Composition of air

Composition of air

  • In dry air,
    • 78%78\% nitrogen
    • 21%21\% oxygen
    • 0.04%0.04\% carbon dioxide
    • 0.9%0.9\% argon
    • other noble gases
      • neon, helium, krypton
    • Water vapor varies between 0%0\% and 5%5\% based on location

Air pollution

Air pollution is defined as the condition in which air contains a high concentration of certain chemicals that may harm living things or damage non-living things. Air pollutants can come from both natural and man-made sources. Sources of natural pollutants include volcanoes. Sources of man-made pollutants include car engines and coal-fired power stations.

PollutantFormationHarmful effects
Carbon monoxide, CO

Colourless and odourless gas, insoluble in water		Forms when there is incomplete combustion of carbon-containing substances in fossil fuels such as coal, petroleum, and natural gas due to a limited supply of oxygen in the internal combustion engine, or poor maintenance of the engine, or a short reaction time in the engine.Poisonous even in low concentrations, can cause headaches, loss of consciousness, and death
CO binds very strongly with blood's haemoglobin, forming a stable complex that oxygen cannot bind to, preventing haemoglobin from carrying O₂ around the body
Methane, CH₄

Colourless and odourless gas, insoluble in water		Produced from the bacterial decay of organic matter like dead organisms and manure Also produced from cattle burpingMethane is highly flammable and may be explosive in air It also contributes to the greenhouse effect and global warming
Oxides of nitrogen, NOₓ (NO and NO₂)

NO is a colourless and odourless gas, neutral, sparingly soluble in water

NO₂ is a brown, pungent, acidic gas, soluble in water		Forms when nitrogen gas and oxygen gas in air react together in the presence of high temperatures of vehicles' internal combustion engines, or during lightning activity
The high energy provides provides enough energy for the surrounding air's nitrogen and oxygen to react and combine to form \ceNO\ce{NO}, which is further oxidised by oxygen in the air to form \ceNO2\ce{NO2}

\ceN2(g)+O2(g)−>2NO(g)\ce{N2(g) + O2(g) -> 2NO(g)}
\ce2NO(g)+O2(g)−>2NO2(g)\ce{2NO(g) + O2(g) -> 2NO2(g)}		Can irritate eyes, trigger coughing, shortness of breath, asthma attacks, and aggravate respiratory diseases such as bronchitis.

Reacts with rainwater and O₂ to form acid rain, turning rivers and lakes acidic, killing fish and aquatic molluscs Acid rain makes soil acidic, leaching nutrients from soil, and harming crops and forests Acid rain reacts with metal/limestone/marble, corroding metal towers/bridges/buildings and marble/limestone buildings (containing CaCO₃)
Ozone, O₃

Very pale-blue gas, pungent (and at ground-level, is a major component of photochemical smog)		(enrichment) Forms during photochemical reactions of NO₂, O₂, and UV light / sunlight Other reactions involving NO, O₂, H₂O, unburnt hydrocarbons, and UV light / sunlight help ozone accumulate by forming NO₂(in syllabus)
Can irritate eyes and cause dry eyes
trigger coughing, shortness of breath, and asthma attacks
aggravate respiratory diseases such as bronchitis

can damage chloroplasts in plants, reducing crop yields
can also cause plastic/rubber to crack by reacting with \ceC=C\ce{C=C} bonds
Sulfur dioxide, SO₂

Colourless and pungent gas, acidic, soluble in water		Volcanic activity (due to sulfur existing in Earth's mantle and crust) or during combustion of fossil fuels that contain sulfur (from biological molecules in the organisms that gave rise to fossil fuel, and from sulfur compounds in the rocks surround the fossil fuel), where sulfur (sulfur compounds) react with oxygen to produce sulfur dioxideCan irritate eyes, trigger coughing, shortness of breath, and asthma attacks, and aggravate respiratory diseases such as bronchitis
Unburnt hydrocarbonsHydrocarbons in petrol that have not been burnt in vehicle engines and escape into the atmosphereCan irritate eyes, trigger coughing, shortness of breath, and asthma attacks, and aggravate respiratory diseases such as bronchitis
some can cause cancer

