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Evaluation of techniques for sulfuric acid and particulate strong acidity measurements in ambient air
ooo4-698l/80/0501 -0559 $02.00/O
ArmosphericEnuironmenf Vol. 14, pp, 559-563. ” Pergamon Press Ltd. 1980. Printed in Great Britain
EVALUATION OF TECHNIQUES FOR SULFURIC ACID AND PARTICULATE STRONG ACIDITY MEASUREMENTS IN AMBIENT AIR B. R.
APPEL,
S. M. WALL,
M. HAIK,
E. L. KOTHNY
and Y. TOKIWA
Air and Industrial Hygiene Laboratory Section, Laboratory Services Branch, California Department of Health Services, 2151 Berkeley Way, Berkeley, CA 94704, U.S.A. (First received 2 July 1979 and in jnalform 25 September 1979) Abstract - A laboratory and field study was conducted to evaluate measurement methods for sulfuric acid and strong acids in atmospheric aerosols. Selective extraction of HzSO., with benzaldehyde and titrimetry for strong acids were evaluated and compared using laboratory-generated mixtures of < 0.3pm diameter H,SO,, (NH&SOL, and NH,HSO, aerosols on clean and atmospheric particulate-loaded filters. Filter media were selected based on filtration efficiency and acid recovery with 5 0.3 pm H,SO, aerosols. An ammonia denuder was employed in field sampling together with removal of non-respirable particles, to increase the stability of H,SO, following collection. Simultaneous gas phase ammonia measurements were made to assist in interpreting the particulate sample results. The presence of atmospheric particulate matter was shown to reduce recoveries of laboratory-generated H,SO, sharply, but recoveries of total strong acid usually remained 2 60 %. Anhydrous benzaldehyde was found to extract NH4HS04 to a substantial degree in the absence of atmospheric particulate matter. Samples collected in Pittsburg, California showed correlation between sulfuric acid and particulate strong acid measurements. As much as 0.6ygme3 H,SO, and 1.6pgmw3 acidity, expressed as H2S04, was found. However, on the basis of recovery studies we believe these represent lower limit values.
1. INTRODUaION California’s air quality standard for sulfate, 25 pg mm3 for a 24-h sample, is based on the premise that a significant fraction of the water-soluble sulfate exists in the atmosphere as H2S04. The present study was undertaken to evaluate techniques for monitoring H2S04 and strongly acidic constituents in atmospheric aerosols. Numerous methods have been proposed for such measurements (Newman, 1979). For the present study, selective extraction of H$O, from filter samples with benzaldehyde (Leahy et al., 1975; Tanner et al., 1977) followed by quantitation as sulfate by the AIHL micromethod (Hoffer et al., 1979) was chosen. Determination of strong acids by microtitration (Brosset and Ferm, 1972) was also evaluated and compared to H,S04 measurements. The authors of the benzaldehyde extraction technique found 70-100% recoveries of H2S0, with < 5% extraction of (NH4)$504 and NH,HSO,. However, recoveries of H2S04 in the presence of atmospheric particulate matter were not reported. Substantially higher solubility for NH,HSO., in reagent grade benzaldehyde was recently observed (Richards et al., 1978), again in the absence of atmospheric aerosol. Barrett et al. (1977) have demonstrated that atmospheric particulate matter caused negative interference in determining H,SOL, because of partial neutralization by basic aerosol constituents. The present study has extended these evaluations of the benzaldehyde extraction and strong acid measurement techniques by including studies of H,S04-
(NH&SO,, H,SO*-NH*HSO.+, H,SO,-particulate and H,SO,-NH,HSO,-particulate interactions. To minimize interference from atmospheric aerosols, sampling employed cyclones to remove particles of aerodynamic diameter greater than 2 or 3 pm, likely to contain much of the reactive material (e.g. carbonates in soil, NaCl from sea salt). To minimize neutralization by ammonia, sampling was performed with an ammonia denuder ahead of a low volume aerosol sampler (Stevens et al., 1979). Interferences caused by the interaction of HISO., with nitrate (Harker et al., 1977) or sulfate salts would not be reduced by the sampling scheme used. The present study included laboratory evaluation of the above procedures as well as ambient air sampling for H,S04, particulate strong acidity and NH,. The sampling location (Pittsburg, California) is in proximity to stationary emission sources for SOI and H,SO*. Particulate samples were also analyzed for NH:, SOi- and NO;. Nitric acid method evaluation and atmospheric results from Pittsburg, California in relation to particulate nitrate are reported elsewhere (Appel et al., 1979a, 1980).
