Atmospheric Chemistry Flashcards
Topic 1 - Alberto Martinez
Atmospheric profiles
Exosphere, thermosphere, mesosphere, stratosphere, and troposphere
Composition of gases in the atmosphere
70% O2, 20% N2, 10% others
Purpose of the atmosphere
Protects the earth’s surface from cosmic and solar radiation.
Why are altitudes of atmospheric profiles slightly variable?
As the earth isn’t fully spherical, some areas are closer to the sun as it rotates, thus causing gas particles to expand with increased temperature.
Why do most chemical reactions occur in the mesosphere?
Because it experiences solar emissions from above and thermal/chemical emmisions from the surface.
Absorption
The conversion of electromagnetic radiation into atomic or molecular internal energy
Photoionisation
Removal of an electron from an atom or molecule, requires high levels of energy
Photodissociation
Photon-induced decomposition of a molecular species, breaking bonds.
Fluorescence
Emission of a photon when excited molecules return to their normal state. The wavelength of the admitted light is the same or longer than the absorbed wavelength.
Collisional quenching
Where the internal energy of an excited molecule that could be released as light is translated into other forms (usually heat) through collisions with other molecules.
Intermolecular energy transfer
Where collisions cause an excited molecule to transfer its energy to another molecule, however, it is not translated into other forms; the other molecule can now potentially fluoresce instead.
Black body radiation
The emission of electromagnetic radiation by an opaque, non-reflective object that has a constant temperature and is held in equilibrium with its surroundings.
Thermosphere
Absorbs the harshest radiation from the sun, being the furthest out.
Thermosphere: distance from earth
over 80km
Thermosphere: chemical comosition
N2 and O2
Thermosphere: temperature change and pressure
-92 to 1200 degrees from down to up, lowest pressure of all layers due to few molecular collisions.
Thermosphere: temperature gradient explanation
Caused by the absorption of short wavelength radiation. When gases absorb photons with energy that exceeds the ionisation energy, the gases become ionised and gain energy, and therefore temperature increases. Higher up in the thermosphere, where there are higher levels of radiation, it is warmer as a result.
Thermosphere: radiation
Due to few molecular collisions, energy is mainly dissipated by radiative loss. In lower layers where molecular collisions are more frequent, energy becomes translated and passed on to other molecules, keeping energy moving and temperature generally constant across the region. In the thermosphere, however, where energy cannot be easily passed around, it tends to get released directly as radiation. As a result, there are sharp temperature changes between day and night due to lack of solar radiation and very little air flow, which increases the contrast further.
Solar wind
Charged particles are emitted from the sun, interacting with the outer edges of the atmosphere
Cosmic rays
High-energy charged particles that come from outside the solar system, often distant stars, supernovae and cosmic events.
Van Allen belts
Regions (inner and outer ring) around the earth that are filled with charged particles, mainly from solar wind. They trap the charged particles to stop them hitting the earth directly and prevent harm. They are stronger when there are higher levels of solar activity. The earth’s magnetic field keeps these charged particles in place
Inner and outer Van Allen belts
The inner belt is filled with protons, and the outer belt is filled with electrons.
Example: Aurora Borealis
Charged particles in the thermosphere collide with gases, causing excitation of electrons. This leads to fluorescence, thus forming the aurora. The colour relates to the energy involved in the collision and the nature of the gas involved (N2, O2)
Example: Radio signals
The thermosphere has the important quality of bouncing radio signals transmitted from the earth.
Why is Aurora most visible near the poles?
The earth’s magnetic field lines converge at the geomagnetic north and south poles. Magnetospheric electrons can be accelerated by various processes and hit the atmosphere as they flow along magnetic field lines in polar regions.
Mesosphere
The second-highest atmospheric profile; not many chemical reactions here due to a lack of ions. The concentration of molecules is slightly increased.
