- Email: [email protected]
Retention of sulfur dioxide by nylon filters
Atmospheric Environment 33 (1999) 2423—2426
Short Communication Retention of sulfur dioxide by nylon filters J.E. Sickles, II *, L.L. Hodson U.S. Environmental Protection Agency, National Exposure Research Laboratory, MD-56, Research Triangle Park, NC 27711, USA Research Triangle Institute, Research Triangle Park, NC 27709, USA Received 19 September 1998; received in revised form 17 November 1998; accepted 17 November 1998
Abstract Based on laboratory studies, recovery efficiencies of sulfur dioxide (SO ) were determined for nylon filters. The nylon filters used in these experiments were found to retain SO . A relatively uniform amount (1.7%) was recoverable from each nylon filter, independent of relative humidity. An appreciable portion of SO was unrecoverable, and this increased from 5 to 16% as the RH increased from 28 to 49%. This unrecoverable SO may account for previous reports of a low bias for SO determinations employing filter packs using nylon filters. Additional characterization of nylon filters is recommen ded prior to their future deployment as an integrative sampling medium for ambient air. 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Sulfur dioxide; Nylon filter; Filter pack; Ambient air sampling
1. Introduction Filtration techniques employing multiple filters are frequently used to sample ambient air for acidifying species. This method, sometimes known as the filter pack (FP) method, usually collects particles on the first filter and gases on subsequent filters. A common configuration calls for a Teflon filter to collect sulfate (SO\), nitrate, and other particles; a nylon filter to collect nitric acid; carbonate-impregnated filters to collect sulfur dioxide (SO ); and sometimes a citric (or oxalic or phosphoric) acid-impregnated filter if ammonia is of interest. Although retention of SO on the Teflon filter is not likely to present a problem, except in the presence of very high loadings of alkaline particles, retention of SO by
*Corresponding author. E-mail: [email protected].
nylon has been reported by several investigators (Japar and Brachaczek, 1984; Anlauf et al., 1986; Batterman et al., 1997). In some air monitoring networks, the recognition of this retention of SO by nylon has resulted in the extraction of sulfur from the nylon filter and addition to that collected on subsequent filters for the determination of ambient SO . Both the US and Canadian acid deposition monitoring networks, CASTNet and CAPMoN, have adopted this approach (Clarke et al., 1997; Sirois, 1997). Sampler evaluation studies have reported 14—15% higher ambient SO concentrations using the annular denuder system versus the FP, suggesting a low bias for SO with the FP (Dasch et al., 1989; Sickles et al., 1990). One possible explanation for this apparent low bias is retention of SO by the nylon filter as an unex tractable entity. The object of the current study was to examine the interaction of SO with nylon filters to test this hypothesis.
1352-2310/99/$ - see front matter 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 4 2 6 - 9
2424
J.E. Sickles, II, L.L. Hodson / Atmospheric Environment 33 (1999) 2423—2426
2. Experimental
prepared using 25 g Na CO , 10 ml glycerin and water to 100 ml total volume. Each experiment was duplicated at the two test humidities. Quantitative SO collection by each FP was verified using the sulfur analyzer. Nylon filters were extracted in 20 ml of 0.003 N NaOH, and the carbonate-impregnated filters were extracted in 20 ml of buffer solution (0.003N NaHCO /0.0024 N Na CO ). Two drops of 30% H O were added to the extracts to ensure complete conversion of SO to sulfate prior to analysis. Samples were sonicated for 30 min prior to analysis by ion chromatography (IC). In each experiment, the sulfate recovery from the nylon-carbonate FP was compared to that from the paired carbonate FP.
