AIRWATCH - Sampling for Nitrogen Dioxide

by P.C. Manins and M.L. Finn

CSIRO Atmospheric Research, Aspendale 3195 Australia

SUMMARY

Nitrogen dioxide (NO₂) is an important urban air pollutant generated by motor vehicles and other combustion sources such as gas stoves. With care, NO₂ can be measured with great sensitivity using the CSIRO kit of experimental equipment for the Airwatch schools and community program of hands-on learning and investigations of air pollution issues. This article describes in detail how nitrogen dioxide sampling is done with the CSIRO kit.

INTRODUCTION

A companion paper, AIRWATCH – The CSIRO Kit (Manins et al., 1998), describes
the Airwatch program and the various experiments in it. By the addition of a second
filter holder, the equipment used to measure fine particle concentrations
(Manins and Finn, 1998) can be extended to monitor nitrogen dioxide
gas concentrations. The steps described here are

• collecting NO₂ gas by a low volume active sampling method onto a filter paper
  coated with an iodide solution that reacts with NO₂ to form a nitrite compound
  on the filter,
• extracting nitrite from the filter sample,
• comparing the colour of the sample with a printed calibration sheet showing
  the concentrations, or comparing with a set of prepared standards of known
  concentration,
• calculating the air concentration of NO₂ and comparing against air quality
  standards.

EQUIPMENT

The equipment needed and possible sources are discussed in Appendix 1.
Figure 1 shows the essential components of the sampling setup. The first filter collects
most particles and prevents them from contaminating the second filter that is treated
chemically to capture NO₂. The flow meter measures how much air is passes through
the filters and the pump sucks the air into the tubing. The battery is necessary for
operation away from mains power. If power is available and one of the recommended
battery trickle chargers is used, the pump can be powered directly from the charger
(a fast charger would probably damage the pump.).

Figure 1. Particle and NO₂ sampling equipment for Airwatch

1. a plain filter holder followed by a treated filter holder; 2. a small gas flow meter; 3. a 12 volt fish tank pump; 4. deep-discharge battery.

PROCEDURAL HINTS

The sampling strategies for monitoring nitrogen dioxide can be similar to those used for monitoring fine particles. However, the sampling time should be shorter since the method is
quite sensitive and the range of calibration can be exceeded. Sampling times can be as
short as an hour or two if the ambient NO₂ concentrations are around 20 ppbv or more. If analysis of the sample indicates a very high concentration, it would be best to repeat the sampling, using a shorter sampling time. Alternatively, the extracted sample can be diluted and the calculation of concentration adjusted to take this into account.

An iodide coating solution is used to treat the filter. The reaction to trap NO₂ on the filter can be expressed as:

Care must be taken to prevent contamination of the filter from the time it is coated, during the sampling period and during the laboratory analysis. Very small concentrations of NO₂ occur in the samples; therefore even the slightest mistake in procedure can result in a large error in the determined concentration of NO₂ due to contamination. Washing all equipment in high grade purified water is an important step; it is tempting to avoid this because of the cost of such water, but it is essential for good results. It would be a good idea to include ‘blanks’ along with the measurement filters for all stages of the analysis. These are filter papers that are coated and handled in exactly the same way as the measurement filter papers, but they are never exposed to NO₂. Analysis should give zero concentration if all is well with the handling and analytical procedures.

Analysis of ambient NO₂ concentration relies on a comparison of the colour of an extracted sample liquid against a precision calibration chart of pink bars. The 'pink scale', shown in Manins et al. (1998: Figure 3), was matched against a set of standards of known NO₂ concentration. When comparing a sample with the pink scale it is important to follow the viewing procedure outlined here.

School students at senior levels may reproduce their own standards, rather than just use the 'pink scale' provided in the kit. The methods for this are given in Appendix 2. Then a calibration curve can be constructed and a densitometer or colorimeter employed to make objective determinations of concentration, but description of this is beyond the scope of the present paper.

