Aerosols Flashcards
Define an aerosol
An aerosol is a dispersion of liquid droplets or solid particles within a gas phase; refers to the combination not just the dispersion
In what ways are aerosols stable/unstable
They are thermodynamically unstable but kinetically stabilised
Why are aerosols thermodynamically unstable
Large surface area:volume ratio. Surfaces are high in energy
What is involved in aerosol dynamic processes
Rapid coupling of condensed particle and gas phase. Many processes with wide range of time and length scales
What influences aerosol motion (5)
- Brownian diffusion
- Turbulence
- Gravitation
- Particle charge
Temperature gradients
What is the process of aerosol delivery
Pressurised container –> fluid –> stream –> bubble generation –> bubble bursting –> liquid droplets, contain active ingredients (AI) –> liquid droplets with AI –> AI only
What motivates hygroscopic growth
Ease of fabrication of hierarchically structured organic and inorganic nanodomains and mesostructure at different lengthscales
Applications of aerosols (7)
Catalysis, sensors, controlled release, therapeutic carriers, optics, photonics, separation
What is a sol precursor
Dispersed dense/porous nanoparticle with inorganic/hybrid polymers and surfactants
What is the process of using a sol precursor
Sol –> atomizer (gas input) –> droplets –> oven –> macro-micro-patterns –> film and particles
What does PM2.5 mean and what is the other common aerosol measurement
Particulate matter 2.5 microns and smaller
PM10
What is the typical size and concentration of a rain drop
1 mm, 1 /L
What is the typical size and concentration of a cloud drop
10 um, 1000 /L
What is the typical size and concentration of a haze drop
1 um, 1000 /L
What is the typical size and concentration of pollution
0.1 um, 10,000 - 100,000 /L
What is the typical size and concentration of nebuliser
0.1 - 5 um, 10,000 - 100,000 /L
What are primary particles?
Emitted into the air directly by resuspension of material
What are secondary particles?
formed in the air: gas-to-particle conversion (nucleation, condensation etc)
How are secondary organic aerosols formed?
oxidation of VOCs
What is the definition of fine aerosol
nucleation mode and accumulation mode
What is nucleation mode
Aerosols 0.001 - 0.1 um in size. Largest by number but just a few % by mass
What is accumulation mode
Aerosols 0.1 um - 1 um, largest by surface area, significant part of mass of total aerosol. Particle removal least efficient here.
What is coarse mode
1 - 100 um aerosol. Removed by mechanical disturbance. Sedimentation causes falling with fast velocity
What is different about coarse and fine aerosol? (6)
Origination, transformation, removal, composition, optical properties, deposition patterns in respiratory tract
Anthropogenic sources of primary aerosol (2)
industrial dust (fine and coarse), soot (fine)
Anthropogenic sources of secondary aerosol (4)
sulfates from SO2 (fine), biomass burning (fine), nitrates from NOx (coarse), organics from VOCs (fine)
Natural (dominates global emissions) sources of primary aerosols (4)
soil dust mineral, sea salt, volcanic dust, biological debris, all coarse
Natural sources of secondary aerosol (3)
Sulfates from bioorganic gases and volcanoes (fine), organic matter from VOCs (fine), nitrates from NOx (fine and coarse)
What sort (size) of aerosol particles are produced over deserts?
Large particles that are around 100% mass
Where is the cleanest air globally?
The polar regions
What is the lifetime of nucleation mode aerosols
less than 1 hour
What is the lifetime of accumulation mode aerosols
Days
What is the lifetime of coarse mode aerosols
minutes –> days
What counters gravity
Bouyancy
What does drag force assume in Stokes Law?
gas phase is continuous for particles > 1 um
What is the Cunningham slip correction factor?
For low gas number densities, the gas doesn’t look like a fluid; Cc added to Stokes law equation for drag
What is the Knudsen number?
It is a constant for a particle which is used to calculate Cc. Smaller particles can “slip” between molecules easier and this takes that into account
What does X (chi) take into account?
