Ground-Based Measurements

We undertake lab and field experiments to better understand fire behavior and impacts, and develop methods to best assess these.  We make ground measurements (often linked to airborne or even satellite observations), and work with collaborators such as the Canadian Forest Service, Northumberland Fire & Rescue Service, and South African National Parks Service to measure at experimental/prescribed burns where conditions are reminiscent of wildfire behaviour.  It is hard work but often very enjoyable as well.  Examples of just a few of our methods are below.

Thermal Imaging

We use thermal IR imaging to measure the radiative heat output from fires, and whilst this is often done from aircraft sometimes we can mount the cameras on a nearly viewing platform or hillside, or even occasionally use them obliquely.  Below you can see two of our thermal cameras deployed on an experimental burn in Dorset, UK.

In addition to larger scale burns, we sometimes use smaller laboratory-scale fires to study fire properties in more detail.  For example we have done much work to link radiant energy emissions from fires to fuel consumption rates.  This potentially allows the fuel consumption (and thus smoke emission) rate from a fire to be estimated from thermal remote sensing methods – for example those applied to satellite observations made by instruments such as MODIS, Meteosat SEVIRI and GOES.

Below are two videos, one showing the thermal camera imagery obtained in the middle IR (3.9 µm) when viewing a small fire burning across the fuel bed, and before that the matching optical camera imagery.  The fuel bed was mounted on a set of digitally logged scales, enabling us to link fuel consumption rate to the release rate of radiant heat.  The set up was something like that shown in the photos below – where in some cases we first had to visit the forest or savannah grassland to make initial tests and/or collect relevant fuels for conducting the experiment.  An early paper on this type of work can be found here.

Field Spectroscopy – Gases

Field spectrometers measure light of different wavelengths, and can be used to assess the properties of the atmosphere through which that light passes, and/or the properties of the surface that is reflecting the light.  We use VIS, IR and UV spectrometry –  for example for assessing the absorption of different wavelengths of light by the gases in wildfire smoke.  One approach we use is Fourier Transform Infrared (FTIR) spectroscopy, since a multitude of atmospheric gases have absorption features in the IR region, including key greenhouse gases (e.g. CO2 & CH4, see below).  We use a Fourier spectrometer to measure the IR spectrum at a high spectral resolution (0.5 cm^-1) and identify the presence of these gases, and use retrieval code (software) to estimate their amounts.  From this we can derive emissions ratios and emissions factors of the relevant gases for the particular type of fire under study.  A couple of papers describing and applying the method are here and here.

The following image shows an IR spectrum covering the wavenumber region 500 to 5000 cm^-1 – equivalent to 2 to 20 µm.  The FTIR spectrometer has been set to view an IR source at a distance of some tens or hundreds of meters, through which in this case smoke from a prescribed open vegetation fire has been allowed to pass.  “Notches” in the spectrum represent areas where atmospheric species such as water vapour and carbon dioxide are absorbing most of the IR energy emitted by the source, whereas “lines” in the spectrum represent spectral regions where the absorption is not so strong.  Particular molecules absorb at particular wavelengths, and by examining the spectral location and depth of the absorption features the type of molecule and its concentration in the atmospheric path between the source and the spectrometer can be derived.  This allows us to probe the smoke constituents, and ultimately tell how much of each species the fire emits per kg of vegetation burnt.

We use a small, portable FTIR spectrometer, which can be used to measure the absorption of IR radiation from a number of different IR sources.

These include portable IR lamps for making horizontal measurements (below left) and the sun for making observations of lofted plumes (below right).

The first video below shows what a typical path between the FTIR spectrometer and the IR lamp look like during a typical experimental burn, and the second includes a display of the resultant IR spectra which changes shape and exhibits different absorption features as varying amounts and types of smoke pass between the spectrometer and IR lamp (notice the dips in signal around 2000 cm-1, this is caused by CO2 and CO).

Another method we have applied is Ultra Violet Differential Absorption Spectroscopy (UV-DOAS).  Certain gases have absorption features in the UV spectral region (e.g. formaldehyde, sulphur dioxide and nitrogen dioxide, see left) which can be probed by this technique.  UV-DOAS spectrometer uses scattered skylight, so you do not have to have a fixed source as per the FTIR spectroscopy described above.  The equipment is also small and very portable (center), and can be mounted on a moving vehicle with the telescope looking upwards at the sky (right), being traversed back and forth below a smoke plume for example.