3.3  Sampling of nitrogen dioxide

A manual method based on absorption of nitrogen dioxide on a sodium iodide impregnated glass-sinter has been developed by Ferm and Sjödin (1993). Due to the reasons mentioned below, only the sodium iodide method has been included in this manual.

Several methods, both manual and continuous have been used for the measurement of nitrogen dioxide in ambient air. In urban air, the chemiluminescence method have replaced the manual absorption solution methods, and is introduced as an ISO standard (ISO, 1985a). The chemiluminescence method for NO2 is based on reduction to NO by a heated catalytic converter and calculation of the concentration as the difference between (NO+NO2) and NO (the signal without converter). For clean air sites commercial monitors are usually not sensitive enough, and since other reducible nitrogen compounds (e.g. HNO3 and PAN) may exist in the same concentration level as NO2, the method is not specific. However, monitors with selective photolytic converters may be used if the sensitivity is adequate.

Also the liquid phase NO2-luminol chemiluminescence reaction has been used in a commercial monitor for nitrogen dioxide at low levels (Schiff et al., 1986). This monitor has been shown to give almost interference-free values for NO2 (e.g. Gehrig and Baumann, 1993). However, a small interference from ozone has been observed by Kelly et al. (1990) and Hesterberg and Neftel (1993) found non-linearities under 1 ppb in addition to other systematically errors due to pressure variations in the inlet system and temperature changes in the reaction chamber. Since this monitor work with a liquid phase reaction, it needs more regular service than the ordinary chemiluminescence instruments.

The manual absorbing solution method based on direct Griess reaction during sampling (Saltzman method) has also been appointed an ISO standard (ISO, 1985b). This method is sensitive and more selective than the chemiluminescence method, but the colour to be measured spectrophotometrically is developed during sampling, and the measurement have to be performed immediately after sampling due to instability. This makes the method unsuitable if the exposed absorbing solution has to be transported to a chemical laboratory far from the sampling site, particular if temperature and light exposure cannot be controlled. Field inter comparison studies has shown that the Saltzman method is not suitable at concentration levels below 1  µg/m3 (EMEP, 1999).

Other absorbing solutions have been used in which nitrogen dioxide is absorbed and transformed to nitrite (EMEP, 1977). These methods are usually not sensitive enough in background areas, and also have the problem of instability of the exposed absorption solution during transport when the temperature and sunlight cannot be controlled. There has also been considerable uncertainty about the absorption efficiency of the absorbing solutions, and to which extent this varies with concentration. The use of experimentally determined absorption efficiencies has shown to be inadequate (Fähnrich et al., 1993).

3.3.1  Determination of nitrogen dioxide using the iodide absorption method

3.3.1.1  Introduction

This method (Ferm and Sjödin, 1993) is based on the same principle as the method developed by Pavlenko and Volberg (1979, 1991). This method is recommended at EMEP stations with low concentrations of NO2 and where the analysis has to be performed in a laboratory far from the sampling site.

3.3.1.2  Principle

Ambient air with a flow rate of about 0.5 l/min is drawn through an air intake (inverted funnel) and a glass filter impregnated with sodium iodide (NaI) and sodium hydroxide (NaOH). Nitrogen dioxide is absorbed in the filter and the iodide reduces NO2 to nitrite (NO2-). The hydroxide is converted to carbonate during sampling due to uptake of carbon dioxide. The nitrite formed on the glass filter is extracted with deionized water. After extraction the nitrite concentration can be determined photometrically by the Griess method described in Section 4.11.

This method can be used for measurement of nitrogen dioxide on a 24 h basis in ambient air within the range 0.1-10 µg NO2-N/m3, assuming an air sample of 0.7 m3 and an extraction volume of 4 ml. Exposed samples are stable for several weeks and can be transferred to a laboratory for chemical analysis.

3.3.1.3  Sampling efficiency and interference

The sampling efficiency at a flowrate of 0.5 l/min and a relative humidity of 15% is higher than 98%. With a relative humidity higher than 60%, the sampling efficiency is higher than 98% even at a flowrate of 4 l/min. (Ferm pers. comm.).

Interference studies showed negligible formation of nitrate on the NaI/NaCO3-substrate. Nitric oxide formation was also never observed behind the filter and no oxidation of nitrite by ozone is found. The absorption of PAN (peroxyacetyl nitrate) and the subsequent formation of nitrite on the alkaline NaI filter have been demonstrated with about 20% absorption (Ferm and Sjödin, 1993). This cause a positive interference which can be severe if the PAN-concentration is higher than the NO2-concentration. This may happen in very remote areas, but not at most of the EMEP-sites.

3.3.1.4  Sampling equipment

Figure 3.3.1 shows the components of a suggested sampling system. The Figure shows an automated system, a simpler set-up with manual exchange of samples may also be used. The main components are:

Air inlet

An inverted funnel made of PTFE teflon, polypropylene, borosilicate or polyethylene should be used in order to prevent entrance of precipitation at the sampling point.

