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Context

March 2020, the coronavirus spreads around the world like a wildfire. The outbreaks of contamination are increasing, there are hundreds of thousands of recognized cases, the global economy is stopped spatially and temporally, humankind is frozen.

However, in many places in the world, nature has returned and is taking back possession of its property. The population no longer goes out to work, so the cars remain in the garages but above all, the planes are firmly landed on Earth (80% of the flights are canceled in Milan on 03/14 1).


This image has an empty alt attribute; its file name is airTraffic_milanAirport-1024x344.png

Fig.1 – Daily variation in percent of the flights maintained in Milan airport

Fig.2 – Comparative pictures of the number of flights over Northern Italy on 12 January and 12 March 2020

Everyone hears that the air is breathable again in China and that the air quality is visibly improving, or that fish can be seen in the canals of Venice 2 where birds are landing a water more clear than it has been for many years.

Simple flu virus or signal from our planet Earth? What is certain is that COVID-19 and its proliferation have a seemingly consequential impact on pollution through the collective measures that the states enforce in their cities and countries.

At Murmuration-SAS, we use satellite measurements of NO2 to correlate the pollution and the coronavirus advance, to be followed in this article.


NO2 as a reflection of human pollution

As you may have guessed, the proxy we’re using for this study is the level of NO2 in the air. A proxy is a variable which is not significant in itself, but which replaces a useful but unobservable or unmeasurable variable (here, the pollution). This concentration allows us to gauge pollution and its evolution in European hot spots, capitals or large industrial zones.What are the sources of NO2, the major pollutant in the troposphere? Mainly, when located in the troposphere, NO2 comes from engine combustion and therefore from means of transport. Airplanes, cars, ships and coal-fired power plants all play a major role in the accumulation of this gas in our air.

As you may have guessed, the proxy we’re using for this study is the level of NO2 in the air. A proxy is a variable which is not significant in itself, but which replaces a useful but unobservable or unmeasurable variable (here, the pollution). This concentration allows us to gauge pollution and its evolution in European hot spots, capitals or large industrial zones.

What are the sources of NO2, the major pollutant in the troposphere? Mainly, when located in the troposphere, NO2 comes from engine combustion and therefore from means of transport. Airplanes, cars, ships and coal-fired power plants all play a major role in the accumulation of this gas in our air.

Our friends the satellites

Copernicus is the Earth observation program of the European European Space Agency. It is an ambitious and unique program that looks at our planet and its environment for the benefit of all European citizens and the wider international community. It offers information services based on satellite Earth observation and in situ (non-space) data (more information here).

Copernicus has enabled the launch of a group of satellites called “the sentinels”. Among these observers, it is the 5P sentinel that will interest us here. 

The 5P Sentinel was put into orbit on October 13, 2017 by the European Space Agency to measure air pollution. Its onboard sensor is called Tropomi (TROPOspheric Monitoring Instrument), it is this layer of the atmosphere that interests us.

The data we are working on are accessible from several sources, but we have opted for access on Google Earth Engine 3 where there are two dataset from S5P:

  • NTRI: For Near Real Time, that’s the one we use.
  • OFFL : For “Offline”.

The data is pre-processed to remove any value with quality assurance (QA) lower than 50. This last operation facilitates the calculations afterward and the display of the results.

All this for what results?

Like said previously, we used Google Earth Engine to do our calculations. They offer a code editor 4 freely available on the web that allows access to the data described above.  Then, in pseudo-Javascript, it is possible to use these data to display measurements on a world map or to calculate particular values in a predefined area. 

The first case study for us was to visualize the concentration of NO2 around the different contamination sites by means of a color code. We have chosen economically strong cities or regions to observe the environmental impact of the measures enforced by different governments. Thus, we chose to take two equal in length image-acquisition periods that could highlight our hypotheses. 

  • First period: from October to December 2019.
  • Second period: from January to March 2020.

(later we chose two more recent periods of only a week to highlight the fast, short term reactions of the states)

Here is an example of the results of this first analysis:

NO2 concentration in the industrial area of Northern Germany

NO2 concentration around Milan

The results are very clear. The decreasing number of flights and the closing of a multitude of factories has a very positive impact on air quality in the areas concerned. We have images showing the same phenomenon in many places in Europe.

The second analysis concerns the detection of a percentage decrease in concentration in these areas. Thanks to Google Earth Engine, we can draw shapes (in the example below, a polygon) and calculate statistical values within these shapes.

         Fig.4 – Example of a drawn area in Earth Engine

We were, therefore, able to access values such as the average concentration in particular areas and thus, once again, make a “before and after”. It was therefore easy to obtain percentages of decrease in NO2 concentration in the homes of interest to us.

And so there you have it: a graph of these percentages in the 4 largest European outbreaks of contamination by comparing two pairs of different periods.

Fig.8 – NO2 evolution in cities with important contamination outbreaks

This graph shows a fairly significant reduction which coincides with the policies of total or partial confinement enforced by the respective governments. The orders of magnitude show a clear decrease of pollution in line with the significant decrease in human activity. 

The current crisis makes concrete and quantitative the necessary efforts to sustainably reduce the negative impact of human activity on the environment. 

Sources

  1. : https://www.eurocontrol.int/Economics/DailyTrafficVariation-Airports.html
  2. : https://www.google.com/search=eauVeniseCoronavirus
  3. : https://earthengine.google.com/

: https://code.earthengine.google.com/

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