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Physical > Aerosols > Flashcards

Aerosols Flashcards

1
Q

Define an aerosol

A

An aerosol is a dispersion of liquid droplets or solid particles within a gas phase; refers to the combination not just the dispersion

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2
Q

In what ways are aerosols stable/unstable

A

They are thermodynamically unstable but kinetically stabilised

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3
Q

Why are aerosols thermodynamically unstable

A

Large surface area:volume ratio. Surfaces are high in energy

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4
Q

What is involved in aerosol dynamic processes

A

Rapid coupling of condensed particle and gas phase. Many processes with wide range of time and length scales

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5
Q

What influences aerosol motion (5)

A
  • Brownian diffusion
  • Turbulence
  • Gravitation
  • Particle charge
    Temperature gradients
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6
Q

What is the process of aerosol delivery

A

Pressurised container –> fluid –> stream –> bubble generation –> bubble bursting –> liquid droplets, contain active ingredients (AI) –> liquid droplets with AI –> AI only

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7
Q

What motivates hygroscopic growth

A

Ease of fabrication of hierarchically structured organic and inorganic nanodomains and mesostructure at different lengthscales

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8
Q

Applications of aerosols (7)

A

Catalysis, sensors, controlled release, therapeutic carriers, optics, photonics, separation

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9
Q

What is a sol precursor

A

Dispersed dense/porous nanoparticle with inorganic/hybrid polymers and surfactants

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10
Q

What is the process of using a sol precursor

A

Sol –> atomizer (gas input) –> droplets –> oven –> macro-micro-patterns –> film and particles

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11
Q

What does PM2.5 mean and what is the other common aerosol measurement

A

Particulate matter 2.5 microns and smaller

PM10

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12
Q

What is the typical size and concentration of a rain drop

A

1 mm, 1 /L

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13
Q

What is the typical size and concentration of a cloud drop

A

10 um, 1000 /L

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14
Q

What is the typical size and concentration of a haze drop

A

1 um, 1000 /L

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15
Q

What is the typical size and concentration of pollution

A

0.1 um, 10,000 - 100,000 /L

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16
Q

What is the typical size and concentration of nebuliser

A

0.1 - 5 um, 10,000 - 100,000 /L

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17
Q

What are primary particles?

A

Emitted into the air directly by resuspension of material

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18
Q

What are secondary particles?

A

formed in the air: gas-to-particle conversion (nucleation, condensation etc)

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19
Q

How are secondary organic aerosols formed?

A

oxidation of VOCs

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20
Q

What is the definition of fine aerosol

A

nucleation mode and accumulation mode

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21
Q

What is nucleation mode

A

Aerosols 0.001 - 0.1 um in size. Largest by number but just a few % by mass

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22
Q

What is accumulation mode

A

Aerosols 0.1 um - 1 um, largest by surface area, significant part of mass of total aerosol. Particle removal least efficient here.

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23
Q

What is coarse mode

A

1 - 100 um aerosol. Removed by mechanical disturbance. Sedimentation causes falling with fast velocity

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24
Q

What is different about coarse and fine aerosol? (6)

A

Origination, transformation, removal, composition, optical properties, deposition patterns in respiratory tract

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25
Q

Anthropogenic sources of primary aerosol (2)

A

industrial dust (fine and coarse), soot (fine)

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26
Q

Anthropogenic sources of secondary aerosol (4)

A

sulfates from SO2 (fine), biomass burning (fine), nitrates from NOx (coarse), organics from VOCs (fine)

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27
Q

Natural (dominates global emissions) sources of primary aerosols (4)

A

soil dust mineral, sea salt, volcanic dust, biological debris, all coarse

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28
Q

Natural sources of secondary aerosol (3)

A

Sulfates from bioorganic gases and volcanoes (fine), organic matter from VOCs (fine), nitrates from NOx (fine and coarse)

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29
Q

What sort (size) of aerosol particles are produced over deserts?

A

Large particles that are around 100% mass

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30
Q

Where is the cleanest air globally?

