Intro to Aerosol Flashcards
What are the sources of aerosol?
Vehicle emissions, bulk to particle conversion
What are the common examples of aerosol?
black carbon, dust particles, pollen, sea salt
how does cloud droplets form?
aerosol compounds attract water and become bigger as more vapor condenses
what are the two types of effects of aerosol?
direct and indirect
what kind of aerosol purely scatters light?
white particles like sea salt
what kind of aerosol cause heating?
black aerosols like black carbon absorbs heat
what is indirect effect?
aerosols can alter cloud’s albedo such that more aerosols means more scattering of light hence cooling
what kind of scattering is for molecules and atoms?
rayleigh scattering
what kind of scattering for particles about the same size of visible light wavelength?
mie scattering
what kind of scattering for larger particles like water droplets?
geometric scattering
what is the size range for aerosols?
1 nm to 100 microns
what is the size range for ultrafine?
1 nm to 100 nm
what is the size range for fine particles?
100 nm to 1 microns
what is the size range for coarse particles?
1 micron to 100 microns
what are the four modes in the atmosphere?
course, accumulation, aitken, nucleation
what is the size range of coarse mode
1 micron to 100 microns
size range of accumulation mode?
100 nm to 1 micron
size range of aitken mode?
10 nm to 100 nm
nucleation mode?
1 nm to 10 nm
what are the examples of spherical aerosols?
droplets, fly ash, inorganic salts
what is the mean free path?
distance travelled by a molecule before colliding to another molecule
What is drag force?
forced exerted by the stagnant gas on a moving body
what is the correction factor to compute for drag force for small particles i.e., <100 microns
cunningham correction factor
what happens to mechanical mobility if we add the cunningham correction factor?
It increases because the particles experience less drag force
what is mechanical mobility?
the ratio of particle velocity and drag force
what is sedimentation velocity?
it is when drag force is equal to the gravitational force
what is the trend between particle size and setting velocity and relaxation time
lower particle means lower settling velocity and lower relaxation time
what happens to drag force when the dynamic shape factor is applied?
the drag force increases
What are the different types of particle diameter?
stokes, aerodynamic, optical
what is the diameter of a sphere that has the same density and settling velocity as the particle?
stokes diameter
what diameter of a sphere whose density is same as water which settles in still air at the same velocity as the particle in question?
aerodynamic diameter
what are latex particles
particles that have known stokes diameter and refractive index
what can we do to be able to compare different concentrations in size bins with different size widths?
divide N (height) by log (delta D_p)
What describes the log-normal distribution?
the geometric mean diameter and standard deviation
what is zeroth moment? area under the curve of dN/dlog (Dp)
total particle number concentration
what do we use in bipolar diffusion charging?
we ionize gases surrounding particles by alpha or beta radiation or x-ray
what are the forces involved in bipolar diffusion charging?
coulumb force, image force, diffusion
which charge is greater in fraction in bipolar charge distribution?
negative charges hence shifted to the left
which ions have higher mean electrical mobility?
negative ions
what are the advantages of bipolar charging?
time independent, narrow for small particles than 300 nm, simple to calculate
what are the disadvantages of bipolar charging?
broad for big particles, greater than 300 nm, small particles are hard to charge
what bipolar model only considers diffusion and is for greater than .1 microns?
gunn distribution
what is Fuchs-distribution
takes into account diffusion and electrostatic, valid for entire submicron range
what is used in unipolar diffusion charging
corona needle
what are the advantages of unipolar charging?
works well for ultrasize range
what are the disadvantages of unipolar?
time dependent, broad distribution for fine range
what is electrical mobility
ratio of particle velocity and drag force
what dictates electrical mobility?
number of charge and particle diameter
what diameter dictates the lower detection efficiency of a CPC?
kelvin diameter where evap = condensation
higher temperature difference of saturator and condenser means?
smaller lower detection efficiency
what happens inside a modern CPC?
we let the aerosol to attract the condensable species such that it becomes bigger and easier to count
how many particles are inside a room?
5000 - 10000 particles
what is the limit of efficiency of a cpc?
around 10 nm low efficiency is observed
where do we use water cpc?
It is applied for hygroscopic materials such that how does aerosol grow in increasing RH
Where is the CPC first calibrated against?
NIST, NPL, PTB
what is the true value of electrometer measurement?
0.95 due to internal losses
What is the initial test for CPC performance?
plateau efficiency at 40 nm and cut diameter around 10 nm
what should be done if the CPC fails its test?
adjust condenser temp to set the cut diameter to 10 nm then cleaning of CPC
what is the area of dma transfer function?
0.5
What is the DMA transfer function dependent?
sample flow ratio and particle size
what is the transfer probability of the mean electrical mobility?
unity
Where does the size resolution for a DMA depends on?
the ratio of aerosol flow and sheath air
What defines the particle size range of a DMA
its geometry
What are the disadvantages of DMA?
at ultrafine particle range, the transfer function becomes broader and maximum transfer decreases
What does monodisperse aerosol generator consist of?
polydisperse aerosol generator, bipolar charger, DMA
what consists monodisperse aerosol?
single charge of smallest particle or multiple charge but larger particles
What does TDMA do
measure the mixing state of a defined particle size and is applied for hygroscopic models, volatility
which is bigger, stokes diameter or aerodynamic diameter?
aerodynamic diameter
if a spherical particle of unknown size has the same particle scattering, which diameter is equal?
optical diameter
what happens to the bipolar charge distribution when you increase the diameter size?
the distribution becomes broader
what is the relationship of electrical mobility and temperature
inversely proportional
by how much is the temperature of the butanol-aerosol mixture decreased in the condenser?
17-27 degC
how are impactors designed?
nozzle - impaction - plate config
