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SPASCIA Introduction
Analysis of NO2 from S5P TROPOMI for monitoring
pollution level at city scale
SPASCIA
Pascal Prunet, Olivier Lezeaux, Claude Camy-Peyret, Hervé Thevenon
Work supported by PHIDIAS
Work supported by AERIS/ICARE and CNES

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SPASCIA Project objectives1-SLIDE SUMMARY
➢ From ESA S5P measurements of the air pollutant NO2 from space, we have assessed the impact of
the human activity reduction on air pollution by comparing the first trimester periods of 2019 and
2020 on a daily, weekly and monthly basis for 4 major cities in Europe
➢ Reductions in the pollution level (using NO2 as a proxy) have been observed at the end of March
2020 (20-50% for Paris; 30-70% for Milan region; 30-60% for Madrid; 40-60% for the last week for
Athens), as compared to the same period in 2019
PARIS
2019
15-31
March
averages
MILAN MADRID ATHENS
2020
15-31
March
averages

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SPASCIA Project objectivesOBJECTIVE AND DATA
➢ We have assessed the impact of the human activity reduction on air
pollution from ESA S5P TROPOMI high spatial resolution measurements of
the pollutant NO2
by comparing the first trimester periods of 2019 and 2020 on a daily, weekly
and monthly basis (whenever possible),
for 4 major cities in Europe i.e. Paris, Madrid, Athens and Milan (within the Po
valley area)
➢ We use the TROPOMI NO2 tropospheric column Level 2 product distributed
by the European Space Agency (ESA)
downloaded from the Copernicus open data access hub
(https://s5phub.copernicus.eu) by the ICARE/AERIS French data centre
(ftp.icare.univ-lille1.fr)
imported and arranged at SPASCIA in a database (Postgresql 11.6, PostGIS 3.0 3)
We generated data images centered on 4 cities and 2 periods of 3 months: from
January to March 2019 and 2020
S5P L2 NO2 pixels are filtered each day : quality assurance value higher than 0.5,
cloud fraction below 40%

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SPASCIA Project objectivesDEDICATED PROCESSING AT CITY SCALE
➢ Dedicated processing of daily samples at city scale
Consider filtered pixels within a disk of 240 km diameter (Paris, Athens), 300 km
diameter (Madrid), or 500 km diameter (Milan)
Identify “valid” pixels (Figure 1a) : filtered pixels plus the spatial interpolation
of the missing pixels where and when it is possible : if the position of this
missing pixel is surrounded by at least 3 neighbouring filtered pixels
Select daily datasets only when valid pixels are higher than 80%
Total NO2 mass of the plume : integrating only the valid pixels higher than a
background value (Figure 1b)
Uncertainty : difference between the total NO2 mass and that for smaller disk
(75% of the nominal size) (Figure 1c)
1a : interpolated (valid) pixels 1b : valid pixels above background for
NO2 plume mass
1c : smaller pixel for uncertainty

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SPASCIA Project objectivesRESULTS : PARIS
➢ Examples of daily maps of NO2 tropospheric columns over Paris area for the
end of March 2019 (top) and same days for the March 2020 (bottom)
Only 33% of valid days on the January-March periods (frequent cloudy data)
19/03/2019
19/03/2020
26/03/2019
26/03/2020
30/03/2019
30/03/2020
31/03/2019
31/03/2020

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SPASCIA Project objectivesRESULTS : PARIS
Qualitative comparison of AirParif estimates with S5P NO2 daily maps for 19 March
➢ Daily maps suggest pollution level reductions consistent with independent
estimates. But weakly/monthly averages are required for reliable results
AirParif NO2 pollution quantification from 17th to 19th March in normal situation and confined situation
S5P data of NO2 tropospheric column over Paris region, 19th March 2019 (left) and 19th March 2020 (right)

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SPASCIA Project objectivesRESULTS : PARIS
➢ 15-day average and weekly average time series of total NO2 plume mass from
January to March 2019 and 2020
Lack of valid data for building reliable averaged quantities in some periods
Large temporal variability of uncertainty bars in January and February, possibly due
to the impact of local meteorology and atmospheric transport when computing
averages based on small numbers of daily values
Robust averages available for mid-March periods
Dates of 15-day periods from 01 January
to 31 March
Bi-weekly (15-day) averages of NO2 plume mass Weekly (7-day) averages of NO2 plume mass
Dates of 7-day periods from 01 January to
31 March

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SPASCIA Project objectivesRESULTS : PARIS
➢ Monthly averaged maps of tropospheric NO2 column for 2019 and 2020
Comparisons for January suffer from reduced number of valid data in 2019
Comparisons for March appear more reliable
January February March
2019
2020

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SPASCIA Project objectivesRESULTS : PARIS
➢ Robust results for second half of March show evidence of NO2 level reduction
from 2019 to 2020
Comparison of 15-31 March averaged maps of NO2 tropospheric column
A 20-50% reduction of the NO2 pollution level at the end of March 2020 is observed,
as compared to the same period in 2019
Average on 15-31 March 2019
8 daily values
Average on 15-31 March 2020
10 daily values

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SPASCIA Project objectivesRESULTS : MILAN AND PO VALLEY
➢ Examples of daily maps of NO2 tropospheric columns over Milan and Po valley
region for the end of March 2019 (top) and 2020 (bottom)
50-70% of valid days on the January-March periods allow to derive reliable averages
16/03/2019
16/03/2020
21/03/2019
21/03/2020
24/03/2019
24/03/2020
28/03/2019
28/03/2020

