Field test of a porous-metal denuder sampler

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Field Test of a Porous-Metal Denuder Sampler

Chuen-Jinn Tsai a , Cheng-Hsiung Huang b , Yao-Chi Lin a , Tung-Sheng Shih c & Bow-Huei Shih a

a

Institute of Environmental Engineering , National Chiao Tung University , Hsinchu, Taiwan b

Department of Environmental Engineering and Health , Yuanpei University of Science and Technology , Hsinchu, Taiwan

c

Institute of Occupational Safety and Health , Council of Labor Affairs , Taipei, Taiwan Published online: 30 Nov 2010.

To cite this article: Chuen-Jinn Tsai , Cheng-Hsiung Huang , Yao-Chi Lin , Tung-Sheng Shih & Bow-Huei Shih (2003) Field Test of a Porous-Metal Denuder Sampler, Aerosol Science and Technology, 37:12, 967-974, DOI: 10.1080/02786820300901

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American Association for Aerosol Research ISSN: 0278-6826 print / 1521-7388 online

DOI: 10.1080/02786820390233188

Field Test of a Porous-Metal Denuder Sampler

Chuen-Jinn Tsai,

_{Cheng-Hsiung Huang,}

_{Yao-Chi Lin,}

_{Tung-Sheng Shih,}

and Bow-Huei Shih

1_{Institute of Environmental Engineering, National Chiao Tung University, Hsinchu, Taiwan}

2_{Department of Environmental Engineering and Health, Yuanpei University of Science and Technology,} Hsinchu, Taiwan

3_{Institute of Occupational Safety and Health, Council of Labor Affairs, Taipei, Taiwan}

In this study, acidic/basic gas and particle concentrations de-termined with a porous-metal denuder and other samplers, that is, silica gel tube, impinger, honeycomb denuder system (HDS), Marple personal cascade impactor, and filter cassette, were com-pared in the ambient environment and different industrial sites in Taiwan. Results show that the concentrations determined by the denuder are very close to those of other samplers, with excellent correlation. The denuder also was tested successfully to determine H2SO4 size distribution in a lead-acid factory. One-way ANOVA

shows no significant differences (P > 0.05) between the porous-metal denuder (PDS) and other samplers. The denuder is compact in size and suitable for sampling in the workplace and ambient environment.

INTRODUCTION

A diffusion denuder is a sampler that removes gases from an aerosol stream to measure their concentrations separately. Gas or vapor molecules diffuse rapidly to the wall of a diffusion sampler and adsorb onto the wall coated with a suitable material. The gas concentration can be determined by extracting the coated substrates and analyzing the samples (Koutrakis et al. 1993; Poon et al. 1994; Possanzini et al. 1983; Pui et al. 1990; Sioutas et al. 1996).

Various denuders were designed and reported in the last 20 years. Pui and colleagues (1990) designed a compact coiled denuder and compared the performance with an annular de-nuder (Possanzini et al. 1983). Koutrakis and colleagues (1993)

Received 16 November 2001; accepted 15 July 2002.

The authors thank the Taiwan Institute of Occupational Safety and Health (IOSH), Council of Labor Affairs, for the financial support of this project under contract number IOSH89-A102.

Address correspondence to Chuen-Jinn Tsai, No. 75 Poai St., Hsinchu, Taiwan. E-mail: [email protected]

and Sioutas and colleagues (1996) developed a glass honey-comb denuder/filter pack system to collect atmospheric gases and particles. The system is considerably smaller than the annu-lar denuder system and can be used easily for annu-large field studies. Poon and colleagues (1994) developed a high-efficiency com-pact diffusion denuder using porous-metal discs. The smaller size of the denuder makes it possible to design a compact atmo-spheric and/or indoor denuder sampling system. Using the same porous-metal discs (diameter: 2.54 cm, pore size: 100µm, thick-ness: 0.317 cm, P/N 1000, Mott Inc., Farmington, Connecticut), Tsai and colleagues (2001a) designed and tested a PDS in the laboratory. The entire casing and substrate support were made of Teflon, and sampling flowrate was fixed to be 2 L/min. The sampler consists of a two-stage cascade impactor (having cutoff aerodynamic diameters of 9.5 and 2.0µm, respectively) to lect liquid particles followed by two porous-metal discs to col-lect basic and acidic gases, respectively. The denuder was tested for gas collection efficiency and capacity at gas concentration two times the permissible exposure limit (PEL, promulgated by Taiwan IOSH, Institute of Occupational Safety and Health). The test data indicated that the gas collection efficiency was high and the capacity was sufficient for the acidic/basic gas sampling in the workplace.

