Kulmala, 2003; Holmes, 2007; Kulmala and Kerminen, 2008). The effects of secondary aerosols are of special concern due to their long lifetime in the atmosphere. They undergo various chemical and/or physical changes during their lifetime; generation of condensable species by atmospheric reactions of gaseous precursors, nucleation, growth by coagulation and/or condensation of condensable species, transportation by diffusion and convection, and dry or wet deposition. Because the dynamics of aerosol, including sedimentation and diffusion, depend on the size of aerosol particles, effects of pressure and temperature on both generation and growth of aerosol need to be investigated. Sulfuric acid is one of the most influential gaseous chemical species in atmosphere, and is key species on secondary aerosol generation. Though ion-induced nucleation (Enghoff and Svensmark, 2008; Munir et al., 2010) and ternary nucleation (Kim et al., 1997; Nagato et al., 2005; Sorokin and Arnold, 2007; Berndt et al., 2010) may enhance the nucleation rate (Kirkby et al., 2010), the homogeneous binary 1 College of Environmental Technology, Muroran Institute of Technology Mizumoto-cho 27-1, Muroran , Japan * Corresponding Author. fjmt@mmm.muroran-it.ac.jp (T. Fujimoto) nucleation remains important. Since recent progress of aerosol diagnostic tools enable to detect new particle generation (Weber et al., 1999; Kulmala et al., 2004), many groups have tried to reproduce new particle formation in laboratory experiments; binary homogeneous nucleation from sulfuric acid vapor generated from a liquid sample and water (Wyslouzil et al., 1991; Viisanen et al., 1997), and binary homogeneous nucleation from sulfuric acid generated by the reaction of SO 2 and OH radicals (Ball et al., 1999; Kobara et al., 2000; Berndt et al., 2006; Young et al., 2008; Benson et al., 2008; Sipilä et al., 2010; Brus et al., 2010; 2011). In these studies, experiments were carried out at room temperature and ordinary atmospheric pressure by using a flow tube reactor because they have mainly focused on the binary homogeneous nucleation rate derived by dividing aerosol concentration by aerosol generation time. Condensation particle counters (CPCs) are used to measure the generated particle concentration (Ball et al., 1999; Brus et al., 2010; 2011). Differential mobility analyzers (DMAs) coupled with various CPCs are also commonly used; Kobara et al. (2000) used a SMPS (TSI, Model 3934), Young et al. (2008) and Benson et al. (2008) used a Nano-DMA/UWCPC, Berndt et al. (2006; 2010) used a Vienna-type DMA and a butanol-based UCPC (TSI 3025). They allowed measuring particle size distribution (PSD) to obtain the growth rate of aerosol particles; however, condensation was the dominant growth mechanism since aerosol concentration was too low for coagulation. Vol. 28No

2 Previously, we have reported aerosol generation and subsequent growth via binary homogeneous nucleation in a sulfuric acid-water system at ordinary atmospheric pressure (Fujimoto et al., 2012). In this work, we have carried out experiments at ordinary atmospheric pressure and reduced pressures. For the experiments at reduced pressures, a lowpressure differential mobility analyzer (LPDMA) combined with a Faraday-cup electrometer (FCE) (Seto et al., 1997) was employed as an aerosol diagnostic tool, because CPCs do not function at reduced pressures. Munir et al., 2010 also used FCE as an aerosol diagnostic tool at low temperature and/or low pressure conditions. Nanometer-sized aerosol particles are easily captured by filter element in the FCE, and then detected as electric current result from the electric charges of aerosol particles. The current was so small that the electrical noise made our detection difficult. The LPDMA- FCE system enabled us to investigate the sulfuric acid aerosol generation at reduced pressure, however it set limit our experimental conditions. Our experiments on the generation of sulfuric acid aerosol were carried out at the SO 2 concentration of 1 5 ppm, that was much higher than background concentration of SO 2 (1 3 ppb) observed at Fukue island (Seto et al., 2013). As pointed out by Wyslouzil et al. (1991), the nucleation process is sensitive to temperature. Hence the reaction chamber was placed in a water bath connected to temperaturecontrolled circulators; a pressure control system and a vacuum pump were used to reduce the reactor pressure. The experimental system had good reproducibility and enabled us to investigate the effects of pressure on the generation and growth of sulfuric acid aerosol by changing the pressure of reactor and residence time. In order to understand the growth of sulfuric acid aerosol in the atmosphere, the effects of temperature and simultaneous effects of both temperature and pressure should be investigated. Investigations of these effects and quantitative discussions are our future works. 