Catalytic converter

  • A catalytic converter is attached to the exhaust system of a car
    • Removes pollutant gases by oxidizing or reducing them
    • Exhaust gases are passed over a catalytic bed containing rhodium, palladium, and platinum
    • Catalytic converters only work with unleaded petrol because lead destroys the catalytic action of the catalyst in the converter
      • Fuels containing sulfur compounsd also poison catalysts
  • Harmful gases, COCO, NOxNOx, and unburnt hydrocarbons, are converted, by chemical reactions, into less harmful gases, CO2CO_{2}, N2N_{2} and water vapour
\ce{2NO2(g) + 4CO(g) &-> N2(g) + 4CO2(g) }\\ \ce{2NO(g) + 2CO(g) &-> N2(g) + 2CO2(g)} \\ \ce{2CO(g)+O2(g) &-> 2CO2(g)} \\ \ce{C_{x}H_{y} + (x + 0.25y)O2(g) &-> xCO2(g) + (0.5y) H2O(l)} \end{align} $$ # Photochemical smog - Photochemical smog, a brownish haze, is a mixture of nitrogen oxides, unburnt hydrocarbons, ground-level ozone, peroxyacyl nitrates(PANs) and volatile organic compounds(VOCs) such as **aldehydes** - It is most common in urban areas with high traffic and sunlight as factories and vehicles release nitrogen oxides and unburnt hydrocarbons into the atmosphere as by-products of fuel combustion - Photochemical reactions - Under UV light / sunlight, $NO_{2}$ breaks down, forming $\ce{NO}$ and an $\ce{O}$ atom (photodissociation), which is also a radical $\ce{O^{.}}$ - $\ce{O2}$ reacts with the $\ce{O}$ atom to form ozone - Under UV light/sunlight, ozone and water vapour react to form an $\ce{OH^.}$ radical - In a series of reactions, the $\ce{OH^.}$ radical reacts with unburnt hydrocarbons, oxygen and $\ce{NO}$, which produces $\ce{NO2}$, **aldehydes** and regenerates the $\ce{OH^.}$ radical for further reactions - Ground level ozone accumulates as more $\ce{NO2}$ is formed, and less $\ce{NO}$ is available to react with ozone - Under UV light/ sunlight, unburnt hydrocarbons react with $\ce{NO2}$ and $\ce{O2}$ to form peroxyacal nitrates (PANs) # Sulfur dioxide and flue gas desulfurisation - Power plants that burn coal and fuel release **flue gas**, which contains high amounts of sulfur dioxide - $\ce{SO2}$ emissions from power stations can be reduced by - refining the coal or oil to remove sulfur content before combustion - Removing sulfur dioxide from the exhaust gases before it is released into the atmosphere through **flue gas desulfurisation** - Sulfur is removed from flue gas by being **precipitated** as a solid residue - $\ce{CaO(s)}$ or $\ce{Ca(OH)2(s)}$ or $\ce{CaCO3(s)}$ in aqueous suspension (slurry) can be used - $\ce{CaCO3}$ is the best - $\ce{CaSO3}$ is further oxidised by oxygen to $\ce{CaSO4}$ - as if $\ce{CaSO3(s)}$ was released into a landfill, $\ce{CaSO3 + H2SO4 -> CaSO4 + H2O + SO2}$ - This would release $\ce{SO2}$ and result in acid rain again - in the following reactions $$\begin{align} \ce{CaCO3(s) + SO2(g) &-> CaSO3(s) + CO2(g)} \\ \ce{CaO(s) + SO2(g) &-> CaSO3(s)} \\ \ce{Ca(OH)2(s) + SO2(g) &-> CaSO3(s) + 2H2O(l)} \\ \ce{2CaSO3(s) + O2(g) &-> 2CaSO4(s)} \end{align}$$ # Acid rain - Rainwater is naturally slightly acidic due to dissolved $\ce{CO2}$ in the air - Acid rain can be formed when air pollutants react with rainwater and oxygen to form strong acids $$\begin{align} \ce{2SO2 + 2H2O + O2 &-> 2H2SO4}\\ \ce{4NO2 + 2H2O + O2 &-> 4HNO3} \end{align}$$ > [!info]- > This is not the full equation for acid rain formation, full eq is > $$\begin{align} \ce{S + O2 &-> SO2} \\ \ce{2SO2 + O2 &≤>[NO2 (catalyst)][or V2O5] 2SO3} \\ \ce{2SO3 + 2H2O &-> 2H2SO4} \\ \end{align}$$ - Other mechanisms, involving $\ce{H2O}$, $\ce{H2O2}$, $\ce{O3}$ or the $\ce{OH^.