The analysis method for NO₂ concentration is well proven. It is the same as used for the passive gas measurements that CSIRO has used for several years.

PROCEDURE

Sampling for NO₂ is carried out 'in-line' with particle sampling, as shown in Figure 1. The second filter holder and filter collects NO₂. This holder should be a clip-down type so that a slightly moist filter does not get torn when the holder is assembled.

The steps are:

1. In the laboratory before the monitoring is begun, prepare several filters with the coating solution as described in Appendix 2.
   
2. Clean and dry the filter holders before sampling.
   
3. Load the first filter holder (screw-down type) with a plain filter for particle sampling. The filter must be present to remove particles from the air stream.
   
4. Using clean tweezers load the second filter holder (clip-down type) with a filter paper that has been treated with the coating solution.
   
5. Follow the sampling procedure as outlined for particle sampling (Manins and Finn, 1998), filling in Table 1 for each sample.
   
6. After sampling, carefully remove the filters from the holders, place in separate new plastic bags, label and seal.
   
7. Back in the laboratory, follow the analysis method described in
   Appendix 2.
   

Table 1. Table of results for nitrogen dioxide sampling

CALCULATION FOR NO₂ IN ppbv

The concentration of NO₂ is usually reported in units of parts per billion (thousand million) or parts per million by volume. This can calculated from the Pink Scale graph. Each colour of the Pink Scale corresponds to a NO₂- concentration ranging from 2 µM to 100 µM. The relationship found using the laboratory methods described in Appendix 2 is shown in Figure 2. If a Pink Scale number near the top of the range is determined, say 8 or 9, the accuracy of the estimate can be improved by diluting the sample to two times the volume and obtaining a new Pink Scale reading. Then the calculated concentration will need to be doubled.

Figure 2. Relationship between pink scale number and concentration of nitrogen dioxide in solution.

If the recommended procedure and extraction volume have been followed, then approximately:

(Other extraction volumes and the effects of temperature can be included — see Appendix 2.)

EXAMPLE:

A Pink Scale reading of 7 is obtained from a sample collected over 12 hours. The volume of air sampled was 2.4 m₃. From Figure 2, the NO₂ concentration in solution is approximately 40 µM. So the NO₂ concentration measured in the air is:

It should then be possible to compare findings with those from local EPAs and with national and international standards. The new Air NEPM for Australia (NEPC 1997) specifies a 1-hour standard for NO₂ of no more than 0.125 ppmv (125 ppbv), so even though in the example the sampling period was 12 times as long, the result almost certainly indicates very clean conditions.

REFERENCES

Manins, P.C. and Finn, M.L. (1998), ‘AIRWATCH – Particle sampling’, this publication.

Manins, P.C., Finn, M.L. and Anderton, J. (1998), ‘AIRWATCH – The CSIRO kit’, this publication.

NEPC (1997), ‘Draft National Environment Protection Measure and Impact Statement for Ambient Air Quality’, National Environment Protection Council Service Corporation, Adelaide, November 1997.

APPENDIX 1: Equipment for Nitrogen Dioxide Sampling

In addition to the equipment required for particle sampling, the following will be needed. Prices mentioned are in Australian dollars; they and the suggested suppliers are given as a guide only.

FILTER HOLDER

For NO₂ sampling a clip-down type of filter holder needs to be used to save tearing the filter if it is still damp. There are several suppliers. One is:

Clip-Down Poretic Filter Holder 47 mm single unit, CAT No. 91250 $65 each ($46 each for 10 or more)

SUPPLIER MEDOS Co., PO Box 717 Mt Waverley Vic 3147
PHONE +61 (3) 9562 6888

Another is:
Clip-Down Filter Holder MFS pp 47 mm Part No. 501200 $36 (Mention Airwatch involvement)

SUPPLIER BONNET EQUIPMENT, PO Box 212 Jannali NSW 2226
PHONE +61 (2) 9524 1277
FAX +61 (2) 9524 1447

PLASTIC VIALS

Square section plastic disposable solution cells are used so that light reflections are not a problem - Part No: 1937/9002 capacity 3.5 ml

SUPPLIER: Starna Pty Ltd, PO Box 113, Thornleigh, NSW 2120
PHONE +61 (2) 9484 0033
FAX +61 (2) 9484 0055
AUS FREE CALL 1800 252 284

The plastic vials scratch very easily — care must be taken when washing them for reuse.

