Microchemical Journal 96 (2010) 132–138

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Microchemical Journal

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Development and validation of passive samplers for atmospheric monitoring of SO2,NO2, O3 and H2S in tropical areas

Vania P. Campos a,⁎, Lícia P.S. Cruz a, Ricardo H.M. Godoi b, Ana Flávia L. Godoi b, Tania M. Tavares a

a Analytical Chemistry Department, Chemistry Institute, Federal University of Bahia, Salvador – Bahia, Brazilb Department of Environmental Engineering, Federal University of Parana, Curitiba – PR, Brazil

⁎ Corresponding author.E-mail address: vaniaroc@ufba.br (V.P. Campos).

0026-265X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.microc.2010.02.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 November 2009Received in revised form 23 February 2010Accepted 23 February 2010Available online 1 March 2010

Keywords:Passive samplersSulfur dioxideNitrogen dioxideOzoneAtmospheric monitoring

Traditionally the monitoring of atmospheric pollutants is aimed at managing accidents and short-termactions for the protection of human health on local and regional scales. These monitoring networks requiredcontinuous measurements with high time resolution, involving costly equipment, operation andmaintenance. More recent simulations with mathematical models and scenario-building on regional,continental and global scales, as well as studies of environmental personal exposure are demanding threemain types of monitoring design, where need for special resolution surpasses the one of temporal resolution:remote places, where no electric power nor trained human resources are available; validation of dispersionsimulation by mathematical modeling on regional to larger scales; indoor measurements where people areacting under noise restriction such as residences, school and hospitals. To meet these demandsenvironmental passive samplers for atmospheric gaseous pollutants have been developed and applied fordifferent situations and environments of Brazil. Passive samplers were developed based on moleculardiffusion of gas through a static layer of air with fixed filters impregnated with a specific solution for SO2,NO2, O3 and H2S and used in the following situations: in remote forest area, urban areas with differentcharacteristics – near the sea, with clean ocean air masses coming in and in areas under direct industrialinfluence. The passive samplers showed good performance when exposed to different types of areas andduring different periods: accuracy of measurements, based on standard deviation, were within therecommended limit by the European Union of ±25% for this type of device and precision below themaximum of 20% cited in the literature, with variation of 2.0 to 16% in the case of these work. The passivedevices were able of measuring concentrations very low as those found in the Amazon region (0.58±0.05 µg m−3 SO2 and 1.10±0.07 µg m−3 NO2) and higher concentrations in industrial areas (20.0±1.3 µg m−3

SO2 e 21.0±1.3 µg m−3 NO2).

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Human activities have contributed effectively to the increasingemissions of pollutants into the atmosphere at levels that canpromote change on a global scale, with possible catastrophicconsequences for the humanity, for example, the depletion of thestratospheric ozone and the increase of the greenhouse effect.

Problems related to the deterioration of air quality have also beenseen on a regional scale as a result of emissions of pollutants such asnitrogen oxides, sulfur compounds (including SO2 and reduced sulfurcompounds) and volatile organic compounds that participate invarious chemical and physical transformations in the atmosphere,which result in several consequences, such as the formation oftropospheric ozone, secondary particulate matter and acid deposition.Furthermore, problems at the local level as those related to odors have

also been frequently reported. For example, the H2S has veryunpleasant odor, which is perceptible to the human sense at verylow concentrations, making bad smelling areas near the emissionsources.

For a more effective control of the pollution and to assess thepotential effects of the atmospheric pollutants on human health andon the environment, it is necessary to develop alternative strategiesfor monitoring, that are economically viable, easy to operate, whichcan be used in different areas including remote areas, and providingreliable information on of the pollutant concentrations.

Active sampling techniques are usually applied, where the air issucked into the sampling device with the aid of vacuum pumps. Thisimplies logistical difficulty, especially in remote areas, since frequentairflow calibration and electric power are required, where oilgenerators should not be used due to pollutant emission and commonbatteries are not able to provide the required power.

Additionally, the equipments used are generally expensive,involve complex operations and require constant maintenance,making it difficult to achieve the non intermittent air monitoring

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data. These difficulties also limit the number of monitoring sitesrequired within large areas under different influences.

Passive sampling has proven to be a viable alternative for theatmospheric monitoring at low cost and high spatial resolution, if thedesired monitoring does not require high time resolution (instanta-neous values).