}$ radical can generate sulfuric acid and nitric acid to form acid rain - $\ce{NO2}$ also catalyses the formation of $\ce{SO3}$ from $\ce{SO2}$, which then dissolves in water to form $\ce{H2SO4}$ $$\begin{align} \ce{SO2 + NO2 &-> SO3 + NO} \\ \ce{2NO + O2 &-> 2NO2} \\ \ce{SO3 + H2O &-> H2SO4} \end{align}$$ - Acid rain - turns rivers and lakes acidic, killing fish (e.g. salmon) and aquatic molluscs (e.g. snails, clams) - makes the soil acidic, which leaches nutrients from the soil, and harms crops and forests - Reacts with metal/limestone/marble, corroding metal towers/bridges/buildings and limestone/marble buildings (containing $\ce{CaCO3}$) - Some methods are used to reduce the emission of pollutants ($\ce{SO2}$ and $\ce{NO_{x}}$) into the atmosphere - use fuels which contain less sulfur or use natural gas which does not contain sulfur - For lakes and soil affected by acid rain, use calcium carbonate to react with the acids - Catalytic converters - Flue gas desulfurisation # The ozone layer - The ozone layer is 15-30km above the Earth's surface, in the stratosphere - chemical reactions in the ozone layer $$\begin{align} \ce{O2 &-> O^.}\\ \ce{O2 + O^. &≤> O3} \end{align}$$ 1. In a photochemical reaction, oxygen molecules absorb UV light from the sun, splitting the molecules into oxygen atoms (which are radicals with two unpaired electrons each) 2. One oxygen atom combines with an oxygen molecule to form ozone and excess energy from the reaction is transferred to another particle (often a nitrogen or oxygen molecule) 3. In another photochemical reaction, an ozone molecule can absorb UV light from the sun the split into an oxygen molecule and an oxygen atom - The products of step 3 can further react to repeat steps 2 and 3, resulting in the ozone layer absorbing significant UV light from the sun, reducing the amount of UV light that reaches the Earth's surface - Above the ozone layer, due to the thinner atmosphere, there are too few molecules/atoms to collide to result in enough chemical reactions to accumulate ozone Below the ozone layer, there is naturally too little UV light passing through to trigger enough chemical reactions to naturally accumulate ozone - The ozone layer has been depleted by reaction with compounds such as chlorofluorocarbons (CFCs), containing chlorine, fluorine and carbon, resulting in more UV light reaching the Earth's surface, which would - increase the risk of skin cancer, sunburns, and eye cataracts in humans - Worsen growth of crops and forests as UV light can cause the deaths of plant cells - Impair photosynthesis in phytoplankton of the oceans, disrupting marine ecosystems ## Chlorofluorocarbons - CFCs are chemically stable, odourless, colourless, non-flammable, and low in toxicity - Their chemical stability meant they were less reactive and resulting in them not being readily biodegraded or chemically destroyed after use on Earth's surface, instead slowly diffusing towards the stratosphere - The more abundant UV light provides enough energy to break the weaker $\ce{C-Cl}$ bonds in CFCs, creating highly reactive chlorine free radicals ($\ce{Cl^.}$) - These react with $\ce{O3}$ to form chlorine monoxide radicals ($\ce{ClO^.}$), which then react with oxygen radicals to regenerate $\ce{Cl^.}$ $$\begin{align} \text{Chain initiation:}&\quad \ce{CF2Cl2 -> CF2Cl^. + Cl^.