An atmosphere of approximately 40 ppb SO was pre pared dynamically using clean air and SO from a calib rated cylinder of compressed gas. The concentration was verified with a Monitor Labs Model 8450 Sulfur Analyzer. Two humidity conditions were obtained by passing scrubbed, dry air through impingers containing deionized, distilled water. The relative humidity (RH) was verified with a Thermoelectric Model 880 Dew Point Hygrometer to be 28 and 49% for the two sets of experiments. The experiments involved simultaneous parallel sampling at 10 l min\ for 130—140 min through two FP comprised of multiple-stage all-Teflon Savillex filter holders configured for 47 mm diameter filters. One FP contained two or three nylon filters in front of two carbonateimpregnated cellulose filters; the other contained three carbonate-impregnated filters in series. Gelman Nylasorb 1.0 lm pore size nylon filters were used. Whatman 41 cellulose filters were impregnated with a solution
3. Results and discussion Paired experiments were performed at two relative humidities, and the results are summarized in Table 1.
Table 1 Results of laboratory experiments of the retention of SO on nylon filters RH (%)
Trial
Filter Pack A
Filter Pack B
FP A / FP B Sulfate (per nylon filter)
Flow
Filter
Rec
Flow
Filter
Rec
Recovered (%)
Unrecovered (%)
Unrecoverable (lg)
28
1
10.58
N1 N2 N3 C4 C5 &
5.56 4.06 4.14 177.70 3.42 194.88
10.37
C1 C2 C3 — —
238.35 1.50 0.57 — — 240.42
0.79
1.9
6.9
16.8
28
2
10.58
N1 N2 N3 C4 C5 &
4.95 4.39 3.58 198.56 1.22 212.70
10.37
C1 C2 C3 — —
leak — — — — —
0.87
1.8
4.4
10.9
49
1
10.09
N1 N2 C3 C4 &
4.20 1.95 149.70 1.24 157.09
10.16
C1 C2 C3 —
229.34 0.60 1.32 — 231.26
0.68
1.3
15.8
36.2
49
2
9.95
N1 N2 C3 C4 &
3.97 4.32 139.96 0.36 148.61
10.09
C1 C2 C3 —
225.16 0.24 !0.65 — 224.75
0.67
1.9
16.5
36.5
Flow is in standard l min\ (760 mm Hg and 0°C). Rec is SO recovered, expressed as lg SO\. Ratio is adjusted for flow differences. N is for nylon filter, where average blank loading is 2.18 lg SO\ (n"4). C is for carbonate-impregnated Whatman 41 filter.
J.E. Sickles, II, L.L. Hodson / Atmospheric Environment 33 (1999) 2423—2426
Although in the 28% RH experiment Trial 2 FP B developed a leak, both of the challenge SO concentrations as determined by the sulfur analyzer and the measured flow rates were identical in Trials 1 and 2. As a result, the sulfate recovered from FP B Trial 1 was assumed to be equivalent to that for Trial 2 and was compared with the results of FP A in Trial 2. At 28% RH, 80 and 87% of SO recovered from the carbonate FP was recovered from the nylon-carbonate FP. This suggests that between 4.4 and 6.9% of SO delivered to each nylon filter (or between 11 and 17 lg SO\) was unrecoverable. At 49% RH, 68 and 67% of SO recovered from the carbonate FP was recovered from the nylon-carbonate FP. This suggests that approximately 16% of SO delivered to each nylon filter (or about 36 lg SO\) was unrecoverable. There was no appreciable difference in SO recovered for the 10 nylon filters tested (4.1$0.9 lg SO\). Thus, these findings at a fixed SO concentration and two different RHs suggest that a relatively uniform amount of SO was recoverable (about 1.7% per filter), but that a larger amount was unrecoverable, increasing from 5 to 16% as the RH was increased from 28 to 49%. Furthermore, the findings at 49% RH are in good agreement with earlier field studies that reported SO determined with annular denuders to be 14—15% higher than corresponding results using FPs containing nylon and carbonate-impregnated filters (Dasch et al., 1989; Sickles et al., 1990). Japar and Brachaczek (1984) found artifact sulfate formation from SO on Ghia Nylasorb nylon filters ranging from less than 1% at RH below 20% to above 5% at RH above 55%. The largest artifact (10% occurred at 85% RH), but they found no strong dependence on SO concentration or on RH within the studied ranges. Anlauf et al. (1986) using Membrana 1 lm pore size nylon filters found SO retention that was seasonally dependent, ranging from 5% in the fall/spring to 11% in the summer. Differences in RH was given as a possible cause for the