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Air pollution: nitrogen oxides and sulfur oxides, effects of acid rain

4. FOSSIL FUEL COMBUSTION, Air Pollution and Climate Change

AIR POLLUTION - sources and effects of sulfur and nitrogen oxides

Anthropogenic means any environmental change or pollution due to human activity and there is a lot of it about on 'Planet Earth'

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4D Fossil fuel combustion air pollution, sulfur oxides, nitrogen oxides, acid rain and how catalytic converters remove pollutants from vehicle exhausts

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4A Fossil fuel air pollution - incomplete combustion, carbon monoxide & soot particulates

4B Pollution, Accidents and Economic Aspects of the Petrochemical Industry

4C Greenhouse effect, global warming, climate change, carbon footprint from fossil fuel burning

A local acid rain project!!!

There were three coal-fired power stations here, the last coal-fired Ferrybridge power station C closed in 2016.

4D AIR POLLUTION - sulfur oxides and nitrogen oxides

non-renewable fossil fuel coal oil gas diagram electricity power generation turbine generator transformer power lines

  • AIR POLLUTION from SO2 NO, and NO2 - other than the effects of carbon dioxide and global warming

    • There are many reasons why we should do our best to control atmospheric pollution, and there are plenty of examples described in the next few paragraphs.

    • Unless strategies are put in place, pollutants build up and create environmental problems for most plants and animals.

    • In particular, the air of many of the world's cities is highly polluted, causing ill-health e.g respiratory diseases like asthma.

    • In principle, the easiest way to reduce pollution (or global warming) is to use less electricity, so burning less fossil fuels, but easier said than done! Alternative fuels and other forms of electrical energy generation are being developed (see above discussion), but are they being developed fast enough and to meet consumer demands?

    • We all like our convenient central heating and reliance on electricity for running our homes. Just think of al we use that runs off electricity - cooker, TV and video systems, washing machine, shower, computers, electric blanket (its cold on the North York Moors in winter!) etc. etc.

    • Apart from carbon monoxide, pollutants such as sulfur dioxide and oxides of nitrogen cause respiratory problems in humans and cause acid rain which damages plants and buildings.

      • Pollution and health problems from sulfur oxides and nitrogen oxides are dealt with below,

      • and soot, particulates and carbon monoxide are dealt with in section 4A incomplete combustion.

      • Oxides of non-metals tend to be acidic and in any high temperature combustion situation non-metals are oxidised to the oxide.

      • Therefore any nitrogen (from air) or sulfur (in a fossil fuel) will undergo an oxidation reaction e.g. to give nitrogen monoxide and sulfur dioxide respectively.

      • When these oxides enter the atmosphere they get oxidised further and dissolve in rain clouds giving weakly acidic solutions.

      • Scientists around the world in developed countries are monitoring air pollution, but unless governments act and enforce regulation and encourage the use of cleaner green energy, simply measuring the concentration of air pollutants doesn't reduce their concentration!

ACID RAIN and dealing with sulfur dioxide

  • ACID RAIN (from sulfur oxides and nitrogen oxides):

    • Fossil fuels contain compounds of the element sulfur.

    • When the fuel is burned the sulfur compounds also burn to form sulfur dioxide, an oxidation process.

    • This is an acidic gas and dissolves in rainwater, it then reacts with water and oxygen to form a very dilute solution of sulfuric acid.

    • sulfur + oxygen ====> sulfur dioxide

      • S(in fuel molecules) + O2(g) ====> SO2(g)

      • Actually in oil, it isn't pure sulfur but compounds like CH3SH, so a more realistic equation for the formation of polluting sulfur dioxide is:

      • CH3SH  +  3O2  ===>  CO2  +  2H2O  +  SO2

    • Sulfur dioxide is a harmful gas and lung irritant, increasing respiratory problems and contributed to 5000 extra deaths in the great 'London Smog' in the 1950's as well as being a major acid-rain gas.

      • Sulfur dioxide reacts with oxygen (in air) and water (rain) and gets oxidised to form very dilute sulfuric acid - the origin of acid rain, and the overall change is represented by the equation below.

      • SO2(g-air) + O2(g-air) + 2H2O(l-rain) ====> 2H2SO4(aq-rain)

    • The formation of acid rain has several bad effects on the environment e.g.