2. EXPERIMENTAL
Anhydrous redistilled benzaldehyde was stored and dispensed under argon. Its benzoic acid content was 0.06-0.6 % w/w. Filter samples were stored inside sealed plastic bags, and sectioned for analysis in an ammonia-free chamber under N,. For H,SO& determinations, I 17cm’ filter sections were extracted in 1Oml benzaldehyde in Teflon-lined, screwcapped glass test tubes under argon by mechanical shaking
559
B. R.
560
M. HAIK, E. L. KOTHNY and Y. Tok~ws
APPFL, S. M. WAN,
for 60 min. Samples were not vacuum dried before extraction, to minimize losses of relatively volatile species. Recovery of H,SO, was unaffected, within 10 “,$experimental uncertainty, by omitting sample drying. Following extraction, the solutions were centrifuged and 8ml aliquots of the clear supernatant withdrawn and extracted with 3 ml H,O under argon. This mixture was shaken for 30 min, briefly centrifuged and an aliquot (5 10ml) removed for sulfate analysis (Hoffer rr al., 1979). The sulfate method was shown to be free of interference from residual benzaldehyde and its oxidation products if the standards were shaken with benzaldehyde as above, Aqueous extractions were done in polystyrene tubes by mechanical shaking for 60min at room temperature. This procedure was shown to extract sulfate from Teflon and glass fiber filters with 95 and 100 ‘I, efficiency, respectively (Appel Ed ul., 1979b). For measurement of strong acid, sufficient standard nitric acid to produce a pH of 4.000 in water was added to 5 ml of the extract. The samples were titrated back to this pH with O.OlN NaOH, in the absence of CO,, using a Radiometer Autoburette ABl2 (Brosset and Ferm, 1978). Analysis of these aqueous extracts for NH; and NO;, as well as the NH, sampling and analysis technique are described elsewhere (Appel et al., 1979a, 1980). Sulfuric acid aerosols were generated from SO, and humidified air (Wall, 1977 ; Wall and Appel, 1978) such that > 99”” of all particles produced had optical diameters < 0.3 am. Ammonium sulfate and acid sulfate aerosols were generated with a nebulizer to produce particles of < 0.3 pm diameter after water evaporation. Atmospheric sampling employed a respirable particulate hi-volume sampler, described elsewhere (Appel et ul.. 1980) with acid-washed Pallflex Quartz 2500 QAO 20cm x 25cm filters. The washing procedure was that of Tanner et ul. (1977). In addition, particulate matter was sampled sequentially through an ammonia denuder (Stevens et u1., 1979), an anodized-aluminum cyclone and Teflon filters mounted in anodized-aluminum filter holders (Environmental Research Corporation, St Paul, Minnesota). At the flow rate used, 22 1min _ I, the cyclone removed particles > 2 ilrn. Both 1 pm pore size Fluoropore (Millipore Corporation) and Zefluor (Ghia Corporation) were used. The denuder was shown to remove > 99 Y0 of ammonia at 100 ppb. Sampling was done for 3 days in February 1979 in Pittsburg, California. Respirable particulate, low volume samples were collected for 8-h periods. Respirable hi-vol particulate samples were collected simultaneously for 2 Rh periods 3. RESULTS AND IWXXJSSIO?;
3.1 Filter The
eoaluation
quartz
and
and su(firic
acid
Teflon filters were shown Table
study
recocery
1. Percentage
to form
sulfuric
negligible artifact sulfate from SOI at relative humidities up to 9Op,, at 2O’C. The three filters employed showed > 98 :‘;, retention for < 0.3 /lrn H,SO, aerosol. These findings are consistent with prior rtudies using room air dust (John and Reischl, 1978) and 0.03- 1 pm dioctyl phthalate particles (Liu and Let. 1976). In the absence of other materials the recovery oF 1Opg sulfuric acid aerosol from four acid-washed quartz filters by benzaldehyde extraction averaged 60 + 10 7: (one a) which compares with 75 :‘,, recovery reported by Tanner et ul. (1977) for similar conditions. With four Zefluor filters, recoveries averaged 74 + 5”<,. The recoveries of strong acid by titration of aqueous extracts from four filters were 94 k 6 and 88 IfI 8 I’<,for the quartz and Teflon filters, respectively. Recovery of the acid from clean filters was not altered by storage for up to 2 weeks in disposable Petri dishes inside sealed polyethylene bags. With the titrimetric method, hexanedioic acid (pK, = 4.4) and aluminum sulfate (pK, = 5) showed no interference, benzoic acid (pK, = 4.2), an 181,, positive interference, while ferric sulfate (pK, - 3) and ammonium acid sulfate (pK, = 2.0) titrated as strong acids, To evaluate H2S0, recoveries in the presence of atmospheric particulate matter, so 1OOpg 2 0.3 pm H,SO, aerosol was added to quartz and Fluoropore filters previously loaded with atmospheric aerosol collected in Berkeley. The samples on quartz were cut from a single, 20 x 25 cm filter and contained respirable (< 3.5 pm) particles only, while those on 47 mm Fluoropore filters were collected without size segregation. Samples were stored from 42 to 2 16 h and analyzed for sulfuric acid and total acidity. The results given in Table 1 indicate that little H,SOL was recovered by benzaldehyde extraction. However, the recovery of total strong acid averaged with the about 60 ?;. These results are consistent reaction of H,SO, aerosol with particulate matter components yielding titratable strong acid(s) of minimal solubility in benzaldehyde. The reduced recovery of strong acid relative to that found with H?SO, on clean filters may reflect the extent of
acid recovery from atmospheric loaded filters*t H 2SO 4 recovery
HISO, Filter
added
(pg)
Benzaldehyde extraction ~~~ _..~~.
