Atmospheric aerosol L1-3

Question 1

What is the earths climate controlled by

Answer 1

The earths climate is controlled by the amount of solar radiation intercepted by the planet and the fraction of that energy that is absorbed

Question 2

What is fundamental in determining how warm or cool the planet is

Answer 2

The balance between energy absorbed by the earth and energy reflected back into space

Question 3

Define solar flux / solar constant

Answer 3

The average solar electromagnetic radiation (total solar irradiance) per unit area is a flux density

Question 4

What is the average value of the solar constant

Answer 4

The average value of the solar constant over space and time is 1368 W.m-2. This is not a
physical constant and is measured by satellites above the Earth’s atmosphere.

Question 5

What is the albedo

Answer 5

Of the incoming solar radiation the fraction that is reflected back to space is called the albedo

Question 6

What can albedo range between

Answer 6

Albedo can range between 0 (no reflectance) and 1 (complete
reflectance—like a perfect mirror)

Question 7

What is the earths albedo know as and denoted as

Answer 7

The Earth’s albedo is denoted here as 𝛼𝑝, known to be 0.31

Question 8

What contributes to the planetary albedo

Answer 8

Clouds and scattering by air molecules

Question 9

What is a black body

Answer 9

A black body is a theoretical concept useful in estimating
the maximum absorption and emission of a surface.

Question 10

What does the black body emissive power depend on

Answer 10

Surface temperature

Question 11

What is the stefan Boltzmann law

Answer 11

The Stefan-Boltzmann law (𝐹𝐵 = 𝜎𝑇^4) gives the total emissive power of a blackbody
in units of W.m-2

Question 12

Define greenhouse effect

Answer 12

the heating of the Earth’s near-surface atmosphere by the trapping of out-going infrared radiation by atmospheric gases.

Question 13

Define global warming

Answer 13

the increase of the near-surface average global
temperature

Question 14

Define radiative forcing

Answer 14

describes the net energy in units of W.m-2 made available to the Earth associated with the increase in the concentration of each greenhouse gas, since 1750.

Question 15

Define aerosol

Answer 15

An aerosol is a dispersion of liquid droplets or solid particles within a gas phase, the term refers to the combination of the gas and liquid/solid state not just the dispersed particles

Question 16

Describe aerosols in terms of thermodynamic and kinetic stability

Answer 16

As with all dispersion colloids, they are thermodynamically unstable but kinetically stabilised

Question 17

Aerosol: give an example of a suspension of liquid droplets in the gas phase

Answer 17

Fog, mist and spray. Processes can occur in the bulk of the droplet as well as at the surface

Question 18

Aerosol: give examples of a suspension of solid particles in the gas phase

Answer 18

Smoke, dusk and fumes. Only processes occurring at the surface need be considered

Question 19

Give examples of aerosols in the atmosphere

Answer 19

• sulphate aerosol and ash (volcanoes)
• organic aerosol
• biological aerosol
• dust
• sea sprays
• smoke and smog

Question 20

Give examples of aerosols in terms of appliances

Answer 20

• household and cosmetic products
• metered does inhalers
• crop spraying
• cloud seeding

Question 21

What key characteristics of aerosols need to be quantifies

Answer 21

• particle size distribution
• how many particles there are and how closely spaced they are
• a measure of the composition of the aerosol and there phase

Question 22

How many orders of magnitude can particle diameter span and how many orders of magnitude in volume does this correspond to

Answer 22

5 orders of magnitude
15 orders of magnitude in volume

Question 23

What are the three different types of mode

Answer 23

Nucleation mode
Accumulation mode
Coarse mode

Question 24

What does Nucleation mode account for

Answer 24

Accounts for the largest number of particles but usually no more than a few percentage by mass

Question 25

What does accumulation account for

Answer 25

Accumulation mode accounts for most of surface area and significant part of mass. Partial removal least efficient in this size range

Question 26

How are coarse mode particle primarily removed

Answer 26

By mechanical disturbance their sedimentation velocity is fast

Question 27

What is the approximate particle diameter range for Nucleation mode particles

Answer 27

Around 0.001 to 0.01 µm

Question 28

What is the approximate particle diameter for accumulation mode particles

Answer 28

Around 0.01 to 1.0 µm

Question 29

What is the approximate particle diameter for coarse mode particle

Answer 29

Around 1.0 to 100 µm

Question 30

Describe the differences in coarse and fine aerosols

Answer 30

- originate separately - transform separately - removed by different mechanisms - have different chemical compositions - have different optical properties - differ in their deposition patterns

