Topic 6 - Atmosphere

composition of the earths atmosphere

• nitrogen 78%

• oxygen 21%

• argon 0.93%

• neon 0.002%

• helium, krypton and xenon

• water

• carbon dioxide

• ozone

the atmosphere is maintained by earths…

gravitational forces which create air pressure

layers of the earths atmosphere

• troposphere

• stratosphere

• mesosphere

• thermosphere

troposphere

• closest to the earth’s surface, extends up to about 10km above sea level

• The earth surface absorbs heat from the sun

• Wind speeds increase with height

• Most of the atmospheric mass is found, includes nearly all the water vapour, clouds and pollutants

• Most of our weather occurs

• Humans and other organisms have most interaction through exchange of gases or through introduction of pollutants

• The greenhouse effect occurs and helps to regulate the temperature of the earth

what is the greenhouse effect?

atmospheric heating phenomenon in which the Earth experiences rise in temperature because of greenhouse gasses in the atmosphere allow incoming sunlight to pass through but trap heat radiated from the earth's surface

energy from the sun enters as…

short wave radiation

heat energy is radiated as…

long wave radiation

green house gasses absorb…

long wave radiation

albedo effect

when solar energy is reflected back into the atmosphere through clouds, ice and snow

greenhouse gasses

• water vapour

• carbon dioxide

• methane

• nitrous oxide

• CFC and HCFC

• Perfluorocarbon

• Sulphur hexa-fluoride

water vapour

• the most abundant GHG

• A rise in temperature results in more water vapour leading to further warming, which allows more water to evaporate

• positive feedback cycle continues

Carbon dioxide

• concentrations are increased by burning of fossil fuels, respiration, volcanic activity and deforestation

methane

arises from emissions from livestock, anaerobic decomposition of waste, rice cultivation and fossil fuels

nitrous oxide

sources include fertilizers, combustion and industrial processes

CFCs and HCFC

have been used as liquid coolants (in refrigerators and air conditioning systems), in the production of plastic foam and as industrial solvents

Perfluorocarbon

is used in production of aluminium

Sulphur hexa-fluoride

is used in production of magnesium

Stratosphere

• extends from 10 to 50km above sea level

• Stratospheric ozone absorbs UV radiation from the sun.

• Temperature is constant at about -60°C in the lower part of the stratosphere, which is shielded by the ozone layer but then increases with altitude

• The air is dry

• Winds increase with height

mesosphere

• ranges from about 50 to 80km

• Without the presence of ozone or other particulates to absorb UV radiation, the temperature declines with height

• It is the coldest part of the atmosphere with temperatures falling to -100°C.

• There are strong winds with speeds up to around 3,000km/h

thermosphere

• This layer extends beyond about 80km to between 500km and 1,000km

• UV and X-radiation from the sun is absorbed which breaks apart molecules into atoms (oxygen, nitrogen and helium atoms are the main components in the upper thermosphere).

• The temperature increases with height and can reach beyond 2,000°C. This heat can cause the layer to expand causing variation in depth overtime from 500 to 1,000km

ionosphere

located within the thermosphere and comprises of an area in which the particles are electrically charged

what absorbs the most amount of UV

• ozone in the stratospheric layer

which types of UV rays can ozone absorb

• UV-B (some is absorbed)

• UV-C (all is absorbed)

UV-A wavelength

• longest wavelength

• 315-400nm

UV-B wavelength

280-315nm

UV-C wavelength

• shortest wavelength with highest energy

• 100-280nm

which is the most dangerous UV to humans?

UV-B

Ozone levels are measured in …

• Dobson Units (DU)

• which is the depth the ozone molecules would occupy at standard temperature and pressure (STP) of 0°C (273 Kelvin) and 1 Atmosphere

1DU =

= 0.01mm of ozone

thereats to staratosphere

• CFC’s

• Natural emissions of ozone depleting substances (volcano eruptions, release nitrogen oxide and sulphate particles)

management of stratospheric ozone

• international agreements (montreal protocol 1987)

secondary pollutants

pollutants formed when primary pollutants react in the atmosphere

types of secondary pollutants

• acid rain

• ozone

how is acid rain formed

formed when nitrogen oxide and sulphur dioxide react with water

how is ozone formed?

• UV radiation splits oxygen molecule (O2) into two oxygen atoms (O + O)

• oxygen atom (O) reacts with oxygen molecule (O2) and forms ozone (O3)

• Ozone is lost by a reaction with other ozone molecule or oxygen atom or gasses (like chlorine)

what heats up the stratosphere?

process of UV radiation used to form ozone converts UV radiation into thermal energy, which heats up the stratosphere

effects of UV-B on humans

• Skin cancer

• Skin ageing

• Immune-suppression, leads to impaired function of the immune system

effects of UV-B on plants and animals

• animals: cancer + eye damage

• impaired growth of plants (example: rice, soybeans)

• reduced phytoplankton growth, less primary production in the ocean, uptake of carbon through the ocean reduced

• reduction in forest productivity, reduces carbon uptake

why were CFC’s thought to be a good chemical to use?

