 |
|
A winning proposal for the Innovative Research Program, 2007: Measurement of Nitrogen Dioxide by cw Cavity Ring-down SpectroscopyInvestigators: Eric Williams,1, 2, 3 Bill Dubé,1, 2 Steve Brown1 Objective: Design and build a prototype instrument to provide versatile, fast-response, absolute, direct measurements of nitrogen dioxide (NO2) in ambient air via the application of recently-developed techniques for high-sensitivity, direct absorption spectroscopy. Background: The accurate determination of NO2 in ambient air is critical for understanding the photochemistry that produces ozone (i.e., photochemical smog) in the troposphere. Currently, the most widely used research technique is photolysis of NO2 to nitric oxide (NO) followed by measurement of the increase in NO via chemiluminescence (see, for example, [Ryerson, et al., 2000]). While this technique is both accurate and sensitive, it has several limitations. First, it requires two independent NO detectors for high-frequency (i.e., 1 min or less) measurements of both NO and the sum, NO + NO2 (=NOx). Because the NO2 measurement is the difference between two signals, it suffers from potential artifacts due to timing fluctuations and changes in background levels, particularly near the detection limit. Second, since the detection principle is the reaction of NO with excess ozone to produce electronically excited (i.e., luminescent) NO2, the instrument has requirements for compressed gases, corrosive materials (e.g. concentrated O3), pumping capacity and exhaust gas disposal. The latter requirements also serve to increase the size, weight and power consumption, making the instrument less versatile during field deployments (e.g., aircraft, ships, etc.). Development of a method for direct detection of NO2 (i.e., not based on conversion to NO) would address many of these difficulties. Proposed Approach: Recent advances in absorption spectroscopy using high finesse cavities [Brown, 2003] are directly applicable to the measurement of NO2, which has absorption bands throughout visible region of the absorption spectrum. Cavity ring-down spectroscopy (CRDS) is one such technique. It is based on the measurement of the time rate of decay of light intensity from an optical cavity (in the simplest case consisting simply of a pair of high-reflectivity mirrors) rather than on a change in light intensity over a fixed path length, as in conventional absorption spectroscopy. Its sensitivity comes from the long effective path length that the light travels within the cavity, which is on the order of 10-100 km. Its sensitivity and dynamic range allows for NO2 detection over nearly its entire atmospheric concentration range, from mixing ratios = 0.05 to well over 100 ppbv, with a time response of 1 s or less [Osthoff, et al., 2006]. Because the instrument calibration is based on an absorption cross section, it is an inherently absolute measurement, although validation via the use of calibration standards will be an important component of the instrument development. The detection principle also allows an instrument design that is low-cost, lightweight, compact and has low requirements for power, compressed gases and hazardous materials relative to the standard photolysis-chemiluminescence method.   There has been considerable development of CRDS instruments based on pulsed lasers within the Chemical Sciences Division at NOAA, including a recent demonstration of NO2 detection in the green (532 nm) [Osthoff, et al., 2006] that has been incorporated into an existing instrument for optical detection of other reactive nitrogen species [Dubé, et al., 2006]. However, pulsed laser sources are relatively large and have high power consumption, making them cumbersome for use in field instruments. Furthermore, NO2 detection in the green suffers from interference due to O3, which also has visible absorption bands. Recent developments in diode lasers make them attractive light sources due to their low cost, compact construction and low operating power. Diode lasers with performance characteristics suited to CRDS are now available at wavelengths in the blue region of the visible spectrum, near the peak in the NO2 absorption. Detection of NO2 in this region has the added advantage of removing the interference due to optical absorption by O3, which absorbs only weakly in this region. The main challenge to the use of such continuous wave light sources in CRDS is the suppression of interference effects that are inherent to the coupling of cw light source to high-finesse optical cavities. Several interference suppression techniques for CRDS with cw lasers have been reported in the recent literature (e.g. [Baer, et al., 2002]), and their application to NO2 detection will be an important part of this IRP. Research Plan: We propose to build a prototype instrument using a combination of components purchased through the CIRES IRP and components already available in our laboratory. We request funding for the purchase of a diode laser, high reflectivity mirrors, detectors, associated electronics, a data acquisition computer and machining and fabrication of custom mounting components. The prototype will be constructed on an existing optical bench available for design and testing of optical systems within the CSD laboratories. Initial design of the optical system, characterization of the sensitivity and precision, calibration of the NO2 absorption cross section using existing standards (already available at CSD), design and testing of a method for accurately zeroing the measurement, and assessment of potential interferences will take place with the breadboard prototype. Once these tasks have been completed, design of a compact, portable system suitable for field use will begin. Benefits: The development of a small, fast, sensitive and versatile NO2 detector will be of enormous value to existing field measurement efforts at NOAA. It may also open up the possibility for additional, new scientific investigations, some of which could be highly interdisciplinary. For example, the rapid time response of this instrument would make it applicable to the measurement of deposition fluxes for reactive nitrogen, which are of interest both to the atmospheric and the biogeochemical sciences. The small size might enable deployment in remote locations and on non-traditional platforms, such as light aircraft, balloons, and tall towers. The development effort is in itself highly interdisciplinary, bringing together expertise from the fields of laser spectroscopy, photonics, analytical chemistry and atmospheric science. Finally, detection of NO2 can be viewed as a means to further ends; development of this expertise within the NOAA and CIRES communities will likely lead to ideas for measurements of other atmospheric trace gases and/or optical properties of the atmosphere via. CRDS and related techniques. References
|
216 UCB