Atmospheric Chemistry

Question 1

Describe the changes in the earths atmosphere with approximate dates.

Answer 1

1. 3 billion years ago, O₂ appears as primitive life grew in the ocean, rest of atmosphere was toxic methane.
2. Delayed O₂ build up as rock surfaces were oxidised, afterwards the O₂ build up can continue.
3. First land plants appeared around 450 million years ago after O₃ layer formed protecting from the suns UV rays.

Question 2

Why is the ozone layer important in the development of cells?

Answer 2

The UV rays between 200 and 300 nm cause cell mutations.

Question 3

How has global temperature changed over the last 400 thousand years? How can we track this?

Answer 3

There has been 4 long ice ages (80% of time) with 4 short interglacial periods with a difference in temperature of around 8 ºC changing every 90 thousand years or so with smaller 20 thousand year fluctuatuons.

Data is estimated from ice cores and fossil plankton.

Question 4

What is a Milankovitch cycle?

Answer 4

3 effects all associated with the earths orbit.

1. The elliptical cycle - the earths orbit isn’t centered around the sun, the distances at the long and short axis depend on the gravitational pull of the other planets. Cycle is between 100k and 400k years.
2. The axial tilt - the tilt depends on the plane of the orbit of earth, tilt is between 22 and 24.5°. This changes every 41k years or so and changes the amount of sunlight reaching earth, amplifiying seasons.
3. The wobble - every 26k years the earth precesses in the direction of the earths axis (getting closer and further from the sun) influenced by tidal forces.

Question 5

How do Milankovitch cycles affect climate?

Answer 5

They influence warming and cooling by changing the amount of sunlight reaching earth. Warming occurs when the orbit is elliptical and the northen hemisphere (mostly land) tilts more towards the sun.

Cooling occurs in more circular orbits with less tilt and more wobble cause summers to be cooler.

Question 6

How do climate predictions from Milankovitch cycles compare to the temperature record? What else must be considered?

Answer 6

The observations show climate behavoir is much more intense than calculated from the cycles. Notably, the elliptical cycle of the orbit is predicted to have the smallest actual effect yet the glacial frequency matches its time period.

A probable sequence of events is as a result of the orbit changes and warming, CO₂ released from the ocean provides more warming by trapping the sunlight. CO₂ is an amplifier of climate.

Question 7

Given the mass of carbon burnt, how can you work out the fractional increase in atmospheric CO₂ due to human activity?

Answer 7

Work out the number of CO₂ molecules released from the mass. Work out the average mass of an air molecule and hence work out the number of atmospheric air molecules from the mass of the atmospheere (given). Find the ratio of new CO₂/air molecules.

Question 8

Explain how much the ocean absorbs CO₂ and why the carbon cycle is no longer in balance.

Answer 8

The ocean absorbed approximately 26% of the CO₂ released from 2002-2011 but cannot absorb any more due to the slow mixing of the dissolved CO₂. The atmospheric CO₂ will take 150 years to be taken up by the ocean.

Question 9

Describe the two cycles causing the uptake of CO₂ in the ocean.

Answer 9

The solubility cycle relates to the CO₂ dissolving and forming ions that will only slowly reform CO₂. The overall process is:

CO₂ + CO₃^2- + H₂O ⇔ 2HCO₃^-

Where the carbonate ions come from limestone and are required for the process.

The biological cycle is where dissolved CO₂ are converted to dissolved organic material by phytoplankton which sinks into the deep ocean.

Question 10

How does the oceans ability to dissolve CO₂ change with temperature?

Answer 10

1. It decreases as the temperature increases, with a 20% efficiency when the temperature increases from 0 to 40 °C.
2. When surface temperature increases, the wind mixes the water less, limiting infusion of fresh carbonate required for the solubility.
3. The stagnent water supports fewer phytoplankton which slows the biological aspect.

Question 11

How much sunlight does the planet surface absorb? How is this determined?

Answer 11

The surfaces Albedo determines how much of the incoming radiation is reflected and how much is absorbed. Snow has a high albedo for example. The global average is 0.3 so 70% of the radiation is absorbed.

Question 12

Which light wavelengths come from the planet and which come from the sun? What is the effect of changing CO₂ concentrations and why?

Answer 12

The sun emits light between 0.2 and 2 μm. The planet is between 10 and 15 μm in emission. The atmosphere is ‘optically thick’ where CO₂ absorbs light so the radiative effect of changing its concentration is logarithmic.

Question 13

What is radiative forcing?

Answer 13

The balance between radiation absorbed by the planet and the radiation reflected over a certain time period. To maintain the temperature they must be equal. It is measured in W m^-2. The greatest contributers are CO₂ and other greenhouse gases.

Question 14

Why does the time period that radiative forcing is measured over matter? What other factors are important?

Answer 14

The impact of CO₂ can be greater than other gases because of how long it stays in the atmosphere. The radiative forcing also depends on the strength and location of the radiation absorption.

Question 15

What is climate sensitivity?

