Abstract
External contour distributions for gas temperatures and for the concentrations of carbon monoxide, carbon dioxide, hydrogen and oxygen are reported at successive times in the smoking cycle. The sidestream gases leave the coal in a vertical column centering at about 3 mm in front of the paper burn line. The levels of oxygen and the carbon oxides outside the coal are not primarily determined by any diffusion gradient originating in the inner coal. Rather, the external levels are determined partly by a convective current set up by the hot coal, and influenced by buoyancy effects. The oxygen in the convection stream forms the oxides of carbon on the hot surface of the coal, and the convected air is transported around the coal in an upwards direction. On the other hand, the hydrogen levels outside the coal result from diffusion from the inner coal, although much of this outward diffusing hydrogen is oxidised to water on the surface of the coal. The visible sidestream smoke column is about 3 mm in diameter and its centre is about 5 mm behind the sidestream gas column, becoming visible when the temperatures in the gas column are below about 150°C. The vapour which eventually condenses to form mainstream and sidestream smoke is released in the general pyrolysis / distillation region of the cigarette, just behind the paper burn line. Some of this vapour diffuses to the outside of the cigarette, through the partially degraded paper, and it condenses to form the sidestream smoke particles once outside the coal.