Aerosol Microphsyics 1 Flashcards
What groups can secondary atmospheric particles be divided into?
- New particle formation (NFP) from gaseous precursors
- Condensation of gases on already existing surfaces
What sizes must newly formed and particles grow to before they influence climate?
Over 50-100nm in diameter
What does NFP stand for?
Newly formed particles
What is aerosol Nucleation?
The Nucleation of trace substances and water from the vapour phase to the liquid or solid phase
What is aerosol Nucleation the first step in?
The phase transition process
What are the requirements/steps for the production of new particles by gas-to-particle conversion?
- Critical embryo/thermodynamically stable cluster ~1nm must be formed
- Once formed these clusters can grow rapidly through condensation and/or coagulation to quasi-stable sizes of 3-4nm Dp
- Survival beyond this size is rare and depends on numerous competing processes
What is supersaturation of vapour phase required for?
Nucleation
What is supersaturation?
Relative humidity > 100%
What does RH stand for?
Relative humidity
What happens to molecules and clusters in supersaturated vapour?
There’s a higher conc of molecules and clusters
How are molecular clusters formed?
By gas phase collisions and attachment of molecules to reach a critical size at which they can grow further by condensation
What is homogeneous Nucleation?
The self-Nucleation of one or more species in the absence of a pre-existing surface
What is heterogeneous Nucleation?
The Nucleation of one or more species onto a pre-existing surface
What is the most common Nucleating agent?
Sulphuric acid
What happens in NPF after stable clusters are formed?
Other substances such as low-volatility organics can take part in the growth process
Once particles grow to a big enough size in NPF what can happen?
They can act as nuclei on which cloud droplets then form by condensation of water vapour
What conditions are needed for Nucleation to occur?
Needs to be high enough concs of nucleating vapours produced from photo-oxidation of atmospheric gases
(E.g. sulphur dioxide and volatile organic compounds)
What does the low volatility of sulfuric acid lead to?
It’s easily supersaturated and the gaseous sulphur dioxide can begin to condense
When does NPF start?
Around midday
Why does NPF start around midday?
Because it requires sunlight
The fact that NPF starts around midday causes the NPF growth graph to look like what?
A banana
Where are aerosol Nucleation events frequently observed?
In the free troposphere and under remote, urban, forested, and marine environments of the lower troposphere
What two main processes control the transformations of particles?
Condensation and coagulation
Do all Nucleation events create clouds?
No
Give an example of a Nucleation event that doesn’t lead to a cloud
Local kelp and algae blooms that create aerosols but particles don’t get high enough or high enough conc to form cloud
Describe the process of coagulation
- Aerosol particles collide with one another due to relative motion between them
- They become so close that Van-der-waals forces bind them together
- They form a larger particle
What is kinematic coagulation?
When relative motion arises from external forces I.e. gravity, electrical forces, aerodynamic effects
What is thermal coagulation?
When the relative motion is due to brownian motion
How does coagulation affect number of particles?
Decrease no. Small particles, doesn’t affect no. Big particles
What does the rate of coagulation depend on?
-diameter of larger particles
-diffusion rate of smaller particles
-conc of large and small particles
Define particle formation rate
The flux of growing nanoparticles through a certain diameter size barrier
What are the 2 macroscopic scale properties relevant for analysis particle growth?
Particle formation rate (J)
Particle growth rate (GR)
What does particle growth rate mean?
Reasonably well-separated mode of particles undergoing a growth into larger sizes
Under most atmospheric conditions, what is the dominant removal mechanism for atmospheric Nucleation mode particles?
Coagulation with larger particles
What is the removal of Nucleation mode particles strongly tied to?
Timescales for growth to larger sizes by condensation and self-coagulation
What happens in slow nuclei growth?
Particles are quickly scavenged away and have practically no atmospheric relevance
What happens in fast nuclei growth?
Significant fraction of particles can survive and eventually modify the entire particle population
what is turbulent diffusion?
turbulent eddies bring material close to the surface
what is brownian motion? (dry deposition)
random brownian motion causes them to collide with any nearby surfaces
what is gravitational settling?
large particles are pulled to surface due to gravity
what are two mechanisms that produce sea salt?
-direct production of sea-salt through spume
-indirect production of sea-salt aerosol through bubbles
