PGAQ #71 by Vic Steblin, Jan 26, 2009

2570 Laurier Crescent, Prince George, BC, 250-564-1123

Enrico Fermi was a famous scientist who was respected for his direct approach to problems. Once when asked about aliens he simply responded “Where are they?” He also introduced a fascinating way of calculating large numbers now called Fermi approximations. He would quickly work out the main influence of things by roughly taking into account a few basic factors.

A Fermi approximation can be applied to air quality and wood. We could consider two main factors of smoke, the starting mass of the wood and the quality of the burn. Mass can be measured directly and quality is the efficiency percent that is commonly given to the type of burn. For simplicity ignore any additional mass that oxygen adds when combining with the carbon and hydrogen in the wood because the carbon dioxide and water vapour are relatively harmless.

Since 1 kilogram is one billion micrograms(ug) this means that one kilogram of wood can raise by one microgram the particulate level of one billion cubic metres (1 cubic kilometre). Although some analysts would reduce this particulate load by the efficiency percent of the wood’s quality and the temperate of the burn, it also makes sense to keep all the potential particulate in the calculation since all particulate can be considered dangerous to health. In any case, with no wind twenty-five kilograms (55pounds) of wood can bring a billion cubic metres (1 cubic km) of air substantially up to the 25 ug level. The Prince George air shed (20km x 20km x 100m up) has 40 cubic km in it, or 40 billion cubic metres. This means that about 40 wood stoves burning 55 pounds each can bring the air shed substantially up to 25 ug. This often occurs when there is no wind or when an inversion traps the air. One mill could also do it!

To make these rough estimates more scientific and believable one could use a better volume of the Prince George air shed and a better estimate of wood burned. It seems obvious that burning wood can have quite an effect depending on the mass burned. Besides wood stoves just imagine what the mills burn!