Topic 1 - Physical Atmosphere

Question 1

What is atmospheric pressure?

Answer 1

The force exerted by the gravitational attraction of the mass of the atmosphere over a unit area.

Question 2

How does altitude affect pressure?

Answer 2

Pressure is highest at the surface and decreases exponentially with altitude.

Question 3

What factors affect the temperature of the atmosphere?

Answer 3

Radiative heating and cooling.

Adiabatic cooling.

Ozone chemistry.

Question 4

What temperature variations occur in the troposphere?

Answer 4

The temperature decreases with altitude.

Question 5

What is radiative heating and cooling?

Answer 5

The lower atmosphere is heated by the infra-red radiation given off by the Earth. At lower altitudes, the higher pressure provides better insulation and therefore higher temperatures. At higher altitudes and lower pressures, heat loss is higher which results in lower temperatures.

This change in temperature is rapid and proportional to the pressure.

Question 6

What is adiabatic expansion?

Answer 6

Air flows from high to low pressure which causes air parcels to rise and also increase in volume. This expansion is adiabatic and the work done results in lower temperatures inside the air parcel.

This results in a linear decrease with altitude.

Question 7

How is tropospheric convection formed?

Answer 7

Adiabatic expansion causes a linear decrease in temperature with altitude. However, radiative cooling has a faster drop in temperature with altitude until about 11 km. At this point, the cooling rates flip and the air temperature of the parcel will be lower than the outside air. This causes the air parcel to return to the surface.

Question 8

What temperature variations occur in the stratosphere?

Answer 8

The temperature rises with altitude due to the absorption of solar radiation by ozone.

The stratosphere is also layered due to a lack of vertical mixing.

Question 9

What temperature variations occur in the mesosphere?

Answer 9

Temperature decreases with height due to a lack of ozone and radiative cooling taking over.

Question 10

What temperature variations occur in the thermosphere?

Answer 10

The temperature rises with no final temperature. This is because at higher altitudes, high energy photons have not yet been filtered and they induce photo dissociations. As the pressures are low, collisional relaxation is slow and the molecular fragments retain excess energy which is translated to kinetic energy.

Question 11

What causes horizontal transport in the troposphere?

Answer 11

There is a large spatial variation in the incoming solar flux which leads to temperature and pressure gradients. This causes an overall air flow from the equatorial regions to the poles.

Additionally, Earth’s rotation leads to the Coriolis effect which results in strong zonal winds (east and west). Meridional winds (north and south) are much slower.

Question 12

What is the lifetime of zonal air mixing?

Answer 12

2 weeks.

Question 13

What is the lifetime of meridional air mixing?

Answer 13

2 months.

Question 14

What is the lifetime of hemispherical air mixing?

Answer 14

12 months.

Question 15

How can chemicals be lost in the atmosphere?

Answer 15

Many chemicals can undergo chemical loss processes such as photolysis or reaction with OH radicals or O₃.

Chemically stable molecules can undergo physical loss processes such as dry deposition into land or water or wet deposition into rain droplets.

Question 16

What does steady state concentration depend on?

Answer 16

The emission and loss rates.

Question 17

What are the features of stratospheric/tropospheric mixing?

Answer 17

There is very little mixing between the troposphere and the stratosphere due to the temperature inversion at the tropopause.

However, compounds that are inert to chemical and physical processes have long lifetimes and can eventually escape into the stratosphere.

Question 18

How can the lifetime of a compound be determined?

Answer 18

It is the inverse of the total loss rate coefficient (sum of the rates of losses by chemical reactions, photolysis and deposition).

τ = 1/k

Question 19

When can Planck’s radiation law be applied?

Answer 19

For a black body emitter.

The sun and Earth’s surface can be treated as a black body emitter.

Question 20

What is the solar constant?

Answer 20

The fraction of the radiation intercepted per unit area from the Sun.

Question 21

What is the albedo effect?

Answer 21

Not all of the incident radiation on Earth is absorbed. 25% is reflected by the atmosphere and 4% is reflected by the planet’s surface.

Question 22

How can the intensity after scattering be calculated?

Answer 22

Using the Beer Lambert Law.

Question 23

What is Rayleigh scattering?

Answer 23

Scattering that occurs when the size of the scattering particle is much less than the wavelength of light.

This is relevant for gas molecules such as N₂ and O₂.

Question 24

What is Mie scattering?

Answer 24

Scattering that occurs when the size of the scattering particle is close to the wavelength of light.

The is relevant for water soluble aerosols.

Question 25

How do larger species (size much greater than the wavelength of light) scatter light?

Answer 25

Proportional to their cross sectional area.

Question 26

What is the emissivity of a material?

Answer 26

The ratio of energy radiated by a particular material compared to the energy radiated by a black body at the same temperature.

Question 27

What is the thermal balance of the Earth?

Answer 27

Radiation absorbed by the planet results in an increase in surface temperature. As the surface temperature increases, thermal emission increases which results in a steady state temperature.

Question 28

What can the power input to Earth be expressed as?

Answer 28

P_in = πr²(1 - A)F_s

This assumes that the area exposed to the incident radius is a disc.

Question 29

What can the power output of Earth be expressed as?

Answer 29

P_out = 4πr²σT⁴.

This includes the Stefan-Boltzmann law and assumes a spherical emitter.