Passive samplers are devices able to take samples of gas or vaporpollutants from the atmosphere without involving air active move-ment through them. The sampling rate is controlled by a physicalprocess such as diffusion or permeation [1] and the averageconcentration of gases in the atmosphere during the exposure periodof the passive sampler is calculated by integration of Fick's first law[2]:

C−C0 =mLDAt

ð1Þ

where C is the external (environmental) gas concentration (µg m−3),m is the total collectedmass (µg), L is the diffusion length (m), D is thediffusion coefficient (m2 h−1), A is the cross-section area of diffusioncourse (m2) and t is the sampling time (h). If an efficient collectionmedia is used, the gas concentration on the collector surface (C0) canbe assumed to be zero; Eq. (1) reduces to:

C =mLDAt

ð2Þ

The diffusion coefficient in air for the sampled gas must becorrected to average ambient temperature during sampling period.

These types of devices were originally developed for application inthe monitoring of personal exposure in workplace, where concentra-tions of pollutants are high [3,4]. However, since the 80s somemodelsof passive samplers for air contaminants began to be optimized andused for external environmental monitoring, where concentrations ofpollutants are much lower and meteorological conditions have greatinfluence.

The application of passive samplers in the monitoring of gases andvapors offers significant advantages when compared with activetechniques: they are simple, low cost, thus allowing simultaneoussampling in large areas of interest; they are small and lightweight,making it easy to transport to areas of difficult access, such as forests,mountains and small islands; often combine the steps of sampling,analyte isolation and pre-concentration in one step; do not requireuse of pumps, and therefore electricity is not necessary; then they canbe used in remote or rural communities, where no electricity isavailable and do not require trained technical staff at the samplingsite. In addition these devices do not require measurements ofsampled air volume, because the results are calculated as averageconcentration for a fixed period by integration of Fick's first law [5,6].Different models of passive samplers, with different geometries andcollection media, have been reported [7–14]. However, few passivesamplers have been validated for atmospheric monitoring in tropicalclimate, where high temperature require more stable reactions andhigh humidity require less hydrophilic reaction products for fixation.Additionally, in the case of atmospheric monitoring in the SouthHemisphere (SH), more sensitive reactions are generally required forremote areas, since concentrations tend to be lower than in the NorthHemisphere (NH), where these devices have beenmostly applied. Thepassive sampler used in the present work has a configurationsuggested by Ferm and Svanberg [15]. The new developmentconsisted of the use of new reagents for the determination of thegaseous pollutants and validation in different tropical environments –urban, industrial and remote, both maritime and forestry – bycomparison with data obtained simultaneously with traditionalcontinuous monitoring. The advantages of homemade monitorscompared to commercial ones are essentially the lower cost and theeasy accessibility, avoiding importation of commercial devices which

frequently involves long lasting and costly bureaucratic procedures inmany countries.

The object of this study was to apply passive samplers in a SHtropical climate and validate by intercalibration with well establishedand adopted commercial continuousmonitors for outdoormonitoringof NO2, O3, SO2 and H2S in different types of Brazilian environments.

2. Materials and methods

2.1. Sampling

At each sampling site, three sets of passive samplers for eachmeasured gas were exposed and fixed with double-sided sticky tapein a plastic dish support for rain and sun protection, at 2 m height andapart for at least 1 m from buildings, trees or other obstructions. Afterthe exposure period, the samplers were sealed, placed in polyethylenevessels, taken to the laboratory and stored in the refrigerator at 4 °Cuntil chemical analysis. To determine whether contamination oc-curred during preparation, transportation and analysis, two samplersof each type were used as field blanks completely sealed inpolyethylene containers, following the same trajectory in the fieldof the exposed passive sampler, ie they were transported to thesampling sites and returned to the lab after the exposure periodwithout exposure.

These samples have been exposed to sites with different sources ofatmospheric emission and environments: Salvador city, capital ofBahia state, Brazil, with approximately three million inhabitants(13°00′ S and 38°30′ W) and situated in a peninsula with the AtlanticOcean and the All Saints bay, receiving Atlantic air masses of tradewinds most of the time; a mature petroleum field in Buracica,Alagoinhas, State of Bahia, 80 km N of Salvador (12°13′20″ S and38°28′44″W) and in Camaçari, 50 kmN of Salvador (12°40′ S e 38°20′W) where the largest petrochemical complex of the South Hemi-sphere is located, concentrating about 50 petrochemical plants, onepulp mill, a copper metallurgy, and various fine chemicals industries;a remote rural area in the Amazon region in the state of Rondônia, in aforest area converted to pasture, about 200 km from the Ji-Paraná city(10°45′44″ S and 62°21′27″ W) and in two residential areas, withmedium traffic density, at two primary schools in the Curitiba city,capital of Paraná State, in south of Brazil, with approximately1.8 million inhabitants (28°17′33″ S and 49°13′22″ W).