Question 31

What are primary particles

Answer 31

Primary particles are emitted into the air directly from a source by re-suspension of material. Released without undergoing significant chemical change

Question 32

What are secondary particles

Answer 32

Secondary particles are formed in the atmosphere from chemical reactions involving gases e.g Nucleation, condensation, growth

Question 33

Examples of primary particles

Answer 33

Soot Sea salt Dust - from deserts

Question 34

Examples of secondary particles

Answer 34

Volcanic gases, organic aerosols

Question 35

How do we define coarse particles

Answer 35

Coarse particles are >1 µm in diameter

Question 36

How do we define fine particles

Answer 36

Fine particles are < 1 µm in diameter

Question 37

What is the log normal size distribution

Answer 37

Particle sizes can span from nm to micro meters the easiest way to capture this information is it show the distribution on a scale of LogP D-diameter

Question 38

Compare and contrast aerosol data displayed using log scale and linear scale for particle diameter

Answer 38

Aerosols typically have number distributions such as those with the characteristic bell shape of a normal distribution in log (D) space. On a linear scale the distributions on a linear scale are asymmetric and shaded to large particle sizes

Question 39

Describe how we calculate the total aerosol distributions

Answer 39

we typically have isolated distributions for Nucleation, accumulation and coarse modes so the total aerosol distributions can be written in terms of a sum of distribution functions

Question 40

Describe concentration, surface and volume distributions

Answer 40

different measures of the aerosol size distribution may be required depending on which property is under investigation

Question 41

What is aerosol number distributions usually dominated by

Answer 41

the Nucleation mode

Question 42

What is the surface area distribution usually dominated by

Answer 42

the surface area distribution usually the most important is dominated by the accumulation mode

Question 43

Define particle lifetime

Answer 43

refers to how long various particles remain suspended before they are removed by natural processes

Question 44

What does particle lifetimes depend on

Answer 44

particle size, composition, attitude and weather conditions

Question 45

What is the lifetime of nucleation mode particles

Answer 45

Hours to days

Question 46

What is the dominant removal process for nucleation modes

Answer 46

coagulation

Question 47

What is the lifetime of accumulation mode particles

Answer 47

days to weeks

Question 48

What is the dominant removal process for accumulation modes

Answer 48

Wet deposition (rain and wash out)

Question 49

What is the lifetime of coarse mode particles

Answer 49

hours to days

Question 50

What is the dominant removal process for coarse modes

Answer 50

Dry deposition (sedimentation)

Question 51

What happens if the number density of gas molecules becomes very small

Answer 51

The gas around the particle now no longer looks like a continuous fluid and the moving aerosol particles is able to slip through the surrounding gas

Question 52

What is stoke’s law

Answer 52

Stokes' Law describes the drag force experienced by small, spherical particles moving slowly through a viscous fluid. It assumes continuous fluid flow around a particle

Question 53

What is the Cunningham slip correction factor

Answer 53

The Cunningham slip correction factor is used to correct the drag force experienced by very small particles moving through a gas, like air. The Cunningham factor accounts for this non-continuum flow behavior.

Question 54

What is the Knudsen number

Answer 54

The Knudsen number (Kn) is a dimensionless number that compares the distance between gas phase collisions to the radius of the particle. It tells us essentially, whether gas behaves like a continuous fluid or not

Question 55

If the particle is not spherical how can we adapt the equation to determine the terminal velocity

Answer 55

If the particle is not spherical, this can be taken into account by introducing a shape factor, χ, into the calculation of the settling velocity.

Question 56

Describe the brownian diffusion of particles

Answer 56

refers to the random, erratic motion of very small particles surrounded in a fluid caused by collisions with individual gas or liquid molecules. This motion leads to a net spreading of particles from regions of high concentrations to low

Question 57

For what type of particle is brownian diffusion most important for

Answer 57

Important process for small particles (< 1µm)

Question 58

Define coalescence

Answer 58

Coalescence is the process where two or more aerosol particles come into contact and merge to form a larger particle