The Sensitivity of Lidar Systems used in the Detection of Tropospheric Volcanic Emissions: A Review

During the Icelandic eruption of Eyjafjallajökull in April and May of 2010, extensive in-situ and remote sensing measurements were taken of the ash cloud and other volcanic emissions such as sulphur dioxide (SO₂). This has led to a substantial increase in the amount of data available on the microphysical and chemical properties of volcanic emissions that have been transported long distances (e.g. greater than one thousand kilometres) from source.

Globally agreed requirements on aviation safety limits and approved monitoring techniques are yet to be determined but, inevitably, observational measurements will be exploited as far as is possible to help establish the location, type, and concentration of volcanic emissions where these may pose a risk to air traffic.

The potential for disruption to air traffic extends far from the source of the eruption. While dispersion models are generally effective at forecasting horizontal transport of aerosols, challenges remain due to the numerous complex processes which occur over time, e.g. aggregation, sedimentation, interactions with localised meteorological phenomena, chemical transformations etc. It is therefore important to be able to utilise measurements at some distance downwind of the eruptive plume. These data can be used for near real-time validation against other types of observation or model forecasts, can enable forecasters to ‘add value' to raw model output, can be used for post-event R&D and in the future be increasingly integrated into dispersion models through inversion and/or data assimilation techniques.

In-situ measurements e.g. via instrumented aircraft, are an effective method of characterising volcanic emissions, but are rather limited in their spatial and temporal coverage. Therefore, for near continuous monitoring of large geographical areas in a volcanic emergency, remote sensing methods are widely used, in particular satellite-based instruments, but increasingly lidar and national ceilometer networks. In cloud free conditions, or where aerosol layers are below cloud, ground-based lidars and ceilometers can provide supplementary measurements to those made by satellite-based instruments, including the structure (top and bottom heights) of multiple aerosol layers, aerosol layers which are close to ground-level and increased boundary layer aerosol concentration. More thorough assessments of the effectiveness, and thus limitations of remote-sensing measurement techniques for different types of volcanic emission, should help to determine assumptions that need to be made, and help establish which combination of measurements may be used in order to reduce uncertainties.

Sulphur dioxide (SO₂) and sulphuric acid (H₂SO₄) aerosols are significant products of volcanic eruptions, with the latter forming over time due to chemical reactions in the atmosphere. These products can remain in the troposphere for weeks, transported thousands of kilometres from source. The capabilities of satellite-based sensors for monitoring SO₂ and volcanic aerosols have been investigated and documented extensively. The use of lidar and ceilometer networks to monitor volcanic emissions is not yet routine or operational in many parts of the world, but the differentiation of aerosol types using lidar technology is an area of science which has been making significant advances in recent years.

The focus of this review is to assess, from recent studies of observations during volcanic eruptions, the capabilities of lidar systems in differentiating volcanic products e.g. ash and sulphate aerosols. The objective will be to provide an overview of the sensitivity, and thus limitations of lidar systems, at detecting and distinguishing the types of volcanic emissions which may be present in the troposphere at long distances from source. An additional aim is to indicate the types of lidar technology and complimentary techniques which have been used most successfully for the characterisation of volcanic emissions.

The potential for complicated interactions of these volcanic emissions e.g. with clouds or other aerosols, which may add to the uncertainty of the measurement, will also be documented.

This review aims to inform forthcoming ICAO International Airways Volcanic Watch Operations Group (IAVWOPSG) meetings, where the definition of ‘discernible ash' with respect to in-situ and remote sensing techniques will be under discussion.