2.2. Description and preparation of the passive sampler

The passive samplers built and used (Fig. 1) was based on gasmolecular diffusion through static air layer and consists of acylindrical polyethylene tube (internal diameter 21 mm, length12 mm) sealed at the bottom. A Teflon membrane (pore 0.5 mm,diameter 25 mm, hydrophobic) and a stainless steel screen formembrane protection (thread diameter 0.08 mm and mesh size0.125 mm) were used at the air inlet, except for the case of H2S wherethe screen has been eliminated to avoid losses of the gas due to itshigh reactivity and easy adsorption on surfaces. After the diffusionlayer there was a cellulose filter (Whatman 40) which wasimpregnated with a specific reagent to trap the diffused gas.

Cellulose filters cut to a diameter of 25 mm to adjust at the bottomof the passive sampler were washed three times with deionized waterin ultrasound and once with ethanol for 10 min and then dried in anoven at 50 °C. All parts of the samplers, and also the polyethylenecontainers used for transport received the same cleaning treatment.For the preparation of the sampling devices, aliquots of 200 μL of animpregnation solution were added to the filters, which were thendried for 48 h in a desiccator containing silica gel. Then, the samplersweremounted, sealed with Parafilm to avoid contamination, placed inpolyethylene containers, sealed and stored in plastic bags properlyidentified until the beginning of the sampling. For each measurement

Fig. 1. Schematic representation of the passive sampler.

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period three samplers receiving the same treatment were kept in thefield without exposure for blank control. Table 1 shows the solutionsused for impregnation of filters and the corresponding analyticalmethods.

2.3. Analysis

After the period of passive samplers exposure, each filter wastransferred to a microtube (Eppendorf, 1.5 mL capacity); the product ofthe gas reaction with the impregnating reagent was extracted inultrasonic bath for 15 min and centrifuged for 5 min (13,500 rpm). ForSO2, 1.5 mL of a 1.0×10−2 mol L−1 H2O2 solution was used to ensurecomplete oxidation to SO4

2−, followed by ion chromatography deter-mination [16,17]. TheO3 reaching the impregnatedfilter corresponds tothe amount of nitrite in the filter oxidized to nitrate, which wasextracted with 1.5 mL of deionized water; the nitrate concentration inextracts was analyzed by ion chromatography [18,19].

In both cases above, the analysis conditionswere: anion separationcolumn Transgenomic ICSep AN1 (250×4.6 mm), eluent NaHCO3

1.7 mmol L−1/Na2CO3 1.8 mmol L−1, flow 1.0 mL min−1 and ionsuppresser Altech DS-PLUS. To determine the precision of the analysis(defined as relative standard deviation), each standard solution ofsulfate or nitrate calibration curve was analyzed four times. Anaverage of 2% was obtained for the precision. The accuracy wasestimated as 4% from the analysis of artificial rain reference sample:Rainwater ID/AES-02 – Unitech, USA.

NO2 absorbed on impregnated filters as nitrite was extracted with1.5 mL of deionized water and determined by molecular spectropho-tometry (λ=540 nm) using the modified Griess–Saltzman method[20]. H2S trapped in impregnated filter [21] was determined as sulfideby molecular spectrophotometry (λ=670 nm), using the methyleneblue method [22].

The passive samplers were validated in the industrial area bycomparing values with the average of those obtained with parallelmeasurements of SO2, NO2 and O3 continuousmonitors manufacturedby Environnement, and calibrated with a dynamic gas calibrator

Table 1Filter impregnation solutions for the passive sampler and analytical methods used.

Sampled gas Impregnation solution

SO2 1.0×10−2 mol L−1 Na2CO3 [16,17]O3 1.5×10−1 mol L−1 NaNO2/2.0×10−1 mol L−1 Na2CO3/1.0 mol L−1

NO2 5.0×10−1 mol L−1 KI/2.0×10−1 mol L−1 KOH in methanol [15]H2S 5.5×10−2 mol L−1 Zinc acetate/1.0×10−1 mol L−1 NaOH/1.0×10−

(Environnement, Model EV-3M). In the forest area the continuousmonitors used were from Thermo Environment for monitoring NO2

(Mod 42C) and O3 (Mod 49C). Tests for exposure of H2S passivesamplers in field were validated with parallel measurements of a H2Snon-commercial monitor, consisting of a flow system (FIA) coupled toa spectrofluorimeter HITACHI F1000 [23], based on the effect ofsuppression of fluorescence of solution acetate mercury-fluorescein(FMA) resulting from the presence of sulfide.