      • the low pH of acid rain causes plant damage, inhibiting growth and killing some, particularly trees,

      • acid rain kills certain life forms and so damages eco cycles and food chains in rivers or lakes harming wildlife like trout,

      • increases the 'weathering' corrosion rates of building stone and statues, particularly those made of limestone in medieval times, the industrial revolution has devastated many of them!


    • Smoke from fossil fuel power stations can be treated to remove most of the acidic sulfur dioxide, but we do demand our electricity supply and its not always easy to balance environmental impact versus consumer demand.

    • All fuels are processed at the oil refinery to reduce the concentration of sulfur/sulfur compounds

      • The is known as the desulfurisation process, but this adds to the cost and not all the sulfur is removed. The sulfur removal process uses energy, so a bit more fossil is burned to remove the potential pollutant!

      • However, low sulfur content petrol and diesel fuels have been introduced.

      • Of course, the less fossil fuels we burn, the less pollution results, so strategies to design more efficient road vehicles, using less energy in the home, renewable energy resources etc. will all help in the long run to reduce pollution.

      • Power stations can be fitted acid gas scrubbers eg removing the acidic sulfur dioxide with an alkaline mixture of water mixed with powdered lime/limestone.

        • An alkaline slurry of calcium hydroxide (calcium oxide + water) is sprayed into the flue gases from the power station furnaces.

          • (A slurry is mix of a powdered solid dispersed in a liquid medium.)

        • In a neutralisation reaction, the sulfur dioxide reacts with the calcium hydroxide to make the neutral salt calcium sulfite - initially a waste product, but much of the acidic sulfur dioxide is removed, so less acid rain damage to the environment.

          • The calcium sulfite is harmful but is readily oxidised to harmless calcium sulfate which is actually a useful commercial product called 'gypsum' that can be crystallised out of the gas scrubbing system.

            • Gypsum is used to make plaster board for lining interior walls and ceilings.

            • Gypsum is calcium sulfate dihydrate CaSO4.2H2O

        • The process is called flue gas desulfurization (FGD).

        • Chemistry of the desulfurisation process:

          • Calcium oxide (CaO) is made by heating limestone (calcium carbonate, CaCO3)

            • CaCO3(s)  ===>  CaO(s)  +  CO2(g)

          • You can do a 'dry' scrubbing reaction between powdered calcium carbonate and sulfur dioxide gas to form calcium sulfite and displace carbon dioxide gas.

            • CaCO3(s)  +  SO2(g)  ===>  CaSO3(s)  +  CO2(g)

          • For 'wet' scrubbing, the calcium oxide is mixed with water to make the gas scrubbing solution - actually a slurry of calcium hydroxide.

            • CaO(s)  +  H2O(l)  ===>  Ca(OH)2(aq)

          • When the flue gases are passed through the solution, calcium sulfite is again formed.

            • calcium hydroxide  +  sulfur dioxide  ===> calcium sulfite +  water

            • Ca(OH)2(aq)  +  SO2(g)  ===>  CaSO3(s)  +  H2O(l)

          • The resulting calcium sulfite solution/slurry can be oxidised with oxygen from air to form calcium sulfate hydrate (gypsum).

            • calcium sulfite  +  oxygen  ===> calcium sulfate

            • CaSO3(aq/s)  +  1/2O2(g)  ==>  CaSO4(s) 

            • Calcium sulfate is not very soluble and crystallises out as CaSO4.2H2O(s)

              • This is sold for use in the building industry as the commercial product gypsum which can partly off-set the extra cost of the desulfurization process.

              • Gypsum can be used to make plasterboard for lining interior walls

            • To show the overall process of gypsum formation you can write the final equation as:

            • CaSO3(s)  +  1/2O2(g)  +  2H2O(l) ==>  CaSO4.2H2O(s)

OTHER POLLUTANTS including dealing with nitrogen oxides and particulates using catalytic converters

Carbon monoxide and carbon particulates were initially dealt with in section 4A,

but they are mentioned here again in the context of three way catalytic converters


    • High temperature combustion also produces other pollutants including ...

    • Nitrogen oxides collectively denoted by NOx:

      • NO is formed in car engines and changes to NO2, which is acidic with water, contributing further to acid rain (above), and are also involved in the chemistry of 'photochemical smog' - which produces chemicals harmful to respiration, irritating to eyes and lungs, causes headaches and tiredness and contributes to acid rain.

