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Core 6 > Fundamentals of Atmospheric Chemistry > Flashcards

Fundamentals of Atmospheric Chemistry Flashcards

1
Q

Do we have a reducing or oxidising atmosphere?

A

Billions of years ago the atmosphere was weakly reducing today is strongly oxidising

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2
Q

How do we reconstruct the past atmosphere?

A

Global temperature and some composition data available from 1850 to the present

Tree rings and climate proxies used to reconstruct temperature from 1000-2000 yrs ago

Ice cores allow us to look at atmospheric composition and temperature over the last 800,000 yrs

Geological evidence - millions of years

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3
Q

Why did it take so long for life to migrate to land from the oceans

A

Oldest evidence of life - 4 billion years ago

Build up of oxygen required - produces ozpone used for UV protection

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4
Q

What are Milsnkovitch cycles?

A

Natural cycles that influence earth’s climate:

The obliquity cycle

The eccentricity cycle

The precession cycle

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5
Q

What is the obliquity cycle?

A

The axial tilt - vaies the plane of the earth’s orbit

Typically 23.5º but can vary from 22 - 24.5º over a period of 41,000 years

When obliquity increases summers in both hemispheres receive greater radiative flux from the sun and winters less

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6
Q

What is the eccentricity cycle?

A

The earth’s orbit is eliptical (a ≠ b)

a and b vary according to gravitational pull from other planets

Longest cycle about every 100,000 and 400,000 years

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7
Q

What is the precession cycle?

A

Wobble

trend in the direction of the Earth’s axis of rotation relative to the fixed stars and occurs every 26,000 years

This gyroscopic wobble of the Earth’s axis is driven by tidal forces which are influeneced by our sun and moon

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8
Q

How does warming and cooling of the earth occur?

A

When the orbit of the earth is eliptical and the northern hemisphere is tilted towards the sun (increased obliquity) - chance to melt out of ice age

We attain a more circular orbit + combined effect of tilt (decreasing obliquity) and wobble cause cooler summers - going back to ice age

Currently in decreasing obliquity phase

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9
Q

Do milankovitch cycles fully explain temperature record?

A

Observations show climate behaviour is much more intense than the calculated variations in the intensity of solar radiation

Gacial frequency matches eccentricity variations but these have much smaller effect on solar forcing than obliquity andhence might be expected to produce weakest effects

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10
Q

What other factors could affect the climate?

A

Sequence of events:

Changes in obliquity and eccentricity cause Earth to warm

Warming oceans causes CO2 to rise about 200-500 years later

CO2 furtehr warms the rest of the planet leasding to further CO2 release

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11
Q

How much CO2 have humans released and what are the main sources of CO2 from humans since 1750?

A

2,000 gigatonnes - 2000 billion tonnes

Coal - 34%

Oil - 25%

Gas - 10%

Cement - 2%

Land use change - 29%

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12
Q

How do we calculate ratio of CO2 to molecules in the atmosphere?

A

2,000 gigatonnes = 2 x 1015 kg CO2

NCO2 = MCO2 / MCO2 molecule

= 2 x 1015 kg / 1.66 x 10-27 x 44.01 kg mol-1

2.74 x 1040 molecules of CO2

Natmosphere = Matm / Mair

= 5.2 x 1018 kg / 4.82 x 10-26 = 1.11 x 1044 molecules

Fractional increase in CO2 = NCO2 / Natm =

2.74 x 1040 / 1.11 x 1044 = 247 ppm

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13
Q

How do we calculate Mair?

A

Mair = (fN2 x MN2 + fO2 x MO2) x u

= ((0.79 x 28) + (0.21 x 32)) x 1.67 x 1027

= 4.82 x 10-26

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14
Q

What determines how much CO2 the oceans can obsorb?

A

uptake of anthropogenic carbon by the ocean is determined by ocean circulation and carbonate chemistry

CO2 lifetime in ocean = 150yrs

Although oceans as a whole have a large capacity for absorbing CO2, ocean mixing is too slow to have spread the rapidly increasing atmospheric CO2 into the deep ocean

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15
Q

What is the overall process of CO2 inthe ocean?