2. Experimental Schematic diagram of an experimental setup is shown in Fig. 1. Our experimental setup has been described elsewhere (Fujimoto et al., 2012), and is described briefly here. It consists of three main parts: i) a continuous flow gas-generation system, ii) a reactor, and iii) an aerosol diagnostics system. Flow rates of compressed oxygen, SO 2 standard gas of 50 ppm balanced by nitrogen, and nitrogen were set with mass flow controllers MFC1, MFC2, and MFC3, respectively. A part of nitrogen flow was passed through a water bubbler filled by deionized water, which was placed in a water bath to saturate with water vapor. All flows were mixed with an in-line type static mixer (G1/4-6, Noritake Co., Ltd.). The flow rate through the reactor was controlled by MFC4 at the outlet of the static mixer; gas stream required for the experiments was delivered to the reactor, and unused gas was delivered to SO 2 analyzer (model i43, Thermo Scientific), and excess gas was purged through a purge valve. The purge valve was used to elevate the pressure upstream the MFC4 up to approximately 1.2 atm to provide the differential pressure required by the MFC4. The dew point of the SO 2 /H 2 O/O 2 /N 2 gas stream was measured by a chilled mirror hygrometer (Dew Star S1, Shinei) Fig. 1 Schematic diagram of experimental apparatus used to investigate the effects of pressure on the growth of sulfuric acid aerosol

3 and was converted to relative humidity taking into account the pressure measured by a pressure transducer (PTX7511-1, Druck Industrial). The reactor and a coiled stainless steel tube (1/8 inch in outer diameter and 1 m in length) immediately near the reactor were placed in a water bath filled with cooling water. The temperature of cooling water was kept at 25 C. Details of the reaction chamber are shown in Fig. 2. The reaction chamber equipped with two opposite windows made of fused silica is cross-shaped in a top view, and is electrolytically plated on the inside. The light ray from a UV source (SX-UID500MAMQQ, Ushio Inc.) passes through a window and activates radical reactions and subsequent atmospheric reactions, then leaves through another window. The power was monitored by a UV power meter during the experiments and was kept at constant value of 4 W. The generated sulfuric acid aerosols pass through short outlet part made of 1/4 stainless steel tube into the diagnostics part directly. They were charged by a bipolar diffusion charger with a 241 Am source. They were size classified and detected by a low-pressure type differential mobility analyzer and a Faraday cup electrometer system (LPDMA-FCE; type III, Wyckoff Co.). A sheath flow of the LPDMA was circulated by a diaphragm pump through tubing that supplies two air filters and two 10-L surge tanks both upstream and downstream of it. Downstream of the FCE, another 10-L surge tank was connected in order to dampen vibration of the sample flow. The pressure of the reactor and LPDMA-FCE system was controlled by a pressure control valve (model 653B, MKS Instruments) coupled with a pressure control unit (model 600, MKS Instruments) and a pressure gauge (Baratoron Type 626B, MKS Instruments, Inc.). UV radiation of wavelengths less than 242 nm as well as 320 nm was emitted by the UV source used in this study (Fig. 3). Thus, the possible reactions (Berndt et al., 2006) for the formation of H 2 SO 4 in our experiments are as follows: 1) oxygen atoms are generated by the photo dissociation by UV ray (λ242 nm), and ozone molecules are generated by a reaction of oxygen atoms with oxygen molecules, 2) ozone molecules are also dissociated by UV ray (λ 320 nm), and excited oxygen atom are generated, 3) OH radicals are generated by reaction of the excited oxygen atoms and water molecules, 4) OH radicals attack SO 2 producing HSO 3, and 5) HSO 3 transform to H 2 SO 4 via reactions with O 2 and H 2 O. Since the reaction 4) is the rate limiting step (Eisele and Tanner, 1993), we assumed that the formation of gas phase H 2 SO 4 depends on UV flux and are independent of the pressure. Conditions of the experiments conducted to investigate the effects of pressure are listed in Table 1. The temperature was kept at 25 C, the reactor pressure was reduced from 1013 hpa to 600 hpa. Total flow rate at 1013 hpa, Q 1013 was set at 2.0 SLM, 1.0 SLM, and 0.50 SLM (volumetric flow rate at 0 C and 1013 hpa). The actual flow rate, Q was calculated by the following equation in order to keep residence time constant while reducing pressure: P Q Q , (1) where Q (SLM) and P (hpa) are the flow rate and the pressure, respectively. Here, the actual volumetric flow rate Q (L min -1 ) at 25 C and under the pressure of P was kept constant at Q' Q P P Q1013 (2) P Q Fig. 2 Detail of a reactor used in the experiments. Fig. 3 Spectrum of UV light. Vol. 28No