A

The polar regions

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31
Q

What is the lifetime of nucleation mode aerosols

A

less than 1 hour

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32
Q

What is the lifetime of accumulation mode aerosols

A

Days

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33
Q

What is the lifetime of coarse mode aerosols

A

minutes –> days

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34
Q

What counters gravity

A

Bouyancy

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35
Q

What does drag force assume in Stokes Law?

A

gas phase is continuous for particles > 1 um

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36
Q

What is the Cunningham slip correction factor?

A

For low gas number densities, the gas doesn’t look like a fluid; Cc added to Stokes law equation for drag

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37
Q

What is the Knudsen number?

A

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

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38
Q

What does X (chi) take into account?

A

Non-spherical particles

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39
Q

What sort of particles are preferentially deposited by impaction & what are the uses of this

A

Large particles; this is used to sample aerosols in cascade impactors

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40
Q

What is Brownian diffusion caused by

A

Particles bombarded by air and imbalance of transient forces

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41
Q

What is the size region of particles for which Brownian diffusion is most important

A

< 0.1 um

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42
Q

What is the primary removal process for coarse mode aerosol

A

Sedimentation

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43
Q

What is the primary removal process for nucleation and accumulation mode aerosol

A

Diffusion and coalescence

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44
Q

What are the challenges in aerosol analysis (4)

A

Wide size and mass range means variation in sensitivity, concentration range: sampling and rates of loss processes, phase variation, sample perturbation before analysis

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45
Q

What are the main things we try to characterise about aerosols (4)

A

number, size distribution, SA conc and mass conc

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46
Q

Why are PM2.5 and PM10 routinely measured

A

Inhalable coarse mode particles between 2.5 and 10 um

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47
Q

What is the general process for measuring aerosols

A

Record mass loading of filters, control T and humidity, for a known volume and set time period

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48
Q

How do impactors work (a column of angry faces)

A

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

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49
Q

What does a CONTINUOUS AMBIENT PARTICULATE MONITOR measure and how

A

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

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50
Q

What does a CONDENSATION PARTICLE COUNTER measure and how

A

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

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51
Q

What does aerodynamic size depend on

A

Passage in gas, defined by size, shape and density

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52
Q

What does electrical mobility size depend on

A

size and shape. Not density

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53
Q

What does optical size depend on

A

Shape, size and refractive index

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54
Q

How does an AERODYNAMIC PARTICLE SIZER work and what are the pros (2)

A

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.

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55
Q

How does a DIFFERENTIAL MOBILITY SIZER work (also called electrostatic classifier)
Pro (1), Con (1)

A

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

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56
Q

Why is composition determination difficult

A

Sensitive to artefacts in sampling, transport and storage, sensitive to loss of volatiles, gas-particle and particle-particle reactions within sample

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57
Q

What techniques are used to determine particle composition (7)

A
  • Atomic emission/abs spec
  • HPLC
  • GC-MS
  • Laser microprobe MS
  • Laser induced breakdown spec
  • Laser induced plasma spec
  • Capillary zone electrophoresis
58
Q

Why is average composition determined by MS (3)

A

in situ, real time, sub micron aerosol

59
Q

Define relative humidity

A

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

60
Q

What is the cloud experiment

A

Nucleation rates are measured in a chamber

61
Q

What are the two big players in atmospheric nucleation

A

ammonia and sulfuric acid

62
Q

What occurs in the heterogeneous nucleation of ammonia/sulfuric acid droplets and what does it result in?

A

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

63
Q

What arises from heterogeneous nucleation at low temperatures?

A

Ice particles

64
Q

How do cloud droplets form?

A

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
65
Q

What is a(H2O)

A

The activity of water in solution

66
Q

What does Raoult’s Law assume?

A

Ideal behaviour of water in solution such that a = x = pressure

67
Q

What is denoted by non-ideality of water activity in solution?

A

a(H2O) = x(H2O)gamma(H2O)

68
Q

What inorganic molecule is often used to calibrate an instrument measuring hygroscopic growth and why?

