Mirai
Watanabe
a,
Takejiro
Takamatsu
b,
Masami K.
Koshikawa
*b,
Kazunori
Sakamoto
c and
Kazuyuki
Inubushi
c
aGraduate School of Science and Technology, Chiba University, 648 Matsudo, Chiba, 271-8510, Japan
bSoil Science Section, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan. E-mail: mkanao@nies.go.jp; Fax: +81-29-850-2576; Tel: +81-29-850-2440
cFaculty of Horticulture, Chiba University, 648 Matsudo, Chiba, 271-8510, Japan
First published on 25th November 2005
We developed a method to analyze atmospheric SOx (particulate SO42− + gaseous SO2) and NOx (NO + NO2) simultaneously using a battery-operated portable filter pack sampler. NOx determination using a filter pack method is new. SOx and NOx were collected on a Na2CO3 filter and PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl) + TEA (triethanolamine) filters (6 piled sheets), respectively. Aqueous solutions were then used to extract pollutants trapped by the filters and the resulting extracts were pre-cleaned (e.g. elimination of PTIO) and analyzed for sulfate and nitrite by ion chromatography. Recoveries of SO2 and NOx from standard pollutant gases and consistency of the field data with those from other instrumental methods were examined to evaluate our method. SOx and NOx could be analyzed accurately with determination limits of 0.2 ppbv and 1.0 ppbv (as daily average concentrations), respectively. The sampler can determine SOx and NOx concentrations at mountainous or remote sites without needing an electric power supply.
Active samplers such as filter pack (FP) methods are widely used for sampling atmospheric pollutants.8–10 FPs have been employed in acid deposition monitoring networks in various countries, such as in EANET,11 CASTNet,12 and EMEP.13 Although FPs provide sensitive measurements and are relatively inexpensive and simple, sampling artifacts often occur in distinguishing particulate (SO42−, NO3− and NH4+) and gaseous (SO2, HNO3 and NH3) forms.14–16 To overcome these problems, annular denuder17 and impactor/honeycomb denuder/filter pack18 systems were introduced. Although these denuder systems are sophisticated and precise, they are relatively expensive, complex, and more labor-intensive than the more usual FPs.10
Active samplers are usually operated using a fixed power supply, and field-oriented samplers operated by batteries are limited;19–21 although many personal samplers to monitor occupational atmospheric environments have been developed and are usefully employed.22–27
We developed a battery-operated portable FP sampler to simultaneously collect atmospheric SOx and NOx at mountainous sites without the need for a power supply, and to obtain daily (or hourly if concentrations are relatively high) average concentrations. In mountainous areas, NOx concentrations can rapidly fluctuate depending on mobile sources (automobiles), local meteorological (e.g. formation of a temperature inversion layer) and weather conditions.1,2,28 They may also fluctuate as a result of NO, which is naturally emitted from the forest soils.29,30 On the other hand, SOx concentrations often change daily or hourly under the influence of volcanic gases (a specific condition in Japan). Forest health may be more seriously affected by short-term high concentrations of the pollutants than by long-term averages. Therefore, our FP sampler may provide a new tool for information gathering, one that would be available to monitor and diagnose forest health.
We analyzed SOx and NOx as total concentrations without distinguishing particulate SO42− and gaseous SO2, or NO and NO2 due to the following reasons: (1) SOx and NOx deposited to vegetation are converted rapidly to SO42− and NO3−, respectively; (2) The analytical precision for SOx is decreased by speciation, because of the very low SOx concentrations in Japan and unexpected artifacts (partial adsorption of SO2 on a pre-filter to collect particulate pollutants14,31); (3) NOx concentrations may be a useful index of air pollution, because most of NOx from anthropogenic sources is emitted initially as NO, and then converted to NO2. In Japan, NOx emissions (especially from automobiles) have been controlled under effluent standards (e.g. NOx and PM Act); and (4) All acidic pollutants including NO should be removed and so facilitate clean exhaust emission from our sampler to a small field chamber32 (details are to be published elsewhere).
The novelty of our method is highlighted by the following points: (1) Determination of NOx using a FP sampler is new, although such samplers (and personal samplers) have been applied for NO2.15,27,33; (2) Our FP sampler is a field-oriented type, which can operate continuously for periods of several days at mountainous sites; and (3) The system is completely air-tight from inlet to outlet; thus, clean exhaust at a constant flow rate can be supplied to a small field chamber, for example, for studying the material leaching from tree leaves.
