course work 2 diffusion tubes

8/6/2019 Course Work 2 Diffusion Tubes

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Environmental PollutionAssignment 2

The monitoring of Nitrogen Dioxideusing diffusion tubes

INTRODUCTION

Adverse effects on health and the natural environmentfrom air pollution have been recognised and reduced inthe last century by effective legislation, and pollution

control. However, there is emerging evidence that thetoxic effects of new photochemical pollutants such asNitrogen Dioxide are apparently related to respiratoryinfections, (Chauhan & Johnston, 2003) of which accountfor an estimated 4 million deaths worldwide from 1997 to1999 (World Health Report, 1997). Currentepidemiological evidence suggests that much of themorbidity and mortality, especially in young children isrelated to both sources of indoor and outdoor pollution

(Hester and Harrison, 1998).

In terms of outdoor pollution, recent evidence frommonitoring data and research has inferred that NO2emissions have increased, and are much higher thaninitially thought particularly due to one sector, roadtransport (e.g. AQEG, 2004). Furthermore, recent studiesshow that NO2 emissions are the main source of pollutionin Bristol (Bristol City Council Air Quality Data, 2009). In

particular NO2 concentrations are higher closer tomotorways and decline with distance, perhaps elevatinglocal vicinity background concentrations (Roorda-Knape,M.C., et al. 1999).

In order to protect human health, outdoor concentrationsof ambient NO2 have been regulated throughout Europesince 1985, with the EC Daughter Directive (1999/30/EC)setting Value Limits to be achieved by 2010 (EuropeanCouncil, 1999). In response to attempt to achieve thesetargets is the UK Governments National Air Quality

Student ID: 07502832 March 14th 2009 Page: 1
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Strategy (NAQS), providing objectives for ambient NO2(DETR, 2000) concentrations as purposed in Table 1below. As stated in LAQM. TG(03) the NAQS annual meanobjective applies to air at locations which are situated

outside of buildings or other natural or man-madestructures, above and below ground, and where membersof the public are regularly present. The particular sitedoes accommodate regular members of the public. As theexposure period in the experiment was two weeks, resultscannot be standardised with shorter averaging means (i.e.1 hour mean) only annual means. However Air QualityConsultants state that if an annual mean concentrationexceeds 60 g/m then it is likely that the NAQS 1-hour

mean objective would have been exceeded (Air QualityUnit, 2008).

Table 1: NAQS objectives for NO2 to be completed by December 2005

(Source: Defra, 2000)

Background NO2 concentrations for the surveyed area on

the Bristol, South Gloucester boundary for 2009, asestimated by Bristol City Council (2007) is ~ 16-40 g/m.

Figure 1 A) Regional annual mean of recorded NO2 in 2007 B) Sampling sitelocation

(Source: Bristol City Council, 2007)


A B
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To model roadside NO2 concentration from traffic relatedair pollution, passive diffusion tube samplers wherelocated as in Figure 2 C), on the South Gloucester

boundary near the M32 motorway. The M32, is the principallink between the National Motorways and the City network,carrying an annual average daily traffic (AADT) volume of ~85,000 vehicles per day including medium and heavy goodsvehicles (Bristol City Council, 2006).

Figure 2 A) National geographical area B) Regional geographical area C) Samplingsite location

(Source: Google Earth, 2009)

THEORETICAL PRINCIPLE OF DIFFUSIONTUBES

Since their introduction in the late 1970s by Palmes andGunnison (Palmes et al., 1976) the method has beenvalidated for outdoor use (Atkins et al., 1986) allowing

passive diffusion tubes to be used for indicativemeasurements of time weighted average concentrationsof nitrogen dioxide in ambient air (Gatrell & Lytnen1998). At present it is undertaken on a wide scale in theUK for spatial and temporal measurements in the contextof Local Air Quality Management (AEA, 2008). Typicalcomponents of a passive diffusion sampler are shown inFigure 3.

Figure 3: Typical components of a diffusion sampler, consisting of an acrylic or

polypropylene tube (length 7.1cm and internal diameter of 1.1cm), two or three


B

A

C
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stainless steel inert meshes coated with triethanolamine and two polyethylenecaps

(Source: AEA, 2009)

As the name suggests passive diffusion tubes absorb thepollutant to be monitored directly from the surrounding

air, without the need of a power supply. It is designed andbased on molecular diffusion along a fixed length tube intoan efficient absorbing medium as depicted in Figure 4.