Tsai and colleagues (2001b) compared the collection effi-ciency and capacity of the denuder with the silica gel tube method and an additional impinger method at gas concentration two times the PEL in the laboratory. The collection efficiencies of the PDS coated with 5% sodium carbonate/glycerin on the PDS, for nitric acid, hydrogen chloride, and hydrogen fluoride were higher than those of the silica gel tube and the impinger when the sampling time was less than 3 hrs. For ammonia, the performance of the PDS, silica gel tube, and impinger was al-most the same.

The PDS is considerably more compact, simpler in design, and easier to handle than the annular denuder system and the HDS for field sampling. In addition, the PDS can measure

967

968 C.-J. TSAI ET AL.

individual concentrations of gas and particulate simultaneously. In comparison, the Marple personal cascade impactor and fil-ter cassette usually are used for particulate concentration mea-surement, and silica gel tube and impinger usually are used to measure gas concentration only. The silica gel tube is used for the collection of six inorganic acids including hydrofluo-ric, hydrochlohydrofluo-ric, phosphohydrofluo-ric, hydrobromic, nithydrofluo-ric, and sulfuric acids in a single sample, and the analysis is done by ion chro-matography at the National Institute of Occupational Safety and Health (NIOSH 1994). The impinger is used in Method 2401 for sampling ammonia gas at the Taiwan Institute of Occupational Safety and Health (TIOSH 1994).

In this work the new PDS was designed and tested for mon-itoring acidic/basic gases and aerosols at the ambient environ-ment around the Hsinchu Science Park and different industrial sites in Taiwan. The measured concentrations were compared with those obtained with other sampling techniques, that is, sil-ica gel tube, filter cassette, impinger, and HDS. The denuder was tested also for size distribution of sulfuric acid particles in a lead-acid battery plant and compared with that measured by the eight-stage Marple personal cascade impactor (Rubow et al. 1987).

TEST SITES AND METHODS

Sampler Design

The denuder tested has a five-stage cascade impactor in front of two denuder discs. The first porous-metal disc collects acidic gases, while the second collects basic gases. The schematic di-agram is shown in Figure 1. The inner diameter of the denuder is 30.6 mm, and the total length is 136 mm. Each stage of the cascade impactor has a single round nozzle whose diameter is 7.2, 4.8, 3.6, 2.6, and 1.9 mm for stages 1 to 5, respectively. The cutoff aerodynamic diameter of stages 1 to 5 was tested to be 9.5, 6.7, 4.8, 3.2, and 2.0µm, respectively. The sampling flowrate is 2 L/min. The inlet of the denuder is an annular slot based on the design of Wistchger and colleagues (1997). The gap of the slot is determined by the thickness of four pins.

Figure 1. Schematic diagram of the porous-metal denuder sampler.

Ambient Environment

To compare the ambient concentrations of inorganic acidic/ basic aerosols and gases, the new PDS and HDSs were collo-cated at 1 m height at the surroundings of the Hsinchu Science Park in Taiwan, for a sampling period of 12.0 hrs. In the study, the substrates of the five-stage impactor were coated with silicon grease to prevent particle bounce. The impactor samples were not analyzed for ionic concentrations because of contamination by silicon grease. Instead, the concentration of fine particles col-lected after the filter of the denuder was analyzed and compared with those determined by the HDS.