Non-spherical particles
What sort of particles are preferentially deposited by impaction & what are the uses of this
Large particles; this is used to sample aerosols in cascade impactors
What is Brownian diffusion caused by
Particles bombarded by air and imbalance of transient forces
What is the size region of particles for which Brownian diffusion is most important
< 0.1 um
What is the primary removal process for coarse mode aerosol
Sedimentation
What is the primary removal process for nucleation and accumulation mode aerosol
Diffusion and coalescence
What are the challenges in aerosol analysis (4)
Wide size and mass range means variation in sensitivity, concentration range: sampling and rates of loss processes, phase variation, sample perturbation before analysis
What are the main things we try to characterise about aerosols (4)
number, size distribution, SA conc and mass conc
Why are PM2.5 and PM10 routinely measured
Inhalable coarse mode particles between 2.5 and 10 um
What is the general process for measuring aerosols
Record mass loading of filters, control T and humidity, for a known volume and set time period
How do impactors work (a column of angry faces)
Particles are accelerated through a nozzle at substrate. They must have a certain inertia (size) to cross streamlines and impact substrate (fibrous/porous membrane filters). Smaller particles follow deflected streamlines with no impaction. They can be designed with different size dependent collection efficiences
What does a CONTINUOUS AMBIENT PARTICULATE MONITOR measure and how
Continuous PM2.5 and PM10. Tapered glass tube fixed at the bottom & oscillates naturally at top. Particles drawn through filter on tip of fibre, added mass changes oscillation frequency, mass calculated
What does a CONDENSATION PARTICLE COUNTER measure and how
Number conc as low as 10^4 /cm3 and higher. Particles undergo rapid growth in butanol environment, detected by light scattering, counts scattering events occuring on laser beam, individual particles < 10 nm - 10 um can be counted
What does aerodynamic size depend on
Passage in gas, defined by size, shape and density
What does electrical mobility size depend on
size and shape. Not density
What does optical size depend on
Shape, size and refractive index
How does an AERODYNAMIC PARTICLE SIZER work and what are the pros (2)
Acceleration through a nozzle, rate increases with decreasing size and density, velocity from time of flight analysis (2 lasers) means you can calculate size. Smallest size is 0.2 um, high resolution, real-time measurements.
How does a DIFFERENTIAL MOBILITY SIZER work (also called electrostatic classifier)
Pro (1), Con (1)
Aerosol exposed to cloud of positive and negative ions in bipolar charger and go to a CPC detector.
3 nm - 1 um concs determined
Takes 10 mins
Why is composition determination difficult
Sensitive to artefacts in sampling, transport and storage, sensitive to loss of volatiles, gas-particle and particle-particle reactions within sample
What techniques are used to determine particle composition (7)
- Atomic emission/abs spec
- HPLC
- GC-MS
- Laser microprobe MS
- Laser induced breakdown spec
- Laser induced plasma spec
- Capillary zone electrophoresis
Why is average composition determined by MS (3)
in situ, real time, sub micron aerosol
Define relative humidity
Relative humidity is the actual partial pressure of water in the gas phase compared to what it would be for a saturated gas phase of water above pure liquid water at the same temperature. Also called S, degree of saturation
What is the cloud experiment
Nucleation rates are measured in a chamber
What are the two big players in atmospheric nucleation
ammonia and sulfuric acid
What occurs in the heterogeneous nucleation of ammonia/sulfuric acid droplets and what does it result in?
Heterogeneous nucleation on organic or inorganic aerosols is the condensation of water
When they are a few tens of nm in size it gives cloud droplets
What arises from heterogeneous nucleation at low temperatures?
Ice particles
How do cloud droplets form?
Heterogeneous nucleation on organic/inorganic aerosol a few tens of nm in size
- Warm air mass rises above cold air and cools with approximately constant partial pressure although there is a slight change with temperature
- at some point, the temperature of the air causes the actual pressure of water in the air to surpass the saturation vapour pressure leading to SUPERSATURATION CONDITIONS
- water condenses onto small aerosol particles forming cloud droplets
What is a(H2O)
The activity of water in solution
What does Raoult’s Law assume?
Ideal behaviour of water in solution such that a = x = pressure
What is denoted by non-ideality of water activity in solution?
a(H2O) = x(H2O)gamma(H2O)
What inorganic molecule is often used to calibrate an instrument measuring hygroscopic growth and why?
H2SO4 because it absorbs/desorbs water continuously
What parameter characterises response to relative humidity
Mass growth factor
The phase and size of a particle of hygroscopic aerosol depends on what?
The vapour phase
What does hygroscopic aerosol do?
Hygroscopic aerosol absorbs water dramatically over 80 % relative humidity leading to dilution of solute
Explain aerosol hysteresis
Transitions occur at particular relative humidities and therefore their phase depends on RH history.