4 Table 1 Flow rate and humidity at the experiments carried out at the reduced pressure conditions Pressure [hpa] (a) τ2.31 s Flow Rate, Q [SLM] (b) τ4.62 s (c) τ9.25 s H 2 O Fraction [ppm] 9,000 9,000 9,000 9,000 9,000 Partial Pressure of H 2 O [hpa] Relative Humidity 28.8% 25.6% 22.7% 19.9% 17.0% Mean residence time estimated by dividing the substantial volume of reactor that included the volume of photo reaction (42.6 cm 3 ) and aerosol growth (41.5 cm 3 ) as shown in Fig. 2 by volumetric flow rate, Q were 2.31, 4.62, and 9.25 s at the flow rate Q 1013 of 2.0, 1.0, and 0.5 SLM, respectively. In our experiments, the fraction of oxygen was 20%. Volumetric fraction of SO 2 was set at 5 ppm or 1 ppm, and H 2 O vapor fraction was set at 9000 ppm. Considering the uprising of an air mass, the partial pressures of SO 2 and H 2 O vapor were reduced as pressure was reduced. Saturated water vapor pressure of H 2 O, p* (hpa) was calculated by following equation (Tetens, 1930) as a function of temperature, T (K), 75. T p T * (3) At 1013 hpa, the partial pressure of H 2 O vapor was set to 9.12 hpa (9,000 ppm) and relative humidity was 28.8 RH%. At the reduced pressure conditions, H 2 O vapor fraction was maintained at 9,000 ppm, and was decreased; it was 5.4 hpa (17.0%RH) at 600 hpa. 3. Results and Discussion 3.1 Reproducibility of measurements First of all, the reproducibility of measurements of PSDs was evaluated. Fig. 4 shows the measured PSDs at 1013 hpa and 600 hpa. Experiments were conducted three times in 3 weeks at the total flow rate Q 1013 of 1.0 SLM and SO 2 fraction of 1 ppm. Both positively and negatively charged aerosol particles were measured by the LPDMA-FCE system, and PSDs were obtained by inverse transformations. As shown in Fig. 4, a fluctuation of three PSDs measured at 1013 hpa are not significant; however, one can see considerable fluctuation due to the relatively small S/N ratio in detection of low concentration aerosol by the FCE at the pressure of 600 hpa. Diameters of average volume and total number concentrations measured in three experiments and their coefficients of variation (CV) were listed in Table 2. Coefficients of variation of both diameter of average volume and total number concentration were not significant. 3.2 Effects of pressure PSDs measured at the SO 2 fraction of 1 ppm and 5 ppm are shown in Figs. 5 and 6, respectively. These experiments Fig. 4 Particle size distributions measured three times in 3 weeks at the residence time of 2.31 s. were carried out between 1013 and 600 hpa, and the residence time of 2.31, 4.62, and 9.25 s. The dependences of total particle concentration and diameter of average volume on the pressure are also shown in Figs. 7 (a) and (b), respectively. The Brownian coagulation coefficient β(d pi, D pj ) is expressed as 2 ( Dpi, Dpj) Dpi Dpj cij (1) 4 in free molecular regime and ( Dpi, Dpj) 2 Di Dj Dpi Dpj (2) in continuous regime. Here D pi and D pj are particle diameters of i and j particle, respectively. D i and D j are diffusion coefficients of i and j particle, respectively. c ij is the thermal velocity between i and j particle. Fuchs (1964) interpolated between them as ( D, D ) 2 pi pj Di DjDpi Dpj D pi pj i j Dpi Dpj gij cij Dpi Dpj 1 D 8 D D 2. (3)

5 Table 2 Diameter of average volume and total number concentration measured in three experiments and the coefficient of variation Diameter of average volume, D V [nm] Total number concentration, N [cm -3 ] Pressure [hpa] Pressure [hpa] Run No C.V Charging polarity Charging polarity Charging polarity Charging polarity Neg % Pos % Neg % Pos % Neg % Pos % Neg % Pos % Fig. 5 Change in particle size distributions by changing the residence time and pressure at the SO 2 fraction of 1 ppm. Fig. 6 Change in particle size distributions by changing the residence time and pressure at the SO 2 fraction of 5 ppm. Vol. 28No

6 Fig. 7 Pressure dependence of a) total number concentration and b) diameter of average volume. Here, g ij is Fuchs length between particles i and j written as g ( g g ) ij i j. (4) Fuchs length g i is ( D l) ( D l ) gi 3Dl pi i pi i pii D pi. (5) Here, l i is mean free path of particle. By using Eq. (3), the Brownian coagulation coefficient was increased as reducing pressure due to increase in diffusion coefficients in the continuous regime and transient regime as shown in Fig. 8. At the mean residence time of 2.32 s shown in Figs. 5 (a) and 6 (a), particle number concentrations were very high, and pressure had only a small effect on the PSDs; the effect was almost negligible at the SO 2 fraction of 1 ppm (Fig. 5 (a)), and the number of nanometer-sized particles is lower at reduced pressure at the SO 2 fraction of 5 ppm (Fig. 6 (a)). The total particle number concentration N, calculated by adding each particle-size fraction, was approximately 