30 Air Monitoring
AN INSTRUMENT FOR SAMPLING VOLCANIC PLUMES FROM A UAV
Figure 1. Plumes of gas and ash during an eruption of Stromboli in September 2019. Image: Jean-François Smekens
Volcanic ash poses a threat on a range of scales, ranging from ashfall in local communities to regional risks to aviation. Volcanic gases pose a further threat to the surrounding area, stemming largely from emissions of sulphur dioxide. For example, “vog” – volcanic smog – is a recurring issue in Hawaii, formed as volcanic SO2 interacts with the surrounding atmosphere to produce sulphuric acid aerosols.
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mall eruptions of the kind common at volcanoes such as Stromboli (Italy) and Kilauea (Hawaii) produce plumes reaching up to hundreds of metres in the atmosphere, which drift in the direction of the prevailing wind [Figure 1]. Whilst plume dispersion models can predict the direction of travel of plumes, local measurements are needed to constrain the volume of pollutants present and verify the predictions of models.
Although many active volcanoes have established monitoring networks, a network of ground-based sensors is unlikely to be suffi ciently dense to fully monitor the development of a moving plume; and monitoring networks may not be present at all around newly active volcanoes. Additionally, it is desirable to be able to measure pollutants at altitude within the plume and along its dispersal axis, as these may later affect communities further downwind.
UAV-mounted sensors provide a fl exible, mobile solution to track plumes as they travel nearby populated areas. A collaboration between Dr Jean-François Smekens from the University of Oxford Department of Earth Sciences, Dr Cunjia Liu from Loughborough University and STFC worked to develop a new sensor package that can be deployed rapidly in response to a developing volcanic crisis, and provide the capability to validate models of plume dispersal and improve forecasts to protect local communities.
STFC RAL Space operates several UAV platforms, including fi xed wing, multirotor, and a vertical take-off and landing UAV. The STFC UAVs are operated by the RAL Space Radiometry group who have three qualifi ed pilots and CAA approval for commercial operations. With the expertise that has been developed in RAL Space there now exists an opportunity to operate the STFC UAVs for the wider academic and industrial community in a scientifi c context, including in the development of this package. RAL Space
IET JANUARY / FEBRUARY 2023
STFC Scientifi c Computing Department supported the project via Computational Fluid Dynamics (CFD) modelling. CFD models around a representative airframe were used to evaluate the best positioning of the aerosol sampling inlet with the aim of achieving isokinetic particle sampling. [Figure 2].
The instrument package developed consists of gas sensors in addition to an aerosol optical particle counter to provide the aerosol size distribution [Figure 3]. The gas sensor package consists of an NDIR CO2 sensor and electrochemical SO2 and H2S sensors.
Figure 2. Screenshot of a sample CFD result, showing the surrounding velocity magnitude for a cruise speed of 18 m/s around the UAV body.
also have the expertise in miniaturisation of instrumentation to fi t within the mass and power constraints of the UAV platform.
Sampling from turbulent airfl ows introduces biases to particulate measurements - a particular concern on multirotor UAVs. For this project a push prop fi xed wing platform was chosen to ensure clean air fl ow around the UAV. To tackle the problem of turbulence induced by the body of the UAV the sampling inlet is extended in front of the UAV [Figure 4].
The aerosol sensor inlet is connected to an isokinetic sampling probe, optimised to minimise aerosol loss at the UAV cruise velocity. The sensor package is suffi ciently fl exible that it can be extended with other gas sensors depending on the application needs and UAV payload capacity. Example results from the instrument package are shown in [Figure 5], demonstrating the response of the sensor to events in a domestic environment.
Sensor data is measured in real time by a microcontroller and data logged via a microcomputer. Provision is made to send live data to monitor instrument responses during plume sampling. The full data logs are recorded to an SD card as a time stamped data stream for offl ine analysis.
Figure 3. Assembled instrument with a prototype inlet for isokinetic sampling in fl ight
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