Atmospheric aerosol L1-3 Flashcards
What is the earths climate controlled by
The earths climate is controlled by the amount of solar radiation intercepted by the planet and the fraction of that energy that is absorbed
What is fundamental in determining how warm or cool the planet is
The balance between energy absorbed by the earth and energy reflected back into space
Define solar flux / solar constant
The average solar electromagnetic radiation (total solar irradiance) per unit area is a flux density
What is the average value of the solar constant
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.
What is the albedo
Of the incoming solar radiation the fraction that is reflected back to space is called the albedo
What can albedo range between
Albedo can range between 0 (no reflectance) and 1 (complete
reflectance—like a perfect mirror)
What is the earths albedo know as and denoted as
The Earth’s albedo is denoted here as 𝛼𝑝, known to be 0.31
What contributes to the planetary albedo
Clouds and scattering by air molecules
What is a black body
A black body is a theoretical concept useful in estimating
the maximum absorption and emission of a surface.
What does the black body emissive power depend on
Surface temperature
What is the stefan Boltzmann law
The Stefan-Boltzmann law (𝐹𝐵 = 𝜎𝑇^4) gives the total emissive power of a blackbody
in units of W.m-2
Define greenhouse effect
the heating of the Earth’s near-surface atmosphere by the trapping of out-going infrared radiation by atmospheric gases.
Define global warming
the increase of the near-surface average global
temperature
Define radiative forcing
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.
Define aerosol
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
Describe aerosols in terms of thermodynamic and kinetic stability
As with all dispersion colloids, they are thermodynamically unstable but kinetically stabilised
Aerosol: give an example of a suspension of liquid droplets in the gas phase
Fog, mist and spray. Processes can occur in the bulk of the droplet as well as at the surface
Aerosol: give examples of a suspension of solid particles in the gas phase
Smoke, dusk and fumes. Only processes occurring at the surface need be considered
Give examples of aerosols in the atmosphere
- sulphate aerosol and ash (volcanoes)
- organic aerosol
- biological aerosol
- dust
- sea sprays
- smoke and smog
Give examples of aerosols in terms of appliances
- household and cosmetic products
- metered does inhalers
- crop spraying
- cloud seeding
What key characteristics of aerosols need to be quantifies
- 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
How many orders of magnitude can particle diameter span and how many orders of magnitude in volume does this correspond to
5 orders of magnitude
15 orders of magnitude in volume
What are the three different types of mode
Nucleation mode
Accumulation mode
Coarse mode
What does Nucleation mode account for
Accounts for the largest number of particles but usually no more than a few percentage by mass
What does accumulation account for
Accumulation mode accounts for most of surface area and significant part of mass. Partial removal least efficient in this size range
How are coarse mode particle primarily removed
By mechanical disturbance their sedimentation velocity is fast
What is the approximate particle diameter range for Nucleation mode particles
Around 0.001 to 0.01 µm
What is the approximate particle diameter for accumulation mode particles
Around 0.01 to 1.0 µm
What is the approximate particle diameter for coarse mode particle
Around 1.0 to 100 µm
Describe the differences in coarse and fine aerosols
- originate separately
- transform separately
- removed by different mechanisms
- have different chemical compositions
- have different optical properties
- differ in their deposition patterns
What are primary particles
Primary particles are emitted into the air directly from a source by re-suspension of material. Released without undergoing significant chemical change
What are secondary particles
Secondary particles are formed in the atmosphere from chemical reactions involving gases e.g Nucleation, condensation, growth
Examples of primary particles
Soot
Sea salt
Dust - from deserts
Examples of secondary particles
Volcanic gases, organic aerosols
How do we define coarse particles
Coarse particles are >1 µm in diameter
How do we define fine particles
Fine particles are < 1 µm in diameter
What is the log normal size distribution
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
Compare and contrast aerosol data displayed using log scale and linear scale for particle diameter
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
Describe how we calculate the total aerosol distributions
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
Describe concentration, surface and volume distributions
different measures of the aerosol size distribution may be required depending on which property is under investigation
What is aerosol number distributions usually dominated by
the Nucleation mode
What is the surface area distribution usually dominated by
the surface area distribution usually the most important is dominated by the accumulation mode
Define particle lifetime
refers to how long various particles remain suspended before they are removed by natural processes
What does particle lifetimes depend on
particle size, composition, attitude and weather conditions
What is the lifetime of nucleation mode particles
Hours to days
What is the dominant removal process for nucleation modes
coagulation
What is the lifetime of accumulation mode particles
days to weeks
What is the dominant removal process for accumulation modes
Wet deposition (rain and wash out)
What is the lifetime of coarse mode particles
hours to days
What is the dominant removal process for coarse modes
Dry deposition (sedimentation)
What happens if the number density of gas molecules becomes very small
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
What is stoke’s law
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
What is the Cunningham slip correction factor
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.
What is the Knudsen number
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
If the particle is not spherical how can we adapt the equation to determine the terminal velocity
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.
Describe the brownian diffusion of particles
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
For what type of particle is brownian diffusion most important for
Important process for small particles (< 1µm)
Define coalescence
Coalescence is the process where two or more aerosol particles come into contact and merge to form a larger particle