Mesosphere: distance from earth
50-80 km
Mesosphere: chemical composition
N2 and O2
Mesosphere: temperature change and pressure
-2 - -92 at the top, low pressure
Mesosphere: Temperature gradient implications
The warmest temperature is at the bottom of the mesosphere. Due to this inversion air currents (wind) are formed.
Mesosphere: Gases
The gases are thick enough to slow down meteors hurtling towards the atmosphere, where they burn up.
Stratopause
The divide between the mesosphere and the stratosphere
Stratosphere
The chemistry in this region is mainly ozone photochemistry
Stratosphere: distance from earth
15-50 km
Stratosphere: chemical composition
mostly N2, O2, O3, CFCs and trace gases
Stratosphere: temperature gradient and pressure
-56 to -2 degrees, reasonable pressure
Stratosphere: temperature gradient implications
Indicates that there is photochemistry occuring in the region e.g., O2 related chemisrty, in particular absorption of UV light by the stratospheric ozone.
Stratosphere: radiation
exceeds 220 nm (UV radiation)
Mesosphere: radiation
Though the amount of high-energy radiation is reduced, chemical composition is similar, so little heating in this region from solar absorption. Some radiation is absorbed in this region.
Mesosphere: temperature gradient implications
Due to the temperature inversions in this atmospheric profile, wind occurs
Stratosphere: photochemical pathways
N2, H2O, CH4: no photochemistry when the wavelength > 180 nm
O2: O + O when wavelength > 240 nm
0* + O for higher wavelengths
O3: O2 + O
O2 + O*
Photochemical pathways
A chemical reaction that is triggered when light energy is absorbed by molecules. In the ozone layer, ozone particles split into O2 and O when hit by UV light, which can then react with one another to form ozone particles again. Dynamic balance: ozone-oxygen cycle.
Ozone chemistry
When ozone molecules absorb photons, their internal energy increases and must release/translate this energy.
Ozone chemistry: the Chapman Cycle
Where ozone dissociates into oxygen and diatomic oxygen. Clearly a destructive process, means that the ozone in the stratosphere would rapidly disappear. The process is constantly in a state of dynamic equilibrium.
Ozone chemistry: ozone layer
Assuming the destructive processes are constant, ozone concentrations will be highest where the formation processes are most frequent. [O2] decreases with increased altitude and pressure whereas UV-C levels increase. This means that there is a layer of ozone that is created in the lower stratosphere.
Ozone chemistry: Catalytic destruction
It is suggested that there are other mechanisms for ozone destruction because the chapman cycle alone would result in too-high a concentration of ozone. Other mechanisms for ozone destruction are described as catalytic as the concentration of whatever reacts does not change. They convert either O3 or O into O2.
Stratospheric OH radicals
Formed via the reaction between UV and ozone, highly unpredictable and reactive species. They then react with other species e.g., H2O, CH4, NO2 through free radical substitution.
Stratospheric halogen radicals
There are a few natural sources of Cl or Br radicals in the atmosphere, however some form from oceanic emissions. Most halogen radicals are caused by human activity.
CFCs
Chlorofluorocarbons: long atmospheric lifetimes, chemically and biologically inert, low bp, non toxic/flammable. Only forms of loss are deposition and transport.
Implications of CFCs in the stratosphere
In the stratosphere, CFCs can be photolysed to produce Cl* which can catalytically destroy ozone.
Antarctica: CFC timeline
In the early 1980s, there was little knowledge of stratospheric ozone destruction.
1982: first noticed a decrease in ozone concentrations.
1989: large hole burnt in the ozone layer at 14-22 km altitude, caused 98% reduction of ozone.
This hole has since spread over the entire continent towards Australia and New Zealand.
Abnormal Antarctic chemistry
In the polar vortex temperatures (-80) freezing chemicals are formed containing (HCl, ClONO2, NO2, CH4)
These species react readily on cloud surfaces, converting cast amounts of reservoir species into atmospherically hazardous halogenated compounds.
