When the airborne particles are subjected to some obstacles in their way, they impact to the obstacle surface with some force and are deposited. Impaction is defined as collision by inertial forces of a small, airborne particle with an obstacle or surface in the air stream, usually at right angles to the mean direction of flow. Since wind speeds are generally much greater than gravitational settling rates, most small airborne particles travel a nearly horizontal course. Their mass and velocity give them an inertial force, which resists changes in speed and direction. When a particle approaches a physical obstacle, the air molecules surrounding the particle divert and flow around the obstacle. If the particle has sufficient inertia, it will continue on its original course or on a path somewhere between this and the path of the air molecules and may strike an obstacle.
In the atmosphere, the efficiency of impaction (the percentage of particles approaching an obstacle that actually strike it) is a direct function of the size, mass and velocity of the particle and an inverse function of the size of the obstacle. Besides efficiency of impaction, the efficiency of retention is also important. A particle, upon impact, may either stick to the obstacle or rebound from it and re-enter the air stream. A sampling surface must be coated with a good adhesive to insure adequate retention. Sampling efficiency is a product of impaction efficiency and retention efficiency and can be determined experimentally in a wind tunnel. Wind, which carries the spores when it comes across the cylinder, the rays deviate away and later converge behind the cylinder. The inertia of the particle helps in impaction of the particle to the cylinder.
Particles may impact on obstacles of any shape, but vertical cylinders are most commonly used as impaction samplers since they are horizontally symmetrical and their impaction efficiency can be calculated. The relationship between efficiency of impaction and cylinder size is illustrated in Fig. 13.1 and is given by the equation below.
Where E = efficiency of impaction
D = cylinder diameter from which particles impact d = crosswind diameter from which particles impact
Thus, ratio of d/D is larger for the smaller cylinder; indicating that a smaller cylinder is more efficient than a larger one, the other entire variable being equal.
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