A

Carbonic acid releases h

= ions which combine with carbinate in seawater to form bicarbonate

CO2 + CO32- + H2O <—> 2HCO3-

As temperature increases the ability of water to absorb CO2 decreases

As surface waters warm the harder it is for winds to mix the surface layers with deeper layers - limits infusion of fresh carbonate rich waters from below

The stagnant water supports fewer phytoplankton so CO2 uptake from photosynthesis slows

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16
Q

What determines the radiation balance?

A

To keep temperature constant energy balance demands radiation coming in is equal to outgoing radiation

Radiation reflected by surface and the atmosphere is deterined by the overall reflectivity of a planet called the ALBEDO

The fraction absorbed is 1-A

The radiation effect of changing the CO2 is a logarithmic function of [CO2]

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17
Q

What are the advantages of showing the effects of emissions?

A

It is emissions that can be directly controlled

It allows many of the indirect effects to be seen - the forcing by emission for CH4 is twice as large than its forcing by concentration because of the indircet effect on ozone and H2O

It shows that air quality (CO, VOCs and NOx) affects climate via indirect effects on ozone

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18
Q

What does species-specific radiative forcing index depend on?

A

Strength and spectral location of absorption of IR radiation

Atmospheric lifetime

Time period over which you’re going to calculate radiative forcing

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19
Q

What is climate sensitivity?

A

Mean change in global temperature that occurs in response to specific forcing.

DT/DF

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20
Q

What is equilibrium climate sensitivity?

A

Equilibrium change in global temperature that occurs in response to doubled CO2 since pre-industrial era

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21
Q

What are global warming potentials?

A

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

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22
Q

What does the big picture of the atmosphere tell us?

A

The atmosphere is strongly oxidising

Most atmospheric chemistry is done with trace species

The most abundant species are nitrogen, oxygen, argon and carbon dioxide in decreasing order

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23
Q

What are mixing ratios and there units?

A

the ratio of molecules (or volumes) of gaseous species to the number of molecules (or volumes) of dry air.

pats per million by volume - ppmv - parts in 106

parts per billion by volume - ppbv - parts in 109

parts per trilion - pptv - parts per 1012

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24
Q

How do you convert ppmv, ppbv and pptv to molecules cm-3?

A

Using the ideal gas law

The number of molecules in 1 L of air at standard atmospheric pressure amd 298K is 2.46 x 10-19 molecules cm-3

So 15 ppbv = (15 x 10-9) x (2.46 x 10-19)

= 3.69 x 1011 molecules cm-3 at STP

For atmospheric particulate matter usually μg (10-6 g) m-3 are used

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25
Q

What implacts the mixing ratio?

A

Up to 100km thermal mixing of gases means that they are well mixed - not separated according to molecular weight

Above 100km the gravitational settling time becomes the same order as mixing time scales - lighter gases enriched

The verticle distribution of non noble gases is also strongly controlled by photochemistry and pressure dependent reaction

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26
Q

How does the temperture of the atmosphere change?

A

The sun does not really warm the atmosphere as it is not very absorbing

Mainly warms the Earth’s surface - the troposphere is cold

Areas of the atmosphere are defined by where temp stops changing

27
Q

What are the characteristics of the troposphere?

A

accounts for 80% of the mass of the atmosphere

Essentially all the water vapour, clouds and precipitation are found here

These provide important mechanisms fro scavenging pollutants from the atmosphere

Temperature decreases with height at abount 9 K km-1

28
Q

What are the characteristics of the stratosphere?

A

Contains about 20% of the mass of the atmosphere

Region of pronounced stability

Temperature increases with height ata rate of 2K km-1

Air in the strophere is stratified - slow mixing as warmer air is above colder air - stagnant

29
Q

Why does temperature decrease with height in the troposhphere?

A

Most heat aborbed by the earth’s surface

Warmer air at the surface rises and cools

30
Q

Why does temperature increase with height in the stratopsphere?

A

Absorption of solar radiation by ozone reaches a maximum in this area

O3 + hv —> O2 + O(1D) λmax = 200-310 nm

The excess energy available after absorption up to threshold value equiv to 310 nm is released as heat

Heat is also released by:

O2 + hv —-> O + O, λ < 240 nm

O + O2 + M —> O3 + M

31
Q

What controls photon flux (no. photon per second per unit of area) at the surface?

