Compact quartz-enhanced photoacoustic sensor for ppb-level ambient NO2 detection by use of a high-power laser diode and a grooved tuning fork

A compact quartz-enhanced photoacoustic sensor for ppb-level ambient NO₂ detection is demonstrated, in which a high-power blue laser diode module with a small divergence angle was employed to take advantages of the directly proportional relationship between sensitivity and power, hence improving the detection sensitivity. In order to extend the stability time, a custom grooved quartz tuning fork with 800-μm prong spacing is employed to avoid complex signal balance and/or optical spatial filter components. The sensor performance is optimized and assessed in terms of optical coupling, power, gas flow rate, pressure, signal linearity and stability. A minimum detectable concentration (1σ) of 7.3 ppb with an averaging time of 1 s is achieved, which can be further improved to be 0.31 ppb with an averaging time of 590 s. Continuous measurements covering a five-day period are performed to demonstrate the stability and robustness of the reported NO₂ sensor system.

Parts-per-trillion-level detection of nitrogen dioxide by cantilever-enhanced photo-acoustic spectroscopy

We present a simple and highly sensitive cantilever-enhanced photo-acoustic sensor for detection of nitrogen dioxide. A noise equivalent detection limit of 50 parts-per-trillion in 1 s is demonstrated. The limit was reached with an average optical power of 4.7 W using a continuous-wave laser at 532 nm. The achieved normalized noise equivalent absorption coefficient was 2.6×10(-10)  W cm(-1) Hz(-1/2).

Monitoring of atmospheric nitrogen dioxide by long-path pulsed differential optical absorption spectroscopy using two different light paths

Measurements of the local distribution of atmospheric nitrogen dioxide (NO(2)) by long-path pulsed differential optical absorption spectroscopy (LP-PDOAS) in Tokyo during August 2008 are presented. Two LP-PDOAS systems simultaneously measured average NO(2) temporal mixing ratios along two different paths from a single observation point. Two flashing aviation obstruction lights, located 7.0 km north and 6.3 km east from the observation point, were used as light sources, allowing spatiotemporal variations of NO(2) in Tokyo to be inferred. The LP-PDOAS data were compared with ground-based data measured using chemiluminescence. Surface wind data indicated that large inhomogeneities were present in the spatial NO(2) distributions under southerly wind conditions, while northerly wind conditions displayed greater homogeneity between the two systems. The higher correlation in the NO(2) mixing ratio between the two LP-PDOAS systems was observed under northerly wind conditions with a correlation factor R(2) = 0.88. We demonstrated that the combined deployment of two LP-PDOAS systems oriented in different directions provides detailed information on the spatial distribution of NO(2).

Modification of a commercial cavity ring-down spectroscopy NO2 detector for enhanced sensitivity

Nitrogen dioxide (NO(2)) plays a central role in atmospheric chemistry, air pollution, and biogeochemical cycles. Many analytical techniques have been developed to detect NO(2), but only chemiluminescence-based instruments are commonly, commercially available. There remains a need for a fast, light, and simple method to directly measure NO(2). In this work we describe the modification and characterization of a small, commercially available cavity ring-down spectroscopy (CRDS) NO(2) detector suitable for surface and aircraft monitoring. A metal oxide scrubber was added to remove NO(2), and provide a chemical zero, improving the detection limit (3sigma of the background noise) from several parts per billion by volume (ppbv) to 0.06 ppbv, integrated over 60 s. Known interferences by water and particles were removed using Nafion tubing and a 1 microm Teflon filter, respectively. A 95% response time of 18+/-1 s was observed for a step change in concentration. The CRDS detector was run in parallel to an ozone chemiluminescence device with photolytic conversion of NO(2) to NO. The two instruments measured ambient air in suburban Maryland. A least-squares fit to the comparison data resulted a slope of 0.960+/-0.002 and R of 0.995, showing agreement within experimental uncertainty.

Trace detection of atmospheric NO2 by laser-induced fluorescence using a GaN diode laser and a diode-pumped YAG laser

We report on the development of a highly sensitive detection system for measuring atmospheric NO(2) by means of a laser-induced fluorescence (LIF) technique at 473 nm using a diode-pumped Nd:YAG laser. A GaN-based laser diode emitting at 410 nm is also used as an alternative fluorescence-excitation source. For laboratory calibrations, standard NO(2) gas is diluted with synthetic air and is introduced into a fluorescence-detection cell. The NO(2) LIF signal is detected by a photomultiplier tube and processed by a photon-counting method. The minimum detectable limits of the NO(2) instrument developed have been estimated to be 0.14 ppbv and 0.39 ppbv (parts per billion, 10(-9), by volume) in 60 s integration time (signal-to-noise ratio of 2) for 473 and 410 nm excitation systems, respectively. Practical performance of the instrument has been demonstrated by the 24 hour continuous measurements of ambient NO(2) in a suburban area.

Continuous wave cavity ring-down spectroscopy measurement of NO2 mixing ratios in ambient air

The performance of a continuous wave diode laser based cavity ring-down (CRD) spectrometer is demonstrated for retrieval of NO2 mixing ratios in ambient air. Measurements are made at wavelengths around 410 nm, a region free from other spectroscopic interferences, and the instrument is shown to be capable of quantitative and precise NO2 mixing ratio determinations without any requirement for calibration. The minimum concentration of NO2 that can be measured by the spectrometer is estimated to be 0.1 ppbv (parts per billion by volume) in 50 s at atmospheric pressure. Using dynamic dilution of a standard NO2 sample, the performance of the spectrometer has been validated for NO2 measurements over the range of 0.2 to 80 ppbv. NO2 measurements performed directly on laboratory air are compared with data obtained with a calibrated chemiluminescence instrument and shown to be in excellent agreement.