particulate-
(” ,I)
Titratton ~~~
Pallflex quartz 2500 QAO
46 82
< 21 < 11
72 f 18 65 i 6
Fluoropore, pore size
54 96
t6 33 + 18
50 + 9 65k 18
1 Km
*Results are mean f 1 (r for five samples stored for 42-216 h at room temperature, and are corrected for observed H,SOL and total H+ in the atmospheric particulate. tFilters contained 206 + 4 and 60 + 13ag SOiper 47mm disc for quartz and Fluoropore filters, respectively. from atmospheric particles.
561
Sulfuric acid and particulate strong acidity measurements in ambient air
reaction of H2S04 with atmospheric particulate matter constituents to form relatively weak acid products (e.g. HCO;). To determine the degree of interaction between (NH&SO, and H,SO, aerosols on a filter, Teflon filters, pre-loaded with about 6 pgcm-’ (NH&SO, were loaded with about 3pgcm-’ H,SO+ aerosol. Added H,SO, was determined by the difference between the total water-soluble sulfate and known quantities of (NH&Sod. Results (Set 1 in Table 2) show recoveries of strong acid as well as apparent H*SO, (benzaldehyde-soluble sulfate) to be increased in the presence of (NHJ2S04. The increased recovery of sulfate in benzaldehyde may be indicative of NH,HSOL formation. The extractability of NH,HSO, aerosol in benzaldehyde, both alone and in the presence of H,SO, aerosol, is shown for filter sets 2 and 3 in Table 2. In the absence of HzS04, 45 + 3% of the NH,HSO, was extracted in benzaldehyde. With mixtures of the acid sulfate and H2S04, and assuming that 100% of the H,SO, was recovered from the acidic filter surface, 48 k 20% of the NH,HSO, was also extracted into benzaldehyde. No relationship between the H,SOI level and the proportion of NH,HS04 extracted is evident. It may be emphasized that, while the benzaldehyde used was dried and distilled, the aerosol samples were extracted without pre-drying. Barrett et al. (1977) employing benzaldehyde without purification, have reported that the water content of NH,HSO, aerosol influenced its solubility in benzaldehyde, at least in the absence of other materials. The interference of NH4HS04 on HzS04 measurement in the presence of atmospheric particulate matter was evaluated by procedures similar to those described above. However, the level of H,S04 recovered by benzaldehyde extraction was below the level of H,S04 added, making measurement of positive interference by NH,HSOI impossible. In contrast to the experiment described in Table 2, no NH,HSO, was extracted by benzaldehyde. This is consistent with the findings described in Table 1; assuming that the strong acid formed by reaction of H,SO, and atmospheric particulate matter was NH,HSO,, this strong acid was not extracted into benzaldehyde under these conditions. Accordingly, the significance of the positive interference by NH4HS04 in HISO, measurement observed in the absence of atmospheric aerosol remains unclear.
_
H?quortz
filters,
Hi-voll
H’bflon filters, lo-vol) H$O,lteflcn filters, lo-
-c
-
0 80
NH,(oxahc -filters)
r.