10 7 cm -3, and the diameters of average volume of particles were approximately 20 nm and 28 nm for the SO 2 fraction of 1 ppm and 5 ppm, respectively as shown in Figs. 7 (a) and (b). From the resulting high particle number concentration, it could be that aerosol particles were undergoing coagulation; the coagulation rate was approximately 10 5 cm -3 s -1 by the product of the coagulation coefficient (approx cm -3 s -1 as shown in Fig. 8) and the square of the particle number concentration (approx. (10 7 cm -3 ) 2 ). For the particles smaller than several 10 nm, the Brownian coagulation is dominated by particle motion in free molecule regime and is independent of pressure. At the initial SO 2 fraction of 5 ppm (when the nucleation rate can be assumed to be higher than 1 ppm), average volume increases, despite total number concentration slightly Fig. 8 Change in the Brownian coagulation coefficient at reduced pressure calculated by Fuchs formula increasing. This can be explained by the rapid coagulation at high particle number concentration. The PSDs obtained at the mean residence time of 4.62 s by decreasing the flow rate Q 1013 to 1.0 SLM are shown in Figs. 5 (b) and 6 (b). The simultaneous decrease in aerosol number concentration and growth of particles were observed, and decrease in the total number concentration and increase in the diameter of average volume were also shown in Figs. 7 a) and b). These are characteristic for coagulational growth and enhanced in lower pressure. As shown in these figures, large part of particles was in the transition regime. The coagulation coefficient in the transition regime is large, and enhanced in reduced pressure as shown in Fig. 8. Therefore, marked change in PSDs along with pressure was observed, and coagulation was enhanced at lower pressure conditions. The PSDs at the mean residence time of 9.25 s, obtained by decreasing the flow rate Q 1013 to 0.5 SLM are shown in Figs. 5 (c) and 6 (c). Comparing with the results at the mean

7 residence time of 4.62 s, the total number concentration decreased and the average volume increased; however, the effects of the pressure are immaterial. Since the total number concentration went down to around 10 5 cm -3, the coagulation rate was also reduced to around 10 cm -3 s -1 ( ). The resulting PSD had rapid coagulation in initial stage of aerosol generation, and growth and subsequent uptake of water and sulfuric acid vapor. The total number concentrations measured in our study were much higher than those of Berndt et al. (N10 4 cm -3, 2010) and Brus et al. (N10 5 cm -3, 2010); coagulation of particles was not dominant in their experiments. Young et al. (2008) measured the PSD with a SMPS/UWCPC at the SO 2 initial condition of at level identical to ours. They did not show the total number concentration in their paper explicitly; however, we can estimate their maximum concentration from one of their figures. The highest aerosol concentration they observed was cm -3 at the residence time of 54 s at initial SO 2 fraction of 11 ppm and relative humidity of 23%. Their particle size distributions were in the free molecular regime, and ranged approximately 5 12 nm. Benson et al. (2008) carried out similar experiments and measured PSDs, and we can estimate their maximum concentration as cm -3 at the residence time of 38 s and relative humidity of 30% at their maximum initial SO 2 fraction of 7.7 ppm. Their particle size distributions were also in the free molecular regime, and ranged approximately 5 12 nm. The total number concentrations measured in our experiments were maximum ones; previous studies described above were carried out at the conditions where aerosol concentrations were not enough to be affected by coagulation. They showed that aerosol size was proportional to the logarithm of RH, because they measured the particle growth solely by condensation. 4. Conclusions Laboratory experiments on the photo oxidation of sulfuric dioxide and subsequent particle generation and growth were carried out by using a unique experimental system, which enabled us to conduct reproducible experiments. By changing the pressure of the reaction chamber, the effects of pressure on the generation and growth of sulfuric acid aerosol were investigated. The reduction of pressure enhanced the growth by coagulation, since the coagulation rate coefficient under both transition and continuous regime increased. In our experiment, we observed a rapid coagulation of sulfuric acid aerosol particles; however, the rapid growth of aerosol was not observed in previous studies and aerosol growth was dominated by water condensation. This is because the earlier work was carried out at low aerosol concentrations, with a focus on the measurement of the binary homogeneous nucleation rate. 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