A

H2SO4 because it absorbs/desorbs water continuously

69
Q

What parameter characterises response to relative humidity

A

Mass growth factor

70
Q

The phase and size of a particle of hygroscopic aerosol depends on what?

A

The vapour phase

71
Q

What does hygroscopic aerosol do?

A

Hygroscopic aerosol absorbs water dramatically over 80 % relative humidity leading to dilution of solute

72
Q

Explain aerosol hysteresis

A

Transitions occur at particular relative humidities and therefore their phase depends on RH history.

73
Q

What can a hysteresis loop result in

A

solution droplets can be supersaturated in solute when dried from high relative humidity

74
Q

What is the deliquescence point

A

the deliquescence point occurs when water activity in the gas phase = water activity in saturated salt solution formed when solute dissolves

75
Q

What is the efflorescence point

A

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

76
Q

At what point does most growth in diameter occur and what does this depend on

A

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

77
Q

What technique is used to measure hygroscopic growth of accumulation mode particles

A

Hygroscopic tandem differential mobility analysis which measures water uptake for nearly monodisperse aerosol

78
Q

How does hygroscopic tandem differential mobility analysis work

A
  • 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
79
Q

What is the Kelvin Effect?

A

Because droplet surfaces are curved, they have a higher vapour pressure than flat surfaces

80
Q

Why is a pure water droplet never stable in size?

A

The partial pressure is greater at the surface than at a large difference therefore condensation reduces the vapour pressure resulting in more condensation

81
Q

In the Kelvin equation, what does the Kelvin term describe? What is the size of this parameter dependent on?

A

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

82
Q

What other factors does a lower surface tension correspond to?

A

lower critical supersaturation and activation

83
Q

What are small aerosols before activation?

A

Condensation nuclei (CN)

84
Q

What are aerosols that activate

A

Cloud condensation nuclei (CCN)

85
Q

Give Henry’s Law

A

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

86
Q

What units is Henry’s coefficient given in?

A

Molar /atm

87
Q

How do you work out concentration from Henry’s coefficient?

A

just multiply by partial pressure
Ha x pa = [A]
Molar /atm x atm = molar

88
Q

What is atomisation

A

Atomisation is vaporisation of liquid droplets and it is important for combustion

89
Q

Give 6 examples of dynamic processes employing aerosols

A
  • 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
90
Q

What governs unsteady evaporation

A

coupled differential equations govern heat and mass transfer

91
Q

How does steady evaporation arise

A

slow mass transfer: rate of energy deposition/removal slow and the process is isothermal; heat and mass transfer are decoupled

92
Q

Under what circumstances can heat and mass transfer be treated by quasi-steady theory

A

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

93
Q

What is the unsteady period in mass transfer

A

mass flux changes as a function of time

94
Q

what is the steady period in mass transfer

A

mass flux balanced in two directions

95
Q

What is the wet bulb temperature

A

when rapid initial evaporation leads to surface cooling, the wet bulb temperature is reached

96
Q

How do structures like hollow spheres, doughnuts, buckled, deformed, folded form

A

Radially unhomogeneous aerosol gives enhanced surface saturation

97
Q

What is the Peclet number

A

peclet number = evaporation rate/diffusion rate

98
Q

How does folding occur

A

Evaporation induced self-assembly

99
Q

How do doughnuts occur

A

evaporation induced micelle packing

100
Q

Give 6 advantages of using aerosols in industry

A
  • cheap
  • scalable
  • 1 pot
  • ease of distribution control
  • access to metastable states
  • tuneable particle size
101
Q

What is the correction factor used for flux values, to obtain true flux, because Jc does not equal Jk

A

gamma measured. It is a measured flux correction

102
Q

What are the two factors that govern the size of gamma measured

A

mass accommodation (alpha) at the interface and diffusion to the interface in the gas phase

103
Q

What is the mass accommodation factor?

A

alpha = number of molecules absorbed/number of collisions with the surface
It is a probability/statistical value

104
Q

What governs flux in large particles?