CHEMICALS

Available from Scientific suppliers. For example:

SUPPLIER: Selby Scientific Limited, 2 Clayton Rd., Clayton, Vic.
PHONE +61 (3) 9013 2990 or 9013 2991
AUS FAX 1800 622 991

Or

SUPPLIER: Sigma - Aldrich Pty Ltd. Unit 2, Anella Ave., Castle Hill NSW 2154.
AUS PHONE 1800 800 097 or +61 (2) 9899 9977
AUS FAX 1800 800 096

HPLC GRADE WATER

The water used in the chemical analysis for NO₂ must be HPLC grade and can be bought from chemical suppliers. Other scientific suppliers listed in the yellow pages also stock this item.

SUPPLIERS: Sigma - Aldrich Pty Ltd. Catalogue number 27, 073 - 3 Water HPLC grade.
1 litre $30.10, 6 × 1 litre $135.00, 2 litre $39.90,
4 × 2 litre $110.50, 4 × 4 litre $199.50.
Crown Scientific Pty Ltd, 1 Florence St Burwood, Vic
ph: +61 (3) 9808 0366; AUS FREECALL: 1800 134 175

How much water is required each time samples are analysed?

Preparing filters and analysing using the NO₂ pink calibration chart will require ~ 3 litres of HPLC water. If the standards are to be made up then ~ 5 litres is required.

PINK-SCALE CALIBRATION SHEETS

These are accurately calibrated printed reproductions of the calibration filters. Original printed sheets must be used. Any other rendering of the calibration sheets such as by photocopying will make the accurate determination of concentrations impossible.

Contact the AIRWATCH organisers to obtain copies of the calibration sheets.

APPENDIX 2: Nitrogen Dioxide Analysis Methods

It is important that protective clothing, gloves and eyewear are worn and that relevant Material Safety Data for each chemical is understood. See chemical supplier literature or information on the web such as http://www.dar.csiro.au/airwatch/index.html or a related link for Material Safety Data for the chemicals needed for this work. Dispose of all used chemicals by diluting with a large volume of water and flushing the lot to waste.

The following methods must be carried out in a clean environment. All equipment such as pipettes, beakers, plastic vials, measuring spoons etc., must be washed with soapy water then completely rinsed with HPLC grade water (this is extremely high grade purified water). Shake any excess water off the equipment.

PREPARING FILTERS FOR SAMPLING

HOW TO MAKE THE COATING SOLUTION

The coating solution is made using HPLC grade water and:

1. All equipment must be cleaned carefully with HPLC grade water.
   
2. Weigh NaOH (~ 4 pellets) into a glass flask or dish.
   
3. Dissolve NaOH in a little HPLC grade water (5 ml or less. If the solution goes cloudy in step 4. then not enough water has been used to dissolve the NaOH.).
   
4. Place the flask on a balance, zero it, and add the NaI, then the methanol.
   

COATING THE FILTER

Repeat this procedure for a number of filters. They can be stored for as long as necessary so long as they are individually sealed.

1. Pipette ~2 ml of coating solution (enough to completely soak the filter) onto the filter immediately after removing it from its sealed package with tweezers.
   
2. Make sure the entire filter is soaked in the solution. Add some more solution if really needed.
   
3. Dry the filter in a nitrogen gas atmosphere, in a dessicator or by directing an electric hair dryer onto the filter for a few minutes in a room free of smoke fumes and gas heating equipment and well away from external NO₂ sources.
   
