Observation of turbulent dispersion of artificially released SO2 puffs with UV cameras
Authors: Dinger Anna Solvejg , Stebel Kerstin , Cassiani Massimo , Ardeshiri Hamidreza , Bernardo Cirilo , Kylling Arve , Park Soon-Young , Pisso Ignacio , Schmidbauer Norbert , Wasseng Jan , Stohl Andreas 
Published: November 2018

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.