Industrial Test Sites

(i) Lead-Acid Battery Factory

The new five-stage impactor PDS was tested with a silica gel tube and filter cassette for total sulfuric acid concentrations at the forming area of a lead-acid battery factory. The sulfuric acid droplets were collected on various impactor stages of the denuder, and the total concentration was the sum of the con-centrations at the five stages. The Zefluor filter with a 37 cm diameter (2.0µm pore size, Gelman Science) was used to col-lect acidic aerosol on the filter cassette at a flowrate of 2 L/min. The samplers were placed 0.5 m away from the forming trough and a distance of 1 m above the ground, for a sampling pe-riod of 1.0 hr. The size distribution of liquid sulfuric acid also was sampled simultaneously by a Marple personal impactor and compared with that determined by the PDS. Since liquid particles do not bounce, substrates were not coated (PE filters for the Marple personal impactor, and porous-metal substrates for the five-stage impactor of the denuder) and were extracted by the deionized distilled water. The Marple personal impactor was an eight-stage impactor with cut points ranging from 0.5µm to 21µm in aerodynamic diameter. The after filter of the Marple personal impactor was a PVC filter. The sampling flow rate is 2 L/min.

(ii) Sulfuric Acid Production Plant

This factory mainly produces 98% industrial and 125% fum-ing sulfuric acid. The samplers, that is, the new denuder, silica

gel tube, and filter cassette were placed about 1 m away from the storage tank of the sulfuric acid and 1.5 m above from the ground for a sampling period of 2 hrs. In this factory, the con-centration of sulfuric acid mist was comparatively low, because it was an outdoor sampling.

(iii) Semiconductor Factory

The samplers, including the new PDS, HDS, and silica gel tube were set up near the wastewater treatment plant, outside a semiconductor factory. Hydrofluoric acid was used to clean the wafer in the factory. For the treatment of wastewater, this factory uses CaCl2as a coagulator to capture F−ions and thus

form CaF2. Hydrochloric acid is added to the wastewater treating

process in order to increase the coagulation efficiency in the pH range 5 to 7. Therefore, at the field site some acidic gases like HCl and HF apparently existed. The samplers were placed about a distance of 1 m above and away from the hydrofluoric acid wastewater reaction tank for 8-hr sampling.

(iv) Fertilizer Factory

This fertilizer factory produces melamine by injecting the urea into the reactor filled with ammonia gas. The samplers, including the new PDS, HDS, and impinger were placed near the ammonia storage tank with a distance of 1 m away and 1 m above the ground for 1-hr sampling. The study was mainly aimed at comparing the ammonia gas concentrations of different samplers.

Laboratory Analysis

(i) Porous-Metal Denuder

The porous-metal discs were coated with different solutions. For acidic gases, 10 ml, 5% (w/v) sodium carbonate, 1% (w/v) glycerol in 1:1 (v/v) methanol/water solution was used. For am-monia gas, 10 ml, 4% (w/v) citric acid in ethanol was used. The coating solution concentrations were higher than that of the de-nuder used for atmospheric sampling (Poon et al. 1994; Sioutas 1996) since the latter was found to be insufficient for the high gas concentration in the workplace. After coating, the porous-metal discs were dried by passing nitrogen gas through them. After sampling, the porous-metal discs were extracted with deionized distilled water in a pressure chamber (at 0.2 atm). The low-pressure chamber was used to help drive out air bubbles trapped in the porous-metal discs so that the adsorbed species can be extracted completely.

(ii) Honeycomb Denuder System

The components of the HDS include an impactor with the cutoff aerodynamic diameter at 2.5µm, a glass-transition sec-tion, two honeycomb denuders, a spacer, and a filter pack. The flowrate of the HDS is 10 L/min. The honeycombs of the HDS were coated using 1% (w/v) sodium carbonate, 1% (w/v) glyc-erol in a 1:1 (v/v) methanol/water solution for acid gases. For ammonia gas, 1% (w/v) citric acid in ethanol was used. A

three-stage filter pack was placed downstream of the denuders. The filter pack consists of a Teflon filter (Gelman Science, 2-µm pore size) to collect fine particles, a nylon filter (Gelman Sci-ence, 1-µm pore size) to collect HNO3and HCl, and a glass fiber

filter (AP40, Millipore Inc.) coated with citric acid to collect NH3 that volatilized from the collected particles on the Teflon

filter. The concentrations of the samples were determined by an ion chromatograph (Model 4500i, Dionex Corp., California).