seasonal difference. At 40% RH and very high SO concentration (approximately 500 ppm) Gel man nylon filters sorbed almost 30% of the SO initially present (Batterman et al., 1997). In addition to the variable sorption characteristics for SO , nylon filters have been found to sorb various amounts of NH (Possanzini et al., 1992; Masia et al., 1994; Karakas and Tuncel, 1997). This feature apparently depends on modification of the nylon polymer from chemical reaction with other trace gases (i.e., HNO and HCl) that occurs during sampling. Thus, it appears that nylon has a substantial capacity for SO that is influenced by RH. The nature of the nylon collection surface may be altered by reaction with other atmospheric trace gases. Continuous use of filters for one or more diurnal cycles in daily (e.g., CAPMoN) or weekly (e.g., CASTNet) applications will subject the filter surface
2425
to a wide range of changing temperatures, RH, and pollutant concentrations. Nylon filters have been produced by different manufacturers (i.e., Ghia, Membrana, Gelman, Sartorius, Pall, and Schleicher and Schuell). It is likely that differences in fabrication as well as differences from a given manufacturer across production batches may influence the collection properties of nylon surfaces. Differences in filter pretreatment (e.g., washing) and extraction protocols may also influence the collection and extraction efficiencies of nylon filters. It is therefore recommended that: these factors be examined to determine as completely as possible their influence on the sampling and recovery characteristics of nylon; a more thorough sulfate extraction method be sought prior to application of nylon filters in future field studies; and regular quality control testing of the sampling and recovery characteristics of nylon filters be incorporated where nylon filters are used in network applications involving atmospheric sampling.
Acknowledgements The EPA, through its Office of Research and Development (ORD), funded and performed the research described here under contract number 68-02-4544 to the Research Triangle Institute. This manuscript has been subjected to the EPA’s programmatic review process and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
References Anlauf, K.G., Wiebe, H.A., Fellin, P., 1986. Characterization of several integrative sampling methods for nitric acid, sulfur dioxide and atmospheric particles. Journal of the Air Pollution Control Association 36, 715—723. Batterman, S., Osak, I., Gelman, C., 1997. SO sorption charac teristics of air sampling filter media using a new laboratory test. Atmospheric Environment 31, 1041—1047. Clarke, J.F., Edgerton, E.S., Martin, B.E., 1997. Dry deposition calculations for the Clean Air Status and Trends Network. Atmospheric Environment 31, 3667—3678. Dasch, J.M., Cadle, S.H., Kennedy, K.G., Mulawa, P.A., 1989. Comparison of annular denuders and filter packs for atmospheric sampling. Atmospheric Environment 23, 2775—2782. Japar, S.M., Brachaczek, W.W., 1984. Artifact sulfate formation from SO on nylon filters. Atmospheric Environment 18, 2479—2482. Karakas, D., Tuncel, S.G., 1997. Optimization and field application of a filter pack system for the simultaneous sampling of atmospheric HNO , NH , and SO . Atmospheric Environ ment 31, 1657—1666. Masia, P., Di Palo, V., Possanzini, M., 1994. Uptake of ammonia by nylon filters in filter pack systems. Atmospheric Environment 28, 365—366.
2426
J.E. Sickles, II, L.L. Hodson / Atmospheric Environment 33 (1999) 2423—2426
Possanzini, M., Masia, P., Di Palo, V., 1992. Speciation of ammonium-containing species in atmospheric aerosols. Atmospheric Environment 26A, 1995—2000. Sickles, J.E., II, Hodson, L.L., McClenny, W.A., Paur, R.J., Ellestad, T.G., Mulik, J.D., Anlauf, K.G., Wiebe, H.A., Mackay, G.I., Schiff, H.I., Bubacz, D.K., 1990. Field com-
parison of methods for the measurement of gaseous and particulate contributors to acidic dry deposition. Atmospheric Environment 24A, 155—165. Sirois, A., 1997. Temporal variation of oxides of sulfur and nitrogen in ambient air in eastern Canada: 1979—1994. Tellus 49B, 270—291.