-z-
0600
Nitrate (quartz Sulfate (quartz
NHi(quartz
filters, HI-vol) filters, HI-voll
ocld-Impregnated T filters, Ht-vol
1400 2200 0600 14CXI 2200 0600
215
2/6
2/7 Time,
Fig. 1. Aerosol
I
1400 2200 0600 2/8/79
PST
constituent and NH, Pittsburg, California.
concentrations,
sampler provided four samples exceeding the limit of detection (O.OOGO.008p-equiv m- 3, for HISO,. These correlated approximately with maxima in total acid measurements. The highest level of H2S04 found, 0.012 f 0.002 p-equiv me3, corresponds to 0.6 + 0.1 pg H,SO, mm3. Based on recovery studies described above, atmospheric H,SO, and strong acids may be present at higher levels. Analysis of a representative set of the aqueous extracts for total soluble iron by flame atomic absorption spectroscopy showed that < 10% of the observed strong acid could be ascribed to Fe3+. Comparing the temporal variations for the species shown, elevated particulate strong acidity generally correlated with lower NH, levels. Particulate NH: 3.2 Atmospheric sampling results was in excess with respect to the levels of SOi- plus Results are summarized in Fig. 1 expressed as flu- NO;, consistent with only minor amounts of strong equiv. m- 3. The relatively short sampling times with particulate acids. the hi-volume sampler provided diurnal variations in 3.3 Summary and conclusions total acidity and other aerosol constituents. The precision of these titrimetric results (29% median (1) In the presence of varying levels of atmospheric coefficient of variation) reflected principally the vari- particulate matter, H,SO, aerosol recovery was ability of the quartz filter blank. However, H,S04 < 30% by benzaldehyde extraction, but usually remained below detectable limits (0.002-0.008 p> 60% by strong acid titration using acid-washed equiv mp3) with these samples. The low-volume quartz fiber or Teflon filters. Accordingly, the latter
562
B. R. APPEL. S. M. WAI.L, M. HAIK, E. L. KOTHNY and Y, TOKI~~
Sulfuric acid and particulate strong acidity measurements in ambient air technique may be preferable for atmospheric sampling in spite of its inability to distinguish sources of H+. (2) On the presumption that H2S04 was the dominant source of strong acid measured in the atmospheric samples collected in Pittsburg, California, the peak ambient HzSO, level during the period sampled was 0.033 + O.O08p-equiv mm3 (1.6 f 0.4pgmm3), for a 2-h average. This compares to 0.012 f 0.002/.1equiv me3 (0.6 & O.l~grn-~) HzSO,, for an 8-h average observed by benzaldehyde extraction. The latter corresponds to about 13 % of the water-soluble sulfate. (3) Elimination of non-respirable particles was insufficient to permit accurate measurement of H,SO,, at least for L 10m3 air samples collected on inert filters. The effectiveness of the ammonia denuder in stabilizing HzSO* remains unclear. (4) Additional atmospheric sampling with the techniques described here will be made at locations likely to provide higher levels of HzSO,. Acknowledgement - L. Raftery and J. Benzing provided very capable assistance. Thecyclone used for low volume sampling was designed and calibrated by W. John. The assistance of the staff of the Bay Area Air Quality Management District in providing use of their Pittsburg, California sampling station is gratefully acknowledged. The work was supported by the California Air Resources Board Research Division. The statements and conclusions in this report are those of the authors and not necessarily those of the California Air Resources Board. The mention ofcommercial products, their sources, or use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products. REFERENCES
Appel B. R., Tokiwa Y., Wall S. M., Haik M., Kothny E. L. and Wesolowski J. J. (1979a) Determination of sulfuric acid, total particle-phase acidity and nitric acid in ambient air. Final Report to the California Air Resources Board, Contract A6-209-30.
563
Appel B. R., Hoffer E. M., Wehrmeister W., Haik M. and Wesolowski J. J. (1979b) Improvement and evaluation of methods for sulfate analysis. Final Report, EPA Grant No. 805~447-1. Appel B. R., Wall S. M., Tokiwa Y. and Haik M. (1980) Simultaneous nitric acid, particulate nitrate and acidity measurements in ambient air. Atmospheric Enoironment 14, 549-554.
Barrett W. J., Miller H. C., Smith J. E. and Gwin C. H. (1977) Development of a portable device to collect sulfuric acid aerosol. EPA Report 600/2-77-027. Brosset C. and Ferm M. (1978) Man-made airborne acidity and its determination, Atmospheric Environment 12, 909-916. Harker A. B., Richards L. W. and Clark W. E. (1977) The effect of atmospheric SO2 photochemistry upon observed nitrate concentrations in aerosols. Atmospheric Enuironment 11, 87-91. Hoffer E. M., Kothny E. L. and Appel B. R. (1979) Simple method for microgram amounts of sulfate in atmospheric particulates. Atmospheric Enoironment 13, 303-306. John W. and Reischl G. (1978) Measurement of the filtration efficiencies ofselected filter types. Atmospheric Environment 12, 2015-2019.