Figure 4: Palmes diffusion tube. Co is the NO2 concentration in ambient air,Cout isthe NO2 concentration at the external interface of the membrane, Cin is the NO2concentration at the internal interface of the membrane and Cfin is the NO2concentration at the interface with the sorbet. S is the internal cross-area of thetube and Sout is the internal cross-area of the membrane cap.

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(Source: Gerboles et al., 2005)

The gas molecules undergo what is deemed moleculardiffusion, with regards to the molecules diffusing from aregion of high concentration (open end) to a region of lowconcentration (absorbent end) along the tube, (whichstops particulate material accumulation due to a lowerrate of diffusion) until collection onto an impregnated filteror an absorbent material. The diffusion barrier is believedto maintain the sampling rate. Additionally the fine meshminimises convective transport of gas to the filter. Interms of monitoring nitrogen dioxide the filter is usuallycoated with triethanolamine that retains NO2 as a stablenitrite analyte, as shown in the following reaction.



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(Source: Hangartner, 2007)

LIMITATIONS OF TECHNIQUE

The diffusion tube technique has two main limitations.

Firstly it is an indicative monitoring technique. Therefore,suitable for screening surveys, or identifying locations ofhigh NO2 concentrations, however it doesnt provide thesame level of accuracy as automatic monitoringtechniques. Secondly, the method only provides aconcentration that is averaged over the exposure period(typically 1-4 weeks) reducing temporal resolution as it isnot possible to measure short-term (e.g. hourly)concentrations (Bush et at., 2001). This also means thatresults cannot be compared with air quality standards andobjectives based on shorter averaging periods i.e. 1 hourmeans (AEA, 2008).

Process is reliant on the following underlining assumptionsin determining sample rate;

1.) Molecular diffusion coefficient is known for NO2 in air,along with its dependence on temperature, pressure,humidity (i.e. presence of water vapour)2.) The absorber is perfect.

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3.) The absorbed gas can be measured quantitatively.4.) There is no absorption or adsorption by the tube walls.5.) No chemical reactions occur in the tube duringsampling that would affect the NO2the concentrations.

6). The concentration is maintained constant at theentrance of the tube (Cape, 2005).

General consensus is that most of the originalassumptions are met in practice, with inaccuraciesminimised except under certain well-defined conditions.However these limitations still remain.

Other possible limitations are summarised in Table 2

below.

Table 2: Possible limitations of the passive diffusion technique

Influence of meterological factors on NO2 samplingrates, i.e. changes in temperature and relative humiditycan impact on performance (Plaisance et al., 2004).Furthermore studies have found that a temperaturereduction has a strong effect on NO2 calibration factors(sampling rates) (Sickles and Mitchle, 1984).

Tubes usually manufactured from either acrylic or

polypropylene have the disadvantage of being almostopaque to UV light. This blocking of UV light is thoughtto result in reduced NO2 photolysis in the tube.Slight possibility of secondary reactions or chemicalchanges in the trapped material during sampling and/orstorage i.e. NO and O3 to give NO2 effecting samplingratePossible interference from other gasesInterfering effects of peroxyacetyl nitrate (PAN) apollutant associated with vehicle emissions (Campbell et

al., 1994)Insufficient extraction of nitrite from grids (Heal et al.,2000)Require two consecutive exposure periods to improvesystematic error, reducing turnaround time of results.Colour development determination in methodologysubjective.Risk of losses of sampled material due to back-diffusion(reverse diffusion), especially for longer sampling times.Lack of specificity compared to chemiluminescentdetectors.



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No standard operating procedures existRelatively limited shelf life (1+ year).Laboratory analysis required.