(iii) Silica Gel Tubes

The commercially available silica gel tubes (SKC 226-10-03, SKC, Inc.) were used for sampling inorganic acids. The silica gel tubes contained two sections of washed silica gel (first section: 400 mg placed by a thick glass fiber filter plug, second section: 200 mg retained by a urethane foam plug). The sampling flow rate was 0.5 L/min. The samples of the silica gel were analyzed according to the inorganic acids method (NIOSH Method 7903). In this study, the Quality Assurance and Quality Control (QA/QC) procedure includes establishment of a calibration curve using standard solutions and a method detection limit (MDL), blank analysis, repeated analysis, and spike sample analysis. The MDL was determined as three times the standard deviation of repeated analysis at five times the lowest possible standard con-centration. The MDL was 0.2, 0.11, 0.02, 0.05, 0.07, 0.09 ppb for HF, HCl, HNO2, HNO3, H2SO4, and NH3 gases,

respec-tively, based on 12-hr sampling at 2 L/min. The blank values of porous-metal disc for the ion species were nondetectable. The results of precision analysis showed that the relative bias of detected values was below 5%. The recovery efficiencies were estimated using spike samples with the concentrations of two times the permissible exposure limit promulgated by Taiwan IOSH based on 15-min sampling at 2 L/min. The cor-responding recovery efficiency from the porous-metal disc for the ion species F−, Cl−, NO−₂, NO−₃, SO2−₄ , and NH+₄ were 95.3 ± 2.1, 97.9 ± 0.9, 97.3 ± 0.8, 96.6 ± 0.7, 95.3 ± 1.4, and 96.9 ± 1.3%, respectively.

RESULTS AND DISCUSSION

Particulate Concentrations

(i) Ambient Environment

In Figures 2a–2f we show that fine particle F−, Cl−, NO−₂, NO−₃, SO2₄−, and NH+₄ concentrations measured with the PDS plotted against those measured by the HDS in the ambient air of Hsinchu Science Park. The F−, Cl−, NO−₂, NO−₃, SO2−₄ , and NH+₄ concentrations determined by the PDS and the HDS were highly correlated, withR2_{of 0.992, 0.996, 0.998, 0.999, 0.983,}

and 0.988, respectively. One-way ANOVA showed no significant differences (P> 0.05) for the six replicate samples taken with

the PDS and HDS. The particulate concentrations and mean ratios of PDS and HDS are shown in Table 1. The mean values are close to 1, indicating the measurements are in agreement.

(ii) Lead-Acid Battery Factory

970 C.-J. TSAI ET AL.

Figure 2. Fine particle (a) F−, (b) Cl−, (c) NO−₂, (d) NO₃−, (e) SO2₄−, (f ) NH+₄ concentrations measured with the PDS in comparison with the HDS in the ambient air around the Hsinchu Science Park.

Figures 3a and 3b show the size distribution of the sulfu-ric acid mist in the lead-acid battery factory. The mass median aerodynamic diameter (MMAD) measured with the PDS and Marple personal cascade impactor was 6.92µm (σg = 1.27)

and 6.99 µm (σg = 1.45), respectively. It indicates that the

particle distribution has a single mode and the particles mainly distributed in 6.0–9.8 µm stage of the Marple personal cas-cade impactor in the sampling site. Similarly, the sulfuric acid

(a)

(b)

Figure 3. Sulfuric acid particle size distribution measured

with (a) PDS, (b) Marple personal cascade impactor in the lead-acid battery factory.