What can a hysteresis loop result in
solution droplets can be supersaturated in solute when dried from high relative humidity
What is the deliquescence point
the deliquescence point occurs when water activity in the gas phase = water activity in saturated salt solution formed when solute dissolves
What is the efflorescence point
The efflorescence point is where on drying, supersaturation solutions form as there are no heterogeneous nuclei on which crystallisation can occur
Efflorescence is a kinetically limited phenomena governed by nucleation
At what point does most growth in diameter occur and what does this depend on
There is a very strong growth in diameter when saturation relative humidity is approached and this has a dependence on the hygroscopicity of the solute
What technique is used to measure hygroscopic growth of accumulation mode particles
Hygroscopic tandem differential mobility analysis which measures water uptake for nearly monodisperse aerosol
How does hygroscopic tandem differential mobility analysis work
- Aerosol is dried first to obtain a reference size
- Tge aerosol is exposed to varying relative humidity and the size is remeasured
- water uptake is parameterised in terms of growth in diameter
What is the Kelvin Effect?
Because droplet surfaces are curved, they have a higher vapour pressure than flat surfaces
Why is a pure water droplet never stable in size?
The partial pressure is greater at the surface than at a large difference therefore condensation reduces the vapour pressure resulting in more condensation
In the Kelvin equation, what does the Kelvin term describe? What is the size of this parameter dependent on?
The Kelvin term gives an activation barrier for small aerosol to grow to cloud droplets
Its height is dependent on the amount of soluble material and the particle size
What other factors does a lower surface tension correspond to?
lower critical supersaturation and activation
What are small aerosols before activation?
Condensation nuclei (CN)
What are aerosols that activate
Cloud condensation nuclei (CCN)
Give Henry’s Law
at constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid
What units is Henry’s coefficient given in?
Molar /atm
How do you work out concentration from Henry’s coefficient?
just multiply by partial pressure
Ha x pa = [A]
Molar /atm x atm = molar
What is atomisation
Atomisation is vaporisation of liquid droplets and it is important for combustion
Give 6 examples of dynamic processes employing aerosols
- Atomisation
- Metered dose inhalers
- Interaction of API with propellant can have significant impact on size, morphology and deposition
- Spray drying in particle engineering:composition of air/water interface determines surface
- Crystallinity influenced by evaporation rate
- Gas-particle partitioning of semi-volatiles and chemical aging by oxidation
What governs unsteady evaporation
coupled differential equations govern heat and mass transfer
How does steady evaporation arise
slow mass transfer: rate of energy deposition/removal slow and the process is isothermal; heat and mass transfer are decoupled
Under what circumstances can heat and mass transfer be treated by quasi-steady theory
IF the interface changes slowly with respect to gas phase composition, attaining a “steady state” then heat and mass transfer can be treated by quasi-steady theory
What is the unsteady period in mass transfer
mass flux changes as a function of time
what is the steady period in mass transfer
mass flux balanced in two directions
What is the wet bulb temperature
when rapid initial evaporation leads to surface cooling, the wet bulb temperature is reached
How do structures like hollow spheres, doughnuts, buckled, deformed, folded form
Radially unhomogeneous aerosol gives enhanced surface saturation
What is the Peclet number
peclet number = evaporation rate/diffusion rate
How does folding occur
Evaporation induced self-assembly
How do doughnuts occur
evaporation induced micelle packing
Give 6 advantages of using aerosols in industry
- cheap
- scalable
- 1 pot
- ease of distribution control
- access to metastable states
- tuneable particle size
What is the correction factor used for flux values, to obtain true flux, because Jc does not equal Jk
gamma measured. It is a measured flux correction
What are the two factors that govern the size of gamma measured
mass accommodation (alpha) at the interface and diffusion to the interface in the gas phase
What is the mass accommodation factor?
alpha = number of molecules absorbed/number of collisions with the surface
It is a probability/statistical value
What governs flux in large particles?
Diffusion (slow)
What governs flux in small particles?
surface kinetics with mass accommodation being the limiting factor
As the Knudsen number increases, the diffusion rate factor…
Decreases
What can a very small mass accommodation factor reflect?
Energetics: presence of a barrier
for example solvent reorganisation required for solvation which is an entropic cost
What is an aerosol flow reactor and how does it work?
