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I live in Madrid, where like in most big cities there is a high level of air pollution, especially due to the traffic. The levels of pollution reached the maximum allowable level a couple of months ago and some measures were taken to decrease them. The measures were to decrease the maximum speed in the main roads to 70 km/h and to ban parking in the city centre for non-residents.

The measures made effect and also the rain has helped to maintain a lower level of air pollution during the last month but the situations seems likely to happen again. These measures taken seemed to me that would only alleviate the situation temporarily.

So I was wondering, what measures can be taken in such a big city to permanently decrease the levels of air pollution?

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    Don't forget the 'incoming' pollution from e.g. upwind industry, airfields etc. – Jan Doggen Jan 26 '16 at 13:24
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    Reduce the population of the city. This is accomplished automatically, if pollution levels are high enough and inward migration is stopped. If you prefer a kinder, gentler approach, give people incentives to go live elsewhere, and to reduce their birth rates. – jamesqf Jan 28 '16 at 20:47
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    You basically ask how can we pollute less without reducing the traffic. Why not just attack the traffic? The urban sprawl is the major pollution/resource depletion contributor. Dense city with public transport is 10x more energy/resorce, land and labour efficient. Why do you insist that you need to conserve resources while increasing the village lifestyle in the city? – Valentin Tihomirov Jan 28 '16 at 23:09
  • The question is related to traffic. Hence, I post this as a comment. Heating can be an important contributor to urban air pollution (particularly in winter). The Athens and other Greek cities, the number of wood stoves (and people heating with it) considerably increased in the recent years, which led to a strong increase of particulate air pollution. – daniel.neumann Aug 2 '16 at 12:58
  • It is important to distinguish air pollution by NOx and O3, on the one hand, and air pollution by PM2.5 and PM10 (particulate matter), on the other hand. Some measures reduce the emissions of one pollutant and increase emissions of another. Also reducing NO emissions (thus, reduced NOx) might yield enhanced O3 concentrations (because NO + O3 <=> NO2; NO + NO2 = NOx). Fuel-saving driving behavior reduces CO2 emissions but the combustion processes might be incomplete yielding enhanced PM and VOC emissions. – daniel.neumann Aug 2 '16 at 13:03
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To encourage people to not use of purchase cars develop a public transportation system that is easily accessible by the to public and caters to the needs of the public so that people will use it. Additionally, the energy source for the public transport system must be electricity. Anything that is combustible, even bio fuels, will maintain a certain level of atmospheric pollution, including soot. This may require the re-establishment of former technologies rarely used now, such over head trolley wires for trams and trolley buses.

As an interim measure convert all cars to use gas, such as LPG or LCG, instead of petrol or diesel.

Also, encourage people to ride bicycles instead of using cars by constructing dedicated cycle paths that can handle large volumes of bicycles.

Should battery powered cars and trucks be as reliable and useful as carbon fuelled vehicles, then ban the use the of carbon fuelled vehicles.

The other thing to do is to ban the use of wood or coal for cooking or heating, if they are used.

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    Make sure public transport is cheaper than parking. Also make sure it runs for 24 hours and is clean and safe. Establish Park and Ride stations on every major road into the city. – RedSonja Feb 1 '16 at 9:52
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    Enforce that burners in heating installations etc are up to spec. – mart Feb 1 '16 at 16:12
  • Just please make sure your ban on wood is only in the city. :) – Maxfield Solar Mar 13 '16 at 0:07
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Plant a lot of trees -- assuming you mean partciulate pollution: See for example Estimating the removal of atmospheric particulate pollution by the urban tree canopy of London, under current and future environments, Tallis et al. 2011

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    Stop planing the trees and migrate into vertical building. Dense infrastructure needs orders of magnitude less fuel (because of shorter distances and use of effective public transport, which transports people, not their vehicles). The trees will grow themselves in the abandoned suburbs. – Valentin Tihomirov Feb 15 '16 at 11:24
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    It considerably depends on the type of trees whether they actually improve the air quality in cities. – daniel.neumann Aug 2 '16 at 12:59
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This brochure of the European Environmental Bureau gives 12 good tips to reduce air pollution in a city. Here a summary and some extra information I added:

  1. Create Low Emission Zones (LEZ): charge or prohibit vehicles that do not comply with certain emission requirements. This is already done in for example London, and Berlin. In Madrid they propose to do this starting in 2017, and in Barcelona in 2018.
  2. Encourage cycling and walking: make them safe and convenient, by making for for example bike lanes, and bike parking places like for example in The Netherlands.
  3. Land use planning: when planning land usage, take into account the air pollution, by for example reducing committing distances, and encouraging cycling, and walking.
  4. Congestion charges: charge more in peak hours, like for example in London, and Stockholm.
  5. Lower (urban) speed limits: lowering the urban speed limit to 30 km/h reduces air pollution a bit, and especially increments safety. This article indicates that speed reduction from 100/120 km/h to 80 km/h especially has influence on the entrance ways of a city.
  6. Restrict access to cars: restrict access to areas with low air quality.
  7. Parking management: limit parking spaces in the city to encourage use of other (public) transport. This has been done in Madrid as a temporal solution. Also parking fees can be asked, or fees depending on the car's pollution.
  8. Public procurement: put pollution requirements on the vehicles bought by for example city. Copenhagen and Berlin have a procurement to buy municipal vehicles that comply with the emission limitation standards.
  9. Car sharing: share the same car between different people. Also carpooling reduces the amount of cars used.
  10. Improving public transport: increase the attractiveness of public transport, by availability, accessibility, reliability, pricing, safety, and comfort
  11. Park and ride: relieve the city center by letting people park outside the city, but this requires a good public transport connection.
  12. Promote electro mobility: promote low-emission vehicles, for example by replacing conventional cars by electrical cars. However the city should have enough charging points.

And then, you can also try to reduce the air pollution more indirectly by promoting things like: recycling, don't/reduce the use of woodstoves, plant trees, use green energy, wash clothes with cold water, limit the use of heaters and A/Cs, etc. Also the way of driving influences pollution: keep your car in shape, accelerate smoothly, minimize air conditioning, etc.

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  • Most of these, particularly 1,4,5,6,7&12 are very negative - they penalise people who are forced to travel in the city but who often cannot afford to just go out and buy a new eco-car. Far better to reduce the need to travel in the first place. – Nick C Feb 25 at 10:41
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One key point not mentioned in the other answers is to reduce the need for traffic - if half the population of the city could work from home just one day a week, you'd have 10% fewer people having to travel in the rush hour - make it two days and you've reduced the demands on your city's transport systems by a fifth...

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What could work:

  1. Ban diesels unless they have sophisticated emission treatment devices
    • Yes, this includes even buses and heavy trucks: only allow the most sophisticated emission treatment devices.
  2. Ban studded tires
  3. Ban gasoline cars without three-way catalytic converter
    • Ok, this is kind of irrelevant today as there are so little of these.
  4. Clean the streets often from dust.
  5. Construct more rail transport.
  6. Ban burning firewood. Ban also burning waste.
  7. If district heating is needed due to cold climate, prefer nuclear and geothermal sources instead of burning wood, coal, peat, oil or natural gas. Actually, geothermal heat pump could in this day and age be an option better than district heating.
  8. Require catalytic converters even on the smallest of motorcycles, unless they are electric
  9. Ban small engines like gasoline-powered lawnmowers.
  10. Ban all two-stroke engines without direct injection.
  11. Control forest fires as well as feasible.
  12. If thermal power plants are needed, prefer nuclear, and build the non-nuclear ones far away from population if such non-nuclear plants are needed.

What not to do:

  1. Blindly assume public transportation must be always good. In reality, a gasoline car with three-way catalytic converter will probably have lower emissions per passenger than a heavy bus. On the other hand, electric rail transport has excellent low emission levels.
  2. Place so heavy restrictions or fees on parking, car use, etc. that people and jobs disappear from the city and move to suburbs. A very good public transport system could prevent people from disappearing to suburbs, but such a system could need state subsidies.
  3. Construct so much renewable electricity without adjustable power that the grid becomes unstable, resulting in blackouts often, causing people to buy small gasoline powered generators.
  4. Attempt to control idling of car engines. Yes, it wastes some fuel, and creates some extra emissions, but the amount of emissions that comes from idling is trivial. Seriously, if this is your only measure, you won't achieve nearly any emission reductions.
  5. Construct bio-energy-burning power plants. The massive amount of trucks hauling wood from forests around the city will create lots of emissions, and also, while bioenergy could be claimed to be CO2 neutral in a very long term, it still has large non-CO2 emissions.
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