Environ. Sci. Techno/. 1995, 29, 787-791
Sampling Artifacts in the Determination of Particulate Sulfate and Son(!) in the Desert Southwest Using Filter Pack Samplers DELBERT J. EATOUGH,* LAURA J . L E W I S , MICHELE EATOUGH, A N D EDWIN A. LEWISt Department of Chemistv, Brigham Young University, Provo, Utah 84602
Particles collected in the desert Southwest on Teflon filters using an annular denuder sampling system absorb SO2(g) in a subsequential exposure to Son(!) in the laboratory if the concentration of fine particulate sulfate measured with a filter pack sampling system is greater than the concentration of sulfate measured with the annular denuder sampling system. If the concentrations of sulfate determined using the two sampling systems agreed, no absorption of SO*(!) by the denuder-collected particles occurs. These results show that SOdg) may be collected by Southwest desert fine particles during sampling, resulting in measured concentrations of fine particulate sulfate that are higher than the correct concentrations. The magnitude of this filter pack sampling artifact, 0.1 -0.4 pg S042-/m3, is small. However, the artifact is important relative to the concentration of particulate sulfate present in the desert Southwest, typically 0.5-2 pg S042-/m3. The magnitude of the filter pack sulfate sampling artifact is not related to either the atmospheric particulate sulfate or SO2(g) concentrations.
Introduction Several studies have shown that light scattering by fine particulate sulfate is the major component in a fine particle light extinction budget and is often the dominant factor in non-Rayleigh extinction in the Grand Canyon, Canyonlands, and other pristine areas of the desert Southwest during periods of significant visibility degradation (1-4). The accurate determination of particulate sulfate and SOz(g)in these areas is therefore important in identifying the role that SO, sources, SO;! and sulfate concentrations, and SO2 to sulfateconversion chemistry play in visibility degradation. The measurement of particulate sulfate and SOZ(g) has typically been accomplished using a filter pack sampling system in which fine particulate sulfate (‘2.5 pm) is collected on a Teflon filter and SOz(g) is collected by a subsequent paper or quartz fiber filter that has been saturated with base (5). We have occasionally used an alternative technique employing an annular diffusion denuder with a basic adsorptive surface to collect SOz(g) followed by a Teflon filter to collect particulate sulfate (611). In previous publications (9, I,?), we have compared the total SO,, particulate sulfate, and SOz(g1 concentrations that have been determined using filter pack sampling techniques with concentrations for the same species determined using the annular diffusion denuder sampling technique. The data compared were taken from a number of field studies including the following: the 1986 Winter SCENES Special Study (61,the 1987 Winter Haze Intensive Tracer Experiment (WHITEX) (IO),the 1989 SCENESTracer Study (9),and the 1990 EPIC apportionment study in the Green River Basin (7, 8). In all of these studies, the concentration of Particulate sulfate determined with a filter pack sampling system was consistently higher than the sulfate concentration determined with the annular denuder sampler (Figure 1). The determined concentration of SOz(g) was correspondingly lower, and the total SO, concentrations were generally in good agreement between the two types of sampling systems. For two of these studies (7,111, the concentration of sulfate determined with a diffusion denuder sampling system could also be compared to the results obtained using the IMPROVE (5) sampler. The IMPROVE system determines sulfate collected on a Teflon filter after a2.5-pm particle cut in a cyclone inlet. IMPROVE sulfate concentrations were also, on the average, higher than those determined using the denuder sampler (Figure 1). The consistent high bias seen in the determination of sulfate in a desert environment using a filter pack as compared to an annular denuder sampling system is consistent with the hypothesis that some SOz(g)is removed by the collected fine particles during the sampling process with a filter pack. This artifact would not be seen with the diffusion denuder sampling system because the SOz(g) is removed from the sampled airstream by the diffusion denuder prior to the collection of particles. This hypothesis can be tested. This paper gives the results of the comparison of fine particulate sulfate determined in collocated replicate * Corresponding author; e-mailaddress:
[email protected]. Present address: Hart Scientific, Pleasant Grove, UT 84062.