      • Many of the reactions are initiated by sunlight acting on the oxides of nitrogen and other chemicals in the air.

      • nitrogen monoxide is formed in high temperature combustion situations e.g. car engines, power station furnace burning coal, oil or natural gas (temperatures can be up to 1000+C in a car engine).

        • nitrogen + oxygen ====> nitrogen monoxide

          • N2(g) + O2(g) ====> 2NO(g) 

          • This reaction occurs in all internal combustion engines.

      • and in air the nitrogen monoxide rapidly combines with the oxygen in air

        • nitrogen monoxide + oxygen ====>  nitrogen dioxide (acidic gas)

          • 2NO(g) + O2(g) ====> 2NO2(g) 

          • Note: Nitrogen monoxide is also known as 'nitric oxide' or 'nitrogen(II) oxide',

          • and nitrogen dioxide is known as nitrogen(IV) oxide.

      • The nitrogen dioxide is oxidised to nitric acid by the reaction with oxygen from air when it dissolves in rainwater - contributing, with sulfur dioxide, to making 'acid rain'.

        • The overall process is summarised in the equation below.

        • 4NO2(g-air) + O2(g-air) + 2H2O(l-rain) ====> 4HNO3(aq-rain)

    • Carbon monoxide CO, which is toxic, and also involved in the chemistry of 'photochemical smog' (see 4A incomplete combustion).

      • This is formed by inefficient incomplete combustion of hydrocarbon fuels.

      • There are legal limits on emissions allowed from car exhaust systems and these are checked every year as part of the MOT test (at least in the UK, not sure on other countries).

    • Unburned hydrocarbons, CxHy, which can be carcinogenic and are also involved in photochemical smog chemistry.

      • Some of these hydrocarbons can pass through the engine without being oxidised.

      • Unburned hydrocarbons belong to a group of chemicals known as volatile organic compounds (VOCs).

      • In the atmosphere the VOCs react with nitrogen oxides (e.g. NO2) in sunlight to produce a photochemical smog haze - a very unpleasant and harmful concoction!

        • Photochemical smog contains many chemicals that can irritate the eyes, throat and lungs.

      • But catalytic converters can significantly reduced these three unwanted emissions of CO and NO, and CxHy (x and y are at least 6) gets oxidised to CO2 and H2O).

      • e.g. using platinum-rhodium transition metal catalysts, these are dispersed on ceramic bed to give a big surface area for the best reaction rate.

      • unburned hydrocarbons + oxygen == catalyst => carbon dioxide and water

        • C7H16(g)  + 11O2(g) ===>  7CO2(g)  +  8H2O(l)

      • and the removal of polluting nitrogen oxides by conversion to nitrogen ..

        • 2NO(g)  +  2CO(g)  == catalyst ==>  N2(g)  +  2CO2(g)

        • The catalyst works best at high temperature AND the catalyst is fabricated to give the largest possible surface area to give the maximum rate of reaction - typical rate of reaction factors controlling the speed of this catalysed reaction.

    • There are other indirect pollution problems to do with burning fossil fuels:

    • Nitrogen dioxide (NO2) is a lung and eye irritant, and, along with nitrogen monoxide, it is involved in the complex chemistry of photochemical smogs which can also produce ozone and other harmful chemicals in the air.

    • Lead compounds used to be added to petrol to improve engine performance.

      • This produces lead compound emissions into the environment.

      • Lead compounds are nerve toxins so it is fortunate leaded petrol is being phased out in most countries - in facts its now illegal to use it.

    • Photochemical smog was mentioned in the previous paragraph.

    • However, ultimately, the only way to reduce atmospheric pollution from fossil fuel burning, is to burn less of fossil fuels and develop other sources of energy to generate electricity and power road vehicles etc.

    • More on catalytic converters  (for advanced A level students)

      • A catalyst is a substance that increases the speed of a reaction by providing a pathway of lower activation energy compared to the uncatalysed reaction, but is unchanged at the end of the reaction.

      • In this case catalytic converters use metals like platinum, rhodium and palladium on the catalyst bed.