Liu B. Y. H. and Lee K. W. (1976) Efficiency ofmembrane and Nuclepore filters for submicrometer aerosols. Enuir. Sci. Technol. 10, 345350. Newman L. (1978) Techniques for determining the chemical composition of aerosol sulfur compounds. Atmospheric Enuironment 12, 113-12.5. Richards L. W., Johnson L. W. and Shepard L. S. (1978) Sulfate aerosol study. Final Report to the Coordinating Research Council, Contract No. CAPA-13-76. Stevens R. K., Dzubay T. G., Russwurm G. and Rickel D. (1978) Sampling and analysis of atmospheric sulfates and related species. Atmospheric Enuironment 12, 56-68. Tanner R. L., Cederwall R., Garber R., Leahy D., Marlow W., Meyers R., Phillips M. and Newman L. (1977) Separation and analysis of aerosol sulfate species at ambient concentrations. Atmospheric Environment 11, 955-966. Wall S. (1977) The design and evaluation of a sulfuric acid aerosol generator with emphasis on particle size control. Master’s Thesis, University of California, Berkeley. Wall S. and Appel B. R. (1978) Thedesign and evaluation ofa sulfuric acid aerosol generator. Paper presented at the Pacif: Conf Chem. Spectrosc., San Francisco.
ArmosphericEnuironmenf Vol. 14, pp, 559-563. ” Pergamon Press Ltd. 1980. Printed in Great Britain
EVALUATION OF TECHNIQUES FOR SULFURIC ACID AND PARTICULATE STRONG ACIDITY MEASUREMENTS IN AMBIENT AIR B. R.
APPEL,
S. M. WALL,
M. HAIK,
E. L. KOTHNY
and Y. TOKIWA
Air and Industrial Hygiene Laboratory Section, Laboratory Services Branch, California Department of Health Services, 2151 Berkeley Way, Berkeley, CA 94704, U.S.A. (First received 2 July 1979 and in jnalform 25 September 1979) Abstract - A laboratory and field study was conducted to evaluate measurement methods for sulfuric acid and strong acids in atmospheric aerosols. Selective extraction of HzSO., with benzaldehyde and titrimetry for strong acids were evaluated and compared using laboratory-generated mixtures of < 0.3pm diameter H,SO,, (NH&SOL, and NH,HSO, aerosols on clean and atmospheric particulate-loaded filters. Filter media were selected based on filtration efficiency and acid recovery with 5 0.3 pm H,SO, aerosols. An ammonia denuder was employed in field sampling together with removal of non-respirable particles, to increase the stability of H,SO, following collection. Simultaneous gas phase ammonia measurements were made to assist in interpreting the particulate sample results. The presence of atmospheric particulate matter was shown to reduce recoveries of laboratory-generated H,SO, sharply, but recoveries of total strong acid usually remained 2 60 %. Anhydrous benzaldehyde was found to extract NH4HS04 to a substantial degree in the absence of atmospheric particulate matter. Samples collected in Pittsburg, California showed correlation between sulfuric acid and particulate strong acid measurements. As much as 0.6ygme3 H,SO, and 1.6pgmw3 acidity, expressed as H2S04, was found. However, on the basis of recovery studies we believe these represent lower limit values.
1. INTRODUaION California’s air quality standard for sulfate, 25 pg mm3 for a 24-h sample, is based on the premise that a significant fraction of the water-soluble sulfate exists in the atmosphere as H2S04. The present study was undertaken to evaluate techniques for monitoring H2S04 and strongly acidic constituents in atmospheric aerosols. Numerous methods have been proposed for such measurements (Newman, 1979). For the present study, selective extraction of H$O, from filter samples with benzaldehyde (Leahy et al., 1975; Tanner et al., 1977) followed by quantitation as sulfate by the AIHL micromethod (Hoffer et al., 1979) was chosen. Determination of strong acids by microtitration (Brosset and Ferm, 1972) was also evaluated and compared to H,S04 measurements. The authors of the benzaldehyde extraction technique found 70-100% recoveries of H2S0, with < 5% extraction of (NH4)$504 and NH,HSO,. However, recoveries of H2S04 in the presence of atmospheric particulate matter were not reported. Substantially higher solubility for NH,HSO., in reagent grade benzaldehyde was recently observed (Richards et al., 1978), again in the absence of atmospheric aerosol. Barrett et al. (1977) have demonstrated that atmospheric particulate matter caused negative interference in determining H,SOL, because of partial neutralization by basic aerosol constituents. The present study has extended these evaluations of the benzaldehyde extraction and strong acid measurement techniques by including studies of H,S04-
(NH&SO,, H,SO*-NH*HSO.+, H,SO,-particulate and H,SO,-NH,HSO,-particulate interactions. To minimize interference from atmospheric aerosols, sampling employed cyclones to remove particles of aerodynamic diameter greater than 2 or 3 pm, likely to contain much of the reactive material (e.g. carbonates in soil, NaCl from sea salt). To minimize neutralization by ammonia, sampling was performed with an ammonia denuder ahead of a low volume aerosol sampler (Stevens et al., 1979). Interferences caused by the interaction of HISO., with nitrate (Harker et al., 1977) or sulfate salts would not be reduced by the sampling scheme used. The present study included laboratory evaluation of the above procedures as well as ambient air sampling for H,S04, particulate strong acidity and NH,. The sampling location (Pittsburg, California) is in proximity to stationary emission sources for SOI and H,SO*. Particulate samples were also analyzed for NH:, SOi- and NO;. Nitric acid method evaluation and atmospheric results from Pittsburg, California in relation to particulate nitrate are reported elsewhere (Appel et al., 1979a, 1980).
