Hybrid microfabricated device for field measurement of atmospheric sulfur dioxide

A miniaturized planar-membrane-based gas collector of 800 nL internal liquid volume was integrated with a microfabricated conductivity detector to measure atmospheric SO2. This device is operated with a dilute H2SO4/ H202/2-propanol absorber for a finite integration period (typically, 1.5 min) without liquid flow. During this period, sulfuric acid is formed from SO2 that diffuses into the liquid and accumulates therein. The increase in conductivity with ongoing sampling is measured. The absorber is then replaced with fresh solution, and the process starts anew. The most important factors that govern sensitivity and the detection limit are the choice of the membrane, the nature of the internal collector solution, and the thickness of the solution layer. A porous polypropylene membrane with some 2-propanol (IPA) incorporated in the internal solution was found to be the best combination. The sensitivity was inversely proportional to the solution layer thickness, and a layer thickness of 100 microm resulted in a practical device with good performance characteristics. Greater applied pressure on the gas phase relative to the liquid side also can improve device performance. The system is operated with 12 V DC and does not require a liquid pump. Under optimized conditions, the LOD is 0.7-1.0 ppbv for a sampling period of 1.5 min. The device was field-tested around Mt. Aso in Japan. Changes in ambient SO2 concentrations could be followed with good time resolution. The results are compared with data obtained by a collocated macroscale instrument.