RESULTS

Nitrogen dioxide was measured at sample site (Figure 2C)between the 3rd and the 17th of February, using Palmesdiffusion tubes. As described in the principles section, TEAretains NO2 as a stable NO2 (nitrite) analyte. NO2concentrations can then be inferred from this estimation,

as the quantity of nitrite in the tubes is directly related tothe average concentration of NO2 in the air. Therefore byusing prepared known standard nitrite solutions, acalibration curve can be plotted of of absorbance ofstandards against NO2(Table 3). The indicative meanabsorbance of nitrite in the sampled tubes (Table 4) canthen be used to estimate the concentration directly fromthe curve (Figure 6). This estimation can be improved byrearranging the linear regression formula y = ax +b

to make x (i.e. estimated nitrite concentration) thesubject.

Table 3: Mean absorbance of nitrite standard solutions with statisticalevaluation

Concentration NitriteStandard Solution(s)

[mg/dm3]

MeanSpectrophotometerResponse at 543nm

wavelength

0.0 0.0000.2 0.090

0.4 0.184

0.6 0.352

0.8 0.460

1.0 0.572

Statistical Evaluation

X 0.457



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Y 0.251

A -0.0192

B 0.5911

r-squared 0.992

Figure 6 Calibration curve of nitrite standards

Calibration Curve of Nitrite Standrds

y = 0.5911x - 0.0192r-squared = 0.992

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Nitrite concentration [mg / dm3]

Nitriteabsorbance[54

3

wavelength]

Table 4: Estimated absorbance of nitrite from sampled site including field blanks

Sampled diffusion tubesreplicates

Mean absorption at 453nmwavelength

1 0.2602 0.2683 0.2434 0.241

5 0.2616 0.2267 0.261

Mean 0.251Standard deviation 0.015

Coefficient ofvariance 5.98

Field blanks

Mean absorption at 453nm

wavelength



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1 0.0002 0.0023 0.000

Control blankMean absorption at 453nm

wavelength1 0.001

Figure 7: Estimated absorbance of nitrite from sample site with mean

Estimated mean absorbance of nitrite from sampled site

0.200

0.210

0.220

0.230

0.2400.250

0.260

0.270

0.280

1 2 3 4 5 6 7

Diffusion tube sample

Absorbance

at453

Mean

Field blanks where used and analysed simultaneously withthe exposed samples to check for any possible

contamination i.e. passive diffusion into sealed tube. Thecontrol blank was placed in the fridge to determine anyabsorption or adsorption by the tube walls undercontrolled conditions.

The mean nitrite concentration from the sample site isestimated at 0.457 mg/dm3. Rearranging Fickssecond law (Figure 5) allows ambient nitrogendioxide concentrations (in mass per volume or air

and volume per volume of air) to be estimated totwo decimal places as follows;

Equation 2: Ficks second law, prediction of how diffusion causes theconcentration field to change with time

C = (Q x Z)(D x r2 x t)

C = average concentration NO2 in air (g m3)



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Q = quantity of analyte, Q= 2G g as analysis on 2 cm3

(g /cm3)Z = diffusion path (m)D = diffusion coefficient (m2/sec)

r2

= cross section (mm2

)t =exposure time (s)

= 0.914 x 0.071.46 x 10-5 x 9.5 x 10-5 x 1188600

C = 38.8 g/m3(over two week duration)

Air quality standards require concentration measurementsin units of volume of nitrogen dioxide per volume of air.

This calculation takes into account the variation oftemperature and pressure on molar volume (Figure 5). Inaddition mean wind velocity was estimated at 3.6 mph or1.61 m/s.

Table 5: Daily mean temperature and pressure

Date(s)

Mean temperature

[C] Mean pressure [mb]03.02.09 0.3 992.9

04.02.10 1.9 991.7

05.02.11 0.2 988.6

06.02.12 0.5 991.4

07.02.13 1.1 998

08.02.14 1.7 1001.6

09.02.15 2.5 991.9

10.02.16 3.3 1003.3

11.02.17 3.4 1018.212.02.18 2.4 1024.9

13.02.19 4.9 1023.3

14.02.20 4.6 1028.2

15.02.21 6.2 1027.8

16.02.22 7.1 1026.7

17.02.23 8.6 1027

Mean 3.2 1009(Source: Prodata Weather systems, 2009)



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This means the results must be converted as follows;

Molar volume = 22.41 x 276.2 x 1009

273 1013= 22.57

ppb = C x MVRMM

Relative molecular mass ofNO2 = 46

ppb = 33.8 x 22.5746

ppb = 16.6 (over two week duration)