2. EXPERIMENTAL
Anhydrous redistilled benzaldehyde was stored and dispensed under argon. Its benzoic acid content was 0.06-0.6 % w/w. Filter samples were stored inside sealed plastic bags, and sectioned for analysis in an ammonia-free chamber under N,. For H,SO& determinations, I 17cm’ filter sections were extracted in 1Oml benzaldehyde in Teflon-lined, screwcapped glass test tubes under argon by mechanical shaking
559
B. R.
560
M. HAIK, E. L. KOTHNY and Y. Tok~ws
APPFL, S. M. WAN,
for 60 min. Samples were not vacuum dried before extraction, to minimize losses of relatively volatile species. Recovery of H,SO, was unaffected, within 10 “,$experimental uncertainty, by omitting sample drying. Following extraction, the solutions were centrifuged and 8ml aliquots of the clear supernatant withdrawn and extracted with 3 ml H,O under argon. This mixture was shaken for 30 min, briefly centrifuged and an aliquot (5 10ml) removed for sulfate analysis (Hoffer rr al., 1979). The sulfate method was shown to be free of interference from residual benzaldehyde and its oxidation products if the standards were shaken with benzaldehyde as above, Aqueous extractions were done in polystyrene tubes by mechanical shaking for 60min at room temperature. This procedure was shown to extract sulfate from Teflon and glass fiber filters with 95 and 100 ‘I, efficiency, respectively (Appel Ed ul., 1979b). For measurement of strong acid, sufficient standard nitric acid to produce a pH of 4.000 in water was added to 5 ml of the extract. The samples were titrated back to this pH with O.OlN NaOH, in the absence of CO,, using a Radiometer Autoburette ABl2 (Brosset and Ferm, 1978). Analysis of these aqueous extracts for NH; and NO;, as well as the NH, sampling and analysis technique are described elsewhere (Appel et al., 1979a, 1980). Sulfuric acid aerosols were generated from SO, and humidified air (Wall, 1977 ; Wall and Appel, 1978) such that > 99”” of all particles produced had optical diameters < 0.3 am. Ammonium sulfate and acid sulfate aerosols were generated with a nebulizer to produce particles of < 0.3 pm diameter after water evaporation. Atmospheric sampling employed a respirable particulate hi-volume sampler, described elsewhere (Appel et ul.. 1980) with acid-washed Pallflex Quartz 2500 QAO 20cm x 25cm filters. The washing procedure was that of Tanner et ul. (1977). In addition, particulate matter was sampled sequentially through an ammonia denuder (Stevens et u1., 1979), an anodized-aluminum cyclone and Teflon filters mounted in anodized-aluminum filter holders (Environmental Research Corporation, St Paul, Minnesota). At the flow rate used, 22 1min _ I, the cyclone removed particles > 2 ilrn. Both 1 pm pore size Fluoropore (Millipore Corporation) and Zefluor (Ghia Corporation) were used. The denuder was shown to remove > 99 Y0 of ammonia at 100 ppb. Sampling was done for 3 days in February 1979 in Pittsburg, California. Respirable particulate, low volume samples were collected for 8-h periods. Respirable hi-vol particulate samples were collected simultaneously for 2 Rh periods 3. RESULTS AND IWXXJSSIO?;
3.1 Filter The
eoaluation
quartz
and
and su(firic
acid
Teflon filters were shown Table
study
recocery
1. Percentage
to form
sulfuric
negligible artifact sulfate from SOI at relative humidities up to 9Op,, at 2O’C. The three filters employed showed > 98 :‘;, retention for < 0.3 /lrn H,SO, aerosol. These findings are consistent with prior rtudies using room air dust (John and Reischl, 1978) and 0.03- 1 pm dioctyl phthalate particles (Liu and Let. 1976). In the absence of other materials the recovery oF 1Opg sulfuric acid aerosol from four acid-washed quartz filters by benzaldehyde extraction averaged 60 + 10 7: (one a) which compares with 75 :‘,, recovery reported by Tanner et ul. (1977) for similar conditions. With four Zefluor filters, recoveries averaged 74 + 5”<,. The recoveries of strong acid by titration of aqueous extracts from four filters were 94 k 6 and 88 IfI 8 I’<,for the quartz and Teflon filters, respectively. Recovery of the acid from clean filters was not altered by storage for up to 2 weeks in disposable Petri dishes inside sealed polyethylene bags. With