at Flashcards
Thermosphere
• Temperature (T) increases with altitude (z) (why?)
• Short wavelengths absorbed and photo-ionisation occurs
leading to energetic atoms and ions
• Low air density ~ heat capacity small
Auroral emissions
• Atomic and molecular systems of O2
and N2 predominate
• especially in the auroral green line (O1S→1D) at 557.7nm
• Bombardment by solar particles from above causes ionisation
followed by dissociative recombination with electrons
O2
+ + e- O(1S) + O(3P)
• The red lines in the aurora result from the transition
O(1D) O(3Pj
) + hv (ʎ = 630.4 nm)
• Violet and blue arise from excited states of N2
Mesosphere
The ‘mystery layer’ – heavy metal chemistry
• T falls with z to ~ -80oC at mesopause (P < 0.01 mb)
(why?)
• Lowest ever atmospheric T recorded at mesopause ~ -
140oC
• Polar mesospheric clouds (more on these later!)
Stratosphere
Temperature increases with altitude (why?)
• Very stable and dry air – planes fly up here sometimes to
avoid turbulence
• Where the ozone layer is (we will discuss this in detail)
• Polar stratospheric clouds form here (more later!)
The Troposphere
is the lowest region in the atmosphere, extending approximately
10km…up to 8km (poles) or 17km (equator).
Here, the temperature falls by about 6.5⁰C/km. Therefore, the air is unstable,
causing weather with rapid mixing, especially in the centre (equatorial plane)
where convective thunderstorms are common
Concentration units for gases
bsolute: molecules per cm3 of air; partial pressure
• Computational modelling, calculations etc.
• Relative: Percent of molecules ( percent by mass); mole fraction (or
mixing ratio); and for minor components, ppmv
(number of molecules per
million molecules of air)
• Health guidelines, published data etc.
Pi Vi = niRT
We can convert relative and absolute via ideal gas formula..HOW?
Explain the temperature changing in the different at. layers
Troposphere: As the altitude increases, the air temperature decreases.
The troposphere is hotter near the Earth’s surface because heat from the Earth warms this air. As the altitude increases the number of air molecules decreases, thus the average of their kinetic energy decreases. The results is a decrease in air temperature with an increase of altitude.
Stratosphere: As the altitude increases, the air temperature increases.
The Stratosphere has a layer of ozone, called the ozone layer. This layer absorbs most of the ultraviolet radiation from sunlight. This results in the stratosphere being warmer.
Mesosphere: As the altitude increases, the air temperature decreases.
The Mesosphere, like the troposphere layer, has a decrease in temperature with altitude because of the decreases in the density of the air molecules.
Thermosphere: As the altitude increases, the air temperature increases.
The Thermosphere is warmed by the absorption of solar X-rays by the nitrogen and oxygen molecules in this outer layer. Thus, the temperature of this layer increases with altitude.
• Reservoirs:
Where any defined species is in significant quantities,
for significant periods. e.g. the atmosphere and ocean are both
reservoirs of carbon
where a defined species has accumulated.
• Source:
The origin of a defined species. Biogenic (natural) or
anthropogenic (due to human activity)
where a defined species can be taken from…
Sink
Destination of a substance. Where does it go to? What is its
fate?
where a defined species can accumulate.
The ocean
both a sink (shallow) and reservoir (deep) of CO2
Timescales of transport in the lower atmosphere
Horizontal flow in lower troposphere ~ 5 ms-1
• chemical species can travel 100’s km/day from emission point.
• Atmospheric winds move air parcels (and emissions) across entire
continents in few days
• Zonal winds in mid to upper troposphere move long-lived species far
from origin (across hemispheres)
• The concept of lifetime…important and highly contextual!
Regional distribution
of pollutants
Depends on;
• Source location and type. • Transport processes spreading it around • Chemical mechanisms removing it from atmosphere.
