1. Finishing Stop Distance
The vector position vs. time
Broken down into x and y components
UmB = a particle’s stop distance for this problem
If UmB > y0 then the particle will impact
For this ideal flow, y0 = x0
If the flow is coming from some finite orifice of radius and height L , then particles will be distributed in x fairly uniformly
So we can expect particles to impact over a range of stop distances UmB ~ L L

2. Finishing Stop Distance
For Stokes flow, we can relate the stop distance to the particle’s properties
Stop distance increases with particle diameter due to the greater sensitivity of mass to diameter than mobility
Particles with a stop distance > L are likely to impact. Thus particles with diameters > than a “cutoff diameter” Dcut will impact, and the rest will be transmitted. Dcut is found by setting xs = L L

3. Using Stop Distance to size aerosols Micro Orifice Uniform Depoist Impactor
For each “stage”, there is a distribution of cutoff diameters based on the geometry of the orifice-impactor.
The MOUDI sends air through a series of impactors, each with faster flow and shorter impaction distances

4. Effective Diameters
Particles are not spheres, and we often don’t know their density
So we define “effective” diameters based on what we can measure
We have already defined a particle’s effective volumetric diameter if we have a known volume and number, but no distribution information.
For an impactor, we are forced to define a particle’s diameter as if it were a sphere with 1 g/cc density.
The “classical” aerodynamic diameter is based on the particle’s terminal velocity assuming it is a sphere of water. This is the same as the impaction diameter, but where L/U is replaced with vT/g
Electrical mobility diameter is defined in the same way, but for a DMA. Since there is no inertial dependence, you don’t assume a density