      • These catalyst beds are designed to maximise the surface area over which the car exhaust gases pass - this further increases the rate of the catalysed reactions i.e. the improves the efficiency of pollutant removal.

      • The catalytic converters are fitted into the exhaust system of a motor vehicle.

      • The inside of a catalytic converter is a honeycomb of a high temperature resistant ceramic material coated with thin layers of the metal catalyst.

      • The honeycomb ensures the gases flow over a large surface area to further speed up the reactions.

      • A lambda probe can measure the oxygen level in the exhaust gases.

      • These are called heterogeneous catalysts because the catalyst solid) is in a different phase or physical state than the reactants (gases).

    • A modern three-way catalytic converter involves three reactions - a reduction and two oxidation reactions.

      • The reduction, using a platinum rhodium catalyst, converts nitrogen monoxide to nitrogen and oxygen.

        • 2NO(g)  ===>  N2(g)  +  O2(g)

      • 1st oxidation reaction oxidises carbon monoxide to carbon dioxide using a platinum-palladium catalyst.

        • 2CO(g)  +  O2(g)  ===>  2CO2(g)

      • 2nd oxidation reaction oxidises unburned hydrocarbons to carbon dioxide and water, uses same Pt-Pd catalyst.

        • e.g.   C7H16(g)  + 11O2(g) ===>  7CO2(g)  +  8H2O(l)

      • Catalytic converters can convert at least 90% of the gases to less polluting and harmful gases.

    • The catalysts used in catalytic converters e.g. platinum, rhodium and palladium, are very expensive metals, but they have the advantage of not corroding easily.

      • Cheaper metals like nickel and copper are also good catalysts but are easily oxidised - poisoned, and lose their ability to catalyse the reactions.

      • The honeycombed structure of the catalytic converter can also be contaminated with particles sticking to the surface, again reducing the efficiency of the reactions.

      • For advanced A level chemistry students See Catalysis for more details on heterogeneous catalysts.

A little local acid rain project

For over 25 years. Tom Chadwick and Phil Brown have been monitoring the pH of almost all the rain that has ever fallen on the village of Castleton, set on a ridge, 500 ft up on the North York Moors in north-east England.


Equipment, method and measurements

We use a simple water collector to measure rainfall and a Jenway pH meter that reads to two decimal places.

Jenway is a good brand, but not cheap! Check out eBay! Jenway 370 pH meter

The pH meter is calibrated with pH 4 and pH 7 buffers, which cover the range of most pH values we measure.

The samples are stored for a few days and tested in batches each week.

You need to keep a check on the calibration of the pH meter and probe and instructions are supplied on how to calibrate and store the equipment.

Measuring the rainfall is an optional extra - the more data the better!


Comments on results so far

Generally speaking, there has been a steady decline in 'acid rain incidents' and the average pH has risen.

For electricity production, this would be expected as fossil fuel power stations are coming to the end of their working lives in the UK and coal is being replaced by natural gas, renewable energy - particularly wind turbines.

The figures for the UK in 2017 are: Natural gas 40%, coal 7%, renewables (wind, solar, hydroelectric) 30%, nuclear 21% and 2% from other sources. This is part of a good trend as we become less reliable on fossil fuels in the UK.

BUT, lately, as listed below, some quite acid rain has been falling on the North York Moors!

Therefore, as regards Europe (including the UK) acid rain is still an environmental pollution problem.



1. Wouldn't this make a good, and not too complicated, environment science project for schools?

All you need is:

a rainwater collector - with a wide funnel for low rainfall,

VERY clean plastic sample bottles to temporarily store the samples (do a batch every week),

a 'quality' pH meter and probe - preferably calibrated with pH 4 and pH 7 buffers

and somebody to check the wind direction from a TV/internet weather chart.

Schools could x-check their results with other schools and us.

2. Natural unpolluted rainwater has a pH of 5.65, slightly acidic due to the dissolving of the weakly acid gas carbon dioxide. 'Carbonic acid' ('H2CO3') is considered to be formed which ionises to a very small degree to give the acidic hydrogen ion (H+).

H2O(l)  +  CO2(g)  H2CO3(aq)    H+(aq)  + HCO3-(aq)

The equilibrium position is very much on the left-hand side and the weakly acidic equilibrium pH of 5.65 is the result of the current carbon dioxide level of just over 400 ppm. The pH is temperature dependent.