- Expose accumulation mode particles to ozone
- vary time and relative humidity
- analyse with a range of techniques to study product branching
- mass spectrometry gives the loss of reactants and therefore how rate depends on concentration of ozone and composition
- oxidation of single particles studied with optical trap: probe the loss of reactants and particle size through evaporation of volatiles
- increased oxidation gives increased O:C, decreased vapour pressure, decreased saturation concentration of component
- organic compound goes from volatile- semivolatile - low volatility
Define elastic scattering and give 3 examples
Elastic scattering leads to a change in momentum of light but not energy
- reflection
- refraction
- diffraction
Define inelastic scattering and give 3 examples
Inelastic scattering results in a change in energy of the light
- fluorescence
- Raman
- absorption
What theory is envoked when Rayleigh scattering occurs on small particles that are approximately comparable to the wavelength of light
Mie Theory
What theory is envoked when Rayleigh scattering occurs on small particles that are larger than the wavelength of light
approximation to geometric ray optics
What is the real component of the refractive index N = n + ik
The real component comes from the speed of light in medium (nu) compared to vacuum (c), n = c/nu
What does the imaginary part of the refractive index number tell us about
attenuation of light by absorption
What 3 parameters are are n and k both dependent on
wavelength (dispersion), temperature, density
What are the Kramers-Kronig relations
the real part of the refractive index, related to the differential of the imaginary part
What is the wavelength necessity for Rayleigh scattering
particles < wavelength/20
Describe Rayleigh scattering
light is scattered forward and backward symmetrically and independently of shape. Angular dependence of scattered unpolarised light is composed of parallel and perpendicular polarised terms
What is Mie scattering
scattering by non-absorbing, spherical particles
- refractive index different for particle and dispersion medium and homogeneous
- incident light assumed to be monochromatic and a plane-wave: all surfaces on which disturbance has constant phase form a set of waves all perpendicular to propagation
- first order treatment for non-spherical particles
What is scattering irradiance dependent on
angle in field where there is no overlap with incident field
What is another word for the scattering angle
phase function
What are angular scattering profiles
Angular scattering profiles are polar plots of intensity variation with angle and the scattering pattern is characteristic of particles
What do nephelometers record
Nephelometers record total (7 - 170 degrees) scattering at three wavelengths (450, 550, 700 nm) for integrated backscattering
How do you use a nephelometer to resolve forward and backward scatter
use a “backscatter shutter” to exclude 7 - 90 degrees
What is a problem caused by the limited approach to zero degrees using a nephelometer
Truncation errors which increase with size of particle
What is the scattering coefficient accuracy of a nephelometer
7 %
How do you use scattering intensity to determine size
Intensity increases with contrast to refractive index of surrounding medium
What is the optical cross section for scattering
It is the cross section of the shadow cast by the object given in units of area
What is the total extinction in scattering
sum of scattering and absorption
On a plot of extinction efficiency (Q) against radius of particle, what causes a) broad oscillations and b) fine oscillations
a) out of phase and in phase interference between light passing around and “through” particle
b) ripple fine structure
What sort of size is the optical cross section of water and why?
What does this give rise to?
The optical cross section of water is very very small. Water has a small geometric cross section and its extinction efficiency is much less than 1
This is the reason for rainbows being observed at a fixed angle by an observer with sunlight behind
Why do rainbows form
- Water has a small optical cross section
- this is independent of the size of the droplet
- the angle depends on the refractive index
- reflection and refraction within droplet gives rainbows
- double rainbows form from 2 reflections within the droplet
Why do ripple structures occur at discrete sizes
Ripple structures only occur when wavelength and n give a standing wave
What is the single scattering albedo
The single scattering albedo is the fraction of light extinction attributed to scattering
omega = Qsca/Qext
What is the coalbedo
The coalbedo is the fraction of absorbed light, 1-omega
What is the dominant mechanism in attenuation of absorption
Condensation of scattering material from the gas phase leading to a change in droplet morphology from chain to clump-like carbon
This does not necessarily affect the bulk aerosol single scattering albedo
How does absorption by an organic component change with wavelength
Absorption increases with shorter wavelength
How does elemental black carbon interact with light
It emits it directly back to the atmosphere
How do polycyclic aromatic compounds interact with light
they are refracting
they are coloured organics
How do low molecular weight hydrocarbons interact with light
they are non-refracting
they are colourless organics