4. Using tweezers, place the filter in a small plastic bag which has a press seal.
   
5. The filters are now ready to load straight into the holder at the sampling site.
   
6. Re-use the plastic envelope to transport the filter back to the lab.
   

ANALYSIS METHOD FOR NO₂

(Exposed filters, if kept sealed and cool, can be stored for several weeks to months.)

MAKING THE REAGENT SOLUTION

1. Weigh 0.050 g NEDA onto a glass dish. Zero the balance.
   
2. Weigh 2.0 g sulfanilamide onto the NEDA.
   
3. Carefully pour the powders into a 250 ml volumetric flask. Wash any remaining powders into the flask with HPLC water.
   
4. Two-thirds fill the flask with HPLC water.
   
5. In a fume cupboard very carefully pipette the H3PO4 (it is highly corrosive!) into the flask.
   
6. Put a stopper on the flask and shake the reagent vigorously until the solids have dissolved.
   
7. Label and date the reagent.
   

EXTRACTING THE FILTER

1. Pipette 5ml HPLC grade water into the plastic envelope holding the filter.
   
2. Carefully soak the entire surface area of the filter with the water.
   

MIXING FIELD SAMPLE WITH REAGENT

1. Pipette equal volumes of extract solution and reagent (eg 2 ml each) into the recommended square-section plastic vial.
   
2. Let the solution settle for half an hour to allow mixing to occur.
   
3. The pink sample should now be ready for comparison with the Pink Scale.
   

COMPARISON BETWEEN SAMPLE AND CALIBRATION SCALE

The method for comparing the sample is as follows:

1. Hold the Pink Scale calibration sheet at arm’s length.
   
2. Hold the top of the vial in the other hand so that you are looking through the two clear faces of the vial.
   
3. Place the vial about 50 mm in front of a white section of the calibration sheet, next to the bars of the pink scale.
   
4. Position yourself so that the main lighting is coming from the side, because there must be no shadow of the vial or your hand on the white background immediately behind the vial. Also avoid any bright reflections from the front surface of the vial — this is why a square section for the vial is important.
   
5. Move the vial along the sheet and decide which step on the Pink Scale most closely matches the sample. An intermediate value may be selected. It is worthwhile to take an average of several readings. The result is the pink scale number for the sample.
   
6. Look at the 'Pink Scale' graph to decide the concentration of NO₂ in the sample. (Read from the chosen pink scale number on the horizontal axis up to the curved line then across to the vertical axis for a result in units of µM‡.)
   

CALCULATION FOR NO₂ IN ppbv

A straight line has been fitted to a set of standards using a least squares regression program. The line is used to calculate the NO₂-concentration in the sample solutions. The concentration of NO₂ gas can be calculated in ppbv as follows:

where: [] = µM concentration in solution

The simplified expression presented in the main paper can be used if the laboratory is at about 20°C and the recommended extraction volume of 5 ml is used.

USING YOUR OWN STANDARDS

This description is for interest. It is not necessary to use this procedure, since a calibration colour chart is available. However, it will be necessary if a densitometer or colorimeter is to be used in place of the Pink Scale. The steps in making the standards are as follows:

• Make up the following in a 1 litre flask with stopper:
  

  0.79 g NaI
  1 litre of HPLC grade water

  Label the flask 'NaI'

• A 100 µM standard is made up from which all the other standards will be derived. To make up the 100 µM standard combine 100 ml NaI solution and 100 µl of 0.1 M NaNO₂ solution (available from chemical supplies)

  Label the flask '100 µM STANDARD'

• Mix the standards according to Table 2 in plastic bottles large enough to hold at least 20 ml of solution.
• Into each standard solution pipette 10 ml of sulfanilamide/NEDA reagent. A colour scale of 'pinkness' should now be evident, proportional to NO₂ concentration, from almost clear (the 2 µM NO₂- solution) to a deep magenta pink (the 80 µM NO₂- solution).

Table 2. Standard Concentrations

Contact: peter.manins@csiro.au

Return to The Airwatch Index

Modified: 16 May 2002