Answer 15

The mean change in global temperature divided by the specific forcing. The equlibrium climate sensitivity (ECS) is the equlibrium change in global temperature in response to the doubled CO₂ output since pre-industry. The models suggest that global temperature will increase from 1.5-4.5 °C

Question 16

What are global warming potentials? What is the weaknesses of this model?

Answer 16

GWP: The potential of 1 kg of a compound A to contribute to radiative forcing relative to that of 1 kg of a reference compount R.

GWP = (a_Aτ_A(1 - e^-t_f/τ_A) / (a_CO2τ_CO2(1 - e^-t_f/τ_CO2​) Not expected to learn

Where a is the radiative forcing from 1 kg of a compound, τ is the atmospheric lifetime and t_f is the time horizon. The reference compound is usually CO₂.

The weaknesses are that CO₂ does not have a single lifetime, and only applies to gases which are horizontally and vertically well mixed (not methane for example).

Question 17

Define mixing ratios and how you work them out.

Answer 17

Mixing ratios are the ratio of molecules of gaseous species to the number of molecules of dry air. Mixing ratios are given in parts per million/billion/trillion by volume (ppmv, ppbv, pptv).

The actual number of molecules are worked out from the ideal gas law where n/V is worked out in molecules per cm³ and multiplied by the parts value with the appropriate power of 10. The pressure determines the height at which the gas is worked out.

The ratio units are very important as the density does not fully represent the conditions.

Question 18

Describe how the vertical mixing changes for different molecules.

Answer 18

Up to 100 km up, thermal mixing means that all gases are well mixed but above the gravitational settling time becomes the same order as the mixing time scales. Therefore the lighter gases mix more and the heavier gases mix less. The vertical distribution of non-noble gases also depends on photochemistry and pressure dependent reactions.

Question 19

Give the altitudes and masses of the earths atmosphere regions.

Answer 19

Troposphere: up to 10 km, 80% mass

Stratosphere: 10 to 47 km, 19.9% mass

Mesosphere: 47 to 80 km, 0.1 % mass

Thermosphere: above 80 km

Question 20

Describe how the temperature changes over each of the atmpsphere regions and why this occurs.

Answer 20

The temperature decreases in the troposphere by 9.8 K km^-1, this is because most of the radiation is absorbed by the land and atmosphere with a small amount being reflected. The warm air rises and cools as it rises.

The temperature in the stratosphere increases by around 2 K km^-1, this is because light of wavelengths 310-200 nm is absorbed by ozone to form O₂ and O*. The excess energy is released as heat which increases the temperature. Energy is also released by recombination of O₂ and O to from ozone.

Question 21

What factors affect photon flux at the surface?

Answer 21

1. Absorption of shortwave radiation by ozone and oxygen
2. The scattering and absorption of radiation by gases and particles affected by pathlength through the atmosphere. The air mass, m, measures how much atmosphere the light has to travel through. For angles < 60°, the air mass can be calculated by the pathlength of the light/the pathlength if the path was vertical. As the angle increases, the air mass increases.
3. Diffuse solar radiation, scattered either from the sun or the earths surface
4. Light reflected from the earths surface which depends on the surface albedo

As the wavelength of light decreases, more light is available to the gas from scattered and diffused light than the direct solar radiation.

Question 22

How can air mass be used in a Beer-Lambert type law?

Answer 22

In the form I/I₀ = e^-tm

Where I₀ is the light intensity of a given wavelength at the top of the atmosphere and t is the total attenuation coefficient.

Question 23

What determines the amount of scattering of light that occurs?

Answer 23

1. The pathlength the light takes.
2. The wavelength of the incoming light.
3. The size of the particles or gas molecules doing the scattering.

Question 24

What determines the rate of photon initiated reactions?

Answer 24

The rate equation can be described with the modified rate constant of -j_a, known as the photolysis rate constant. This depends on the wavelength an intensity of light and takes into account the intrinsic strength of light absorption by the molecule at the correct wavelength.

Question 25

What are the main pollutants of concern today?

Answer 25

Particulate matter (PM), oxides of nitrogen (NO_x) and ground level ozone.

Question 26

Define primary and secondary pollutants and give the potential sources.

Answer 26

Primary pollutants are emitted directly into the air, such as hydrocarbons, SO₂, NO, CO and PM.

Secondary pollutants are the result of the chemical transformations of primary pollutants, such as O₃ and NO₂.

Sources include anthropogenic - from human activity, biogenic - biological sources and geogenic -from geological processes.

Question 27

Describe how easy it is to reduce emissions of different primary pollutants?

Answer 27

Pb was very easy to reduce by regulations of petrol as it was the main source. SO₂ was also easy as the main sources were from coal which could be replaced with gas and flue gas desulfurisation treatments.

NO_x and PM are harder to remove as changes in comustion engines aimed at reducing one may cause an increase in the other.

Question 28

Give the sources and trends of the emissions of PM, VOCs and NO_x.

Answer 28

PM is usually emitted by diesel cars that are increasing in number and biomass whose contribution is increasing.

VOCs are NMHC (non-methane hydrocarbons) and oxygenated hydrocarbons such as aldehydes and ketones. Sources are fossil fuels and solvent use, biomass burning and natural sources. Biogenic sources are the largest contributer so there is little global trend.