A

Diffusion (slow)

105
Q

What governs flux in small particles?

A

surface kinetics with mass accommodation being the limiting factor

106
Q

As the Knudsen number increases, the diffusion rate factor…

A

Decreases

107
Q

What can a very small mass accommodation factor reflect?

A

Energetics: presence of a barrier

for example solvent reorganisation required for solvation which is an entropic cost

108
Q

What is an aerosol flow reactor and how does it work?

A
  • 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
109
Q

Define elastic scattering and give 3 examples

A

Elastic scattering leads to a change in momentum of light but not energy

  • reflection
  • refraction
  • diffraction
110
Q

Define inelastic scattering and give 3 examples

A

Inelastic scattering results in a change in energy of the light

  • fluorescence
  • Raman
  • absorption
111
Q

What theory is envoked when Rayleigh scattering occurs on small particles that are approximately comparable to the wavelength of light

A

Mie Theory

112
Q

What theory is envoked when Rayleigh scattering occurs on small particles that are larger than the wavelength of light

A

approximation to geometric ray optics

113
Q

What is the real component of the refractive index N = n + ik

A

The real component comes from the speed of light in medium (nu) compared to vacuum (c), n = c/nu

114
Q

What does the imaginary part of the refractive index number tell us about

A

attenuation of light by absorption

115
Q

What 3 parameters are are n and k both dependent on

A

wavelength (dispersion), temperature, density

116
Q

What are the Kramers-Kronig relations

A

the real part of the refractive index, related to the differential of the imaginary part

117
Q

What is the wavelength necessity for Rayleigh scattering

A

particles < wavelength/20

118
Q

Describe Rayleigh scattering

A

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

119
Q

What is Mie scattering

A

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
120
Q

What is scattering irradiance dependent on

A

angle in field where there is no overlap with incident field

121
Q

What is another word for the scattering angle

A

phase function

122
Q

What are angular scattering profiles

A

Angular scattering profiles are polar plots of intensity variation with angle and the scattering pattern is characteristic of particles

123
Q

What do nephelometers record

A

Nephelometers record total (7 - 170 degrees) scattering at three wavelengths (450, 550, 700 nm) for integrated backscattering

124
Q

How do you use a nephelometer to resolve forward and backward scatter

A

use a “backscatter shutter” to exclude 7 - 90 degrees

125
Q

What is a problem caused by the limited approach to zero degrees using a nephelometer

A

Truncation errors which increase with size of particle

126
Q

What is the scattering coefficient accuracy of a nephelometer

A

7 %

127
Q

How do you use scattering intensity to determine size

A

Intensity increases with contrast to refractive index of surrounding medium

128
Q

What is the optical cross section for scattering

A

It is the cross section of the shadow cast by the object given in units of area

129
Q

What is the total extinction in scattering

A

sum of scattering and absorption

130
Q

On a plot of extinction efficiency (Q) against radius of particle, what causes a) broad oscillations and b) fine oscillations

A

a) out of phase and in phase interference between light passing around and “through” particle
b) ripple fine structure

131
Q

What sort of size is the optical cross section of water and why?

What does this give rise to?

A

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

132
Q

Why do rainbows form

A
  • 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
133
Q

Why do ripple structures occur at discrete sizes

A

Ripple structures only occur when wavelength and n give a standing wave

134
Q

What is the single scattering albedo

A

The single scattering albedo is the fraction of light extinction attributed to scattering
omega = Qsca/Qext

135
Q

What is the coalbedo

A

The coalbedo is the fraction of absorbed light, 1-omega

136
Q

What is the dominant mechanism in attenuation of absorption

A

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

137
Q

How does absorption by an organic component change with wavelength

A

Absorption increases with shorter wavelength

138
Q

How does elemental black carbon interact with light

A

It emits it directly back to the atmosphere

139
Q

How do polycyclic aromatic compounds interact with light

A

they are refracting

they are coloured organics

140
Q

How do low molecular weight hydrocarbons interact with light

A

they are non-refracting

they are colourless organics

Physical (4 decks)

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