The boundary layer
The troposphere consists of the boundary layer and the free troposphere. • In the boundary layer, friction with the surface (of earth) causes highly turbulent mixing close to the surface (wind). The height of the boundary layer over land varies between 0.5-3km during the day and collapses to less than 0.5km at night when the ground cools faster than the overlaying atmosphere. • Over the ocean the BL height is usually about 1km and has less diurnal variation. There is more turbulent mixing at night because the atmosphere cools faster than the ocean
Timelines of pollution diffusion
• Air mixes from the boundary layer to the free troposphere in about 2
days.
• E-W winds blow with an average of 10-30 m/s, so air is transported
around a line of latitude in 1-3 weeks.
• It takes at least 1 year for tropospheric air to cross between
hemispheres.
The distribution of pollutants around the world therefore is highly
dependent on the chemistry of those pollutants and the physical
nature of the environment in which they reside.
What is ‘Air pollution’?… When is
a substance a ‘pollutant’?
Dependant on:
• Concentration • Toxic effects on biological organisms (including people!) • Reactivity and timelines • Context ( i.e. ozone in stratosphere vs troposphere)
Primary (1
o) pollutants:
Oxides of nitrogen(NOx), hydrocarbons and/or other volatile organic compounds (VOCs)- vehicles, factories, towns, agriculture, shipping, volcanoes, wildfires Transform into secondary (2 o) pollutants via photochemical reactions (species formed thereafter as a result of these reactions are also referred to as secondary pollutants)
Formation of the Ozone (O3
) Layer
• 1 billion years ago, blue-green algae used Sun’s energy to split H2O & CO2 to form organics and O2 - photosynthesis • Some O2 reacted with C to form CO2 and the rest accumulated - As O2 CO2 • IN the upper atmosphere, O2 absorbed sunlight to form O radicals which recombined with O2 to form O3 (more on this later!) • O3 protects the planet from UV light & allowed early life to develop
Present Day Atmosphere
• Relatively stable mixture of several hundred gases from different origins. • Gaseous envelope surrounds the planet and revolves with it. • Mass of ~ 5.15 x 1015 tons held to planet by gravity how does this influence atmospheric composition? Major components up to 80 km are ~ O2 (21%), N2 (78%), and Ar (1%) Small amounts of other trace gases.
Atmospheric scale - Troposphere
• The troposphere contains roughly 85% by mass of the entire atmosphere. • Characterized by decreasing temperature with height and strong convective mixing. • The region that most biological activity, weather processes & chemistry occurs.
Chemistry of the troposphere
• The tropospheric environment is oxidising: like a low temperature
combustor, hydrocarbons, CO and H2 get oxidised to CO2
and H2O
• Pollution, mainly in the form of NOx
(NO and NO2
) and hydrocarbons
contributes to photochemical smog, secondary product and aerosol
formation.
• This is dependant on weather conditions, sources, sinks and the
chemical characteristics of the pollutant.
Hydrocarbons
Wide range of sources; can be both anthropogenic and biogenic.
• All species with carbon chain backbones, including unsaturated,
oxygenated and halogenated species.
• Many are highly reactive in the oxidative atmosphere and influence
the chemistry of the air.
• Species in the gas phase at STP are often referred to as VOC’s or
volatile organic compounds.
• Volatile Organic Compounds ( VOCs) contribute to the extreme
complexity of gas phase and aerosol chemistry in the atmosphere
Nitrogen oxides in the troposphere
• NO and NO2
are rapidly interconverted into each other and are
therefore considered as NOx = NO and NO2
• The ratio of [NO]/[NOx
] is about 0.2 at the surface but increases at
higher altitudes.
Lifetime of NOx
• The atmospheric life time of NOx
is short, at the boundary layer
(hours) and increases with altitude (days)
• Lifetime is longer in winter than summer (lower [OH])
• The short lifetimes results in limited transport (vertically and
horizontally) in the NOx
form but other forms are more stable (more
on this later)
distribution- shipping lanes, biomass burning, power plants
