Long term air ion monitoring in search of pre-earthquake signals

Air ion concentrations at ground level have been measured both indoors and outdoors to address a wide variety of questions, such as estimating air quality or assessing the impact of ions on human health. While most studies rely on discrete measurements, other applications require continuous monitoring over extended periods of time. In this paper, we discuss the challenges posed by long term air ion monitoring within the frame of earthquake forecasting. We first provide a brief state of the art, outlining the main hypotheses advanced to explain anomalies in air ion concentrations that were reported prior to earthquakes, namely the p-hole theory and the radon exhalation theory. We then describe the array of Gerdien capacitors deployed by QuakeFinder in the US and abroad with the goal to identify pre-earthquake variations in air ion data: we discuss the characteristics of these instruments and derive the criticial mobility associated with their parallel-plate geometry. We then present the results of a Parallel Sensor Test (PST) during which positive and negative air ion counters were deployed side by side at increasing separation distances. This test provides insight about the sampling rate at which air ion concentrations should be measured with the QuakeFinder air ion counters.