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Smog problems in Paris

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  • Location: The Netherlands
  • Location: The Netherlands

    Smog in Paris


    During the last few days, a few cities in France have been suffering from high levels of smog. The problems are the highest in Paris, where unhealthy levels of air pollution are being reached. Therefore, the government has decided to give free access to public transport and introduced alternative driving days [i.e. half of the cars (even numbers on the license plate) are allowed to drive on one day, while the other half of the cars (odd numbers on the licence plate) are allowed to drive on the day afterward].

    From the BBC:




    Paris restricts car use after pollution hits high

    Alternative driving days are being introduced in the French capital, Paris, in an attempt to tackle dangerous levels of air pollution.

    From Monday, drivers will only be able to use their vehicles every other day.


    The full article can be found below:




    Putting the values in perspective


    Yesterday, critical levels of PM 2.5 were exceeded in Paris. PM 2.5 is a measure for air pollution, defining the concentration of aerosols with a diameter smaller than 2.5 micrometre. The maximum level of PM 2.5 reached yesterday was 193 micrograms per cubic meter in the centre of Paris. When the concentration of PM 2.5 exceeds 150 micrograms per cubic meter, conditions could become dangerous for human health.


    The classification index for PM 2.5 can be seen below:




    The PM 2.5 values in Paris (various stations), as well as other air quality variables, can be found in the link below:




    Of note is that the high levels of PM 2.5 reached in Paris are very common in China. Many cities there rarely have values of PM 2.5 below 100 micrograms per cubic meter. This doesn't indicate, though, that values like this should be common in Europe as well.


    The PM 2.5 values of different cities in China can be seen below:




    Meteorological causes


    The source of the persistent smog in France is persistent high pressure over Western Europe. Due to this high pressure, there is very little flow over France. This persistent lack of flow causes most of the air pollution that develops over Paris simply stays at the same place. For example, check the GFS chart of the wind forecast of today 12Z below:


    Posted Image

    GFS 10m average wind (12Z run)


    What can be seen is that the average wind near Paris during the day is around 10 kt (20 km/h). This is not enough to clean the atmosphere around Paris. 


    A more important cause of the air quality problem might be the stable boundary layer which is caused by the persistent high pressure. Due to the nocturnal cooling near the surface (combined with very high temperatures at about 1000 meter height), a strong inversion (increase of temperature with height) becomes established. Because of this inversion, there is very little mixing of air (convection) in the boundary layer during the night. As a result, the pollution stays near the surface.


    During daytime, it takes a very long time for the inversion to be cleared (sometimes it lasts the whole day), and therefore, the air pollution has very little time to be mixed out of the surface. 


    The inversion strength during daytime can be seen in the Skew-T diagram of La Trappes (close to Paris) below:


    Posted Image

    Sounding of La Trappes as of 12Z (12 UTC, 13:00 local time).


    To keep things simple, only focus on the temperature line (the righter most fat black line) at the lowest levels of the atmosphere. What can be seen is that the temperature increases with height at around 800 meters. The increase in temperature is about 2*C (read the temperature by tracking various points at the temperature line by the blue skewed lines of equal temperature). Though this is not a very strong inversion, it is rather unusual that such an inversion is still apparent during the afternoon in Spring. This also indicates the inversion must have been much stronger at peak strength.


    The forecast from GFS indicates that flow might be increasing in about 5 days when a trough digs southward just to the west of France. Hopefully this will mark the end of the bad air quality conditions in France. 


    Finally, any remarks/additions on the post are always welcome!




    Edited by Vorticity0123
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