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Oslo – less cars but bad air

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Dry weather, blowing dust and buses still running make the PM2.5 values soar in Oslo, Norway

Like a third of the world population, the citizens of Oslo are under corona virus lockdown. This means less traffic and less industrial activity, so what explains the high values of particulate matter the latest days?

PM2.5 values Bygdøy allé, Oslo. Values in μg/m3. Black dashed lines show average values before and after lockdown. The red line marks the lockdown date in Oslo (13 March). Source: OpenAQ.

In the figure above, we noticed unusual high PM2.5 values for Oslo at the end of March and decided to investigate possible explanations.

NO2 values in the same period are actually reduced, as satellite images from ESA can tell. Ground level measurements confirm the reduction, see the figure below.

NO2 levels Bygdøy allé, Oslo. Values in μg/m3. Black dashed lines show average values before and after lockdown. The red line marks the lockdown date in Oslo (13 March). Source: OpenAQ.

Traffic has gone down – but the buses and trucks remain

Bygdøy allé, weekly traffic. Source: Statens vegvesen

The total number of cars on the roads is reduced by 35% after the lockdown. However, the reduction is mainly due to less circulation of personal cars. The buses and trucks operate almost as before the travel restrictions.

The heavy vehicles contribute significantly to poor air quality, both NO2 and particulate matter. Less cars on the roads probably also leads to higher speed, and in turn more air pollution.

Windy, dusty streets and dry weather

Oslo has enjoyed a sunny spring weather the last weeks, with very little rain or snow. The winds, however, have been quite strong. The PM2.5 values peak between 24 and 28 March, as we see there is a strong wind and no precipitation these days.

Oslo precipitation March 2020
Bygdøy allé, Oslo, Norway. Precipitation March 2020 in mm. The red line marks the lockdown date in Oslo (13 March). Source: Met.no
Oslo wind speed March 2020
Bygdøy allé, Oslo, Norway. Wind speed in m/s. The red line marks the lockdown date in Oslo (13 March). Source: Met.no

The dry air combined with strong winds mean that dust and small particles are whirled up. Roads in Oslo have not yet been cleaned after a Nordic winter’s use of sand, salt and gravel and studded tyres. There is plenty of small particles to be caught by the wind and little humidity to weigh them down. Combine with the buses and trucks still running and we get our high particulate matter values.

More on lockdown weather

In our recent article we saw quite another pattern in quarantined Wuhan, China. Here, the PM2.5 level dropped by almost 40% after quarantine measures were enforced. We have not analysed traffic patterns or weather conditions in Wuhan, but can at least say that the Oslo quarantine is less strict than the Wuhan version. People are still allowed to go outside, and enjoy the sunny but somewhat polluted spring air.

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