QUARTZ-ENHANCED PHOTOACOUSTIC SPECTROSCOPY FOR ENVIRONMENTAL TRACE GAS SENSING WITH WSNWS

At present, air pollution monitoring is carried out with highly accurate measurement equipment, which, as a matter of the high cost of the equipment and maintenance, is only possible at low spatial resolution. To achieve high-resolution pollution maps, these point measurements assist dispersion models, which, however, only deliver approximate values of pollution levels. Attempts have been made to increase the spatial resolution of measurements, using dense wireless sensor networks (WSNWs) of low-cost sensors at the cost of limited accuracy. Approaches range from fixed nodes (e.g. [2]), to public transport mounted sensors ([1]). Among the pollutants of highest interest is nitrogen dioxide (NO2). The detection of NO2 in WSNWs is usually done by electrochemical or metal oxide - based sensors, which are cheap but lack long-term stability and suffer from cross sensitivity to other gases as well as limited lifetime. Our approach for sensing NO2 is based on quartz-enhanced photoacoustic spectroscopy (cf. [3]) to overcome the aforementioned drawbacks. QEPAS is able to sense NO2 concentrations in the low ppb range in lab environment.[4] To build a sensor for ambient conditions, we follow a low-cost approach with off-the-shelf components and a simple setup. A schematic of the setup is shown in Figure 1. The lightsource is a commercial blue laser diode (OSRAM PL 450B) with a peak wavelength of 450 nm. At that wavelength, NO2 is highly absorbing, and cross interferences to other ambient gases are small. The beam is shaped with two lenses, an asphere and a cylinder lens. The latter allows us to optimize the beamshape to the shape of the fork. A custom designed two-stage amplifier circuit using standard electronic components amplifies the signal. The quartz tuning fork is detached from an evacuated off-the-shelf watch crystal oscillator of 32.678 Hz resonance frequency. Another challenge is the dependence of the forks resonance frequency on temperature, pressure and humidity on the fork. This is of particular interest at ambient conditions, where temperature, pressure and humidity vary. We therefore use a second, identical fork, to track and control the resonance frequency. This second oscillator circuit relies on a low-cost Pierce oscillator circuit.