Fundamentals of Atmospheric Chemistry Flashcards
Do we have a reducing or oxidising atmosphere?
Billions of years ago the atmosphere was weakly reducing today is strongly oxidising
How do we reconstruct the past atmosphere?
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
Why did it take so long for life to migrate to land from the oceans
Oldest evidence of life - 4 billion years ago
Build up of oxygen required - produces ozpone used for UV protection
What are Milsnkovitch cycles?
Natural cycles that influence earth’s climate:
The obliquity cycle
The eccentricity cycle
The precession cycle
What is the obliquity cycle?
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
What is the eccentricity cycle?
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
What is the precession cycle?
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
How does warming and cooling of the earth occur?
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
Do milankovitch cycles fully explain temperature record?
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
What other factors could affect the climate?
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
How much CO2 have humans released and what are the main sources of CO2 from humans since 1750?
2,000 gigatonnes - 2000 billion tonnes
Coal - 34%
Oil - 25%
Gas - 10%
Cement - 2%
Land use change - 29%
How do we calculate ratio of CO2 to molecules in the atmosphere?
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
How do we calculate Mair?
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
What determines how much CO2 the oceans can obsorb?
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
What is the overall process of CO2 inthe ocean?
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
What determines the radiation balance?
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]
What are the advantages of showing the effects of emissions?
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
What does species-specific radiative forcing index depend on?
Strength and spectral location of absorption of IR radiation
Atmospheric lifetime
Time period over which you’re going to calculate radiative forcing
What is climate sensitivity?
Mean change in global temperature that occurs in response to specific forcing.
DT/DF
What is equilibrium climate sensitivity?
Equilibrium change in global temperature that occurs in response to doubled CO2 since pre-industrial era
What are global warming potentials?
The potential of 1kg of a compound A to contribute to radiative forcing relative to that of 1kg of a reference compound R
What does the big picture of the atmosphere tell us?
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
What are mixing ratios and there units?
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
How do you convert ppmv, ppbv and pptv to molecules cm-3?
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
What implacts the mixing ratio?
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
How does the temperture of the atmosphere change?
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
What are the characteristics of the troposphere?
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
What are the characteristics of the stratosphere?
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
Why does temperature decrease with height in the troposhphere?
Most heat aborbed by the earth’s surface
Warmer air at the surface rises and cools
Why does temperature increase with height in the stratopsphere?
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
What controls photon flux (no. photon per second per unit of area) at the surface?
Absorption of shortwave radiation by ozone and oxygen
Scattering and absorption of radiation by gases amd particles affected by path length through atmosphere
What does the air mass tell us in terms of scattering and how do we calculate it?
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
How can we estimate the reduction in solar intensity due to scattering and absorption through an air mass m?
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
What makes up the total light intensity reaching a given volume of gas?
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
What affects the amount of scattering?
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
How can solar radiation initiate reactions?
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
What affects jA?
wavelegth
intensity of light
Takes into account intrisic strength of light absorption by molecule A in the wavelegth of interest
What is the absorption cross section?
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
What is the quantum yield for the decomposition of a reactant molecule?
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
What are the main pollutants of concern to health?
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
Define sources of pollutants
Primary pollutants
Secondary pollutants
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
How have Pb and SO2 emissions been reduced?
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
What are VOcs and what are the sources?
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)
Where do oxides of nitrogen come from?
NOx = sum of No and NO2
NOx produced when N2 and O2 in air react in high temperature combustion
What are the sources of NOx?
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
Why are there still concerns over PMs?
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)
How have tropospheric ozone levels changed in the 20th century?
Increased
Define wet and dry deposition?
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
How do we estimate rate of transfer of gases and aerosols into clouds
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
Calculation of Ci, gas
What are examples of dry depositon and how is it characterised?
Stomatal uptake by trees and grasses and ocean uptake of soluble gases
characterised by deposition velocity
What is the life cycle of a tropospheric gas?
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
How can tropospheric ozone be formed and waht is the issue?
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
What are the reactions of methane in the atmosphere?
Why is the reaction between OH and CH4 important?
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
What is the rate limitting step of ozone production?
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
How many molecules of O3 can be formedfrom one CH4?
4
Which reactions cause ozone levels to change?
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
How do NOx levels affect ozone production?
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
Hoe is the hydroxyl radical formed?
What is the atmospheric lifetime?
Represents the decay time of a perturbation to a particular gas
Time taken to reach 1/e of concentraton
How is lifetime calculated?
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]
How do we calculate atmospheric gas concentrations?
Use steady state analysis - assumes reactions are happening suffieciently quickly that intermediates = 0 in given time