Increasing carbon dioxide levels in the atmosphere will slightly increase the acidity of seas and oceans and this will have ecological consequences.

Advanced A level chemistry notes:

The pH of a solution is defined a minus the logarithm to the base 10 of the hydrogen ion (H+) concentration in moles/dm3.

pH = -log10[H+(aq)], which translates as [H+(aq)] = 10-pH

The pH is slightly dependent on temperature.

Since the pH scale is based on logarithm base 10, each pH unit represents a factor of x 10 in the concentration of hydrogen ions.

This has implications for how you view the relative acidity of your water sample.

The background pH on unpolluted water is ~pH 5.6.

If an 'acid rain event' happens and the pH of the rain water is as low as pH 3.6, it means the rain is 10 x 10 = 100 times more acidic than unpolluted rain!

3. The optimum pH for most fish life and other aquatic creatures is 6.5 to 9.0.

Salmon prefer a water of pH 7.0 to 8.0. Acid rain falling into, and accumulating in lakes and rivers can have a devastating effect on fish populations.

Fish can adjust to pH values down 6 as long as there is no sudden change or fluctuation in the level of acidity.

Aquatic life becomes seriously affected if the pH of the water falls below pH 5.

Fish begin to die if the water pH falls below pH 4.

4. Acid rain has, still does, cause destruction to forests and corrosion of stone buildings.

5. -

Some recent 'acid rain events' recorded - most rain these days has a pH of 5 to 7

Date pH Wind direction Rainfall mm Comments
Nov 23rd/25th 3.47/3.84 E   Possible air mass from Eastern Europe? - coincidentally, on the BBC news was an item on the large quantities of coal being burned in Poland and other Eastern European countries - its an unfortunate important aspect of their economies!
Nov 26th/27th 4.12/4.15 NE   Origin ?
Dec 2nd 4.19 SSE   Possibly from UK fossil fuel plants in south of Castleton e.g. in Yorkshire and the Midlands?
Dec 6th/7th 3.84/4.13 SW 6.5/6.5 'acid rain' origin southern-midlands UK?
Dec 16th-19th 4.05-4.27 SE 5-17 mm 'acid rain' origin Continent?
Jan 13th 4.28 SW 3 mm 'acid rain' origin southern-midlands UK?
Jan 19th 4.43 SE 15 mm 'acid rain' origin Continent?
Jan 22nd 4.38 NW 4 mm origin of acid rain?
Jan 25th 4.07 S 4 mm fossil fuelled power stations midlands of England?
Oct 27th 4.00 NW    
Nov 1st 4.05 SE    
Nov 8th 3.78 N    
Nov 9th 4.00 SW    
Nov 11th 3.90 SW    
Nov 15th 4.00 N    
2020-2021       No acid rain events worth recording - a good sign in the UK
2022 pH Wind direction   Location: Castleton village, North Yorkshire Moors, England
Nov 8th

Nov 16th

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Sometimes from the E/SE we get polluted air mass from continental Europe - fossil fuel east European countries?

Not sure on the acid rain from the SW, now there are very few fossil fuels power stations in the UK.

The is a biomass power station 60 miles SSW of the village?

There are three active fossil fuel power stations in the UK

West Burton, Nottinghamshire, England, south of my location

Ratcliffe on Soar, Nottinghamshire, England, south of my location

Kilroof, County Antrim, NI, quite some distance west of my location

All three to close by the end of 2024

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14-16 school chemistry, air pollution from sulfur oxides and nitrogen oxides Urban Outfitters, American Outfitters, Target, Lulus, Hollister, Victoria's Secret, Adidas, Forever 21, Nike sports products, air pollution from sulfur oxides and nitrogen oxides, revision study notes for 14-16 school chemistry AQA Edexcel OCR IGCSE/GCSE 9-1 chemistry science topics modules for studying the effects of air pollution from fossil fuel burning producing acidic nitrogen oxides sulfur oxides, what causes acid rain, health issues from oxides of nitrogen and oxides of sulfur, methods of scrubbing flue gases from fossil fuel burning to remove sulfur dioxide and reducing acid rain and corrosion of stone and metal,  how does a three way catalytic converter work to reduce air pollution from motor vehicles

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