The pollutants collected on the filters were analyzed by ion chromatography after extraction into aqueous solutions. The best sampling and analytical conditions to obtain the maximal and reproducible recoveries were investigated using standard pollutant gases. Then, field data obtained by our sampler were compared with those from other instrumental measuring methods to confirm performance.
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Fig. 1 Scheme (a) and picture (b) of a portable filter pack sampler for SOx and NOx. |
PTIO + TEA filter: Cellulose filters (Advantec No. 5C, 47 mm id) were washed with deionized water by sonication for 30 min, and rinsed 3 times with deionized water. The filters were then dried for more than 12 h at 60 °C in an electric oven. 0.5 ml of 0.1 M PTIO and 0.5 M TEA (in acetone) was impregnated homogeneously into the filter. The filters were then shaken gently to evaporate the acetone, stored in an opaque desiccator containing NaOH (or silica gel) as the desiccant, and used within five days (the filters can be stored for more than 1 month in a refrigerator34).
a Dionex. b Hitachi. | |
---|---|
Analyzer: | DX-100a |
Column: | IonPac AS12A/AG12A (4 mm id)a |
Eluent: | 2.7 mM Na2CO3 and 0.3 mM NaHCO3 |
Flow rate: | 1.5 ml min−1 |
Suppressor: | ASRS-ULTRA or ASRS-ULTRA IIa |
Autosuppression recycle mode | |
Sample injection volume: | 250 μl |
Detector: | Conductivity detectora |
Integrator: | D-2500 chromato-integratorb |
All chemicals were obtained from Wako Pure Chemical Industries, except for PTIO (Tokyo Kasei Kogyo). Deionized water was obtained from a Milli-Q system (Millipore).
Alkaline solutions impregnated into filters | Recovery of SO2 (%) | |||
---|---|---|---|---|
Alkali | Solvent | Volume/ml | From SO2 aloneb | From SO2 + NOxc |
Values: means (standard deviations), replicates. The cellulose filters (Advantec No. 5C, 47 mm id) were used.a Tetrabutylammonium hydroxide.b 100 ppmv SO2 in 1 L N2.c 100 ppmv SO2 + 400 ppmv NO + 200 ppmv NO2 in 1 L N2.d Not tested. | ||||
1 M Na2CO3 | Water | 0.3 | 98.7 (0.3), 3 | 25.1 (3.5), 3 |
1 M Na2CO3·1.5H2O2 | Water | 0.3 | —d | 56.7 (6.4), 3 |
1 M NaOH | Water | 0.3 | 98.8 (5.5), 3 | 16.7 (2.1), 3 |
1 M NaOH + 0.2%Ca(OH)2 | Water | 0.3 | 98.2 (0.9), 3 | 12.7 (0.7), 3 |
6% K2CO3 + 2% glycerol | Water | 0.3 | 101.8 (2.1), 3 | 52.0 (1.9), 3 |
0.5 M TEA + 0.1 M PTIO | Acetone | 0.5 | 87.5 (13.8), 13 | 48.7 (2.4), 5 |
10% TBAHa | Methanol | 0.5 | 100.4 (2.5), 3 | 67.2 (3.7), 3 |
10% TBAH + 0.5 M TEA + 0.1 M PTIO | Methanol | 0.5 | — | 44.9 (2.3), 3 |
Aqueous absorbend (0.1 M NaOH) | 0.6% H2O2 | 10.2 | 96.6 (3.0), 13 | 103.1 (0.7), 3 |
The recoveries of SO2 from air samples with SO2 alone (without NOx) were quantitative, irrespective of the filter types, except for the PTIO + TEA filters, with which recoveries varied from 63 to 99% (this variation was not improved by changing the sample volume (1 L or 10 L) or the matrix (N2 or air)). However, when SO2 coexisted with NOx in the air samples, SO2 recoveries decreased in all cases, although SO2 detected in the 2nd alkaline filters, which were installed behind the 1st filters, was just trace (<5%).
These results suggested that SO2 was collected quantitatively on all alkaline filters, but when SO2 coexisted with NOx it was partly transformed to S-species, which were not recoverable by the analytical method (ion chromatography) used. Such a reaction might be promoted when concentrated standard gases prepared in dry N2 matrix are used. Sickles and Hodson16 also reported that a portion of the SO2 retained by nylon filters was unrecoverable. To convert the unrecoverable S-species on alkaline filters to SO42−, the oxidative digestion method37 was applied as the post-sampling treatment. When the air samples contained SO2 and NOx, SO2 recoveries were low (38–69%) by the H2O2 extraction method (described above), but improved to 72–100% when the oxidative digestion method was employed (Table 3). On the other hand, recoveries from air containing SO2 alone (without NOx) were quantitative in both methods. These results suggested that the intermediate oxidized S-species are formed on the alkaline filters during sampling.
