Chemistry of the atmosphere

Explanation

• For about 200 million years, air has contained approximately 80% nitrogen and 20% oxygen, with much smaller proportions of carbon dioxide, noble gases and water vapour.
• Use a percentage as a fraction of 100: volume of a gas $=(\text{percentage}/100)\times\text{total volume}$.
• For example, a $350\,\text{cm}^3$ air sample contains about $0.20\times350=70\,\text{cm}^3$ of oxygen.
• The 80:20 split is an approximation: nitrogen and oxygen do not make exactly 100%, and the proportion of water vapour varies.
• The accepted modern values are close to four-fifths nitrogen and one-fifth oxygen, with argon, carbon dioxide and variable water vapour making up the remainder.

Explanation

• Theories of the early atmosphere have changed and developed as evidence has been interpreted; one theory proposes that intense volcanic activity released the gases that formed it.
• The atmosphere may initially have resembled those of Mars and Venus today: mainly carbon dioxide, with little or no oxygen; volcanic nitrogen accumulated, and small proportions of methane and ammonia may also have been present.
• As the Earth cooled, volcanic water vapour condensed to form oceans; carbon dioxide then dissolved and carbonates precipitated as sediments, lowering its atmospheric proportion.
• Evidence from 4.6 billion years ago is limited, so this is a supported theory rather than a certain, directly observed account; detailed knowledge of other theories is not required.
• Scientific explanations are revised when new geological or planetary evidence becomes available, so conclusions must match the strength of the evidence.

Explanation

• Algae and plants released oxygen by photosynthesis: carbon dioxide and water form glucose and oxygen using light energy.
• Link the biological timeline: algae began producing oxygen about 2.7 billion years ago, oxygen then appeared in the atmosphere, and later plants increased its proportion further.
• Photosynthesis simultaneously removes carbon dioxide and supplies oxygen, so a growth in photosynthetic organisms changes both gases in opposite directions.
• Do not attribute the oxygen rise to volcanoes: in this model the sustained increase came from algae and plants, eventually allowing animals to evolve.
• The photosynthesis equation provides the chemical link between the removal of carbon dioxide and the release of oxygen.

Explanation

• Atmospheric carbon dioxide decreased because algae and plants used it in photosynthesis and because it dissolved in the oceans.
• Trace where the carbon went: dissolved carbon dioxide contributed to carbonate sediments and limestone, while carbon in organisms was eventually locked into fossil fuels.
• Coal formed mainly from buried plant material; crude oil and natural gas formed mainly from buried remains of marine organisms over millions of years.
• A common error is to say carbon disappeared: it was transferred into biomass, sedimentary rocks and fossil fuels rather than destroyed.
• Each mechanism transfers carbon out of the atmosphere while conserving the carbon atoms in another reservoir.

Explanation

• Water vapour, carbon dioxide and methane are greenhouse gases that maintain a temperature on Earth high enough to support life.
• Describe the greenhouse effect by wavelength: short-wavelength radiation from the Sun reaches and warms the surface, which emits longer-wavelength infrared radiation.
• Greenhouse-gas molecules absorb some outgoing long-wavelength radiation and re-emit it in all directions, reducing the rate at which energy escapes to space.
• Do not confuse the greenhouse effect with ozone depletion, and do not claim that greenhouse gases stop all radiation from leaving Earth.
• The natural greenhouse effect is necessary for life; enhanced greenhouse warming concerns changes in greenhouse-gas concentrations.

Explanation

• Burning fossil fuels and deforestation increase atmospheric carbon dioxide; livestock farming, rice cultivation, landfill and decay of organic waste can increase methane.
• For a recall question, give two distinct human activities for carbon dioxide and two for methane, linking each activity to the correct gas.
• A strong climate-evidence evaluation checks sample size and duration, uncertainty, peer review, agreement with other data and whether the source communicates the full evidence.
• A single weather event or a short local record cannot by itself establish a global climate trend; correlation also needs a scientifically plausible explanation.
• Correlation between emissions and climate variables becomes more persuasive when supported by a plausible mechanism and several independent data sets.

Explanation

• An increase in average global temperature is a major cause of climate change, but the consequences differ between regions.
• Use a cause-and-effect chain: warming can melt land ice and expand seawater, raise sea level, alter rainfall and extreme-weather patterns, and change habitats or species distributions.
• For example, sea-level rise may increase coastal flooding, while changed rainfall may produce drought in one region and greater flood risk in another.
• Do not present every projected effect as certain: discuss its scale, likelihood, risk and environmental implication using the evidence supplied.
• Different regions may experience opposite rainfall changes, so answers must use the evidence supplied rather than claiming one universal outcome.

Explanation

• A carbon footprint is the total carbon dioxide and other greenhouse gases emitted across the full life cycle of a product, service or event.
• Include raw materials, manufacture, transport, use and end-of-life when comparing footprints; omitting a stage can reverse a decision.
• Footprints can be reduced through lower energy use, renewable energy, less travel, reduced waste, recycling, methane capture and changes in farming or diet.
• Reductions may be limited by cost, available technology, infrastructure, public acceptance, convenience and incomplete or uncertain life-cycle data.
• Because data across a complete life cycle can be uncertain, carbon-footprint comparisons should state the same boundaries and assumptions.

Explanation

• Burning carbon- and hydrogen-containing fuels can release carbon dioxide and water vapour; limited oxygen can also produce carbon monoxide, soot and unburned hydrocarbons.
• Identify sulfur dioxide from sulfur impurities in a fuel, and oxides of nitrogen from nitrogen and oxygen reacting at the high temperatures inside engines.
• Given a fuel's composition and conditions, first list its elements, then use oxygen supply and combustion temperature to predict the possible gaseous and particulate products.
• Carbon monoxide and carbon dioxide are different products: carbon monoxide and soot indicate incomplete combustion, whereas complete combustion of carbon produces carbon dioxide.
• Incomplete combustion can produce both carbon monoxide and carbon particulates, while unburned hydrocarbons may also escape.

Explanation

• Carbon monoxide is toxic, colourless and odourless, so a dangerous concentration is not easily detected by human senses.
• Sulfur dioxide and oxides of nitrogen cause respiratory problems and acid rain; connect each pollutant to both human-health and environmental effects when asked.
• Particulates damage health and cause global dimming by reducing the amount of sunlight reaching the Earth's surface.
• Do not assign global dimming to carbon dioxide or acid rain to soot: name the pollutant before explaining its specific effect.
• Carbon monoxide's lack of colour and smell increases its danger because toxic exposure cannot be detected reliably by human senses.