Observation of turbulent dispersion of artificially released SO<sub>2</sub> puffs with UV cameras

Abstract

Abstract. In atmospheric tracer experiments, a substance is released into the turbulent atmospheric flow to study the dispersion parameters of the atmosphere. That can be done by observing the substance's concentration distribution downwind of the source. Past experiments have suffered from the fact that observations were only made at a few discrete locations and/or at low time resolution. The Comtessa project (Camera Observation and Modelling of 4-D Tracer Dispersion in the Atmosphere) is the first attempt at using ultraviolet (UV) camera observations to sample the three-dimensional (3-D) concentration distribution in the atmospheric boundary layer at high spatial and temporal resolution. For this, during a three-week campaign in Norway in July 2017, sulfur dioxide (SO2), a nearly passive tracer, was artificially released in continuous plumes and nearly instantaneous puffs from a 9 m high tower. Column-integrated SO2 concentrations were observed with six UV SO2 cameras with sampling rates of several hertz and a spatial resolution of a few centimetres. The atmospheric flow was characterised by eddy covariance measurements of heat and momentum fluxes at the release mast and two additional towers. By measuring simultaneously with six UV cameras positioned in a half circle around the release point, we could collect a data set of spatially and temporally resolved tracer column densities from six different directions, allowing a tomographic reconstruction of the 3-D concentration field. However, due to unfavourable cloudy conditions on all measurement days and their restrictive effect on the SO2 camera technique, the presented data set is limited to case studies. In this paper, we present a feasibility study demonstrating that the turbulent dispersion parameters can be retrieved from images of artificially released puffs, although the presented data set does not allow for an in-depth analysis of the obtained parameters. The 3-D trajectories of the centre of mass of the puffs were reconstructed enabling both a direct determination of the centre of mass meandering and a scaling of the image pixel dimension to the position of the puff. The latter made it possible to retrieve the temporal evolution of the puff spread projected to the image plane. The puff spread is a direct measure of the relative dispersion process. Combining meandering and relative dispersion, the absolute dispersion could be retrieved. The turbulent dispersion in the vertical is then used to estimate the effective source size, source timescale and the Lagrangian integral time. In principle, the Richardson–Obukhov constant of relative dispersion in the inertial subrange could be also obtained, but the observation time was not sufficiently long in comparison to the source timescale to allow an observation of this dispersion range. While the feasibility of the methodology to measure turbulent dispersion could be demonstrated, a larger data set with a larger number of cloud-free puff releases and longer observation times of each puff will be recorded in future studies to give a solid estimate for the turbulent dispersion under a variety of stability conditions.

Can statistics of turbulent tracer dispersion be inferred from camera observations of SO<sub>2</sub> in the ultraviolet?

“…The SO 2 plume is embedded in this domain and is viewed from the side at a distance of about 250 m by the UV camera which is placed 1 meter above the surface. This camera-plume distance is comparable to that used during part of the first COMTESSA field campaign described by Dinger et al (2018).…”

“…With the FOV and a camera resolution of about 0.5 m at the plume, this gives a camera with 400×88 pixels. It is noted that the UV cameras used by Dinger et al (2018) had 1392×1040 pixels.…”

“…During the COMTESSA campaigns both continuous and puff releases of SO 2 are done. For puffs the dispersion increases as the square of the time, t 2 , of flight (see Dinger et al, 2018, and references therein). Taking the horizontal pixel number as a surrogate for time (Taylor's frozen turbulence hypothesis) we note the linear behaviour of the relative dispersion for pixels close to the release point (Fig.…”

“…To achieve this, six UV cameras have been built to measure SO 2 densities from various viewing directions. A series of experiments with puff and continuous releases of SO 2 from a tower have been performed as described by Dinger et al (2018). It is known from measurements of volcanic SO 2 emissions that aerosol and viewing geometry affect the retrieved SO 2 amounts (Kern et al, 2013).…”

Concentration Fluctuations from Localized Atmospheric Releases

“…An alternative remote sensing approach based on multiple cameras was also implemented very recently with the aim of investigating concentration fluctuations, but so far results are limited to relative dispersion statistics (Dinger et al. 2018 ).…”