 

Figure 3.3.1: Sampling system for nitrogen dioxide.

Tubing

The sampling tube connection between the air inlet and the absorption system should be as short as possible, and made of PTFE teflon, polypropylene, borosilicate glass or polyethylene.

Filterholder with prefilter

A filterholder with a filter should be used in front of the absorption system in order to remove particulate matter. The filter must be inert to NO2. A teflon membrane filter with a pore size 1–2 mm or a Whatman 40 cellulose filter or equivalent may be used. The filterholder and the connections to the sampling line must be airtight. The prefilter can be used for one week.

Absorption system

A 4 mm thick sintered glass filter 25 mm i.d. with a porosity of 40–60 mm enclosed in a glass bulb as shown in Figure 3.3.2 is used as a substrate for the impregnation. The glass bulbs should be connected to the sampling line using short pieces of silicon tubing. During transport the silicon tubing must be closed by pieces of glass or plastic rods.

Figure 3.3.2: Sintered glass filter in a glass bulb.

Flow control device

In order to maintain a constant flow through the absorption system, a restrictor (glass capillary or hypodermic needle) or an electronic flow controller should be used.

Pump and gas meter

A membrane pump is recommended. If the pump is placed in front of the gas meter it must be leakproof. A calibrated dry or wet gas meter is recommended for recording the sampled air volume. Accurate air volume readings are most impor­tant for the accuracy of the measurement. Calibration of the gasmeter should be performed at least once a year. If the gas meter is placed in front of the pump, it must be assured that the pressure drop behind the absorption system is negligible.

3.3.1.5  Commercial supply

Pump:
GAST, Model DOA-P101-BN
MEG. Corp., Benton Harbor, Mich, USA.

Gas meter:
FLONIDAN
Gallus 2000 G1.6
Islandsvej 29
DK-8700 Horsens, Denmark

Sinter glass filter in bulb:
Porosity 40-60 µm
Werner Glas & Instrument AB
Västra Rydsvägen 118
S-196 31 Kungsängen, Sweden
Tel.: +46 8 851 700 70, fax: +46 8 581 700 71

3.3.1.6  Site requirements

The site requirements for nitrogen dioxide are as for sulphur dioxide with respect to regional location and point sources. Particular attention should be paid to the possibility of contamination from motor vehicles, tractors, and other machinery with combustion engines. As nitrogen dioxide is taken up by vegetation, the air intake should not be sheltered by vegetation, but be freely exposed. The air intake should be 2–5 m above the ground. The sampling site should be at least 100 m away from any road open to public traffic, but the minimum distance to roads depends also on the traffic volume. This has been discussed in Section 2. The pump and sampling equipment should be placed in a room where the temperature is controlled at 20 °C ± 5 °C.

3.3.1.7  Preparation of the absorption system

Impregnation solution

The impregnation solution consist of 9 g NaI and 1 g NaOH in 90 g methanol or ethanol (7.9 g NaI and 0.88 g NaOH in 100 ml methanol or ethanol). The solution should be made fresh for every new batch of filters to be prepared, due to possible uptake of nitrogen dioxide from laboratory air.

Cleaning of exposed samplers

Used samplers must be carefully cleaned before use. They may be left overnight soaked in deionized water to release old marking labels etc. After that the samplers are then cleaned by flushing deionized water into the back end of the tube ( in the opposite direction of the sampling flow). At 0.5 - 1 litre of deionized water should be flushed through each tube to ensure a proper cleaning. After cleaning the samplers should be dried in a laboratory oven at 100 - 105oC.

Preparation of the sinters

500µl impregnation solution is added to carefully cleaned glass sinters. The sinters should be dried with a flow of NO2-free air. For cleaning the drying air, a NaI impregnated filter should be used. The glass bulbs should be closed as soon as possible after the drying. Well-protected impregnated sinters can be stored for several weeks before, as well as after sampling, preferably in a cool place.

3.3.1.8  Sampling procedure

Assemble the sampling equipment at the site as shown in Figure 3.3.1, and make sure that the glass bulbs are coded with site name and date. Control that the equipment is leak free. Check the initial flowrate with the gasmeter. It should be about 0.5 l/min. Record the gasmeter reading at the start and again at the stop. For a 24-hour sample the total air volume should be about 0.72 m3. If a sequential sampler for one week is used, it is possible to record the total volume for seven samples and then divide by seven, provided the flow rate is kept constant. When using a sequential sampler is it important to check that the right glass bulb is actually exposed.

After sampling the glass bulbs must be sealed and sent to the laboratory for chemical analysis.

3.3.1.9  Preparation of samples and chemical analysis

Preparation of samples

Open the front end of the glass bulb and add carefully 4.0 ml of a 0.001 M (133 µl = 149 mg triethanolamine to 1 litre water) solution of triethanolamine in deionized water. (The triethanolamine is added to reduce the iodine formed in the reaction with nitrogen dioxide to iodide). The open end should be closed again and the bulb shaken for about 15 min. The lower end should then be opened and placed in a vial or test tube. When the upper end is opened the leaching solution flows through the glass sinter and into the test tube. Some of the solution may be removed by blowing air into the open end of the bulb. About 0.5 ml of the leachate will, however, remain in the glass filter using this procedure, and need to be thoroughly washed out when preparing the filter for new sampling. The NO2--concentration can be determined as described in Section 4.11 or by an automatic version of the method either in the flow-injection (FIA) or continuous flow mode.

