week 6 Flashcards
(35 cards)
1
Q
what are aerosols
A
- tiny particles ranging from a few nanometres to tens of micrometres across
important for: - air quality
- the appearance of the sky
- radiative forcing
2
Q
types of aerosol
A
- mineral dust (sandstorms, etc.)
- sea-salt aerosols (from ocean spray)
- volcanic ash
- volatile organic compounds (from plants and some industries)
- sulphates (volcanic emissions, burning fossil fuel, algae)
- black carbon/soot and brown carbon (by-product of combustion)
- pollen, viruses, etc. (natural sources)
- cloud droplets
3
Q
health impacts of aerosols
A
- larger aerosol particles get filtered out before they reach the lungs and cause no harm
- particles below a critical diameter can reach the lungs:
PM10: particles <10um
PM2.5: particles <2.5um - PM 2.5 is small enough to get into the alveoli and cause harm
- many illnesses are linked to aerosol pollution contributing to millions of deaths per year
4
Q
describe fine particles in the body
A
- via nose/lung -> blood stream -> brain
- lung inflammation/ impaired function
- low oxygen in blood
- inflammation
- arterial stiffness and small-vessel damage
- asthma
- lung cancer
- COPD
- slower cognitive processing (?)
5
Q
describe the fall-speed of a particle
A
- the terminal velocity (fell-speed) of a spherical object (Vt):
ππ‘ β πππ π (π^3)/π΄πππ (π^2) β πππππ’π - aerosol particles are very small so they fall so slowly they are effectively suspended in the air
- for larger particles (>microns), this fall speed matters and they can fall at relevant speeds
6
Q
what is specific humidity
A
(q) - the amount of water vapour in a mass of air (Kg water per Kg air)
7
Q
what is saturation water vapour content
A
(qs) - the equilibrium water vapour content of air in contact with a flat surface of water (Kg water per Kg air)
8
Q
what is relative humidity
A
(RH) - the ratio of specific humidity to the saturation water vapour content (%)
9
Q
describe relative humidity and evaporative cooling
A
- the lower the relative humidity, the more easily water evaporates. a wet surface wonβt evaporate into saturated air (RH>100%)
- relative humidity is critical to determining how hot it feels:
- it is harder for sweat to evaporate (taking away latent heat energy) in very humid conditions
- it can feel much warmer in a rainforest than in hot , dry places as the relative humidity is much higher in the rainforest
10
Q
describe relative humidity and condensation
A
- water will try to condense when the relative humidity gets over 100%
- the saturation water vapour content is lower for cooler air
- so as air cools it will condense water vapour where possible
11
Q
what is the clausius-clapeyron relationship
A
- the clausius-clapeyron relationship describes how the saturation water vapour content varies as a function of temperature
- under typical atmoshperic conditions the Clausius-Clapeyron relationship approximates to the water-holding capacity rising at ~7% C^-1
- this relationship is critical to determining many properties of climate
12
Q
what is the dew point temperature of an air parcel
A
- as an air parcel rises and cools, its saturation water vapour content drops, so as an air parcel that is initially unsaturated (RH<100%) will become saturated at
some point (RH =100%) - The dew point temperature of an air parcel with some specific water vapour content is the temperature at which its relative humidity would become 100%.
- Below the dew point temperature
condensation can occur.
13
Q
describe Particle formation from condensable gas
A
- For a liquid droplet to grow from gas molecules it must satisfy 2 energetic constraints:
- The energy difference between a free gas molecule and a bound liquid molecule (favourable for RH >100%)
- The potential energy of resisting surface tension
- The smaller the drop the greater the surface tension energy barrier and the higher the relative humidity must be before it will grow (lower and it will shrink)
14
Q
describe Nucleation, super-saturation and cloud condensation nuclei
A
- Homogeneous Nucleation describes the process of spontaneous particle formation from a condensable gas.
- Water vapour in air with no impurities wonβt form droplets unless the air reaches a RH of 300-400%, i.e. the air must be highly super-saturated (RH β«100%)
- Cloud Condensation Nuclei (CCN) allow droplets to form by Heterogeneous Nucleation at much lower super-saturation levels (RH >100%) and so are critically important in cloud formation.