DATA ANALYSIS

Data validity requires comprehensive intraspecific analysisof both systematic and random errors. In terms ofsystematic errors it is important to appreciate that theresults of the measurements are only as good as theinstrument, which in turn will only give accurate results ifproperly calibrated (Pepper et al., 1996). Therefore thegreatest source of systematic error in the assignment ismost likely attributable to the nitrite standard calibrationespecially at lower concentrations, as shown by high

coefficient of variance in previous classroom experiments.It must be appreciated that for complete statisticalevaluation, a comparison of laboratory results againstcertified reference standards would be required. Thereforethis absence significantly reduces data validity, as analytequality and methodology cannot be accessed. Thestatistical analyse of estimated calibration data (Figure 6)did however produce a high r-squared value (correlationcoefficient) for linearity. This result is equal to the percent

of the variation in one variable that is related to thevariation in the other. This strong correlation suggests



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that the equipment response is universally sensitive andthat no apparent anomies occurred during the procedure.

In terms of sample site data validity, both standard

deviation and coefficient of variance statistics have beenimplemented. These can be used to assess random erroror precision as a measure of reproducibility. The averageamount data deviates from the mean abundance is 0.251 0.015. This low standard deviation suggests greatprecision of reproducibility. Consequently the precision inmeasurements is good, as there is only an estimated5.98%, variation in the data set, with no apparentoutliners. There is however an important caveat in using

this statistic for data interpretation. Comparison ofsampled data to quality control data is required tocalculate percentage deviation. Without this standardisedbenchmark, data could have a desirable coefficient ofvariance but operators could repeatedly make the samemistakes gaining similar but inaccurate results. Reiteratedby Cape (2005), the largest source of error in usingdiffusion tubes for monitoring nitrogen dioxide is inter-laboratory variation. This apparent lack of standardisation

in methodology seriously reduces data validity.

Field blanks and control blanks used in the preparation ofthe calibration curve and in-situ seem to suggest thatthere is no evidence of trace sources of artificiallyintroduced contamination causing bias. Therefore valuesdid not require subtracting from masses of exposedsamplers. However in QC analysis laboratory equipmentshould be calibrated against blank control diffusiontubes from a certified laboratory.

The validity of data is influenced not only by methodology(Bush et al., 2001) but, meterological factors. (Plaisance etal., 2004) Of which the strongest bias on sampling rate iscreated by the effect of wind velocity, causing a reductionin molecular diffusion length (Glair & Penkett, 1995). Thestudy quantified this as a 47% reduction in diffusion lengthfor air velocities close to 2m/s. Consequently, with acalculated mean air velocity of 1.6 m/s during the two



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week sampling period (Prodata Weather Systems, 2009)then a plausible diffusion reduction is 38%.

CONCLUSION

In comparison to Defras NAQS for the UK (Table 1) theroadside sampling site analysed was 3% below theobjective for protection of human health at 38.8g/m3.More specifically for kerbside locations (1-5m from thekerb of a busy road) it is 28% below the annual UKaverage concentration determined by the NO2 diffusiontube network in 1997 (Stevenson et al. 2001). Moreoverthe results reflect ambient findings published by Bristol

City Council in 2007 (Figure 1). Hence it seems that in thelight of scientific understanding about the effects ofnitrogen dioxide on health, levels atthat particular samplesite levels are acceptable. However despite this,concentrations are above new legislation introduced in2007 for protection of vegetation and ecosystems (Defra,2007).

Finally, as an indicative method for screening nitrogen

dioxide in potential problematic locations, evidencestrongly suggests (Gerboles et al, 2005 & Bush et al.2001) that the accuracy of measurement accomplishesthe Data Quality Objective (DQO) laid down in theEuropean Directive, justifying their continued use. Beingcost effective and convenient in mapping spatialdistributions and investigating long-term trends. Howeverdata must be interpretated carefully, with regards toappreciating potential systematic and random error that

could cause either positive or negative bias. Additionalresearch and method standardisation is required to fullyvalidate and clarify these internal and external impacts.

REFERENCES

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Administrations. Available fromhttp://www.airquality.co.uk/archive/reports/cat05/0802141

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