A

Absorption of shortwave radiation by ozone and oxygen

Scattering and absorption of radiation by gases amd particles affected by path length through atmosphere

32
Q

What does the air mass tell us in terms of scattering and how do we calculate it?

A

air mas m, is the path length transversed by solar radiation to reach the earth’s surface

m = Length of path of direct solar radiation / Length of verticle path through the atmosphere

For zenith angles < 60° m ≈ L/h

For the sun directly overhead θ = 0 and the air mass is 1

As θ increases air mass increases

33
Q

How can we estimate the reduction in solar intensity due to scattering and absorption through an air mass m?

A

Using the Beer Lambert law

I/Io = e-tm

Io - the light intensity at a given wavelength incident at the top of the atmosphere

I - the light intensity transmitted to the earth’s surface

t - the total attenuation coefficient

m - the air mass

34
Q

What makes up the total light intensity reaching a given volume of gas?

A

direct solar radiation (I/Io = e-tm)

Diffuse solar radiation - light from the sun or reflected from the earth’s surface that is scattered by gases and particles (does not change intensity or λ but reduces the amount of incoming radiation reaching the earth’s surface

Light reflected from surface - depends on type of surface - snow highly reflective surface albedo of 0.9 - forect albedo = 0.1

As λ decreases more of the light available to given volume of gas comes from scattering or diffuse radiation

35
Q

What affects the amount of scattering?

A

λ of the incoming radiation

the size of scattering particle or gas molecule

Prescence of a large number of particles with size of about 0.5 microns results in shorter wavelengths being preferentially scattered - means sky appears blue and sunset is red

36
Q

How can solar radiation initiate reactions?

A

Through photolysis - photo dissociation of a molecule upon light absorption

A + hv —> B + C,

d[A]/dt = jA[A] - first order process with rate constant jA

jA - photolysis raste constant

37
Q

What affects jA?

A

wavelegth

intensity of light

Takes into account intrisic strength of light absorption by molecule A in the wavelegth of interest

38
Q

What is the absorption cross section?

A

Units - cm2 / molecule

Effective area of the molecule that photon needs to traverse in order to be absorbed

The larger the absortion cross the easier it is to photoexcite the molecule

39
Q

What is the quantum yield for the decomposition of a reactant molecule?

A

unitless

Φ = #molecules decomposed / #photons absorbed

Since not all photons are absorbed productively the typical quantum yield will be less than 1

> 1 are possible for photo-induced or radiation-induced chain reactions in which a single photon triggers a long chain of transformations

40
Q

What are the main pollutants of concern to health?

A

particulate matter, NOx and ozone

PM - particles from vehicles exhaust, chimneys or formed in reactions in the air - no safe exposure level

Ozone - safe limit - 75 ppb averaged over 8 hours - regularly exeeded in urban areas

Air pollution can also afect plants and animals in nature impacting biodiversity and reducing crop yields

41
Q

Define sources of pollutants

Primary pollutants

Secondary pollutants

A

Sources of pollutants: anthropogenic, biogenic, geogenic or combination

Primary: those emitted directly into the air eg hydrocarbons, SO2, NO, CO, Pb, organics and combustion generated PM

Secondary: the result of chemical transformation of primary pollutants e.g. ozone, NO2, secondary organic aerosol

42
Q

How have Pb and SO2 emissions been reduced?

A

Lead was added to gasolene as an antiknock agent - govenment regukations removed it and levels have come down

SO2 was released in coal combustion - reduced by switching fuel use from coal and fitting flue gas desulfurisation in the coal plants

43
Q

What are VOcs and what are the sources?

A

VOC comprises of non methane hydrocarbons and oxygenated hydrocarbons

Sources:

anthropogenic - mostly fossil fuels

biomass burning

natural sources - tree and grass emissions release substantial quantities of reactive ydrocarbons (isoprene)

44
Q

Where do oxides of nitrogen come from?

A

NOx = sum of No and NO2

NOx produced when N2 and O2 in air react in high temperature combustion

45
Q

What are the sources of NOx?

