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Instrumental Determination of NOx via Chemiluminescence

Environmental Instrumental Analysis I

In the preceding chapters, we have seen that nitrogen oxides play a leading role in atmospheric chemistry, both in the stratosphere and at ground level. In this box, we see how the concentration of NO arid NQ2 gases in environmental air samples can be determined using a sophisticated, modern method of analysis.

When chemicals react to produce light, the process is termed chemiluminescence. If conditions for a particular reaction are well known and can be controlled in an analytical instrument, chemiluminescence can be used as a sensitive and selective means of determining the concentration of components in the reaction. A few well-known chemical reactions that produce chemiluminescence are the basis for methods in tropospheric and stratospheric environmental analysis. One of the most common of these methods is the detection of nitric oxide (NO) and nitrogen dioxide (NOz).

The chemiluminescence reaction that produces light in this method is the gas-phase reaction of NO with ozone (O3):

In the NO detector, this reaction takes place in a small steel reaction vessel under controlled conditions. The excited-state nitrogen dioxide created in this reaction, designated by N02*, very quickly returns to ground state by giving off light in the red and infrared range of the light spectrum (600 to 2800 nanometers):

The amount of light produced by this reaction is dependent on pressure and temperature. A constant low pressure is maintained in the reaction vessel by use of a vacuum pump that constantly evacuates the chamber. Typical cell pressures are approximately 1 to 100 torr. The amount of light produced in the reaction is proportional to the amount of whichever reac-tant is not in excess in the reaction chamber. If ozone is provided in excess (from a steady ozone generator in this case), then the light output reflects changes in NO concentration. The light created by the reaction is detected by a photomultiplier tube (PMT) whose output is fed to a computer system. The computer software correlates the amount of light produced with the quantity of reactant by referring to the relation between light intensity and NO concentration, which is obtained from previous calibration experiments. In instruments of this kind, the PMT signal is often integrated over short time periods (e.g., 10 seconds) using what is referred to as a photon counting system, which improves sensitivity.

The schematic diagram of this instrument, shown in the figure below, includes the reaction chamber, ozone generator, PMT, computer, and

(continued on p. 200)

Environmental Instrumental Analysis I

Instrumental Determination of NOx via Chemiluminescence (continued)

vacuum pump. Gas from the atmosphere is sucked directly into the reaction chamber and immediately mixed with an excess of O3 (i.e., more O3 than NO). Typical sampling volumes are 1000 standard cubic centimeters per minute (seem). Some instruments have gold-plated surfaces inside to prevent surface reactions and increase the light-collection abilities of the light chamber. The light produced is detected by the PMT mounted immediately adjacent to the reaction chamber and separated by a transparent window or a filter that can block out light from interfering reactions.

This same instrument can be used to determine N02 by incorporating a chemical reduction step for the incoming air, in which N02 is reduced to NO by a hot metal catalyst before entering the reaction chamber. NO is then determined as before, but now the signal includes input from the presence of the air's NO and N02. If alternating signals are generated in a short time period—one with the atmospheric air stream that has been reduced and the other without reduction—by means of a switching valve, then the concentration of both nitrogen oxides can be determined:

NO concentration = signal generated by unreduced air flow

N02 concentration = signal generated by reduced air flow - signal from unreduced air flow

The limits of detection and selectivity (ability to determine NO or N02 in the presence of interférants) over O3, S02, and CO—all common atmospheric gases—are very good. Less than 1 ppbv NO can be routinely determined using this method (Department for Environment UK, 2004). Interferences from reduced gas-phase components also commonly present, such as NH3, can be minimized by decreasing the temperature in the catalyst chamber described above (Environmental Protection National Service, 2000).

Instruments of this kind have been used on airplane-based stratospheric and tropospheric sampling projects by the National Atmospheric and Space Administration and the National Center for Atmospheric Research. Similar instrumentation has been used for tropospheric studies of urban pollution, using laboratory-based "smog chambers," and in indoor air pollution studies.

The figure below shows the variation in concentration of NO as an airborne scientific expedition equipped with this kind of NO detector flew through cumulonimbus clouds— tall, vertically developed, and actively raining (Ridley etal., 1987).

The researchers were flying level at 9.3-km altitude while sampling the tropospheric air

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This method of NO and N02 measurement has also been used in experiments carried out at Antarctica at the South Pole (Davis et al., 2004). Earlier presumptions were that no nitric oxide was produced in that pristine setting since internal combustion engines, which are the major source of NO in urban environments, were absent. However, in the austral summer of 24-hour daylight, nitric oxide is continuously produced by sunlight photolysis of nitrate anion in the snow pack. And so, unlike diurnal variations of NO such as those seen in urban environments (see figure below), time-course studies over an entire 24-hour

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period at a lab at the South Pole have shown a relatively constant NO concentration at the j snow pack surface. The overall concentrations of NO at that South Polar site were also significantly higher than at other polar sites (sometimes by orders of magnitude) where similar examinations had been carried out, so it appears that the interesting meteorological conditions present at 90° S—including constant sunlight during austral summer—directly affect the production of NO. If you look carefully at the graph, you can see a dip in NO concentration that took place near midnight (0:00 h). This occurred when the shadow from a nearby building temporarily fell on the sampling site, decreasing the production of NO from the snow's nitrate ions. Plotted on the same figure is urban air NO concentration for a sampling site in Houston, Texas on August 17, 2006 (TCEQ, 2006). The common urban diurnal variation of NO is clear.

References: D. Davis, G. Chen, M. Buhr, J. Crawford, D. Lenschow, B. Lefer, R. Shetter, F. Eisele, L. Mauldin, and A. Hogan, "South Pole NOx Chemistry: An Assessment of Factors Controlling Variability and Absolute Levels," Atmospheric Environment 38 (2004): 5375-5388.

Department for Environment UK, "Nitrogen Dioxide in the United Kingdom, 2004," http://www.defra.gov .uk/environment/airquality/aqeg/nitrogen-dioxide/ index.htm

Environmental Protection National Service UK, "Monitoring Methods for Ambient Air, 2000," http://publications.environment-agency.gov.uk/pdf/ GEHOl 105BJYB-e~e.pdf

B. A. Ridley, M. A. Carroll, and G. L. Gregory, "Measurements of Nitric Oxide in the Boundary Layer and Free Troposphere over the Pacific Ocean," Journal of Geophysical Research 92(D2) (1987): 2025-2047.

TCEQ (Texas Commission on Environmental Quality), http://www.tceq.state.tx.us.

PART II

Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

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