the titrimetric method, hexanedioic acid (pK, = 4.4) and aluminum sulfate (pK, = 5) showed no interference, benzoic acid (pK, = 4.2), an 181,, positive interference, while ferric sulfate (pK, - 3) and ammonium acid sulfate (pK, = 2.0) titrated as strong acids, To evaluate H2S0, recoveries in the presence of atmospheric particulate matter, so 1OOpg 2 0.3 pm H,SO, aerosol was added to quartz and Fluoropore filters previously loaded with atmospheric aerosol collected in Berkeley. The samples on quartz were cut from a single, 20 x 25 cm filter and contained respirable (< 3.5 pm) particles only, while those on 47 mm Fluoropore filters were collected without size segregation. Samples were stored from 42 to 2 16 h and analyzed for sulfuric acid and total acidity. The results given in Table 1 indicate that little H,SOL was recovered by benzaldehyde extraction. However, the recovery of total strong acid averaged with the about 60 ?;. These results are consistent reaction of H,SO, aerosol with particulate matter components yielding titratable strong acid(s) of minimal solubility in benzaldehyde. The reduced recovery of strong acid relative to that found with H?SO, on clean filters may reflect the extent of
acid recovery from atmospheric loaded filters*t H 2SO 4 recovery
HISO, Filter
added
(pg)
Benzaldehyde extraction ~~~ _..~~.
particulate-
(” ,I)
Titratton ~~~
Pallflex quartz 2500 QAO
46 82
< 21 < 11
72 f 18 65 i 6
Fluoropore, pore size
54 96
t6 33 + 18
50 + 9 65k 18
1 Km
*Results are mean f 1 (r for five samples stored for 42-216 h at room temperature, and are corrected for observed H,SOL and total H+ in the atmospheric particulate. tFilters contained 206 + 4 and 60 + 13ag SOiper 47mm disc for quartz and Fluoropore filters, respectively. from atmospheric particles.
561
Sulfuric acid and particulate strong acidity measurements in ambient air
reaction of H2S04 with atmospheric particulate matter constituents to form relatively weak acid products (e.g. HCO;). To determine the degree of interaction between (NH&SO, and H,SO, aerosols on a filter, Teflon filters, pre-loaded with about 6 pgcm-’ (NH&SO, were loaded with about 3pgcm-’ H,SO+ aerosol. Added H,SO, was determined by the difference between the total water-soluble sulfate and known quantities of (NH&Sod. Results (Set 1 in Table 2) show recoveries of strong acid as well as apparent H*SO, (benzaldehyde-soluble sulfate) to be increased in the presence of (NHJ2S04. The increased recovery of sulfate in benzaldehyde may be indicative of NH,HSOL formation. The extractability of NH,HSO, aerosol in benzaldehyde, both alone and in the presence of H,SO, aerosol, is shown for filter sets 2 and 3 in Table 2. In the absence of HzS04, 45 + 3% of the NH,HSO, was extracted in benzaldehyde. With mixtures of the acid sulfate and H2S04, and assuming that 100% of the H,SO, was recovered from the acidic filter surface, 48 k 20% of the NH,HSO, was also extracted into benzaldehyde. No relationship between the H,SOI level and the proportion of NH,HS04 extracted is evident. It may be emphasized that, while the benzaldehyde used was dried and distilled, the aerosol samples were extracted without pre-drying. Barrett et al. (1977) employing benzaldehyde without purification, have reported that the water content of NH,HSO, aerosol influenced its solubility in benzaldehyde, at least in the absence of other materials. The interference of NH4HS04 on HzS04 measurement in the presence of atmospheric particulate matter was evaluated by procedures similar to those described above. However, the level of H,S04 recovered by benzaldehyde extraction was below the level of H,S04 added, making measurement of positive interference by NH,HSOI impossible. In contrast to the experiment described in Table 2, no NH,HSO, was extracted by benzaldehyde. This is consistent with the findings described in Table 1; assuming that the strong acid formed by reaction of H,SO, and atmospheric particulate matter was NH,HSO,, this strong acid was not extracted into benzaldehyde under these conditions. Accordingly, the significance of the positive interference by NH4HS04 in HISO, measurement observed in the absence of atmospheric aerosol remains unclear.
