SO

Here, we propose a new method to estimate the propagation direction of the
volcanic plume directly from SO

Prediction and monitoring of volcanic events is highly desirable. Besides
conventional methods, like seismicity or deformation measurements, continuous
monitoring of volcanic gas emissions is a still relatively new method for
predicting volcanic eruptions. The four most common changes in volcanic
behaviour preceding an eruption are earthquakes, deformation, thermal
anomalies and an increase in degassing of the volcano. Moreover, not only can an
increase in degassing behaviour be an indicator of an imminent eruption, but so can a change in the composition of the
volcano's degassing (see e.g.

For short-term as well as long-term monitoring of volcanic degassing
behaviour, in situ and remote-sensing techniques have been developed.
While in situ techniques, such as alkaline traps and MultiGAS

The SO

The propagation velocity of the plume and the distance between the plume and
the camera are two important variables used to determine the SO

In any case, an important prerequisite for the determination of absolute trace
gas flux values is the precise knowledge of the distance between the plume and
observing instrument (usually the SO

This distance is usually more difficult to (precisely) determine than it is
generally assumed: while the geographic locations of the volcanic gas source
(i.e. usually the crater) and the position of the instrument are almost
always precisely known, the plume propagation direction (like the plume
velocity) is not. It is advantageous to know the propagation direction of the
plume to achieve a good estimation of the plume distance. This usually
requires additional measurements, which are often hard to make at volcanoes
due to the limited infrastructure. This paper is about the possibility of
determining the plume propagation direction itself from a time series of SO

The trace gas flux

If the volcanic SO

Schematic view on the influence of the inclination of the plume on
the measured variables for the SO

In a first simplified approach for a small FOV angle of a few degrees, the
inclination deviations are negligible (below 10 % change in SO

In contrast to the apparent underestimation of the plume velocity, the
measured column densities

Usually, SO

For FOV angles of the SO

Schematic sketch of the influence of a large FOV angle on the
deviation of the plume length

The plume length deviation equation (Eq.

Mean deviations of the plume extent in the

These calculations cover half of the actual FOV angle of the camera. If the
plume is inclined with the angle

Usually the SO

Combined deviations of the three variables of the flux determination
for the right half of the image of an SO

Figure

Ratio of the real variables to the measured variables of the
velocity (upper panel), plume distance or diameter (middle
panel) and column densities (lower panel) for an SO

Equations (

With

Deviation of the total true flux from the measured flux (with no
inclination assumed) as a function of the FOV angle of the SO

Schematic drawing of the shift of the best known distance towards the border of the image array. The equations for the perspective correction can be adapted to the respective position of the best known distance.

We used the considerations developed in Sect.

Map of the geometrical set-up of the measurement data set. The
inclination of the plume is 38

Figure

Deviations of the SO

Mean fluxes of seven different cross sections of the measurement data
set as a function of the angle correction. Each cross section is plotted in a
different colour. The SO

Observation of a wind direction change using the apparent flux
ratios of two different cross sections of the plume which were corrected for
the perspective. Depending on the propagation velocity, it is possible to
determine direction changes on a timescale of minutes. For every
inclination towards the image plane, the ratio of the fluxes is unique. On
this data set, there occurred a direction change after phase A at 12:26 UTC.
The wind direction in phase A was 281

If there are changes in the propagation direction of the plume during
SO

We showed that an inclined plume causes apparent spatial flux gradients in
the SO

While the proposed algorithm is applicable to straight and bent plumes (i.e. plumes where the wind direction changes within the field of view), so far the combined effects of this study and radiative transfer issues have not yet been addressed. Additionally, if several plumes are masking each other, the proposed algorithm may not work.

Recent improvements in the velocity determination of volcanic plumes using
image processing methods like optical flow algorithms can be combined with
the proposed perspective correction method for a robust SO

The raw measurement data (spectra and images, more than 1 GB in size) of the SO

The authors declare that they have no conflict of interest.

We acknowledge the work of Sebastian Illing and Marco Huwe on the SO

Further, the authors thank for the financial support from the DFG project “DFG BO 3611/1-2”.

We acknowledge the financial support of the Deutsche Forschungsgemeinschaft and Ruprecht-Karls-Universität Heidelberg within the funding programme Open Access Publishing. Edited by: T. von Clarmann Reviewed by: three anonymous referees