Nitrogen oxides control the photochemical production of ozone 03 in the lower atmosphere and influence the concentration of the hydroxyl radical HO in the troposphere. As an important member of the nitrogen oxide family, N02 plays a significant role in serving as the only known source of ozone (through photolysis) in the lower atmosphere and as sink for HO via formation of nitric acid. Therefore, accurate measurements of nitrogen dioxide are essential for the understanding of the processes that determine the distribution of tropospheric ozone and for assessing the impact of human activity on the troposphere. Although reliable and sensitive techniques for the direct measurement of nitric oxide NO have been developed, these results could not be fully applied to direct N02 measurements. The purpose of the present study was to further develop a low pressure laser-excited fluorescence technique for measuring N02 by introducing the idea of a Two-Chamber-Fluorescence-Cell. Theoretical considerations of the performance of such a cell and the flow regime within the cell were done in order to assess the optimum cell design and the conditions for optimum operation. Measurements on a provided two-chambered test cell were done to verify the theoretical predictions made. This test cell has been explored with a Nd:Y AG pumped dye laser (100mW@564nm, 30Hz) and a supplied N02 concentration of lOOppm. The theoretical results suggest that the concept of a Two-Chamber-N02- Fluorescence-Cell yields an improved sensitivity compared to present direct techniques for measuring N02 In addition, experimental results give a better understanding of how to realize the initial idea of an operating Two-Chamber-N02- Fluorescence-Cell.

Problems associated with instruments used to measure low levels of NO₂ include non-specificity, low sensitivity, and an uncharacterized dependence on environmental conditions. MDA Scientific has recently introduced a continuous colorimetric tape technique (chemcassette) to monitor for pollutant gases. Advantages of this technology include ease of use, fast-response alarm capability, and adaptability to measure different pollutants. This research characterized and compared chemcassette performance for NO₂ with other low-level NO₂ monitoring technologies including the luminox LMA-3 monitor and the EPA reference CSI chemiluminescnt monitor. The test protocol was based on EPA procedures. The chemcassette was found to be sensitive to temperature and relative humidity effects. The chemiluminescent monitor had the best overall performance. The luminox monitor displayed temperature dependence. Variation in stability of the chemcassette optical system, tape paper uniformity, and possible interference from ozone were characterized. (Abstract shortened with permission of author.)

Triethanolamine (TEA) is a semiconducting polymer which exhibits a resistance change when exposed to various gases. The polymer also exhibits a number of reactions with nitrogen dioxide, with the reaction products being heavily dependent on the presence or absence of water vapor. Previous studies have attempted the incorporation of a TEA-carbon nanoparticle composite as the active sensing layer in a chemresistive sensor for detection of NO₂. The incorporation of carbon nanoparticles in the polymer nanocomposite was thought to amplify the sensor's response. There are a number of chemical reactions that can occur between TEA and NO₂, with the reaction products being heavily dependent on the presence and amount of water vapor in the environment. Because of this influence, it becomes necessary to know to what degree the presence of water vapor interferes with the sensing response. In this work we show that the sensor exhibits a reversible resistance change as background humidity changes. This sensitivity to humidity changes is so large that it renders undetectable any resistance change that could be caused by the reaction of TEA with NO₂. Furthermore, we show that the presence of low levels of NO₂ do not interfere with adsorption of water vapor. The detection mechanism is based on measuring resistance changes in the TEA film due to the adsorption/desorption of water vapor. The sensing response can be described by Langmuir adsorption by using a site-based model for the polymer film resistance. Breakdown of the polymer film over time due to continuous adsorption of water vapor, as well as photodegradation of the polymer film, will be discussed. SEM images will also be presented showing growth of crystallites on the electrode walls, as well as experimental results demonstrating degradation of the sensing film during sensor operation.

Nitrogen dioxide (NO₂) is a pollutant of interest for study both because of its controlling role in the oxidant capacity of the atmosphere and the health risks it poses. Concerns about the health effects of NO₂ and its role in forming deleterious atmospheric species have made it desirable to have low-cost, sensitive ambient measurements of NO₂. A continuous-wave laser-diode laser-induced fluorescence (LIF) system for NO₂ was developed here which operates at ambient pressure, thereby eliminating the need for an expensive pumping system. The current prototype system has achieved sensitivity several orders of magnitude beyond previous efforts at ambient pressure (limit of detection of 2 ppb, 60 s averaging time). Ambient measurements of NO₂ were made in Portland, Oregon using both the standard NO₂ chemiluminescence method and the LIF instrument and showed good agreement (r² = 0.92). In addition, investigations into surface mediated chemistry involving oxides of nitrogen (namely, NO₂) have stimulated new inquiry into potential heterogeneous sources of NO₂ as well as challenged the stability of permanent sinks for NO₂. The possibility that surface mediated chemistry plays a significant role in NOy chemistry in urban air has for the past few decades received considerable attention. The AP-LIF NO₂ instrument is uniquely suited to measure surface chemistry under near ambient conditions. The so called 'renoxification' reaction of gaseous NO with surface bound HNO₃ yielding NO₂ (2HNO₃(surface) + NO--> 3NO₂ +H₂O(surface)) was suggested as a potentially important source of NO₂ which also degraded the stability of nitric acid as a sink of active oxides of nitrogen. Yet, there is disagreement in the literature as to the importance of this reaction. The disagreement stems from differing measurements of the rate for the renoxification reaction. Because there are differences in experimental setups no one research group has studied the renoxification reaction under ambient conditions, i.e., at moderate concentrations of NOy and in a static cell held at 1 atm. In this work, the production of NO₂ was measured using a novel AP-LIF. This setup made it possible to measure the rate of production of NO₂ due to the heterogeneous reaction of NO with HNO₃ under ambient conditions. Under these conditions it was found that renoxification due to gas-phase NO on surface HNO₃ is not a significant source of NO₂. However, this study did show the importance of water vapor in the renoxification of surface HNO₃.