Fluorescence detection of atmospheric nitrogen dioxide using a blue light-emitting diode as an excitation source

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.

Direct measurement system of nitrogen dioxide in the atmosphere using a blue light-emitting diode induced fluorescence technique

An instrument for measuring atmospheric nitrogen dioxide has been developed by a light-emitting diode induced fluorescence (LED-IF) technique. Air was introduced into a fluorescence detection cell. A pulsed blue light LED with a peak wavelength of 430 nm was irradiated to excite NO2 molecules in this cell. Fluorescence emitted from excited NO2 molecules was detected by a dynode-gated photomultiplier tube. The current detection limit of the LED-IF instrument was estimated to be 7.0 and 0.91 ppbv (parts per billion by volume) at 1-min and 1-h integration times, respectively, with a signal to noise ratio of 2. This result indicates that this LED-IF instrument can measure sufficiently precise 1-h values of NO2 concentrations in the urban atmosphere. An NO2 test observation and an intercomparison of the LED-IF instrument with an NO2 measurement system based on a photolytic converter/NO-O3 chemiluminescence method were performed in the urban atmosphere. Concentration differences between the two methods were within ±25% for about 90% of the data. It has been demonstrated by these observations that NO2 concentrations can be observed in the urban areas using the LED-IF instrument.