intro, structure & composition of the atmosphere

L1

Why study AQ?

• AQ is

  • the air you breathe

  • the earth’s radiation balance, climate

  • visibility

  • conservation of historic buildings & statues

  • human health, cost of medication and quality of life

  • ecosystem & crop health

  • wildlife, aquatic ecosystems

How does temperature change with changes in atmospheric structure?

• troposphere: T decreases with height

  • surface heating & turbulent mixing

  • tropopause: turbulent mixing stops

• stratosphere: T increases with height

  • stability

  • stratopause: ozone layer, absorption of UV light

• mesosphere: T decreases with height

  • lack of ozone. Without ozone to absorb solar radiation, there is very little heat input

  • mesopause: highest clouds, coldest

• thermosphere: T increases with height

  • photodissociation and ionisation

Why does atmospheric pressure increase exponentially towards the surface?

• ρ = F/A =mg/A

• gravity & compressible fluid

  • gravity pulls air downward, and air is a compressible fluid

• weight accumulation

  • the deeper you go in the atmosphere, the more air molecules exist above you

  • their weight compresses the air below, making air denser near the surface

• feedback loop

  • higher density (ρ) at lower altitudes means pressure increases much faster /m of descent compared to higher up, creating an exponential curve:

    • ρ(z) = ρ₀ e^-z/H

What determines surface pressure?

• total weight of the air column

  • surface pressure (ρ₀) is equal to the total weight of the air column resting directly above a given unit area on the surface, extending all the way to the top of the atmosphere.

• Hydrostatic formula

  • δρ/δz = -ρg

What is the Hydrostatic Balance/equilibrium?

• The exact balance between the net upward force on a layer of air (due to decreasing pressure with height) and the downward gravitational force.

• δρ/δz = -ρg

  • the -ve sin shows that the pressure decreases as height(z) increases.

What was the atmosphere like 3.5 billion years ago?

• consisted mainly of

  • nitrogen

  • large amounts of gg (CO2 & CH4)

  • and small amounts of water vapour

  • almost no O2

• originated from the continuous volcanic eruptions

What was the atmosphere like 2.5 billion years ago?

• various processes led to a dramatic increase in the amount of oxygen (the great oxidation event)

• photosynthesis led to a gradual increase in the amount of oxygen

• so the harmful UV radiation in the atmosphere was absorbed more and more

What is the composition of the current atmosphere?

• By volume not mass:

  • total 99.9%

    • N2: 78%

    • O2: 21%

    • Argon: 0.9%

  • CO2: 0.042%

  • CH4: 0.0002%

  • CO: 0.00001%

  • O3: 0.000004%

What are the roles of the gases occuring in very low amounts? Such as CO2, CH4, NOx etc.

• chemical reaction in the atmosphere

• air pollution and its effects on public health

• cleansing the atmosphere

• protecting against damage by UV radiation

• the greenhouse effect

What are trace gases?

gases which have very small mole fractions.

• examples: CH4, N2O etc.

What is photochemical smog? Include their health effects.

• near the surface of the Earth, O3 is the primary ingredient of photochemical smog.

• summer smog occurs in big cities where chemical reactions occur under the influence of summer sunlight

• winter smog consists of SO2 and H2SO4

• smog causes harmful health effects such as respiratory problems

• negative impacts on vegetation

What does the concentration of pollutants depend on?

• sources

• transport (advection, dispersion)

• chemistry

• sinks

Molar mass (M) formula including mole fraction (C_v,i) formula

M = ∑C_v,iM_i

C_v,i = (number of moles of gas i) / (total number of moles of all gases in the mixture)

Define residence time. Include the general notation.

• the time during which the concentration (the gas density in kgm^-3) of a gas in the atmosphere decreases

• as a result of the removal processes to a factor of 1/e (0.368) of the initial concentration,

• assuming that there is no formation of the substance

• dC/dt = -kC

  • C = concentration of gas density (kgm^-3)

  • k = reaction constant (s^-1)

What is the purpose of the constant k in residence time.

• includes all removal processes

• the equation is a rough approximation because the removal processes depend on many aspects

• 𝛕 = 1/k

Explain 2 methods the residence time of gases in the atmosphere can be estimated.

• 1st method:

  • select a part of the atmosphere in which we know that there are no sources of a given substance (for e.g. a column of air above the ocean)

  • by following the concentration of this column over time (using balloon measurements)

  • can get an idea of the removal rate of the substance.

  • 𝛕 = (t)/ ln(C(0)/C(t)

• 2nd method:

  • applied in a situation in which the concentration is (fairly) constant over time and the sources are known

  • the sum of all removal processes is in equilibrium with the sources

  • turnover time:

    • 𝛕 = VC_e/Q

      • V = the volume of air parcel (m³)

      • C_e = the equilibrium gas density (kgm^-3)

      • Q = the total flux of the substance (kgs^-1)

Describe and explain the relationship between variability and lifetime.

• the larger the variability, the smaller the lifetime 𝛕

How can you measure mole fractions of gases in the atmosphere experimentally?

• by measuring a time series of a specific gas

• can separate the average mole fraction (or conc. C)

• into an average and its variations

What is the box model and mass balance equation?

• dm/dt = E + F_in - F_out + P - L - D + F_v

  • E: emission

  • F_in: horizontal inflow

  • F_out: horizontal outflow

  • P: production

  • L: loss

  • D: deposition

  • F_v: vertical transport

What is the relative (R) and absolute (L) removal rate?

• R: the fractional rate at which a substance is removed per unit time by deposition, chemical loss, or nuclear decay.

• L: scales linearly with concentration/mass,

  • L = R x m

  • 𝛕 = 1/R

What is the lifetime in case of sinks?

• sinks may be:

  • chemical reaction, e.g. with OH: (k x [OH] x m)

  • photolysis: (J x m)

  • dry deposition: (v_d x m)

  • wet deposition: (v_w x m)

• almost all sinks are linear with mass m (or conc.)

• dm/dt = E - k x [OH] x m - J x m - v_d x m - v_w x m

• so lifetime:

  • 𝛕 = 1/ (k[OH] + J + v_d + v_w)

Advantage of using a Eulerian model?

• allow to describe spatial gradients

• differences between the hemisphere and temporal variation related to the general circulation

• understand the CO2 balance in more detail

• compare to observations at different locations on Earth

Which components need a Eulerian model?

shorter lived species, like CO, NH3, NOx