Development and Field Testing of a Small, Efficient, Photolytic Converter for Measurement of Ambient Nitrogen Dioxide (N

1. Development and Field Testing of a Small, Efficient, Photolytic Converter for Measurement of Ambient Nitrogen Dioxide (NO2) Martin Buhr, Air Quality Design, Inc. Golden, Colorado 2008 National Air Quality Conference April 8, 2008

2. Outline • Understanding how NO2 fits into NOx, NOy, and ozone production • The photolysis method for measuring NO2 • Some results from ambient measurements • Use of photolytic converters with chemiluminescent NO monitors

3. Understanding NO2 : Chemistry • Ozone formation 2 NO2 + hn NO + O3 NO + O3  NO2 RH + OH  RO2 NO + RO2  NO2 Net: NO2 + RH  O3 • Acid formation NO2 + OH  HNO3

4. Understanding NO2: Definitions • NOy = NO + NO2 + HNO3 + Organic nitrates + particulate nitrates • NOy can be understood as the sum of oxidized reactive nitrogen, including some final oxidation products. • NOy is best measured using a thermal-catalytic converter in conjunction with a chemiluminescent NO monitor.

5. Understanding NO2: Definitions • NOx = NO + NO2 • NOx can be understood as that portion of NOy that is available to create ozone. • NOx is best measured using a photolytic converter in conjunction with a chemiluminescent NO monitor.

6. Measurement of NO2 using photolysis • NO2 + hn NO + O3 • Photolysis works best at wavelengths between 350-400 nm. • Broadband sources such as Xe or Hg arc lamps may lead to significant interferences. • LED-based light source is fairly narrow spectrally and has minimal interferences.

7. Measurement of NO2 using photolysis • The reverse reaction, NO + O3 NO2, also takes place in the photolytic converter, so it is important to minimize the residence time of the processed sample before the chemiluminescence chamber. • For the remote inlet system we locate the sample critical orifice in the inlet box to accomplish this. • For the same reason, we try to minimize the residence time of the sample in the photolysis chamber to about 1 second.

8. Measurement of NO2 using photolysis • High efficiency (~50-60% NO2 conversion/second) • Highly specific for NO2 • Negligible radiant heating of the sample gas • Long light-source life (estimated >5,000 hours) • Power consumption: 30 W, 24 VDC • Converter weight: 0.6 kg • Converter size: 5 cm (w) x 5 cm (h) x 15 cm (l)

9. Measurement Results • Near the source region NOx and NOy are comparable, i.e. – NOx/NOy ~ 1. • Far from the source region NOx/NOy approaches zero. • In between, measurement of both quantities provides an estimate of how photochemically processed an air mass is, and how much ozone producing capacity it still has. AQ data courtesy of Baylor University

10. Implementation • Implementation works best with the converter located at the sample inlet. • It is also possible to locate the converter within the NO monitor, but care must be taken to minimize the residence time of the ambient sample in the inlet piping. Come have a look at this system at the Teledyne-API booth

11. Summary • Measurement of NO2 is a critical step for understanding the ozone formation potential of an air mass. More important on a regional rather than local scale. • Measurement of NOx and NOy together provides the best view of the photochemical age of an air mass. • Implementation of the solid-state converter has been demonstrated for both surface and airborne applications and is available commercially.