Short Communication Retention of sulfur dioxide by nylon filters J.E. Sickles, II *, L.L. Hodson U.S. Environmental Protection Agency, National Exposure Research Laboratory, MD-56, Research Triangle Park, NC 27711, USA Research Triangle Institute, Research Triangle Park, NC 27709, USA Received 19 September 1998; received in revised form 17 November 1998; accepted 17 November 1998
Abstract Based on laboratory studies, recovery efficiencies of sulfur dioxide (SO ) were determined for nylon filters. The nylon filters used in these experiments were found to retain SO . A relatively uniform amount (1.7%) was recoverable from each nylon filter, independent of relative humidity. An appreciable portion of SO was unrecoverable, and this increased from 5 to 16% as the RH increased from 28 to 49%. This unrecoverable SO may account for previous reports of a low bias for SO determinations employing filter packs using nylon filters. Additional characterization of nylon filters is recommen ded prior to their future deployment as an integrative sampling medium for ambient air. 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Sulfur dioxide; Nylon filter; Filter pack; Ambient air sampling
1. Introduction Filtration techniques employing multiple filters are frequently used to sample ambient air for acidifying species. This method, sometimes known as the filter pack (FP) method, usually collects particles on the first filter and gases on subsequent filters. A common configuration calls for a Teflon filter to collect sulfate (SO\), nitrate, and other particles; a nylon filter to collect nitric acid; carbonate-impregnated filters to collect sulfur dioxide (SO ); and sometimes a citric (or oxalic or phosphoric) acid-impregnated filter if ammonia is of interest. Although retention of SO on the Teflon filter is not likely to present a problem, except in the presence of very high loadings of alkaline particles, retention of SO by
*Corresponding author. E-mail: [email protected].
nylon has been reported by several investigators (Japar and Brachaczek, 1984; Anlauf et al., 1986; Batterman et al., 1997). In some air monitoring networks, the recognition of this retention of SO by nylon has resulted in the extraction of sulfur from the nylon filter and addition to that collected on subsequent filters for the determination of ambient SO . Both the US and Canadian acid deposition monitoring networks, CASTNet and CAPMoN, have adopted this approach (Clarke et al., 1997; Sirois, 1997). Sampler evaluation studies have reported 14—15% higher ambient SO concentrations using the annular denuder system versus the FP, suggesting a low bias for SO with the FP (Dasch et al., 1989; Sickles et al., 1990). One possible explanation for this apparent low bias is retention of SO by the nylon filter as an unex tractable entity. The object of the current study was to examine the interaction of SO with nylon filters to test this hypothesis.
1352-2310/99/$ - see front matter 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 4 2 6 - 9
2424
J.E. Sickles, II, L.L. Hodson / Atmospheric Environment 33 (1999) 2423—2426
2. Experimental
prepared using 25 g Na CO , 10 ml glycerin and water to 100 ml total volume. Each experiment was duplicated at the two test humidities. Quantitative SO collection by each FP was verified using the sulfur analyzer. Nylon filters were extracted in 20 ml of 0.003 N NaOH, and the carbonate-impregnated filters were extracted in 20 ml of buffer solution (0.003N NaHCO /0.0024 N Na CO ). Two drops of 30% H O were added to the extracts to ensure complete conversion of SO to sulfate prior to analysis. Samples were sonicated for 30 min prior to analysis by ion chromatography (IC). In each experiment, the sulfate recovery from the nylon-carbonate FP was compared to that from the paired carbonate FP.
An atmosphere of approximately 40 ppb SO was pre pared dynamically using clean air and SO from a calib rated cylinder of compressed gas. The concentration was verified with a Monitor Labs Model 8450 Sulfur Analyzer. Two humidity conditions were obtained by passing scrubbed, dry air through impingers containing deionized, distilled water. The relative humidity (RH) was verified with a Thermoelectric Model 880 Dew Point Hygrometer to be 28 and 49% for the two sets of experiments. The experiments involved simultaneous parallel sampling at 10 l min\ for 130—140 min through two FP comprised of multiple-stage all-Teflon Savillex filter holders configured for 47 mm diameter filters. One FP contained two or three nylon filters in front of two carbonateimpregnated cellulose filters; the other contained three carbonate-impregnated filters in series. Gelman Nylasorb 1.0 lm pore size nylon filters were used. Whatman 41 cellulose filters were impregnated with a solution
3. Results and discussion Paired experiments were performed at two relative humidities, and the results are summarized in Table 1.
