Air pollution Flashcards
Filtration mechanisms for air pollution (3) ?
Interception - Efficiency increases with increasing particle size
Inertial impaction - efficiency increases with increasing particle size
Diffusion - efficiency increases with decreasing particle size
What is interception?
Since both the particle and the fiber have finite sizes, a particle is collected if it is within a distance of 1/2 particle diameter from the fiber surface
What is inertial impaction?
Because of inertia, particles tend to remain their original trajectory. Although airflow bypasses the fiber, the net result is that some particles, especially the heavier ones, collide onto the fibre
What is diffusion?
Very small particles behave like gas molecules, they diffuse randomly while they are carried to the fiber by bulk flow. The net result is that some particles, especially the smaller ones, collide onto the fiber
What are the problems in a cascade impactor? (3)
- Particle Bounce - solved by applying grease, but for subsequent OC analysis grease cant be used
- Overloading - due to long sampling time. Too much deposit on the substrate would reduce the flowrate and therefore cut off characteristics of the impactor stages
- For collecting smaller particles a higher velocity is needed - can be achieved by operating at lower pressure or by reducing the size of the orifice on top of the impaction plate.
Definition of bioaerosols?
Airborne particles that are living or derived from living organisms
How does a cascade impactor work?
Used for separating and collecting particles of a few size groups.
Particles impact on a flat plate to the next stage. Particles larger than cut size will deviate from the air stream and impact out on the collection plate, while smaller particles will not be collected and will follow the air stream.
What are the different types of infectious diseases?
Communicable - originated from human
Non-communicable - originated from ambient environment
Hospital acquired - endogenous in human, opportunistic infection
Rules of thumb to achieve collection of bioaerosols
Bioaerosols concentrations vary greatly over time.
Sampling times should be long enough to collect a detectable and representative number of particles and organisms, and short enough to avoid overloading and masking of the colonies.
What are the principles of bioaerosols collection? (3)
- Inertial impaction - inertial forces impact the particle onto a surface (agar)
- limited by particle bounce and blow off, and death of organisms due to impact stress - Liquid impingement - inertial forces impact the particles onto a submersed or washed with liquid
- Filtration - Inertial force, interception, gravitational setting, diffusion and electrostatic attraction enhance to deposition on or within filter medium
Advantages of impinger over impactor? (5)
- Helps prevent dehydration
- Measure the collected volume, growth in medium directly or after dilution/concentration
- Avoids errors due to colony overgrowth or crowding that are commonly found in agar impactors
- Clusters of organisms can be separated in the media by gentle shaking
- Particles in liquid can be counted directly with cell counters or concentrates by centrifugation for microscopic investigation
Disadvantages of impinger?
- Physical stress of impaction on bottom, leading to lower biological activities
- Particles may bounce and be carried away with the effluent flow
- Collected particles may also be re-aerosolized from bubbling and thus lost
- Additional sampling process compared with direct agar impactors
How can you modify a TSP sampler so that it is capable of sampling PM10?
By adding a particle size selective inlet (SSI) on top. Can either use an impactor to fractionate particles, cyclone to separate particles or membrane filter?
What is the difference between impaction and filtration?
Impaction is separation by inertia and filtration is separation by size exclusion (sieve)
What can be used to sample PM2.5?
Virtual impactor or cascade impactor