Question 30

What causes a molecule to absorb infrared radiation?

Answer 30

There must be a change in the dipole moment with stretching/bending.

Question 31

What is the natural greenhouse effect?

Answer 31

The radiative forcing from naturally occurring gases (and in naturally occurring quantities).

Question 32

What is radiative forcing?

Answer 32

The difference in incoming energy and outgoing energy.

Question 33

What is a positive feedback system for the Earth’s greenhouse effect?

Answer 33

Mid-IR emission from the surface of the planet → molecules in the atmosphere absorb the some of the mid-IR emission → absorption produces vibrationally excited molecules → collisions convert vibrational energy to translational energy warming the atmosphere → warmed atmosphere heats the planetary surface → mid-IR spectrum moves to shorter wavelengths with increased intensity.

Question 34

What is the Goldilocks Hypothesis?

Answer 34

The term for a planet (Earth) that has conditions that are ‘just right’ for life.

Question 35

Why is Venus not suitable for life?

Answer 35

Thick atmosphere (9 MPa) that consists of mainly CO₂.

Strong greenhouse effect which causes a high temperature.

Water is in vapour form which means it cannot be locked away in rocks through carbonate ions which causes a runaway greenhouse effect.

Question 36

Why is Mars not suitable for life?

Answer 36

Thin atmosphere (670 Pa) although p_CO2 is higher than Earth’s.

Very weak greenhouse effect resulting in a low temperature.

Water is in a frozen form.

Question 37

Why is Earth suitable for life?

Answer 37

Atmosphere with enough p_CO2 to provide a good greenhouse effect.

Moderate temperature which allows liquid water to be found on the surface.

Liquid water can lock CO₂ into rocks which provides a stabilising effect for life to evolve.

Question 38

How can historical records of temperature be accessed?

Answer 38

Using the ¹⁸O / ¹⁶O isotope ratio. As H₂¹⁶O evaporates easier than H₂¹⁸O and the isotope ratio is determined by temperature. Snow falling in the polar regions is enriched in H₂¹⁶O. A layer by layer record of average ocean temperatures can be found by measuring the ¹⁸O / ¹⁶O ratio in deep ice cores at the poles.

Question 39

How else can ice cores give atmospheric data?

Answer 39

Small gas bubbles can become trapped in ice.

Question 40

What is Rayleigh fractionation?

Answer 40

The H₂¹⁶O isotope evaporates more easily than the heavier isotopologue (kinetic isotope effect). The H₂¹⁸O isotope condenses more easily than the light isotopologue (Rayleigh fractionation).

Therefore the δ¹⁸O value depends on ocean surface temperature as well as on the amount of precipitation before reaching the poles.

Question 41

What are factors that could affect the accuracy of the oxygen isotope ratio?

Answer 41

Temperature variation in the atmosphere as the water travels towards the poles.

Compression can change with depth.

Movement of ice can give misleading readings.

Melting of ice can destroy layering.

Question 42

What information can ice cores provide?

Answer 42

Key events such as nuclear explosions (radioactivity) and volcanic eruptions (acidic layer) can be checked.

The ¹⁸O depletion can be checked against other records such as carbonate sediments under oceans.

The amount of ¹⁸O in oxygen trapped in ice bubbles.

Carbon dioxide in bubbles can be radiocarbon dated.

Question 43

How does a 70 ppm increase in CO₂ levels cause a 7 K increase in average global temperatures?

Answer 43

A positive feedback mechanism is created.

A slight increase in global CO₂ levels enhances radiative forcing, slightly raising the temperatures → warmer oceans do not absorb as much CO₂ and result in more H₂O vapour. This further raises temperatures → this process continues until a new equilibrium is established.

Question 44

What were historical sources of carbon dioxide?

Answer 44

The main sources were respiration and the oxidation of decomposing organic matter.

Question 45

Why hasn’t the temperature heated up as much as in the past with current carbon dioxide emissions?

Answer 45

Industrialisation has also caused large amounts of water soluble sulphate particles to be released. These are reflective and increase the albedo of the planet resulting in a cooling effect.

Question 46

What is global warming?

Answer 46

The increase in Earth’s temperature over and above that caused by the natural greenhouse effect.

Question 47

What is the atmospheric window?

Answer 47

The spectral range between 8 to 13 μm where there is very little absorption by gases in the atmosphere.

Question 48

How does the number of atoms in a molecule affect IR absorption?

Answer 48

There are a higher number of vibrational modes which means it is more likely to absorb in the atmospheric window.

Question 49

What is global warming potential?

Answer 49

GWP is the ability of molecules to enhance radiative forcing and global warming. It is calibrated with respect to CO₂ over a certain time period (usually 100 years).

Question 50

What is global warming potential dependent on?

Answer 50

Efficient IR absorption in the atmospheric window and the lifetime of the molecules.

Question 51

What are policy initiatives aimed to reduce global warming?

Answer 51

Net zero policies.

Switch to low carbon energy sources and increasing energy efficiency.

Afforestation and rewilding initiatives to sequester carbon from the atmosphere to the biosphere.

Ban on refrigerant gases based on halocarbons.

Question 52

What does k_r represent?

Answer 52

A chemical reaction rate.

Question 53

What does J represent?

Answer 53

A photolysis rate.

Question 54

What does k_d represent?

Answer 54

A deposition rate.