Table 1 Results of laboratory experiments of the retention of SO on nylon filters RH (%)
Trial
Filter Pack A
Filter Pack B
FP A / FP B Sulfate (per nylon filter)
Flow
Filter
Rec
Flow
Filter
Rec
Recovered (%)
Unrecovered (%)
Unrecoverable (lg)
28
1
10.58
N1 N2 N3 C4 C5 &
5.56 4.06 4.14 177.70 3.42 194.88
10.37
C1 C2 C3 — —
238.35 1.50 0.57 — — 240.42
0.79
1.9
6.9
16.8
28
2
10.58
N1 N2 N3 C4 C5 &
4.95 4.39 3.58 198.56 1.22 212.70
10.37
C1 C2 C3 — —
leak — — — — —
0.87
1.8
4.4
10.9
49
1
10.09
N1 N2 C3 C4 &
4.20 1.95 149.70 1.24 157.09
10.16
C1 C2 C3 —
229.34 0.60 1.32 — 231.26
0.68
1.3
15.8
36.2
49
2
9.95
N1 N2 C3 C4 &
3.97 4.32 139.96 0.36 148.61
10.09
C1 C2 C3 —
225.16 0.24 !0.65 — 224.75
0.67
1.9
16.5
36.5
Flow is in standard l min\ (760 mm Hg and 0°C). Rec is SO recovered, expressed as lg SO\. Ratio is adjusted for flow differences. N is for nylon filter, where average blank loading is 2.18 lg SO\ (n"4). C is for carbonate-impregnated Whatman 41 filter.
J.E. Sickles, II, L.L. Hodson / Atmospheric Environment 33 (1999) 2423—2426
Although in the 28% RH experiment Trial 2 FP B developed a leak, both of the challenge SO concentrations as determined by the sulfur analyzer and the measured flow rates were identical in Trials 1 and 2. As a result, the sulfate recovered from FP B Trial 1 was assumed to be equivalent to that for Trial 2 and was compared with the results of FP A in Trial 2. At 28% RH, 80 and 87% of SO recovered from the carbonate FP was recovered from the nylon-carbonate FP. This suggests that between 4.4 and 6.9% of SO delivered to each nylon filter (or between 11 and 17 lg SO\) was unrecoverable. At 49% RH, 68 and 67% of SO recovered from the carbonate FP was recovered from the nylon-carbonate FP. This suggests that approximately 16% of SO delivered to each nylon filter (or about 36 lg SO\) was unrecoverable. There was no appreciable difference in SO recovered for the 10 nylon filters tested (4.1$0.9 lg SO\). Thus, these findings at a fixed SO concentration and two different RHs suggest that a relatively uniform amount of SO was recoverable (about 1.7% per filter), but that a larger amount was unrecoverable, increasing from 5 to 16% as the RH was increased from 28 to 49%. Furthermore, the findings at 49% RH are in good agreement with earlier field studies that reported SO determined with annular denuders to be 14—15% higher than corresponding results using FPs containing nylon and carbonate-impregnated filters (Dasch et al., 1989; Sickles et al., 1990). Japar and Brachaczek (1984) found artifact sulfate formation from SO on Ghia Nylasorb nylon filters ranging from less than 1% at RH below 20% to above 5% at RH above 55%. The largest artifact (10% occurred at 85% RH), but they found no strong dependence on SO concentration or on RH within the studied ranges. Anlauf et al. (1986) using Membrana 1 lm pore size nylon filters found SO retention that was seasonally dependent, ranging from 5% in the fall/spring to 11% in the summer. Differences in RH was given as a possible cause for the seasonal difference. At 40% RH and very high SO concentration (approximately 500 ppm) Gel man nylon filters sorbed almost 30% of the SO initially present (Batterman et al., 1997). In addition to the variable sorption characteristics for SO , nylon filters have been found to sorb various amounts of NH (Possanzini et al., 1992; Masia et al., 1994; Karakas and Tuncel, 1997). This feature apparently depends on modification of the nylon polymer from chemical reaction with other trace gases (i.e., HNO and HCl) that occurs during sampling. Thus, it appears that nylon has a substantial capacity for SO that is influenced by RH. The nature of the nylon collection surface may be altered by reaction with other atmospheric trace gases. Continuous use of filters for one or more diurnal cycles in daily (e.g., CAPMoN) or weekly (e.g., CASTNet) applications will subject the filter surface