In addition to the continuous monitor, validation of the sulfurdioxide passive sampler in an urban area and in a remote forest wereconducted using denuder tubes (glass tubes 35 cm long×6 mm indiameter) coated with a solution of potassium tetrachloromercurate(TCM) 0.1 mol L−1 with air flow of 40 Lh−1 for 24 h. SO2 fixed andstabilized as dichlorosulphitomercurate was subsequently deter-mined by molecular spectroscopy (λ=548 nm) using the pararosan-iline method [24,25].

3. Results and discussion

3.1. Validation of the passive samplers

The validation of the passive sampler for NO2, O3, SO2 and H2S(Fig. 2) in the field was conducted in an industrial influence areaexposing the devices for different periods beside the continuousmonitors to compare the results considering the calibrated continu-ous methods as a reference. The accuracy of the measurements usingpassive samplers, expressed as percentage relative error, and theprecision based on the relative standard deviation for each setcontaining 3 samplers and different exposure periods were deter-mined. The statistical analysis was performed using the F test toevaluate the accuracy of the methods and the paired t test to comparemeans. Fig. 2 displays examples of different comparative measure-ments at different sites and with different exposure times.

The accuracy of the NO2 passive sampler was determined at threedifferent sites of the area with industrial influence. The statisticalanalysis showed no significant difference between the results of the

Analytical method

Ion chromatography [16,17]Glycerol [18,19] Ion chromatography [18,19]

Molecular spectrophotometry UV–VIS [20]2 mol L−1 trisodium citrate [21] Molecular spectrophotometry UV–VIS [22]

Fig. 2. Atmospheric concentrations for SO2, NO2, O3 and H2S measured simultaneouslyusing passive samplers with different periods of exposure and continuous monitors inarea of industrial influence. Mean Environment temperature and relative air humidity:22–31 °C and 78–100%. Mean value of active sampling during period of exposure ofpassive sampling. – – – – Mean from sets of three passive samplers.

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concentrations obtained by both methods. The correlation betweenthe concentrations obtained with passive samplers for NO2 and thecontinuous monitors is highly significant (R=0.9556, pb0.0001). Forperiods of exposure between 7 and 28 days, the accuracy of mea-surementswith passive samplers varied between 2.0 and 12%, with anaverage of 6.8% for the concentration range 3.5–21 μg m−3 atmo-spheric NO2.

The obtained results with O3 passive samplers and continuousmonitors in two seasons during exposure periods ranging from 1 to4 weeks, were compared and the average accuracy was determinedas 11%. The two methods were statistically compared, concludingthat the average concentrations did not differ significantly withinthe confidence interval of 95%. A study of the correlation betweenthe 2 types of sampling was also done with these data by regressionanalysis and the results show that there is good agreement betweenthe two methods (R=0.8551, pb0.0001). An average of 9.6% wasobtained for the accuracy of measurements with passive samplersfor O3, in the concentration range from 14–31 μg m−3 atmosphericO3.

The accuracy of the SO2 passive sampler measurements wasverified, in the industrial area, by comparison withmeasurements of acontinuous SO2 analyser and in urban and remote forest areas bycomparison with denuder tubes coated with 0.1 mol L−1 TCMsolution. The results of these comparisons for periods of exposurebetween 1 and 2 weeks showed average accuracy of 17 and 9.5% in theindustrial area and in urban areas respectively, representing anaverage accuracy of 15%. The statistical analysis shows that there is nodifference between the average concentrations obtained by passivesamplers and continuous monitors; the linear regression results showthat there is good agreement between the two methods (R=0.9473,pb0.0001). The accuracy of passive samplers for SO2 ranged between2.4 and 11% in the concentration range of 2.8 to 20 μg m−3 inindustrial area, and 4.0–10% in the concentration range of 1.9–3.0 μg m−3 in urban area.

The accuracy of measurements of H2S passive sampler wasdetermined by comparison with the average concentrations obtainedwith a non-commercial continuousmonitor, resulting in an average of19% for exposure periods of 1 to 2 weeks. For the statistical analysis itwas observed that there is no significant difference between theaverage concentrations determined by two methods, and linearregression analysis there is a good correlation between them. Theaccuracy of measurements made with these samplers varied from9.0–15% in the concentration range of 0.5–3.0 μg m−3.

The average accuracy of themeasures with all the passive samplersfor exposure period between 1 and 4 weeks are within the limitrecommended by the European Union Directive of ±25%, andprecision of passive measurements, expressed as relative standarddeviation, is below themaximum value of 20% commonly found in theliterature [8,26–31].

The detection limits obtained for the passive sampler using fieldblanks shown in Table 2 are compatible with passive sampler de-veloped by other authors and reported for 1 and 4 weeks exposure[15,32]. The detection limits of the passive sampler for NO2, SO2, O3

Table 2Passive samplers detection limit (µg m−3) for NO2, SO2, O3 and H2S consideringdifferent periods of exposure.

