Measurements:
We carried out three artificial tracer release measurement campaigns in the summers of 2017, 2018 and 2019 at the Rena military facility in Eastern Norway (Fig. 1). A fourth campaign in 2020 had to be cancelled because of COVID restrictions. During the campaigns, we released SO2 as an artificial tracer that could be measured downwind from a release mast (10-m high in 2017; 60-m high in 2018 and 2019) that was set up at the site. During the first two experiments, SO2 was blown upward from bottles located at the ground through a pipe. We made both continuous releases over extended time periods (~30 min) as well as near-instantaneous puff releases (Fig. 2). To facilitate even shorter puffs, we filled balloons with SO2, pulled them up the mast, and exploded them there during 2019. The release mast, and another mast, were equipped with eddy covariance systems at different heights to measure turbulent fluctuations of the wind. A drone was used to record temperature profiles throughout the ABL.
To detect SO2 in the atmosphere, we built six UV and three IR SO2 camera systems that were connected to computers running self-developed software to control camera operation and record data. All cameras are also equipped with GPS systems and clinometers to record time, camera position and pose. The SO2 cameras were placed around the mast to observe the SO2. Camera distances from the mast ranged from ~160 m to ~1.2 km.
We also carried out a 3-week campaign at Stromboli volcano in spring of 2019 (Fig. 3). The volcano is a strong natural SO2 source and was selected because of its quasi-continuous activity. Cameras were placed at six positions around the whole island, facilitating observations from different angles (Fig. 4). One week after the end of our campaign, an extraordinarily strong eruption killed one person at exactly the location where we operated one of our cameras.
Modelling:
The measurements were compared with state-of-the-art modelling of turbulent tracer transport. Large-eddy Simulation (LES) models are the most advanced models available to resolve turbulence in the ABL. We adapted the freely available open source code PALM (Parallelized Large-Eddy Simulation Model, Fig. 5) and used it to reproduce the conditions at our field site with an extremely high resolution of ~1 m (Fig. 6 and 7). Comparisons with the turbulence measurements and tomographic data are not yet finished but will be published soon.
The LES simulations were also used as input to a 3D Monte Carlo radiative transfer model (RTM) which simulated the signal recorded by UV cameras to assess if images of SO2 plumes may be used to derive plume statistics of relevance for the study of atmospheric turbulent dispersion.
Tomography:
To reconstruct the 3-D tracer concentration distribution based on camera measurements, we applied two methods, one relatively simple, the other more complicated. The more simple one applies simple triangulation and allows determining the total mass of a tracer puff, the trajectory of its mass centre, and the spread of the puff mass around its centre, as a measure of turbulent dispersion. The more complicated method allows a full 3-D reconstruction of the tracer distribution. Figure 8 shows puff observations from five UV cameras and Figure 9 the corresponding 3-D tomographic concentration reconstruction.
Scientific results of our project were so far mostly communicated via scientific publications, and a few more publications are currently in preparation. The results also led to the improvement of PALM and FLEXPART, models that are widely used in the scientific community. The project has considerably advanced our understanding of turbulence, and has also made large technological advances in camera observations and tomography.