15
Q
describe nucleation and growth of sulphate particles
A
- Sulphates condense much more readily than water vapour and can nucleate spontaneously in the atmosphere
- Condensation of more gas on particles and Coagulation of smaller particles creates larger particles
- Above a critical size they can act as cloud condensation nuclei
16
Q
describe other cloud condensation nuclei and ice nuclei
A
- There are many kinds of aerosol particles that can act as cloud condensation nuclei, though some are more effective than others (hydro-phobic particles like oil droplets donβt act as CCN).
- Below 0Β°C a similar process can
occur for the nucleation of ice
particles, though very few
particles act as effective ice
nuclei.
17
Q
describe cloud droplet growth and precipitation
A
Cloud droplets can grow by:
* Condensation if supersaturation high enough
* Collisions between droplets and ice crystals (coagulation)
- As the droplets grow their terminal velocity increases and they can begin to fall out as rain, colliding with more droplets and growing as they fall.
- However above a certain size they break apart as they fall.
17
Q
more precipitation
A
- Much precipitation begins as falling ice crystals which can reach the surface as snow or can melt to become rain.
- Storms create strong updrafts that can keep larger particles suspended. This gives them more time to grow and can produce dramatic hail stones in some cases.
- When warm air rises over a cold
surface freezing rain can occur
17
Q
give a quick overview of cloud types
A
- Clouds form when moist air becomes super-saturated.
- This occurs in different ways:
- Convectionβ e.g. in Tropics
- Uplift β e.g. along fronts
- Cooling β e.g. Winter / Polar
- It also occurs in different contexts, e.g. unconstrained convection or convection under a cap of warm air.
- This produces a wide range of cloud types.
17
Q
describe cloud coverage
A
- Clouds cover much of the Earth and have a large effect on its radiative budget.
- The effect of global circulation can be seen on cloud cover distribution
- Clouds cover and precipitation is greatest in areas of ascent and suppressed in areas of descent
18
Q
describe LW absorbing layer at different altitudes
A
- Temperature drops with altitude.
- As the intensity of black-body radiation is a strong function of temperature the altitude of an absorbing layer is critical.
- A low-altitude absorbing layer has little warming effect
- A high-altitude absorbing layer has a much greater effect
19
Q
describe the radiative effects of clouds
A
- Clouds both scatter light and absorb LW radiation.
- Low-level clouds, like marine strato-cumulus have a net cooling effect as the albedo effect outweighs the LW effect
- High-level cirrus clouds have a net warming effect as they are optically thin but strongly absorbing of LW radiation.
- Mid-level clouds and thick clouds have a mixed effect.
20
Q
describe cloud feedbacks
A
- Global warming triggers various changes in cloud properties that affect the radiative balance of earth
- Together these are believed to have a net positive feedback though it is highly uncertain:
Ξ»ππππ’ππ = +0.27 Β± 0.42 Wm-2K-1
21
Q
describe Aerosol radiative effects β direct and indirect
A
- Aerosols can have a strong direct radiative effect β they scatter light but they also absorb SW and LW radiation, how much of each depends on the particle type:
- Sulphate aerosols are highly reflective though absorb some radiation.
- Black carbon (soot) aerosols strongly absorb light.
- Aerosols also have a powerful indirect radiative effect by modifying cloud properties.
- Anthropogenic aerosols have a strong net negative (cooling) direct and indirect radiative forcing
22
describe direct radiative effects of absorbing aerosols
- Aerosols which absorb a lot of light can have a net warming or cooling effect depending on altitude.
- The lower atmosphere is well-mixed and there is a small temperature difference and so the heat absorbed warms the surface too.
- In the stratosphere absorbing aerosols heat the stratosphere but much of the heat is radiated to space.
23
describe aerosol optical depth
- The transmittance of light through an
aerosol layer is given by:
πΌπ‘ = πΌπ Γ π^βΞ΅ππ§
Where:
* πΌπ‘ is the intensity of light transmitted through the layer
* πΌπ is the intensity of light incident on the layer
* Ξ΅ is the molar scattering or absoptivity coefficient
* c is the concentration of the attenuating species
* z is the path length.