Passive sampler Exposure time

One week One month

SO2 0.26 0.13NO2 0.21 0.060O3 1.1 0.29H2S 0.29 –

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and H2S were calculated taking into account the value of 3 times thestandard deviation of the blanks obtained in field campaigns for eachexposure period. In the calculation, the diffusion coefficient and theparameters (A and L) related to the geometry of the sampler were alsoconsidered.

3.2. Application of passive samplers in other Brazilian areas

3.2.1. Urban areas (Salvador – BA and Curitiba – PR)Measurements of SO2, NO2 and O3 were made in nine sites of

Salvador city with the passive sampler exposed for two consecutiveperiods of two weeks each; the daily temperature ranged between22 and 28 °C and the relative humidity between 65 and 100%.Salvador city is in a privileged position with respect to dispersion ofpollutants: located on the Atlantic coast, with predominance oftrade winds from east (SE, E and NE), receives air masses from theAtlantic during more than 80% of the time. Thus, in all samplinglocations the average concentrations of SO2 (1.8–3.9 μg m−3), NO2

(3.6–12 μg m−3) and O3 (17–37 μg m−3) are far below the Brazilianlegislated standards [33].

The passive monitoring carried out in the atmosphere of thetwo primary schools in residential areas in the Curitiba city showssimilar results to those in the Salvador city: 1.2–1.9 μg m−3 SO2, 6.7–11 μg m−3 NO2 and 19–22 μg m−3 O3 (Fig. 3).

3.2.2. Forest area used as pasture (Ji-Paraná – RO)The passive sampler for SO2, NO2 and O3 were exposed in a forest

area used as pasture in Rondônia (Amazon region), in periods ofheavy rainfall and in the dry season when many fires occur in forestand pasture areas.

The atmospheric concentrations obtained in wet and dry periodswere: 1.10±0.07 μg m−3 and 4.60±0.20 μg m−3 for NO2; 16.0±1.2 μg m−3 and 51.0±3.1 μg m−3 for O3; b0.26 μg m−3 and 0.58±

Fig. 3. Average atmospheric concentrations of SO2,

0.05 μg m−3 for SO2, respectively. This means an increase of about4 times for NO2 and 3 times for O3 in the dry season as a result of firesin the region. These data are consistent with those found for con-tinuous monitoring for NO2 and O3 (Fig. 4), and discontinuous mon-itoring using denuder tubes coated with 0.1 mol L−1 TCM solution forSO2.

3.2.3. Petroleum field (Buracica – BA)H2S passive samplers were exposed for 1 and 2 weeks in four sites

of the petroleum field in Buracica – BA. In petroleum exploration,groundwater contamination with anaerobic bacteria is common,producing H2S, which is emitted into the atmosphere and causing anuisance to workers and to the local communities, who frequentlycomplain of the unpleasant odor. The average concentrations of H2Swere found in range of 0.42–1.3 µg m−3 and as the threshold ofperception of the odor of this gas is 0.70 µg m−3 [34] the complain ofthe population proved to proceed. The accuracy of the passivesamplers was determined between 5.2 and 14%.

4. Conclusions

The passive samplers developed and validated showed adequateperformance when exposed to different geographical regions underthe influence of diverse pollutant sources such as traffic, industries, oilfields and biomass burning during different seasons.

When compared to active monitoring methods under realconditions, the passive sampling used showed accuracy of 10–19%,thus within the limit recommended by the European Union (±25%).The precision of the measurements as relative standard deviation forthree simultaneously applied passive samplers was below themaximum value of 20% cited in the literature, varying between 2.4and 12% for SO2 at the concentration range of 0.58–20 μg m−3, 2.0 to15% for a NO2 concentration range of 1.1–21 μg m−3, 2.5 to 16% for a

NO2 and O3 in Salvador – BA and Curitiba – PR.

Fig. 4. Comparison of average NO2, O3 and SO2 concentrations measured in parallel bycontinuous measurement techniques and passive sampling in a forest area used aspasture, Rondônia, Amazon region, wet and dry season.

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O3 concentration range of 14–51 μg m−3, and 5.2–15% for a H2Sconcentration range of 0.40–3.0 µg m−3. Considering the growingdemands for environmental monitoring, passive samplers are anexcellent tool for low cost atmospheric monitoring.

Acknowledgements

Authors acknowledge CAPES for fellowship and CETREL forsupplying the SO2, NO2 and O3 continuous analyzer's data from thestation located at industrial influence areas.

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