Blanks

All steps in the described procedure, which could contaminate the samples, should be controlled regularly and properly documented.

Before using the impregnation solution, it should be controlled for the content of NO2-. In order to have the same concentration of the iodide reagent in this test sample as in the normal samples, 0.5 ml of impregnation solution is mixed with 4 ml deionized water before analysis. The analysis is performed as described in Section 4.11. The impregnation solution blank value should be less than 0.005 µg N/ml.

When a new batch of impregnated filters have been produced, 5% of the filters should be leached as the exposed samples. The leaching solution should be analysed in the usual way for NO2-. The amount of NO2- found should be less than 0.02 µg N/filter.

In every batch of impregnated filters sent to the sampling site, filters which shall remain unexposed (field blanks) must be included. For daily sampling, one field blank per station per week is needed.

3.3.1.10  Calculation of the air concentration

The concentration C of nitrogen dioxide in the air sample expressed as µg N/m3 is given by:

         

where   a   is the concentration of NO2- in µg N/ml in the leachate,
            v1  is the volume of the leaching solution, normally 4 ml,
            v2  is the volume of the sampled air in cubic meter.

3.3.1.11  References

Aas, W, Hjellbrekke, A.-G., Semb, A. and Schaug, J. (1999) Data quality 1997, quality assurance, and field  comparisons. Lillestrųm. Chemical Co-ordinating Centre, Norwegian Institute for Air Research (EMEP/CCC 6/99).

European Monitoring and Evaluation Programme (1977) Manual for sampling and chemical analysis. Lillestrųm, Chemical Co-ordinating Centre, Norwegian Institute for Air Research (EMEP/CHEM 3/77).

Fähnrich, B., Hanssen, J.E. and Nodop, K. (1993) Comparison of measuring methods for nitrogen dioxide in ambient air. Lillestrųm, Chemical Co-ordinating Centre, Norwegian Institute for Air Research (EMEP/CCC-Report 3/93).

Ferm, M. and Sjödin, Å. (1993) Proposal of an impregnated filter technique for monitoring of NO2 at EMEP stations. In: EMEP Workshop on measurements of nitrogen-containing compounds. Les Diablerets, Switzerland, July 1992. Lillestrųm, Chemical Co-ordinating Centre, Norwegian Institute for Air Research (EMEP/CCC-Report 1/93).

Gehrig, R. and Baumann, R. (1993) Comparison of four different types of commercially available monitors for nitrogen oxides with test gas mixtures of NH3, HNO3, PAN and VOC and in ambient air. In: EMEP Workshop on measurements of nitrogen-containing compounds. Les Diablerets, Switzerland, July 1992. Lillestrųm, Chemical Co-ordinating Centre, Norwegian Institute for Air Research (EMEP/CCC-Report 1/93).

Hesterberg, R. and Neftel, A. (1993) Problems with the Luminox detector LMA-3. In: EMEP Workshop on measurements of nitrogen-containing compounds. Les Diablerets, Switzerland, July 1992. Lillestrųm, Chemical Co-ordinating Centre, Norwegian Institute for Air Research (EMEP/CCC-Report 1/93).

International Organization for Standardization (1985a) Ambient air - Determination of the mass concentration of nitrogen oxides - Chemiluminesence method. Geneve (ISO. International Standard 7996:1985).

International Organization for Standardization (1985b) Ambient air- Determination of the mass concentration of nitrogen dioxide - Modified Griess-Saltzman method. Geneve (ISO. International Standard 6768:1985).

Kelly,T. J., Spicer, C.W. and Ward G. F. (1990) An assessment of the Luminol chemiluminescence technique for measurement of NO2 in ambient air. Atmos. Environ. 24A, 2397-2403.

Pavlenko, A.A. and Volberg, N.S. (1979) Use of solid sorbents for determination of nitrogen oxides. Trudy GGO, 417, 105-112 (in Russian).

Pavlenko, A.A. and Volberg, N.S. (1991) Determination of NO2 in atmosphere using thin film sorbent for sampling. In: EMEP Workshop on quality and comparability of atmospheric measurement data. Weilrod-Neuweilnau, Federal Republic of Germany, April 1991. Lillestrųm, Chemical Co-ordinating Centre, Norwegian Institute for Air Research (EMEP/CCC-Report 5/91).

Schiff, H. I., Mackay, G. I., Castledine, C., Harris, G. W. and Tran, Q. (1986) Atmospheric measurements of nitrogen dioxide with a sensitive luminol instrument. Water Air Soil Pollut., 30, 105-114.


Last revision: November 2001