- Aerosol optical depth (π) is a measure of the total effect of an aerosol layer on incoming light:
- π°π = π°π Γ π^βπ
Ο = Ξ΅ππ§
π = π₯π§(π°π/π°π)
- Measurements of aerosol optical depth are calculated for light passing directly down through an aerosol layer
- We can easily directly observe aerosol optical depth (Ο) but not its components (Ξ΅, π, π§)
24
describe volcanic eruptions and Earth's climate
- Volcanic eruptions have played a critical role in shaping the Earthβs climate on short and long timescales.
~250 million years ago a massive, long-lived eruption created the Siberian Traps
covering ~7 Million Km2 ~0.5 km deep with lava. The CO2 from this eruption is believed to have warmed the Earth substantially and to be responsible for the βgreat dyingβ
~70,000 years ago the Toba supereruption produced a substantial cooling that almost wiped out humans
25
describe the cooling effect of explosive volcanic eruptions
- Eruptions release CO2, ash and SO2.
SO2 reacts to form sulphate aerosols which scatter light and cool.
- The plumes of the most powerful, explosive volcanic eruptions reach the stratosphere where strong winds can spread them globally and particles are clear of clouds allowing them to persist for years.
- Mt. Pinatubo erupted in 1991 producing a cooling of ~0.5 Β°C that persisted for a couple of years.
Tambora 1815 and Krakatoa 1884 had substantially greater impacts
26
describe stratospheric aerosol geoengineering
- Is a proposal to create an artificial stratospheric aerosol layer, mimicking the cooling effects of volcanic eruptions.
- Newly-designed high-altitude jets could deliver the megatons of SO2 needed to the Tropical stratosphere.
- It is estimated that hundreds of aircraft at a cost of billions of dollars per year could produce a substantial cooling effect.
- This is only a small fraction of the costs of mitigation and climate harms
27
describe stratospheric aerosol geoengineering - particle size
- The smaller an aerosol particle is the more light it scatters so long as its not too much smaller than the wavelength of light (visible light is 400 β 700nm)
- This means that the optimum particle size for stratospheric aerosol geoengineering will be ~1 micron.
- One limitation to SO2 is that as the injection rate rises more of the sulphate condenses onto existing particles producing larger, less efficient particles. This means that more mass is needed to produce the next increment of cooling.
28
describe the first indirect aerosol effect - cloud albedo effect
- Aerosols can act as cloud condensation nuclei (CCN).
- Air polluted with aerosols will have a higher concentration of CCN than clean air which will lead to more cloud droplets forming when clouds form.
- Recall that given the same mass of particles, smaller ones will scatter more light
- Aerosol pollution therefore increases cloud albedo
29
describe the second indirect aerosol effect - cloud lifetime effect
- Smaller particles fall more slowly impeding the development of precipitation.
- Therefore, a cloud with more droplets should retain its water content and coverage for longer.
- For low-clouds this should lead to additional cooling.
30
describe the semi-direct effect - cloud adjustments
- Aerosols reduce light reaching the surface and warm the layer of the atmosphere that they are in.
- Recall that relative humidity is a function of temperature, and so is the super-saturation level needed for cloud droplets to be stable.
- The warming of the layers of the atmosphere containing clouds can therefore lead to them burning off, reducing cloud cover.
31
describe ship tracks and marine cloud brightening
- Marine strato-cumulus clouds form extensive areas of low clouds in the sunny sub-tropics.
- Ships can leave distinct ship tracks as they emit sulphates which act as CCN and brighten the clouds (first indirect effect)
- Marine Cloud Brightening is a proposal to deliberately produce this effect on a larger scale using sea-salt aerosols emitted from specially-designed ships.
32
describe cirrus cloud thinning
- Recall Cirrus clouds have a net warming effect so thinning them out would cool.
- Some cirrus forms by homogeneous nucleation producing many light, long-lived crystals, and relatively thick clouds
- If ice nuclei could be introduced then heterogeneous growth could occur leading to heavier, shorter -lived crystals and thinner cirrus clouds. This would produce a cooling effect.
