Research Letter #8

WHERE THERE'S SMOKE ...

... THERE'S CHEMISTRY AND PHYSICS AND ...

Recent discussion of a standard for the acceptable level of smoke in the atmosphere prompted this letter. I have tried to extract some basic information but I have found that there is much to learn!

Smoke is a complex product of combustion. Definitions vary but Drysdale (1985) points out that the inclusion of both the solid and gaseous products in the definition is sensible. This contrasts with the usual situation in which only the solid particles are considered. A further complication is that the ‘solid’ bits may include liquids. Which definition suits probably depends on the context of its use. Despite this, it is as well to remember how smoke is formed.

There may be thousands of products – a “bewildering array” (Chandler et al. 1983), theoretically containing “every possible molecule with a chain length varying from one atom to the length of the original fuel molecules” (Lobert and Warnatz 1993).

Smoke composition is related to the flaming and smouldering phases of the fire (Ward and Radke 1993). Emission rates are inversely proportional to combustion efficiency (Ward and Radke 1993); that is, there will be more smoke from smouldering material than from flaming material. Particulates – the ‘solid’ bits in smoke - include tars, soot and ash (Lobert and Warnatz 1993). The particulates may result from chemical reactions in the smoke plume (e.g. tar) or be the remnants of the complete combustion in flames (e.g. ash). Soot appears to arise from tar; soot may go to CO2 if combustion is complete. Soot particles may grow by aggregation into bigger particles if the particles do not decompose (Drysdale 1985). Particulate matter can arise from fuels even if they are liquids or gases (Drysdale 1985).

According to Chandler et al. (1983) about 90% of the particulate mass has a diameter less than one micron in diameter (i.e. one millionth of a metre, or one thousandth of a millimetre). Only 15% of particulate matter is in the light-scattering range of 0.3 to 0.8 microns diameter. The mid-range diameter of particles is 0.1 micron.

Information on particle sizes as given above can be considered indicative of what may be expected. It is perhaps obvious to a person who has observed fires up close that the numbers and sizes of particles varies widely from positions just above the fire to positions high up in the sky. Near the fire there may be large pieces of airborne bark and leaves but higher up in the smoke plume these are rare.

Literature cited

Chandler, C., Cheney, P., Thomas, P., Trabaud, L. and Williams, D. (1983). Fire in Forestry 1. Forest Fire Behavior and Effects. Wiley, Chichester.

Drysdale, D. (1985). An Introduction to Fire Dynamics. Wiley, Chichester.

Lobert, J.M. and Warnatz, J. (1993). Emissions from the combustion process in vegetation. In: P.J. Crutzen and J.G. Goldammer (eds) Fire in the Environment: the Ecological, Atmospheric and Climatic Importance of Vegetation Fires, pp. 15-37. Wiley, Chichester.

Ward, D.E. and Radke, L.F.(1993). Emissions measurements from vegetation fires: a comparative evaluation of methods and results. In: P.J. Crutzen and J.G. Goldammer (eds) Fire in the Environment: the Ecological, Atmospheric and Climatic Importance of Vegetation Fires, pp. 53-76. Wiley, Chichester.

Malcolm Gill
7 January 1999