NOx

About: NOx is a research topic. Over the lifetime, 26367 publications have been published within this topic receiving 496555 citations.

Impedancemetric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NOx at high temperature

Abstract: A solid-state electrochemical NOx sensor was fabricated by using a closed-one-end yttria-stabilized zirconia (YSZ) tube, an oxide sensing electrode (SE) and a Pt counter electrode (CE). The complex impedance of the device using each of several oxide SEs was measured in the frequency and the temperature ranges of 0.1 Hz to 100 kHz and 600–700 °C, respectively. In most cases, a large semicircular arc was observed in complex impedance spectra (Nyquist plots) in the lower frequency range examined in dry-air flow. Only in the case of the device using ZnCr2O3 SE, the semi-arc shrank to some extent upon exposure to NOx gas. The resistance value (Z′) at the intersection of the semi-arc with the real axis at lower frequencies (around 0.1 Hz) varied with concentration of both NO and NO2 in the sample gas. The impedance value at 1 Hz of the present device was found to vary almost linearly with the concentration of NO (or NO2) from 50 to 400 ppm. The 90% response and 90% recovery times were as short as less than few seconds at 700 °C. Furthermore, it is noted that the sensitivity of NO is almost equal to that of NO2. This indicates that the present device can detect the total NOx at higher temperatures.

Photochemical production of gas phase NO x from ice crystal NO3

Abstract: Recent measurements have demonstrated that sunlight irradiation of snow results in the release of significant amounts of gas phase NOx (NO+NO2). We report here the results of a series of experiments designed to test the hypothesis that the observed NOx production is the result of nitrate photolysis. Snow produced from deionized water with and without the addition of nitrate was exposed to natural sunlight in an outdoor flow chamber. While NOx release from snow produced without added NO−3 was minimal, the addition of 100 µM NO−3resulted in the release of >500 pptv NOx in a 9 standard liter per minute (sLpm) flow of synthetic air exposed to the snow for 10–20 s; the rate of release was highly correlated with solar radiation. Further addition of radical trap reagents resulted in greatly increased NOx production (to >8 ppbv in a flow of 20 sLpm). In snow produced from deionized water plus sodium nitrate, production of NO2 dominated that of NO. The reverse was true in the presence of radical trap reagents; this suggests sensitivity of the NOx release mechanism to pH, as a basic compound was added, or to the presence of free radical scavengers. A mechanism for NOx release from NO−3photolysis consistent with these observations is presented. These results support previous suggestions that surface NOx release may have a significant impact on boundary layer photochemistry in snow-covered regions and that nitrate photolysis on cirrus cloud particles may result in the release of gas phase NOx. A potential for pH-dependent impacts on ice core records of oxidants and oxidized compounds is also suggested.

Low and high NOx tropospheric photochemistry

Abstract: Emission inputs to the troposphere are processed in two fundamentally different ways, leading to two atmospheric states with greatly different properties. We define these states in terms of rates of NOx emissions and radical production. In the low NOx state, trace gas inputs to the atmosphere are removed primarily by oxidation reactions. In the high NOx state the oxidative potential of the atmosphere is greatly diminished and primary pollutants build up, as alternate removal processes are slow. We show here that this dual behavior is due to a titrationlike reaction between NOx and free radicals in which either reactant can be in excess.