Polar stratospheric clouds (PSCs)
ClONO2 + HCl = Cl2 + HNO3
ClONO2 + H20 = HOCl + HNO3
HNO3 will be rained out from these PSCs, however Cl2 and HOCl are stored within the clouds.
With the nitrogen removed through rain, the chlorine species are now free to react with the ozone. When temperatures rise, the chemistry of these regions return to normal.
Dimerisation and its importance
???
Antarctic vs Arctic.
Arctic only found minor ozone loss in the atmosphere as it isn’t cool enough. The arctic vortex is much weaker with more mixing, which leads to fewer PSCs and less ozone depletion overall.
Troposphere
The atmospheric profile closest to the earth.
Troposphere: distance
0-10 km at the poles, 16km at the tropics
Troposphere: chemical composition
Broad composition including 85% of total atmospheric mass.
Troposphere: temperature gradient and pressure
15 - -56 degrees from bottom to top. 1 bar of pressure.
Troposphere: radiation
UV-A, visible light, some infrared radiation.
Troposphere: temperature gradient implications
Inverted temperature profile causes wind, low temperature in the tropopause means that many tropospheric species e.g., water, are frozen out and cannot enter the stratosphere.
Tropopause
An effective temperature barrier between the troposphere and stratosphere.
Troposphere: Human impact
Tropospheric chemistry is heavily influenced by humans and land features (land/sea). Humans don’t induce new chemistry, however tend to push equilibriums too far in one direction.
Key species in tropospheric chemistry
HOx, NOx, SOx, terpenes, aerosols
Terpenes
Acid rain
Caused by a fine balance of CO2 in the troposphere. Equilibrium in gaseous CO2 is maintained by CO2 dissolving into water masses, which can lead to the production of carbonic acid (H2CO3, pH 5.6)
Effects of acid rain
Destruction of marble or limestone buildings.
Leach minerals out of soil, which are essential for growth
Al3+ can poison trees
SO2 attacks foliage
Al3+ and SO42- are toxic to fish and many lakes cannot regulate the pH of their water under 6.
Working out if an ion is hard or soft
charge/radius, hard = high number, soft = low number
Photochemical smog
Formed by SO2, killed 4000 of the London population in 1952.
Made up from UV light, hydrocarbons and NOx.
Causes eyes/nasal/throat irritation and severely reduced visibility.
The origins of smog
Combustion engines produce hydrocarbons and NOx, which are key to the formation of smog. Piston engines also tend to emit unoxidised hydrocarbons, caused by the incomplete combustion near the walls of the engine.
Smog mitigation efforts
Since 2008, emissions for light duty motor vehicles in the USA have been capped at 0.41 g/mile HCs, 0.4 g/mile NO, 3.4 g/mile CO. Catalytic converters have also helped to reduce pollutant emissions.
Indicators of smog formation
Production of tropospheric ozone and peroxyacetylnitrate (PAN).
Degree of toxicity of smog
Unknown as a whole, but O3 is toxic to humans, causing irritation to the respiratory mucous system, and PAN can cause eye irritation, phytotoxicity and mutagenicity.
Source of O3
Tropospheric source: NO2 reacting with UV light.
Source of peroxy radicals
Many exist as a result of photolytic processes between O3/UV or NO*/NO2/H2O (more likely)
Non-photolytic sources include: HONO/UV and alkene/O3
Criegee intermediates
Global warming
Temperature of the earth on aerage is increasing, sea levels are rising and ice caps are melting.
Global warming: effects on earth
Assuming the earth is a black body emitter, the earth could become an emitter with a temperature of 256K, however the earth would become uninhabitable.
Radiative absorption
Real surface temp of the earth is 288K, 32K discrepancy is due to absorption of solar flux. The earth mainly emits long-wavelength infrared radiation.
Greenhouse gases
Absorb emissions from the earth’s surface - effectively trapping it and preventing it from being passed into space. Creates a net warming effect.