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SPASCIA Project objectivesRESULTS : MILAN AND PO VALLEY
➢ 15-day average and weekly average time series of the total NO2 plume mass
from January to March 2019 and 2020
Temporal variability in year-to-year differences from January to early March
Trends of decreasing NO2 levels from early February to end of March observed both
in 2019 and 2020
NO2 level reduction from 2019 to 2020 starting from mid-March
Dates of 15-day periods from 01 January
to 31 March
Bi-weekly (15-day) averages of NO2 plume mass Weekly (7-day) averages of NO2 plume mass
Dates of 7-day periods from 01 January to
31 March

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SPASCIA Project objectivesRESULTS : MILAN AND PO VALLEY
➢ Monthly averaged maps of tropospheric NO2 column for 2019 and 2020
Year-to-year comparisons are reliable for the 3 months and show a NO2 pollution
level reduction starting in March 2020 as compared to 2019
January February March
2019
2020

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SPASCIA Project objectivesRESULTS : MILAN AND PO VALLEY
➢ Robust results for second half of March showing evidence of a significant NO2
level reduction from 2019 to 2020
A reduction of NO2 pollution level of 30 % over the second half of March 2020, as
compared to the same period in 2019, is observed from these data
The average data for the three last weeks of March suggests an increasing reduction
with time, from about 20% (12-18/03 values) to about 65% (19-25/03 and 26-31/03
values)
Average on 15-31 March 2019
15 daily values
Average on 15-31 March 2020
9 daily values

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SPASCIA Project objectivesRESULTS : MADRID
➢ 15-day average and weekly average time series of total NO2 plume mass from
January to March 2019 and 2020
Trends of decreasing NO2 levels from early February to end of March observed both
for 2019 and 2020
NO2 level reduction from 2019 to 2020 from mid-March
A large uncertainty in some case (e.g. 19-25 March 2020) is due to the small number
of valid days for this period
Dates of 15-day periods from 01 January
to 31 March
Bi-weekly (15-day) averages of NO2 plume mass Weekly (7-day) averages of NO2 plume mass
Dates of 7-day periods from 01 January to
31 March

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SPASCIA Project objectivesRESULTS : MADRID
➢ Monthly averaged maps of tropospheric NO2 column (linear scale) for 2019
and 2020
Year-to-year comparisons are reliable for the 3 months : NO2 pollution level
reduction in March 2020 as compared to 2019 is clearly observed
January February March
2019
2020

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SPASCIA Project objectivesRESULTS : MADRID
➢ Robust results for second half of March show evidence of NO2 level reduction
from 2019 to 2020
A reduction of NO2 pollution level of 10-30 % over the second half of March 2020, as
compared to the same period in 2019, is observed from these data
The average data for the last week of March suggests an even larger reduction of
about 40-60 % (26-31/03 values)
Average on 15-31 March 2019
15 daily values
Average on 15-31 March 2020
5 daily values

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SPASCIA Project objectivesRESULTS : ATHENS
➢ 15-day average and weekly average time series of total NO2 plume mass from
January to March 2019 and 2020
No evidence of noticeable NO2 level reduction from 2019 to 2020 in March from 15-
day averages
The NO2 mass reduction from 2019 to 2020 appears significant for the last week of
March
Dates of 15-day periods from 01 January
to 31 March
Bi-weekly (15-day) averages of NO2 plume mass Weekly (7-day) averages of NO2 plume mass
Dates of 7-day periods from 01 January to
31 March

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SPASCIA Project objectivesRESULTS : ATHENS
➢ Monthly averaged maps of tropospheric NO2 column (linear scale) for 2019
and 2020
Year-to-year comparisons are reliable for the 3 months : No evidence of NO2
pollution level reduction in March 2020 as compared to 2019
January February March
2019
2020

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SPASCIA Project objectivesRESULTS : ATHENS
➢ Robust results for last week of March suggest NO2 level reduction from 2019 to
2020
The NO2 mass reduction from 2019 to 2020 appears significant for the last week of
March, suggesting a beginning of the reduction of about 40-60 % (26-31/03 values) of
NO2 levels for this period in 2020 as compared to 2019
The number of valid days available over the Athens area for the last two weeks of
March 2019 and 2020 allows to be reasonably confident in this estimate
Average on 23-31 March 2019
7 daily values
Average on 23-31 March 2020
8 daily values

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SPASCIA Project objectivesSUMMARY
➢ From ESA S5P measurements of the air pollutant NO2 from space, we have assessed the impact of
the human activity reduction on air pollution by comparing the first trimester periods of 2019 and
2020 on a daily, weekly and monthly basis for 4 major cities in Europe
➢ Reductions in the pollution level (using NO2 as a proxy) have been observed at the end of March
2020 (20-50% for Paris; 30-70% for Milan region; 30-60% for Madrid; 40-60% for the last week for
Athens), as compared to the same period in 2019
PARIS
2019
15-31
March
averages
MILAN MADRID ATHENS
2020
15-31
March
averages

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SPASCIA Project objectivesContact
Thank you for your attention
SPASCIA contact:
pascal.prunet@spascia.fr
Tel : +33 (0)6 75 55 10 01; +33 (0)5 34 32 03 91
SPASCIA – Parc Technologique du canal – 10 Avenue de l’Europe
31520 Ramonville – France