• Marnix Hamelberg

Air quality during the corona crisis: follow-up

In the previous blog post we assessed the air quality trends in Amsterdam. It showed a drop in nitrogen dioxide (NO2) during 2020. However, it remains unclear whether the lower NO2 was caused by the corona crisis or increased wind speeds. Expanding our analysis to more sensors in the Netherlands using additional methods could shed some light on this. A total of 64 air quality sensors can be seen in the map below.

Air quality deviations

Let us start with the smoothed NO2 measurements for 10 air quality sensors as can be seen in the graph below. It shows the NO2 during 2020 (solid line) compared to the average NO2 of the last 6 years (dashed line). Again, we can confirm the drop in NO2.

Assessing the Netherlands

But how significant the drop in NO2? We can quantify this by dividing the 2020 values by the historic averages. This gives us the percentage difference. Converting the data to a fraction normalizes the NO2 deviations for each air quality sensor. We do this for all 64 sensors located in the Netherlands. The results can be seen in the heatmap below. Each cell represents the average percentage difference per month (horizontal axis) at each location (vertical axis).

Most cells in the above heatmap are blue, meaning that the majority of locations and months had lower than average NO2 concentrations during the first half of 2020. The measurements at the white cells do not deviate as much, and the red cells have a higher than average NO2. The blue tones are especially dark in the month March, which makes sense as here the corona crisis peaked. Furthermore, looking at the sensor locations, you may suggest that the difference in air quality is more pronounced in urban regions.

Historic deviations

So 2020 was a beneficial year for our longs (unironically). But how does this year compare to previous years? The same method that generated the above heatmap is used for these years, which can be seen below. Now the difference becomes even clearer, where 2020 is in the deep blues. There seems to be an overall downward trend in NO2, which is good to know. However, 2017 is an exception, showing quite a peak in January and February.

Factors at play

In the previous blog post we saw unusually slow wind speeds during early 2017. This may have contributed to the higher NO2 values during this period since there is an inverse correlation. The wind speeds were exceptionally high early 2020, suggesting that wind may be the cause of the healthier air instead of the corona crisis. But we do not leave it to guess work and settle this once and for all by the simple graph below. The blue bars are the average NO2 concentrations for each month and year, and the red line the average wind speeds for the same time intervals. You can see during 2017 in January and February lower wind speeds and higher NO2 values. The inverse is true for 2020 with the addition of March. However, the following months all the way up to Juli still see lower NO2 concentrations in 2020 whilst the wind speed has stabilized. This means that the wind speed was not the only factor in the improved air quality.

Human influence?

Finally, one more check to see if the improved air quality may have had an anthropogenic origin is by comparing NO2 to the amount of cars/trucks/etc on our roads. In the graph below the road traffic really starts to drop from March and up during 2020. The NO2 had already dropped before March because of the higher wind speeds. But when the wind speed stabilizes, the NO2 stays below average together with the road traffic. Could this mean that road traffic has a large impact on the NO2?

Looking at the years before 2020, the relationship between road traffic and NO2 concentrations seem to dissipate. The NO2 has a downward trend while the road traffic stays the same. So normal road traffic behavior may not directly influence air quality, but it is definitely a reflection of how our industry, society and economy operates. The historic drop in road traffic can potentially be an objective measurement of other human factors that contribute to our air quality.

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