• stable compounds (why initially thought to be not harmful to environment)

• long life times between 65 and 110 years

• able to be produced very cheaply

how are CFC’s harmful to the environment?

• once in the air, CFC’s are not soluble, cannot be removed by rain

• after 10-20 years CFC’s move from the troposphere to the stratosphere

• in the stratosphere, UV light breaks down CFC molecules, producing chlorine atoms

• Chlorine atoms react with ozone, breaking it down

• chlorine atoms form hydrogen chloride, which diffuses out into the troposphere and is washed out by the rain

• 1 molecule of CFC can break down 100,000 molecules of ozone

photochemical smog

Photochemical smog occurs when sunlight activates reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) resulting in the formation of ozone and peroxyacyl nitrates (PAN)

what are VOC’s (volatile organic compounds)

VOCs are carbon based compounds with a low boiling point such as propane, butane and formaldehydes

ozone in the statosphere

• “good ozone”

• reflects UV B and UV C rays

• protects life on earth from UV ray damage

ozone in the troposhere

• “bad ozone”

• secondary polutant formed through reaction between nitrogen oxides (NOx) and oxygen molecules driven by solar radiation

• contributes to smog, harmful to health, damaging to ecosystems

policies to reduce air pollution

• Altering human activity that produces air pollution

• Legislation to prevent or regulate the release of air pollutants

• Clean up and restoration of damaged system

acid deposition

refers to the falling of acidic substances from the atmosphere to the Earth's surface

the pH of natural deposition is …

usually acidic at between pH 5 and 6

why is natural deposition acidic?

due to the presence of carbon dioxide in the atmosphere which forms carbonic acid

how does acid deposition occur?

• occurs as a result of primary pollutants of sulphur dioxide and nitrogen oxides reacting in the atmosphere

• form secondary pollutants of sulphuric acid and nitric acid

types of deposition

• dry deposition

• wet deposition

wet deposition

when pollutants are incorporated into the clouds or falling raindrops and result in acidified rain or snow

dry deposition

when atmospheric pollutants are removed by gravity or direct contact under dry conditions (emissions of ash or dry particles from power stations are absorbed directly onto plants and buildings)

The impact of acid deposition on aquatic and terrestrial ecosystems also depends on …

the capacity of the environment to neutralise the acidic input

buffering capacity

alkaline calcium, magnesium compounds, and calcium carbonate (limestone)

atmosphere functions

• provides a shield from meteorites

• protects us from harmful radiation from the sun

• moderates and stabilizes our climate including temperature

• obtain oxygen and carbon dioxide

what maintains the air pressure in the atmosphere

gravitational forces

air pressure changes … with altitude

uniformly

ODS

ozone depleting substances

types of ODS

• CFC’s

• methyl bromide

• halons

• hydro-bromo-floro-carbons

• carbon tetrachloride

• methyl chloroform

methyl bromide

• life span 2 years

• used as a soil fumigator to eradicate pests

• Natural sources include emissions from the ocean and the burning of biomass

• releases bromine in the stratosphere

• bromine 50 times more effective than chlorine at destroying ozone

halons

• similar to CFC’s but contain bromine instead of chlorine

• used in fire extinguishers

HBFC’s (hydro-bromo-fluoro-carbons)

• similar properties to CFCs

• contain either or both bromine and fluorine

• solvents, cleaning agents and as suppressants in fire extinguishers

Carbon Tetrachloride

• solvent, dry cleaning agent, refrigerant and as a propellant for aerosol cans

Methyl chloroform

• used in industrial solvents, degreasing agent, correction fluid, spray adhesive and in aerosols

natural emissions of ODS

• volcanoes

spacial differences

variation in stratospheric ozone levels around the world

Seasonal changes of ozone over the Polar Regions

• lowest ozone levels were found to be at their lowest during spring and early summer

why are ozone levels higher in winter?

• during the dark, cold days of winter, the wind creates a swirling mass of air called a "polar vortex"

• This prevents air from the lower latitudes entering

• Cold winter temperatures lead to the formation of polar stratospheric clouds (PSC) within the polar vortex

• CFCs and ODS molecules react in the PSC and form chlorine atoms and other ozone depleting chemicals

• In the absence of sunlight, they are unable to react with ozone and hence accumulate within the PSC

Montreal Protocol (1987)

• set out a timetable to reduce production of chlorofluorocarbon by 50% and freeze production of halons.