mist distributed in 6.7–9.5 µm stage of the PDS. Figures 4a and 4b show the comparative study of total concentrations of sulfuric acid mist in the lead-acid battery and the sulfuric acid factory sampled with the PDS and the silica gel tube and fil-ter cassette. For the lead-acid batfil-tery, the results indicate that the PDS has high correlation with other samplers with correla-tion coefficients 0.998 and 0.976 for the silica gel tube and the filter cassette, respectively. With the PDS, the sample concen-tration obtained was 1134.2 ± 136.4 µg/m3_{(average± standard}

deviation), whereas with the silica gel tube and filter cassette it was 1151.8 ± 137.9 and 1157.3 ± 135.9 µg/m3_{, respectively.}

Table 1

Comparison of particulate concentrations and mean concentration ratios of PDS and HDS

Average concentration± SD, µg/m3 Species PDS HDS Average concentration ratio± SD PDS/HDS F− 0.82± 0.86 0.90± 0.97 1.00± 0.14 Cl− 2.23± 1.21 2.39± 1.34 0.95± 0.10 NO−₂ 0.43± 0.56 0.43± 0.56 0.96± 0.09 NO−₃ 2.66± 1.40 2.68± 1.39 1.00± 0.11 SO2−₄ 7.17± 3.90 7.13± 3.20 0.98± 0.12 NH+₄ 2.83± 2.13 2.87± 1.99 0.96± 0.10

Figure 4. Total sulfuric acid mist concentration measured

with the PDS in comparison with the (a) silica gel tube and (b) filter cassette in the lead-acid battery factory and sulfuric acid factory.

972 C.-J. TSAI ET AL.

One-way ANOVA showed no significant differences (P > 0.05)

for six replicate samples of the forming area of the lead-acid battery factory, monitored with the PDS, silica gel tube, and filter cassette. The filter of the cassette was extracted with 3 ml

Figure 5. (a) HF, (b) HCl, (c) HNO2, (d) HNO3, (e) SO2, (f ) NH3concentrations measured with the PDS in comparison with the

HDS in the ambient air around the Hsinchu Science Park.

of solution containing 10−4 N HClO4 and 0.04 M KCl for pH

meter. Concentrations of H+ were then calculated by the pH of the sample using a standard calibration curve determined from known concentrations of H2SO4. The nmol/nmol ratios of

2[SO2−₄ ]/[H+] are close to 1, indicating that the field particles mainly were of sulfuric acid mist and the H2SO4existed in the

droplet form.

(iii) Sulfuric Acid Factory

For the sulfuric acid factory, Figures 4a and 4b show that the H2SO4acid mist concentrations measured by the PDS were

found to be very close to the silica gel tube and filter cas-sette. The R2 _{for the PDS with the silica gel tube and filter}

cassette was found to be 0.998 and 0.999, respectively. For the PDS, the sample concentration was 806.2 ± 5.7 µg/m3

(aver-age± standard deviation), whereas for the silica gel tube and filter cassette, the concentrations were 807.9 ± 21.5 µg/m3_and

819.1 ± 13.7 µg/m3_{, respectively. One-way ANOVA showed no}

significant differences (P> 0.05) for six replicate samples of the

sulfuric acid factory, sampled with the PDS, silica gel tube, and filter cassette.

Gas Concentrations

(i) Ambient Environment

The HF, HCl, HNO2, HNO3, SO2, and NH3 concentrations

measured with the PDS plotted against those measured by means of the HDS in the ambient of the Hsinchu Science Park are shown in Figures 5a–5f, and the data obtained were in excellent correlation, withR2 _{of 0.998, 0.998, 0.992, 0.980, 0.998, and}

0.995, respectively. One-way ANOVA showed no significant differences (P > 0.05) for the six replicate samples taken with

the PDS and HDS. The gas concentrations and mean ratios of PDS and HDS are shown in Table 2. The mean values are close to 1, indicating the measurements are in agreement.

