Portable Spectroscopy System for Environmental Monitoring: An SO2 Case Study

In this work, a compact, portable, extended operating life, maintenance-free and remotely operable apparatus for optical absorbance measurements in the ultraviolet region is proposed. The system is useful for concentration measurement of chemical species or biological agents that exhibit ultraviolet light absorption. An apparatus prototype was developed using a commercial LED as photons source at the desired wavelength and a photo-detector with low noise, high sensitivity and visible blindness, opportunely fabricated using Silicon Carbide technology. A suitable electronic for handling the very low-level current of the detector has been designed and built to be robust, portable, low-power, low-cost and with a WiFi user interface. The ultraviolet spectroscopy apparatus application to which this work is aimed, is the low-level concentration measurement of SO2 in volcanic environment, while it can be easily adapted to other molecules detection and higher concentration level. While in previous Authors' work the system feasibility has been explored using laboratory instrumentation, in this work the focus is to design a system that can be used in real harsh volcanic environment, where system lightweight, endurance and chemical strength against aggressive compounds are paramount. Resolution of approximately 1 ppm, compactness, robustness and insensitivity to humidity and temperature variations are required for this kind of environment. The performed laboratory tests and calibration for SO2 monitoring, are reported and discussed. The system shows good sensitivity as high as 8 pA/ppm, a resolution < 1 ppm for SO2 detection and low cross-sensitivity to main components usually present in volcanic gases.