} \\ \text{Chain propogation}&\quad \ce{Cl^. + O3 -> ClO^. + O2} \\ &\quad \ce{ClO^. + O^. -> Cl^. + O2} \end{align}$$ - The regenerated chlorine radical can continue to participate in the chain reaction as a catalyst, accelerating the breakdown of ozone without being consumed in the process, ultimately leading to the depletion of the ozone layer in the stratosphere - For refrigerants, industries used less chemically stable hydrochlorofluorocarbons (HCFCs), then less chemically stable hydrofluorocarbons (HFCs) - However, both were strong greenhouse gases, contributing to global warming - Industries transitioned to using hydrocarbons for creating insulating foam (pentane), and as aerosol spray propellants and refrigerants (both isobutane) - These hydrocarbons naturally break down in the lower atmosphere more easily, contributing much less to ozone layer depletion and global warming, but they are also flammable - This is due to the $\ce{C-H}$ bonds, which allow for oxidation in the lower atmosphere # Carbon cycle - The amount of $\ce{CO2}$ in the atmosphere is almost constant - $\ce{CO2}$ is added mainly through aerobic respiration from plants and animals; and combustion - respiration $$\ce{C6H12O6(aq) + 6O2(g) -> 6CO2(g) + 6H2O(l)}$$ - combustion of fossil fuels or hydrocarbons $$\ce{CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)}$$ - $\ce{CO2}$ is removed from the atmosphere through photosynthesis in green plants $$\ce{6CO2 (g) + 6H2O(l) -> C6H12O6(aq) + 6O2(g)}$$ - Large amounts of $\ce{CO2}$ in the air are also removed when dissolved in the oceans ## Greenhouse effect - Atmosphere acts like a greenhouse - Traps heat to keep Earth warm enough to support life - Long-wavelength UV radiation from the Sun enters the atmosphere - Earth's surface absorbs some of this energy - The rest reflects back into the atmosphere as infrared radiation or heat - Greenhouse gases trap heat - Include carbon dioxide $\ce{CO2}$, water vapour $\ce{H2O}$, and methane $\ce{CH4}$ - Absorb infrared radiation and trap heat within the atmosphere - Any gas molecule with three or more atoms can absorb infrared radiation - Greenhouse effect maintains average surface temperature at around 15°C - Essential for sustaining life and keeping water in liquid form - Without it the average surface temperature would be around −50°C - It helps maintain the natural carbon cycle ## Global warming - Global warming occurs when the greenhouse effect becomes too strong - Usually due to an excessive buildup of greenhouse gases - Higher concentration of carbon dioxide and other greenhouse gases traps more heat, leading to increased global temperatures - Carbon dioxide $\ce{CO2}$ is the most significant greenhouse gas in terms of concentration - Others include methane $\ce{CH4}$, nitrous oxide $\ce{N2O}$, and chlorofluorocarbons (CFCs) - Methane is about 30 times more effective than $\ce{CO2}$ at trapping heat - Its atmospheric concentration is much lower - Human activities since the Industrial Revolution caused a rapid rise in atmospheric $\ce{CO2}$ levels, intensifying the greenhouse effect - Global warming causes various climate-related impacts worldwide - including rising sea levels due to thermal expansion and melting of polar ice caps and glaciers - Atmospheric concentration of carbon dioxide as of 2026 is around 430 ppm ### Effects of global warming - Melting of ice glaciers causes the sea level to rise even further, resulting in the disappearance of low-lying islands and coastal land - More frequent and serious extreme weather events occur globally - including heatwaves, wildfires, droughts, tropical cyclones, and floods - Equatorial countries like Singapore will probably experience more rain - Countries in North America and Europe will probably suffer drought - This drought would result in serious food shortages - Warmer water in the oceans will produce more powerful and destructive tropical cyclones - Some living species may go extinct due to - habitat loss, seasonal timing changes, and ocean acidification - coral bleaching due to warmer water - Further impacts by more frequent and serious extreme weather
Built with LogoFlowershow