(ii) Semiconductor Factory

In the wastewater treatment plant of the semiconductor fac-tory, Hydrogen Fluoride (HF) gas concentrations measured with the PDS, HDS, and silica gel were found to be 0.00357 ± 0.00155, 0.00352 ± 0.00152, and 0.00345 ±0.00145 ppm (average± standard deviation), respectively, from an 8-hr sam-pling, as shown in Figures 6a and 6b. The R2_{for the PDS with}

the HDS and silica gel tube was 0.995 and 0.998, respectively.

Table 2

Comparison of gas concentrations and mean concentration ratios of PDS and HDS Average concentration± SD, ppb Species PDS HDS Average concentration ratio± SD PDS/HDS HF 3.71± 2.44 3.48± 2.25 1.07± 0.06 HCl 4.60± 2.61 4.46± 2.62 1.04± 0.08 HNO2 1.11± 0.76 1.09± 0.70 1.01± 0.09 HNO3 1.48± 1.16 1.52± 1.15 0.99± 0.12 SO2 5.83± 2.81 5.93± 3.01 1.00± 0.08 NH3 11.14± 3.50 11.24± 3.26 0.98± 0.07

Figures 6a and 6b also show that HCl concentrations were 0.0067 ± 0.00321 ppm (average ± standard deviation) for the PDS, 0.00654 ± 0.00301 ppm for the HDS, and 0.00654 ± 0.00293 ppm for the silica gel tube. The R2_{for the PDS with the}

HDS and silica gel tube was 0.998 and 0.995, respectively. One-way ANOVA showed no significant differences (P > 0.05) for

six replicate samples of the semiconductor factory, monitored with the PDS, silica gel tube, and HDS.

(iii) Fertilizer Factory

The results of NH3concentration sampled by the PDS,

com-pared with the HDS and impinger are shown in Figures 7a and 7b, respectively. The results indicate the NH3concentration

mea-sured with the PDS was 2.132 ± 0.049 ppm (average ± standard deviation), whereas with the HDS and impinger it was 2.095 ± 0.054 and 2.155 ± 0.062 ppm, respectively. The R2 _was

Figure 6. Hydrofluoric and hydrochloric acid gas

concentra-tions measured with the PDS in comparison with the (a) HDS and (b) silica gel tube in the semiconductor factory.

974 C.-J. TSAI ET AL.

Figure 7. Ammonia gas concentration measured with the PDS

in comparison with the (a) HDS and (b) impinger in the fertilizer factory.

calculated for the PDS with the HDS and impinger and found to be 0.999 and 0.999, respectively. One-way ANOVA shows no significant differences (P> 0.05) for six replicate samples

of the fertilizer factory, sampled with the PDS, HDS, and impinger.

CONCLUSIONS

A PDS sampler has been designed and tested with collocated silica gel tube, impinger, HDS, Marple personal cascade im-pactor, and filter cassette in an ambient environment around the Hsinchu Science Park and different industrial sites, that is, lead-acid battery factory, sulfuric lead-acid factory, fertilizer factory, and semiconductor factory in Taiwan.

The denuder was compared for the ambient acidic/basic aerosol and gas concentrations, sampled near the Hsinchu Science Park, with those of the HDS and found that the data are highly correlated with correlation coefficient (R2_{) greater}

than 0.980. The results show that total concentrations of sulfu-ric acid mist measured with the new denuder, silica gel tube, and filter cassette in the lead-acid battery and sulfuric acid factory are very close to each other, with correlation coefficients 0.998 and 0.976 for the lead-acid battery and 0.998 and 0.999 for the sulfuric acid factory, respectively. The size distributions of sul-furic acid particles, sampled with the new denuder and Marple personal cascade impactor in the lead-acid battery factory also are very close.

The concentrations of gases, that is, hydrogen fluoride, hydro-gen chloride, and ammonia, determined with the new denuder, HDS, silica gel tube, and impinger near the wastewater treat-ment tanks of the semiconductor and fertilizer factory also are close to one another and highly correlated.

This study shows that the proposed denuder sampler is appli-cable for a wide range of sampling concentrations of acidic/basic gases and aerosols. One-way ANOVA shows no significant dif-ferences (P> 0.05) between the new denuder and other

well-established samplers.

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