Alkaline solutions impregnated into filters | Recovery of SO2 (%) | |||
---|---|---|---|---|
From SO2 alonea | From SO2 + NOxb | |||
H2O2 extraction | Oxidative digestion | H2O2 extraction | Oxidative digestion | |
Values: means (standard deviations), replicates. The quartz fiber filters (Pallflex 2500QAST, 47 mm id) were used.a 10 ppmv SO2 in 10 L N2.b 10 ppmv SO2 + 40 ppmv NO + 20 ppmv NO2 in 10 L N2.c Not tested. | ||||
0.5 M Na2CO3 | 96.7 (2.1), 5 | 96.8 (2.1), 3 | 49.7 (8.3), 4 | 83.7 (3.0), 10 |
1 M Na2CO3 | —c | — | 64.0 (3.9), 3 | 84.6 (10.6), 3 |
1 M NaOH | 99.4 (—), 2 | — | 64.6 (6.9), 3 | 90.2 (7.8), 10 |
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Fig. 2 Recoveries of SO2 at different humidities. Relative humidity: dry, ca. 0%; humid, 25.2 ± 5.4% (18–35%). Concentrations of pollutants: SO2, 5–10 ppmv; NO, 20–40 ppmv; NO2, 10–20 ppmv. Filter: Na2CO3 filter of 47 mm id. |
Recoveries of SO2 from dry air with SO2 and NOx were ca. 50%, whereas those from humid air were ca. 87% (range: 83–92%). This could indicate that moisture accelerates the oxidation of SO2 collected on the Na2CO3 filters to SO42−, even when NOx coexists. Lewin38 and Sickles et al.10 also reported that humidity was an important parameter in the determination of SO2 by the FP method.
Artificially polluted air samples | Filters | Recovery of NOx (%) | ||||||
---|---|---|---|---|---|---|---|---|
Pollutants | Conc./ppmv | Volume/L | Matrix | Size/mm id | Sheets | As NO2− | As NO2− + NO3− | Replicates |
Values: means (standard deviations).a 40 ppmv NO + 20 ppmv NO2 + 10 ppmv SO2.b 400 ppmv NO + 200 ppmv NO2 + 100 ppmv SO2.c Each filter was impregnated with 0.5 ml of 0.1 M PTIO + 0.5 M TEA (acetone solution).d Each filter was impregnated with 0.1 ml of 0.1 M PTIO + 0.5 M TEA (acetone solution). | ||||||||
NO | 40 | 10 | N2 | 47c | 6 | 87.6 (3.6) | 97.4 (5.3) | 11 |
NO | 40 | 10 | Air | 47 | 6 | 103.0 (4.2) | 104.2 (4.5) | 8 |
NO | 20 | 10 | Air | 47 | 5 | 101.3 (2.8) | 103.5 (2.7) | 4 |
NO | 20 | 10 | Air | 47 | 4 | 91.5 (5.6) | 93.6 (5.4) | 4 |
NO | 20 | 10 | Air | 25d | 4 | 70.7 (9.5) | 73.8 (9.7) | 22 |
NO2 | 20 | 10 | N2 | 47 | 6 | 87.7 (5.1) | 104.7 (9.9) | 10 |
NO2 | 200 | 1 | N2 | 47 | 6 | 101.2 (2.3) | 104.7 (2.0) | 6 |
NOx + SO2 | Lowa | 10 | N2 | 47 | 6 | 103.3 (1.9) | 107.4 (1.2) | 4 |
NOx + SO2 | Highb | 1 | N2 | 47 | 6 | 99.3 (6.1) | 102.3 (7.4) | 5 |
NOx was recovered quantitatively using the 6 piled sheets of PTIO + TEA filters. When 40 ppmv NO in the 10 L bag with N2 was sampled, the amount of NO collected on each filter decreased exponentially from the 1st to 6th filters, with 90% of the NO being recovered as NO2− and the remainder as NO3−. However, most (ca. 99%) NO was recovered as NO2− when NO was prepared in an air matrix (with moisture). As shown in Fig. 3, NO is oxidized to NO2 by PTIO, and then the NO2 is disproportionated with TEA and H2O to form NO2− and NO3−.39,40 Therefore, conversion of NO to NO2− may be promoted by moisture. At the lower NO loadings, 5 filter sheets were sufficient for quantitative collection, but the recoveries decreased if the number of filters was reduced further or if smaller filter sizes were used. This may be attributed to the slow reaction between NO and PTIO, and thus a portion of NO may have escaped without oxidative collection. Recoveries were also decreased when coarse filters were used.