NO_x are produced by the combination of O₂ and N₂ at temperatures over 2500 °C. Main sources are vehicles, power plants and notably ships as well as biomass, soil, plants and lightning. Catalytic converters have reduced the emissions but ship emissions are still growing.

Question 29

Describe in detail and give examples of wet deposition.

Answer 29

Wet deposition is where soluble pollutants are dissolved into clouds, fog, rain or snow and deposited with the water droplets when they fall. This is important for SO₂ and HNO₃. The rate of transfer of a soluble gas (g) or a particle/aerosol (p) into rain can be approximated by

W = S_ixC_{i, gas}

Where W is the rate of transport of species i form medium to medium, S_ix is the scavenging coefficient for species i where x represents either the particulate or gas phase and C_i is the concentration of species i in the gas phase.

Note that the equation assumes the reaction is irreversible and independant of the quantity of material previously scavenged.

Question 30

Write a rate equation for wet deposition species concentration and manipulate it into a useful form.

Answer 30

A rate equation for the change in concentration of species i can be written.

dCi/dt = -W + R + E

Where R is reactions gas or aqueous phase and E is emissions to produce the species. W is the rate of wet depostion and is equal to S_ixC_i. If R and E = 0, then the equation can be written as:

dC_i/dt = -S_ixC_i

This can be integrated by the seperation of variables to give the equation:

C = C₀ exp(-S_igt)

The concentration of species i deposited can be calculated by working out C - C₀ and multiplying that by the vertical length of cloud.

Question 31

Describe in detail the process of dry deposition.

Answer 31

Pollutants are transported to the ground level and absorbed/dissolved by material there without being dissolved first. Mechanisms for uptake include stomatal uptake by trees and grasses and by ocean uptake of soluble gases.

Dry deposition is characterised by a depostion velocity, V_g (cm s^-1).

F = -V_g x C

Where F is the net flux of a species (molecule cm^-2 s^-1) to the surface and C is the concentration.

V_g is a positive number and fluxes towards the surface are taken as negative. Dry deposition is usually envisioned as a resistance model, where the overall deposition velocity is the inverse of the total resistance, V_g = (R_T)^-1 where R_T is the total resistance composed of the surface (R_surf), boundary layer (R_b) and aerodynamic resistance (R_a).

Question 32

Outline the three reactions of NO and NO₂ to form and break down ozone.

Answer 32

Formation

NO₂ + hv → NO + O

O + O₂ + M → O₃ + M

Destruction:

NO + O₃ → NO₂ + O₂

Question 33

Give the series of reactions that show how methane, CO and VOCs react to lead to the formation of ozone.

Answer 33

Methane:

CH₄ + OH → CH₃ + H₂O

CH₃ + O₂ + M → CH₃O₂ + M

CO:

CO + OH → CO₂ + H

H + O₂ + M → HO₂ + M

VOCs:

RO₂ + NO → RO + NO₂

CH₃O + O₂ → HO₂ + HCHO

HO₂ + NO → OH + NO₂

NO₂ causes the formation of ozone. The peroxy radicals (RO₂) can oxidise molecules of CO and CH₄ causing the catalytic production of O₃. HCHO can also go on to form two more molecules of HO₂.

Question 34

What is the rate limiting step in the production of ozone?

Answer 34

The limiting step of ozone production is the reaction of peroxy radicals with NO to form NO₂. The NO₂ will react on faster than its production. This leads to the following rate equation:

rate ≈ k[RO₂][NO]

The reaction rate is approximatly 10⁷ molecules cm^-3 s^-1 in daylight.

Question 35

Describe how ozone production depends on VOCs and NO_x concentrations.

Answer 35

Ozone will react to form OH when exposed to light and water.

In low [NO_x], VOCs will form RO₂ which will form ROOH, CO will form HO₂ which will form HOOH. Net ozone destruction occurs.

In high [NO_x], RO₂ and HO₂ will form NO₂ which leads to the formation of ozone. OH will be lost quickly by reaction with NO₂ to form HNO₃ so HO₂/RO₂ + NO becomes a major source of OH.

Peroxides are removed by deposition meaning they are terminating steps. As VOCs are in a large amount of the processes it shows that ozone production.

Question 36

How is OH formed in the troposphere and how can its presence be used?

Answer 36

Ground level ozone is oxidised to form an excited oxygen radical and O₂, approximately 10% of the excited oxygen radicals will react with water to form 2 OH radicals, the rest is deactivated with air. In very polluted air OH can be produced from peroxy radicals and NO.

Abundance of OH can determine where gases are mostly destroyed. The reaction of methane with OH will form CH₃ and water.

Question 37

How do you work out the lifetime of an atmospheric species?

Answer 37

In the reactions where it reacts; if 1st order → add the rate constant, if 2nd order → add the rate constant multiplied by the concentration of the other reacting species taken to be a constant. Add all the rates together to find the effective rate, k’.

The lifetime is 1/k’

Question 38

What methods can be used to study rates of atmospheric reactions and what must be considered?

Answer 38

Steady state analysis can be used however, remember that for some reactions the photolysis rate constant, j, must be used.