_
H?quortz
filters,
Hi-voll
H’bflon filters, lo-vol) H$O,lteflcn filters, lo-
-c
-
0 80
NH,(oxahc -filters)
r.
-z-
0600
Nitrate (quartz Sulfate (quartz
NHi(quartz
filters, HI-vol) filters, HI-voll
ocld-Impregnated T filters, Ht-vol
1400 2200 0600 14CXI 2200 0600
215
2/6
2/7 Time,
Fig. 1. Aerosol
I
1400 2200 0600 2/8/79
PST
constituent and NH, Pittsburg, California.
concentrations,
sampler provided four samples exceeding the limit of detection (O.OOGO.008p-equiv m- 3, for HISO,. These correlated approximately with maxima in total acid measurements. The highest level of H2S04 found, 0.012 f 0.002 p-equiv me3, corresponds to 0.6 + 0.1 pg H,SO, mm3. Based on recovery studies described above, atmospheric H,SO, and strong acids may be present at higher levels. Analysis of a representative set of the aqueous extracts for total soluble iron by flame atomic absorption spectroscopy showed that < 10% of the observed strong acid could be ascribed to Fe3+. Comparing the temporal variations for the species shown, elevated particulate strong acidity generally correlated with lower NH, levels. Particulate NH: 3.2 Atmospheric sampling results was in excess with respect to the levels of SOi- plus Results are summarized in Fig. 1 expressed as flu- NO;, consistent with only minor amounts of strong equiv. m- 3. The relatively short sampling times with particulate acids. the hi-volume sampler provided diurnal variations in 3.3 Summary and conclusions total acidity and other aerosol constituents. The precision of these titrimetric results (29% median (1) In the presence of varying levels of atmospheric coefficient of variation) reflected principally the vari- particulate matter, H,SO, aerosol recovery was ability of the quartz filter blank. However, H,S04 < 30% by benzaldehyde extraction, but usually remained below detectable limits (0.002-0.008 p> 60% by strong acid titration using acid-washed equiv mp3) with these samples. The low-volume quartz fiber or Teflon filters. Accordingly, the latter
562
B. R. APPEL. S. M. WAI.L, M. HAIK, E. L. KOTHNY and Y, TOKI~~
Sulfuric acid and particulate strong acidity measurements in ambient air technique may be preferable for atmospheric sampling in spite of its inability to distinguish sources of H+. (2) On the presumption that H2S04 was the dominant source of strong acid measured in the atmospheric samples collected in Pittsburg, California, the peak ambient HzSO, level during the period sampled was 0.033 + O.O08p-equiv mm3 (1.6 f 0.4pgmm3), for a 2-h average. This compares to 0.012 f 0.002/.1equiv me3 (0.6 & O.l~grn-~) HzSO,, for an 8-h average observed by benzaldehyde extraction. The latter corresponds to about 13 % of the water-soluble sulfate. (3) Elimination of non-respirable particles was insufficient to permit accurate measurement of H,SO,, at least for L 10m3 air samples collected on inert filters. The effectiveness of the ammonia denuder in stabilizing HzSO* remains unclear. (4) Additional atmospheric sampling with the techniques described here will be made at locations likely to provide higher levels of HzSO,. Acknowledgement - L. Raftery and J. Benzing provided very capable assistance. Thecyclone used for low volume sampling was designed and calibrated by W. John. The assistance of the staff of the Bay Area Air Quality Management District in providing use of their Pittsburg, California sampling station is gratefully acknowledged. The work was supported by the California Air Resources Board Research Division. The statements and conclusions in this report are those of the authors and not necessarily those of the California Air Resources Board. The mention ofcommercial products, their sources, or use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products. REFERENCES
Appel B. R., Tokiwa Y., Wall S. M., Haik M., Kothny E. L. and Wesolowski J. J. (1979a) Determination of sulfuric acid, total particle-phase acidity and nitric acid in ambient air. Final Report to the California Air Resources Board, Contract A6-209-30.
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