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0013-936x/95/0929-0787$09.00/0
@ 1995 American Chemical Sociew
VOL. 29, NO. 3,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY 1787
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FIGURE 1. Average of the concentrations of sulfate determined using a filter pack sampling system minus a dinusion denuder sampling system for atmospheric studies in the Southwest United States.
samples using a low-volume filter pack sampling system. an annular denuder sampling system, and on the Teflon filter ofan annulardenudersamplingsystemwith exposure of the filter to SO&) following the sample collection. Results are given for experiments conducted at Canyonlands National Park, UT, and at Meadview. AZ. just west of the Grand Canyon National Park
Experimental Section SO2(@Adsorption Experiments. In order to determine if
the difference in fine particulate sulfate determined using a filter pack as compared to a n annular denuder sampling system may be due to SO&) adsorption by collected fine particles, a series of diffusion denuder and filter pack samples were collected at both the Canyonlands National Park, UT, IMPROVE sampling site and the Meadview, AZ. SCENES sampling site using annular denuder and filter pack sampling systems. Annular Denuder Sampler. The annular denuder sampling system consisted of two NaHC0,Iglycerinecoated University Research Glass (Carboro. NC) annular denuder sections (URG2000-30F) in series followed by a Teflon filter pack (URG2000-TF3)witha47-"Teflon filter (Zefluor. 1pm. Gelman Sciences). The inlet was a Tefloncoated impactor inlet (University Research Glass elutriator jet URG2000-30P coupler impactor URG2000-30BC2 with Teflon seal ring URG2000-30F) with a 2.5-pm cut at a flow of 18 L min-I. In a subset of the samples collected at Meadview, additional samples were collected where the Teflon filter was replaced with a 47-mm Nuclepore filter (0.4-pm pore). Samples from the sodium bicarbonatecoated annulardenudersectionswererecovered bywashing with distilledldeionized water and stored at 4 "C until analyzed. The Teflon filters were stored in either a Monovette (Sarstead 02.263.600) or a Petri dish (Millipore PD1004700)at 4 "C until returned to Provo, UT, where they were stored at -40 "C until analyzed. The Teflon filters stored in the Petri dishes were the samples to be exposed to SO2(@subsequent to sample collection. Field-handled blank samples were obtained for every three samples collected. Filter Pack Sampler. The filter pack sampling system consisted of the inlet used with the annular denuder sampling system, followed by the filter pack used with the ?a8 ENVIRONMENTALSCIENCE &TECHNOLOGY 1 VOL. 29, NO.3,1995
annular denuder sampling system. There were no components in the filter pack sampling system used at Canyonlands designed to collect or be analyzed for S02(g).In the experiments conducted at Meadview. the Teflon filter of the filter pack sampling system was followed by two NaHC03/glycerine-coated fiber filters to collect S 0 2 ( g ) . In a subset of the samples collected at Meadview. additional sampleswerecollected where theTeflon filterwas replaced with a47-mm Nuclepore filter. TheTeflon and carbonatesaturated filter were stored in a Monovette at 4 "C until returned to Provo. UT, where they were stored at -40 "C until analyzed. Field-handled blanksamples were obtained for every three samples collected. Sample Collection. Samples were collected during two periods at Canyonlands National Park. The first samples were collected during March 1991. On three consecutive days, a sampling system employing six replicate annular denuders and two replicate filter packs was run for a 24-h period. Each of the eight samplers was preceded by a URG 2.5-pm impactor inlet. The second set of samples was collected at Canyonlands during June 1991. The protocol was thesame as forthe first Canyonlands sampling program, except that three replicate filter packsamples were obtained. The last sampling program was conducted at Meadview in November 1992. In this program, six replicate denuder samples, triplicate filter pack samples with Teflon filters, and triplicate denuder and filter pack sampling systems with Nuclepore filters were collected each sampling period. A total of eight data sets were obtained at Meadview. Sequential day timelnight time samples were collected for the first six sample sets. A 30-h sample and a 24-h sample were then collected. The sample flow rates for both the annular denuder and the filter pack sampling systems for each sample collection period were nominally 20 L(STPI1 min. Forallstudies. flowwascontroUedwithaTylan (Model FC262) mass flow controller and checked with a calibrated Kurz (Model 1541) mass flow meter and a dry gas meter (Rockwell Model R-200). Sample Analysis. The concentrations of SO&) for each experiment were determined from analysis of the two denuder sections. The NaHC0,Iglycerine coating and collected gases in the denuder annulus was recovered by rinsingwith5 mlofwater. Theextractsolutionswerekept refrigerated (4 "C)until analyzed by ion chromatography
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FIGURE 2. Comparison of the concentrations of sulfate determined on a Teflon filter after a diffusion denuder to remove SOdgl a Teflon filter in a filter pack, and a Teflon filter in a denuder sampling system that is exposed to SOAg) after sample collection. Sample sets collected at Canyonlands (CL-A and CL-B) and Meadview. The average and standard deviation of replicate samples are shown.
