“…An important step was the introduction of a fluorescent-particle tracer (Nickola, 1977), which allowed a sudden increase in the range of dispersion studied, from the few hundred metres allowed by the pitch smoke studies. Other early tracer substances include sulphur dioxide (Barad, 1958;Cramer et al, 1958;Haugen, 1959), uranine dye, zinc and cadmium sulphides (Haugen and Fuquay, 1963;Fuquay et al, 1964;Nickola, 1977), krypton-85 (Nickola, 1977;Heffter et al, 1984), rhodamine B (Nickola, 1977), spores and metal oxides (Vanderborght and Kretzschmar, 1984). A more comprehensive anthology and description of these early experiments are available elsewhere (Islitzer and Slade, 1968;Draxler, 1984).…”