Particle Basics Flashcards
What is the mean free path (λ) of a gas (air)?
The average distance a molecule travels between collisions (air at STP = 66 nm)
What are 5 key pieces of info for describing particles?
- composition 2. avg. size 3. polydispersity 4. distribution shape 5. cumulative vs frequency
What particles will best scatter visible light and why?
380 - 700 nm particles because light is scattered most by particles in the size range corresponding to the wavelength of light (380 - 700 for visible light).
What is the fundamental issue with histograms?
The bar height depends on the bar width.
How to calculate the fraction of particles in a given range from the frequency size distribution?
df = f(dp)*ddp (the fraction of particles from dp to ddp)
What does the area under the curve of the frequency size distribution represent?
1
How to find the fraction of particles from size a to b from the frequency size distribution?
fab = integral of a to b [f(dp)ddp]
If only one size is used, fbb = 0 because the interval width is 0.
How can you calculate the arithmetic mean?
Add all the values together, divide by the number of values.
What is the median?
1/2 of particles are smaller and 1/2 are larger than the median size.
What is the mode?
Highest point of the frequency distribution.
What is the geometric mean? When would it be used?
Used when there is a lognormal distribution. Calculated as the arithmetic mean would be calculated but values (diameter) are converted to ln space for calculations then converted back to real space with e^.
What are the first 3 moments averages of the frequency distribution?
1st: mean diameter
2nd: diameter of average surface area (or avg. settling velocity)
3rd: diameter of average mass
dp = (sum(n*d^p)/N)^(1/p)
How does the 95% interval differ from normal to lognormal distributions?
mean +/- 2σ vs exp(ln dg +/- 2ln σg) or [dg/σg^2, dg*σg^2]
What does σg = 1, 1.45 and 1.3 mean?
σg = 1: monodisperse σg = 1.45: self-preserving number size distribution for brownian coagulation σg = 1.3: self-preserving volume (mass) size distribution for brownian coagulation.
What is Stokes diameter?
Particles which have a settling velocity and density as a sphere with this diameter.
What is Aerodynamic diameter
Particles which have the same settling velocity as a sphere with unit density (1g/cm3, i.e. water) and this diameter.
In general do irregularly shaped particles fall faster or slower than an equivalent sphere (dynamic shape factor > or < 1)?
Irregular particles fall slower (χ > 1).
What is the definition of the feret diameter?
Measure of a particle along two perpendicular length.s
What is the definition of the martin diameter?
Measure of particle length which bisects the projected area. (Splits the projected area into 2 equal parts).
What can fractal dimension (Df) tell us about particles?
Particle formation/growth mechanism.
Df and name of 6 common particle growth mechanisms?
Monomer cluster: “Eden” Df = 3.00, “Vold” Df = 3.00, “Witten-Sander” Df = 2.50
Cluster-cluster: “RLCA” Df = 2.09, “Sutherland” Df = 1.95, “DLCA” Df = 1.80.
Reaction-limited, ballistic, diffusion-limited.
Describe nucleation/inception of particles.
atoms or molecules cluster together to a critical size to form a particle.
Describe condensation/surface growth of particles.
atoms or molecules adhere/bond to the surface of an existing particle thus increasing its size.
Describe evaporation of particles.
The opposite of condensation, individual atoms/molecules leave the surface of a particle reducing its size.