2425
to a wide range of changing temperatures, RH, and pollutant concentrations. Nylon filters have been produced by different manufacturers (i.e., Ghia, Membrana, Gelman, Sartorius, Pall, and Schleicher and Schuell). It is likely that differences in fabrication as well as differences from a given manufacturer across production batches may influence the collection properties of nylon surfaces. Differences in filter pretreatment (e.g., washing) and extraction protocols may also influence the collection and extraction efficiencies of nylon filters. It is therefore recommended that: these factors be examined to determine as completely as possible their influence on the sampling and recovery characteristics of nylon; a more thorough sulfate extraction method be sought prior to application of nylon filters in future field studies; and regular quality control testing of the sampling and recovery characteristics of nylon filters be incorporated where nylon filters are used in network applications involving atmospheric sampling.
Acknowledgements The EPA, through its Office of Research and Development (ORD), funded and performed the research described here under contract number 68-02-4544 to the Research Triangle Institute. This manuscript has been subjected to the EPA’s programmatic review process and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
References Anlauf, K.G., Wiebe, H.A., Fellin, P., 1986. Characterization of several integrative sampling methods for nitric acid, sulfur dioxide and atmospheric particles. Journal of the Air Pollution Control Association 36, 715—723. Batterman, S., Osak, I., Gelman, C., 1997. SO sorption charac teristics of air sampling filter media using a new laboratory test. Atmospheric Environment 31, 1041—1047. Clarke, J.F., Edgerton, E.S., Martin, B.E., 1997. Dry deposition calculations for the Clean Air Status and Trends Network. Atmospheric Environment 31, 3667—3678. Dasch, J.M., Cadle, S.H., Kennedy, K.G., Mulawa, P.A., 1989. Comparison of annular denuders and filter packs for atmospheric sampling. Atmospheric Environment 23, 2775—2782. Japar, S.M., Brachaczek, W.W., 1984. Artifact sulfate formation from SO on nylon filters. Atmospheric Environment 18, 2479—2482. Karakas, D., Tuncel, S.G., 1997. Optimization and field application of a filter pack system for the simultaneous sampling of atmospheric HNO , NH , and SO . Atmospheric Environ ment 31, 1657—1666. Masia, P., Di Palo, V., Possanzini, M., 1994. Uptake of ammonia by nylon filters in filter pack systems. Atmospheric Environment 28, 365—366.
2426
J.E. Sickles, II, L.L. Hodson / Atmospheric Environment 33 (1999) 2423—2426
Possanzini, M., Masia, P., Di Palo, V., 1992. Speciation of ammonium-containing species in atmospheric aerosols. Atmospheric Environment 26A, 1995—2000. Sickles, J.E., II, Hodson, L.L., McClenny, W.A., Paur, R.J., Ellestad, T.G., Mulik, J.D., Anlauf, K.G., Wiebe, H.A., Mackay, G.I., Schiff, H.I., Bubacz, D.K., 1990. Field com-
parison of methods for the measurement of gaseous and particulate contributors to acidic dry deposition. Atmospheric Environment 24A, 155—165. Sirois, A., 1997. Temporal variation of oxides of sulfur and nitrogen in ambient air in eastern Canada: 1979—1994. Tellus 49B, 270—291.