A

Anthropogenic - vehicles, power plants, ships, industry

Biomass burning

Natural sources - soils, plants

Use of catalytic converters leads to a redution in NOx, CO and hydrocarbons

Ship emissions are still growing

Reducing NOx is a challenge for vehicle manufacturers because changes to combustion conditions that reduce NOx can increase PM

46
Q

Why are there still concerns over PMs?

A

PMs are orimary and secondary pollutants

diesel cars areincreasing

emissions fro domestic biomass combustion have increasing contributions

meteorological influences on concentration - climate change

other chemical influeneces (NH3)

47
Q

How have tropospheric ozone levels changed in the 20th century?

A

Increased

48
Q

Define wet and dry deposition?

A

Wet: pollutants sre dissolved in clouds, fog, rain or snow and deposited when these waer droplets impact the earth’s surface

Dry: pollutants transported to ground level and adsorbed/absorbed by materials there without first being dissolved

49
Q

How do we estimate rate of transfer of gases and aerosols into clouds

A

Wgas to rain = SigCi, gas

Waerosol to rain = SipCi, aerosol

W - rate of transport of species i fro medium to medium

Sig and Sip are the scavenging coefficients fro species i in the gas and particulate phase

Ci, gas is the conc of species i in the gas phase

These reactions are only applicable if scavenging is irreversible and if it is independent of the quality of material scavenged previously

50
Q

Calculation of Ci, gas

A
51
Q

What are examples of dry depositon and how is it characterised?

A

Stomatal uptake by trees and grasses and ocean uptake of soluble gases

characterised by deposition velocity

52
Q

What is the life cycle of a tropospheric gas?

A

The major way for pollutants to be removed from the atmosphere is to be oxidised to a more soluble form that can be deposited to the surface

53
Q

How can tropospheric ozone be formed and waht is the issue?

A

Formed in the troposphere by via NO2 photolysis but may react with NO

NO2 + hv –> NO + O

O + O2 + M —> O3 + M

NO + O3 –> NO2 + O2

Net = null cycle

54
Q

What are the reactions of methane in the atmosphere?

A
55
Q

Why is the reaction between OH and CH4 important?

A

Promotes th formation of NO2 which promotes the formation of additional tropospheric ozone

Peroxy radicals convert NO to NO2 whilst preserving acitve radicals that can further oxide CO and CH4 or convet more NO to NO2

This is the catalytic production of tropospheric ozone

56
Q

What is the rate limitting step of ozone production?

A

O + O2 + M —> O3 + M - 1 x 109 molecule cm-3 s-1

NO2 + hv —> O + NO - 1 x 108 molecule cm-3 s-1

RO2 + NO —> CH3O + NO2 - 1 x 107 molecule cm-3 s-1

rate ozone production = k[RO2][NO]

[RO2] = [H2O] + [CH3O] + [C2H5O]….

k ≈ 8 x 10-12 cm3 molecule-1 s-1

57
Q
A
58
Q

How many molecules of O3 can be formedfrom one CH4?

A

4

59
Q

Which reactions cause ozone levels to change?

A

VOC oxidation mainly formaing peroxides - net ozone destruction

ozone formed when nitrogen oxides are present to react with peroxy radicals

HO2 / RO2 + NO becomes important source of OH

Major loss of OH becomes OH + NO2 + M –> HNO3 + M

only important when NO2 is really high

60
Q

How do NOx levels affect ozone production?

A

At low NOx the dominant reaction of peroxy radicals are self reaction to form peroxides which arecremoved by decomposition - radical terminating steps - result in low ozone / ozone destruction

As NOx rises HO2 + NO —> OH + NO2 results in high ozone formation

If too much NOx present it starts to act as a radical sink

61
Q

Hoe is the hydroxyl radical formed?

A
62
Q

What is the atmospheric lifetime?

A

Represents the decay time of a perturbation to a particular gas

Time taken to reach 1/e of concentraton

63
Q

How is lifetime calculated?

A

For a first order reaction lifetime = 1/k

units = s

For second order reaction - treat as pseudo first order

[A] + [B] treat as if B is constant

Lifetime A = 1/k[B]

64
Q

How do we calculate atmospheric gas concentrations?

A

Use steady state analysis - assumes reactions are happening suffieciently quickly that intermediates = 0 in given time

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