ICOS Science Conference 2022: Day 3, Session 22

1. Operation of the ICOS-Cities urban CO2 sensor network in
Zürich, Switzerland
ICOS Science Conference 2022
Stuart K. Grange1,2,*
, Simone Baffelli1
, Christoph Hueglin1
, Pascal Rubli1
, Andrea Fischer1
, and
Lukas Emmenegger1
Presented on September 15, 2022, Utrecht, the Netherlands
1Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland
2Wolfson Atmospheric Chemistry Laboratories, University of York, York, YO10 5DD, United Kingdom
∗stuart.grange@empa.ch

2. Introduction
• Urban carbon dioxide (CO2) emission inventories can be verified by sensor
networks
• The use of low-cost sensors can offer advantages compared to traditional
instrumentation, but a key component of the use of low-cost sensors in
atmospheric monitoring is the treatment and processing of their data1
• Here, the ICOS-Cities Zürich’s urban CO2 sensor network and data processing
pipeline will be presented
• The key objective of Zürich’s urban CO2 sensor network (and data) is to provide
near-real-time, accessible, CO2 observations for emission inventory validation
1
Peltier et al., 2021, WMO- No. 1215, https://bit.ly/3BfxHXm
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 1

3. Zürich’s ICOS-Cities sensor network
• Zürich’s ICOS-Cities CO2 sensor network is formed of 83 monitoring sites
• There are three tiers of sensor/monitoring instrumentation that are currently
active
• Low-cost sensors: Sixty locations where 32 sites have co-located sensors and 28
sites have single sensors – generally at street-level
• Mid-cost sensors: Twenty locations, generally on rooftops away from immediate
emission sources – installed with two reference gas cylinders for daily tests
• CO2 gas analysers: Reference instruments in and outside of Zürich city
• Extra measurements at the Hardau II field site: A high rise building equipped
with a mast for eddy covariance sampling and other monitoring activities in parallel
ICOS projects
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 2

4. 0 2.4 4.8km
Sensor type Gas analyser & low−cost Mid−cost Low−cost Low−cost & mid−cost Extra measurements
Map tiles by Stamen Design (stamen.com), CC BY 3.0. http://creativecommons.org/licenses/by/3.0
Map data © OpenStreetMap contributors, ODbL. http://www.openstreetmap.org/copyright
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 3

5. Photos of a low-cost CO2 sensor (left) and mid-cost CO2 sensor with its two reference gas cylinders (right).
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 4

6. The nearest high-rise building with the mast is the Hardau II field site (Hard, Zürich).
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 5

7. Low-cost sensor characterisation before field deployment
• Before the low-cost sensors were installed in their field locations, they were tested
and their performance characterised with the use of a climate chamber
• Within the climate chamber, the sensors were exposed to different set-points of
CO2 concentrations, ambient temperatures, and ambient humidities
• After the climate chamber characterisation, the low-cost sensors were co-located
with an analyser for two weeks at the Dübendorf-Empa monitoring site
• The field co-location was also done for the mid-cost sensors
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 6

8. Air temp. (° C)
RH (%)
CO2 (ppm)
Apr 25 May 02 May 09
1000
2000
3000
4000
5000
6000
25
50
75
10
20
30
Date
Sensor
response
(various
units)
An example of a sensor’s response while in a climate chamber – CO2 concentrations, air temperature, and
humidity was varied.
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 7

9. Sensor field calibrations
• The different sensors types have different field calibration strategies
• The mid-cost sensors are installed in situ with reference gas cylinders (≈ 400 and
600 ppm) for daily checks
• It is impractical for this approach to be used for the low-cost sensors
• Data-driven field calibration methods are applied based on the network’s
behaviour, for example drift corrections are applied based on network percentiles1
• Currently, 53 % of low-cost monitoring sites have paired low-cost sensors (this
proportion will increase too) and comparisons can be made between the sensors to
help identify anomalous sensor behaviour
1
Delaria et al., 2021, Atmospheric Measurement Techniques, https://doi.org/10.5194/amt-14-5487-2021
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 8

10. Mid−cost sensor ‘436‘ at Wildbachstrasse
Calibration A Calibration B
Jul 15 Aug 01 Aug 15 Sep 01 Jul 15 Aug 01 Aug 15 Sep 01
565
570
367
369
371
373
Date
CO
2
concentration
(ppm)
Dashed lines show mean, lower, and upper control limits
Control chart of reference gas checks for a mid-cost sensor at one monitoring site. Note the different scales on
the y-axes.
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 9

11. Manesseplatz Schule Nordstrasse
Jul 15 Aug 01 Aug 15 Sep 01 Jul 15 Aug 01 Aug 15 Sep 01
150
200
250
170
175
180
Date
Difference
in
daily
CO
2
concentration
between
paired
sensors
(ppm)
Dashed lines show mean, lower, and upper control limits
Control chart of the daily deltas between reported daily means for two sites where paired low-cost sensors are
active. Note the different scales on the y-axes.
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 10

12. Mon Tue Wed Thu Fri Sat Sun
0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20
425
450
475
Hour of day
Mean
CO
2
(ppm)
Site Breite substation Hardau II
410
420
430
440
450
0 5 10 15 20
Hour of day
Mean
CO
2
(ppm)
420
425
430
435
Mon Tue Wed Thu Fri Sat Sun
Weekday
Mean
CO
2
(ppm)
420
425
430
Jan Feb Mar Apr May Jun Jul Aug Sep Oct NovDec
Month
Mean
CO
2
(ppm)
Mean CO2 concentrations at different aggregation periods at for two mid-cost sensors at two contrasting
locations. Shaded zones are 95 % CIs.
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 11

13. Zürich’s ICOS-Cities data processing pipeline
• All observations are harvested and inserted into an ICOS-Cities database from
various APIs and data sources
• The progressive transformations of the CO2 concentrations are named “levels”
and are fully transparent to data users
• The key objective of the data pipeline is to provide near-real-time, accessible, CO2
observations from the ICOS-Cities network for emission inventory validation
• The functionality discussed has been implemented in a code base that has been
developed for easy distribution and reuse
• The knowledge gained from the Zürich sensor network will be of help for our
project partners in Paris and Munich where sister sensor networks will be deployed
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 12

14. Low-cost sensors
Data processing and
sensor calibration
processes
Mid-cost sensors Analysers
Others (options
for the future)
Publicly accessible
code package
Observations:
reported by sensor
Observations:
calibrated
Observations:
calibrated & adjusted
Publicly accessible data
Validation of emission
inventories
Community data
analysis activities
Our analysis activities
A simplified schematic of the Zürich’s ICOS-Cities CO2 sensor network’s data processing pipeline.
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15. Conclusions and outlook
• Careful and transparent data processing is required when using data from
atmospheric sensor networks, especially when employing low-cost sensors
• ICOS-Cities CO2 monitoring network in Zürich city has been carefully designed
with different sensor types to allow for robust sensor calibrations to deliver
near-real-time CO2 concentrations for emission inventory validation
• The different data “levels” resulting from the data processing pipeline are
transparent and the code base is portable for use elsewhere
• Our immediate group look forward to helping our colleagues internally for
inventory validation and the deployment of similar sensor networks in Paris and
Munich
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 14

16. The presentation slides can be found here.
Grange et al., 2022. Operation of the ICOS-Cities urban CO2 sensor network in Zürich, Switzerland 15