UNEP OzonAction Programme

• UN programme

• set up to help developing countries reduce the use of ODS through technical advice and multilateral funds

challenges in dealing with ODS

• long life spans of ODS

• ODS present in discarded equipment

• lack of alternatives

• replacement chemicals are also ODS, but have a shorter life span (example CFC’s replaced HCFC’s)

• black market for CFC’s (due to lack of cheap alternatives)

• lack of policy and enforcement

which ODS have been banned under the Montreal Protocol

• CFC’s

• Methyl Bromide

• Halons

• HBFC’s

• Carbon tetrachloride

• Methyl chloroform

which ODS have been set to be phased out by the Montreal protocol?

• HCFC’s phase out by 2030

why are concentrations of Halons in the atmosphere increasing, after they had been banned by the montreal protocol?

possibly due to emissions from old fire extinguishers

why can nitrogen dioxide be both a primary pollutant and a secondary pollutant?

• If formed during combustion of fossil fuel and emitted into the atmosphere, it is a primary pollutant

• If formed from nitrogen oxide in the air reacting with oxygen, it is a secondary pollutant

primary pollutants

• carbon dioxide

• water vapor

• sulfur dioxide

• nitrogen oxide

• particulates

sulfur dioxide

• Coal and oil contain sulphur

• When fossil fuels are combusted they produce sulphur dioxide gas

• is toxic (eye and lung irritation)

• principal component of acid rain

nitrogen oxide (NOx)

• nitrogen oxide and nitrogen dioxide

• formed during combustion of fossil fuels

• levels tend to be high when concentration of motor vehicles

• yellow-brown gas

• respiratory irritants

• principal component of acid rain

• precursor of photochemical smog and the formation of ozone

particulates

• produced during combustion of fossil fuel and emitted into the atmosphere

• PM10 and PM2.5

photochemical smog is more likely under which conditions?

• High emissions of pollutants from combustion of fossil fuels

• High levels of sunlight

• Calm or light winds which reduces dispersion and dilution and allows pollutants to accumulate at ground level

• Dry weather conditions in which rain does not wash the pollutants out of the air

• Where the topography allows pollutants to accumulated such as a valley surrounded by hills

• thermal inversion (cold air is trapped below a warm layer of air)

impacts of ozone in the tropospheric ozone

• Is a highly reactive gas

• Causes inflammation of the lungs

• Irritates the eyes and nose

• Damages cells in the leaves disrupting photosynthesis and reducing plant growth which affects crops and forest

• Damages fabrics such as rubber and plastics

policies to reduce air pollution

• altering human behaviour (using more energy efficient devices, walking/biking, well insulated homes)

• economic incentives (tax on pollution, subsidies for green energy, road tolls/parking charges)

• technological changes (catalytic converters)

catalytic converters

• Catalytic converters reduce the amount of potential pollutants emitted   

• Reducing NOx to form nitrogen gas and oxygen gas (e.g. 2NO = N₂ + O₂)

• Oxidising carbon monoxide to form carbon dioxide

• Oxidising VOCs to carbon dioxide and water

acid deposition

when precipitation has a pH lower than five 

why is rain pH level between 5 and 6?

carbonic acid is present in rain

carbonic acid is formed from carbon dioxide reacting with water

what primary pollutants cause acid deposition?

• sulfur dioxide, reacts with water to form sulfuric acid

• nitrogen oxide, reacts with water to form nitric acid

deposition can occur as:

• wet deposition

• dry depostion

wet deposition

when pollutants are incorporated into the clouds or falling raindrops and result in acidified rain or snow

dry deposition

when atmospheric pollutants are removed by gravity or direct contact under dry conditions

what can neutralize acid deposition

• alkaline calcium

• magnesium compounds

impact of acid deposition on aquatic systems

• acid deposition can enter aquatic system directly (precipitation), or indirectly (run-off)

• can lower the pH of aquatic environment

• species are sensitive to pH changes

• loss of species can have a knock on effect through the food chain

impact of acid deposition on terrestial ecosystems

• lowering soil pH

• Leaching of plant nutrients (calcium, magnesium and potassium), reduces the nutrients available for plant uptake

• Mobilisation of aluminum ions that can damage plant root systems and can also be leached into nearby watercourses

• Mobilisation of other toxic metals from the soil such as cadmium, lead and mercury

exposure of plants to acid deposition results in …

• amage to the cuticle wax found on leaves which reduces plant photosynthesis

• Lower tolerance to pests, disease and low temperatures

• Reduction in crop yield in agricultural areas

• Loss of biodiversity and reduction in forest areas

impact of acid deposition on materials

• increases the rate of stone erosion and metal corrosion