NO2 was collected more easily, and thus by less filters than NO, probably because the reaction between NO2 and TEA was rapid. For example, NO2 in air samples (20 ppmv in 10 L N2) was collected quantitatively on the 1st to 3rd filters. Sickles et al.33 also recovered NO2 almost quantitatively (87 ± 9%, n = 33) on glass fiber filters with TEA from air containing 5–400 ppbv NO2.
NOx was also recovered quantitatively from air samples with NOx and SO2, and most (>96%) of the NOx was recovered as NO2−. SO2 did not interfere with the NOx determination. Similar results have also been reported by Sickles et al.33
Most of the NOx collected on the PTIO + TEA filters was recovered as NO2− (NO3− was trace), probably because the ambient air contained moisture. Distributions of NO2− recovered from the 1st to 7th filters are shown at different concentrations of NOx in ambient air (Fig. 4). More than 98% of the NO2− was detected in the PTIO + TEA filters (the 2nd to 7th filters). Trace NO2− and NO3−, recovered from the Na2CO3 filter (the 1st filter), might have been nitrous acid and particulate nitrate + nitric acid, respectively.10,33 In other FP and passive sampling methods using TEA as absorbent, most NO2 is retained as NO2− (with trace NO3−) on the filters.33,39,41
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Fig. 4 Distribution of NOx recovered from each filter at different atmospheric concentrations of NOx. 1st filter: Na2CO3 filter, 2nd–7th filters: PTIO + TEA filters. Sampling sites: NIES and Mt. Tsukuba. Period: January–April 2004. |
When ambient SOx was sampled, it was not detected from the PTIO + TEA filters (these can also collect SOx) behind the Na2CO3 filter, or from the other Na2CO3 filter which was tentatively placed behind the first Na2CO3 filter. The side reaction (i.e., from SO2 to unrecoverable S-species) on the Na2CO3 filter was usually negligible under moist conditions. Also, almost quantitative recoveries of ambient SO2 using similar alkaline filters have been reported.9 In the case of ambient NOx, it was recovered mostly from the first PTIO + TEA filter, and recoveries decreased exponentially at the subsequent filters (those from the last filter were negligible) (see Fig. 4). In addition, as described later (Fig. 5), good correlation was observed between the ambient pollutant concentrations obtained by our FP method and those by other instrumental methods. Therefore, even under field diluted conditions we expected good recoveries of SOx and NOx, similar to those observed for standard gases with high pollutants.
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Fig. 5 Relationships between (a) daily average concentrations of SOx obtained by the current FP method and those of SO2 obtained by UV fluorescence, and (b) daily average concentrations of NOx obtained by the current FP method and those obtained by chemiluminescence. Values of slope and intercept: mean ±95% confidence interval. Period: January 2004–February 2005. |
In Fig. 5(a), the pollutants analyzed were different between our FP and the instrumental methods (i.e., SOx in the former and SO2 in the latter); thus, linear regression that did not pass the origin was employed. SOx concentrations obtained by our method correlated well with those obtained for SO2 (not SOx) by the UV fluorescence method, but were always ca. 30% higher, probably due to particulate SO42− also being accounted for in this method, as our sampler was not equipped with a particle collection filter and/or an impactor. The intercept did not differ significantly from zero; suggesting that the SO42− fractions were relatively small and changed with SO2 (i.e., were absent when SO2 was absent). High concentrations of SOx, affected by volcanic gases discharged and transported from Miyakejima Island (located 240 km SSW of NIES) were sometimes observed. This was supported by back trajectory analysis using METEX.42
In Fig. 5(b), the pollutants analyzed were the same between both methods; therefore, linear regression including the origin was employed. NOx concentrations obtained by our method were in good agreement with those obtained by chemiluminescence, showing a regression line with a slope of about one.
During the study period, temperature and relative humidity varied widely over ranges of 2.8–29.5 °C (average: 13.0 ± 8.7 °C) and 29–83% (59 ± 15%), respectively. Therefore, our FP method may enable simultaneous determination of atmospheric concentrations of SOx and NOx, without significant effects from temperature or humidity.
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