(Dionex Model 2000i) for sulfate using a Na2C031NaHC03 eluent. The concentration of for the samples collected at Meadview were also determined by analysis of the carbonate-saturated filters. These filterswere extracted by sonication for 20 min with 4 mL of water, and the resulting solutions were analyzed by ion chromatography. For each sample collection period, the three (or two for the first study at Canyonlands) filter pack filters and the three denuder Teflon filterswere analyzed for sulfate in the normal way. The Teflon filter-collected material was extracted by sonication for 20 min with 4 mL of water, and the resulting solution was analyzed by ion chromatography. The remaining three denuder sampler Teflon filters were exposed to S02(g) prior to analysis for sulfate. These denuder sampler Teflon filters, which were loaded with field-collected fine particles, were placed in a URG Teflon filter pack and then attached to a Teflon sample line running to a 30-m3Teflon environmental chamber (13)containing SOz(g) at a concentration of 10 ppb. The SOZ(g) exposure was accomplished by sampling the chamber gas mixture at a nominal flow rate of 20 L(STP)/min (controlled with a Tylan mass flow controller) for 30 min. The total SOZ(g) exposure in this experiment was comparable to the field exposure of the filter pack filters to the ambient concentration of S02(g)over a period of 24 h. Blank Teflon filters were also exposed to S02(g) in these experiments. The actual concentration of SOz(g) in the chamber was determined by collecting two denuder samples and analyzing the bicarbonate extracts for sulfate by IC. A quarter section of each Nuclepore filters collected in the study at Meadview was analyzed for sulfate using the same procedure as normally used for the analysis of sulfate on Teflon filters. The Nuclepore filters collected at Meadview were also analyzed for elemental content by PIXE (14).
Results and Discussion Complete results for the more comprehensive study at Meadview are available (15). Figure 2 shows the results
from the SO&) exposure experiments. The mean sulfate concentration determined with collocated replicate diffusion denuder and filter pack samplers is given for each of the sampling days in the three studies. CL-A is the study at Canyonlands National Park in March 1991. CL-B is the study at Canyonlands National Park in June 1991, and Meadview is the 1992 study at Meadview. For day 1 of the CL-A study, the filter pack artifact was zero with the denuder and filter pack samplers giving the same sulfate concentration of approximately 2 nmol/m3. In this case, exposure of the three additional filters from the denuder samplers to S02(g)did not result in an increase in the apparent sulfate concentration (Figure 2). Presumably, the particles in this sample were not sufficiently basic to adsorb any S02, and the artifact was therefore zero. On days 2 and 3, however, a significant positive filter pack artifact of approximately 3 nmol of sulfate/m3was observed. In both of these cases, exposure of the three additional filters from the denuder sampler showed that the fieldcollected particles can adsorb SOz(g) and yield an erroneous value for the particulate sulfate concentration. The increase in the sulfate found on the S 0 2 ( g ) exposed denuder filters for days 2 and 3 is, in fact, just about the average for the positive artifact shown in Figure 2. This seems quite reasonable as the SOp(g) exposure was designed to mimic the exposure that particles in a filter pack would receive during a 24-h sampling period at typical ambient concentrations of S02(g). During the CL-B study, comparable results were obtained. However, in the second set of samples collected at Canyonlands, no artifact was seen in the first two collected samples, and an artifact of about 2 nmol of sulfate/m3was seen for the third sample collected. The increase in sulfate on a Teflon filter upon exposure of the field-collected fine particles from an annular denuder sampling system to SO2 was also comparable to the measured difference in the sulfate determined using a filter pack as compared to an VOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1789
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sulfate sampling artifact was seen in both the comparison of the sulfate concentrations determined using the two sampling systems and in the results obtained for exposure of the denuder filter to SO2subsequent to sample collection for sample 7 but not for sample 8 (Figure 2). It has been established that particle losses due to deposition to the denuder wall during passage of sampled air through annular denuders are small,
