the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Sep 2025
ACROPOLIS: Munich Urban CO2 Sensor Network
Abstract. Urban areas are major contributors to anthropogenic CO2 emissions, yet detailed monitoring remains a challenge due to the cost and operational constraints of traditional sensor networks. As a scalable alternative, we established the ACROPOLIS (Autonomous and Calibrated Rooftop Observatory for MetroPOLItan Sensing) network in the Munich metropolitan area, using mid-cost sensors to enable dense, city-scale observation. This work outlines the development of the hardware and software of the system, its performance and the first year of operation, during which more than 70 million CO2 measurements were collected in urban, suburban and rural environments.
The primary goal was to evaluate whether mid-cost Vaisala GMP343 sensors, when combined with manufacturer internal corrections and environmental stabilization, can reliably measure CO2 concentrations with sufficient accuracy to resolve urban gradients. We implemented a fully automated 2-point calibration procedure using synthetic dry reference gases and conducted a multi-week side-by-side comparison with a high-precision Picarro reference instrument to assess sensor performance.
Our results show that, despite inter-sensor variability in temperature sensitivity, the hourly aggregated mean root mean square error (RMSE) of all sensors is 1.16 ppm with a range of 0.57 to 2.58 ppm. For the specific sensor housed in our second-generation enclosure with PID-controlled heating, the performance improved from 0.9 to 0.6 ppm RMSE. Analysis of spatial and temporal patterns reveal distinct seasonal cycles, urban–rural concentration gradients, and nighttime accumulation events, consistent with expected biogenic and anthropogenic activity, and atmospheric transport mechanisms.
We conclude that mid-cost urban networks can provide scientifically valuable, spatially highly resolved greenhouse gas observations when supported by appropriate calibration and stabilization techniques. The open-source design and demonstrated performance of the ACROPOLIS network establish a blueprint for future deployments in other cities seeking to advance emissions monitoring and urban climate policy.
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Status: open (extended)
- RC1: 'Comment on egusphere-2025-4157', Anonymous Referee #1, 26 Sep 2025 reply
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RC2: 'Comment on egusphere-2025-4157', Anonymous Referee #2, 18 Oct 2025
reply
This paper reports the initial results of establishing a CO₂ observation network using the low-cost NDIR-based Vaisala GMP343 sensors at 17 sites across the city of Munich.
The study focuses on two main aspects: (1) how the accuracy of the sensors used in the network was improved, and (2) how the system was applied to capture high-resolution spatial and temporal CO₂ variability within the city.
For the first aspect—sensor accuracy—the study compares the sensitivity of the Vaisala GMP343 sensors to three environmental variables (humidity, pressure, and temperature) against a Picarro reference instrument. Among these variables, temperature had the greatest impact on the NDIR sensors. In the second-generation network, an additional temperature stabilization enclosure was introduced to address this issue. As a result, the RMSE decreased from a maximum of 2.6 ppm in the first-generation system to less than 1 ppm in the second generation, achieving the target accuracy. In summary, the study aimed to enhance NDIR sensor accuracy primarily by controlling the temperature factor.
For the second aspect, the monitoring sites were categorized into three zones—urban, suburban, and rural—to examine spatial variability in CO₂ concentrations. At one specific site (MAIR), a Hampel filter was applied to remove the influence of nearby ventilation outlets. Although the filter effectively removed some peaks, it was not entirely successful in eliminating all local pollution signals. The study found that the classified zones showed clear diurnal and seasonal differences: during summer, rural and suburban sites exhibited greater diurnal variability than urban sites due to photosynthetic activity. This pattern persisted in winter, though the diurnal amplitude was considerably smaller.
Overall, the study is well conducted, but several areas require revision or clarification before publication. Please refer to the comments below:
(Page 6, Line 123)
The paper states that the intake line was extended up to 50 m, with a flow rate of about 0.5 LPM. Is this flow rate sufficient for such a long sampling line? Please provide a proper justification or reference.(Page 9, Line 215)
Calibration was performed only at two points—400 ppm and 520 ppm—for slope/intercept correction. Can linearity across a wide and long-term concentration range (350–600 ppm) be ensured with only two calibration points? Since actual CO₂ levels in different urban zones may fall outside this range, would additional multi-point calibration or slope tracking be necessary?(Page 9, Line 203)
The use of the Wagner equation to calculate water vapor saturation pressure for deriving dry mole fractions seems appropriate. However, since the water vapor data came from an external instrument, that instrument itself likely has some uncertainty. Would this not affect the accuracy of the dry CO₂ mole fraction? Please discuss this potential limitation.(Page 10, Line 230)
Using a long analysis window may risk classifying short-term traffic plume signals as “outliers.” However, such short-term and abrupt fluctuations are key features of urban CO₂ dynamics. Applying too long a window could remove meaningful short-term events as noise. Please provide additional justification or discussion on this issue.(Page 21, Figure 10, Lines 416–427)
To control excessive local pollution, the study applied the Hampel filter used in previous studies. While this method effectively removes extremely high peaks, it does not fully eliminate local contamination. The paper notes that the filter captured the ventilation effects but did not perform particularly well. Moreover, since this station is used as a background site, placing the sensor so close to a ventilation outlet seems questionable. Please provide further explanation or justification for this site configuration.Citation: https://doi.org/10.5194/egusphere-2025-4157-RC2
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Overall this is very good work and consistent with previous findings cited here, that lower cost trace gas sensors have utility if properly calibrated and corrected, but each sensor must be independently corrected as they can vary in their usefulness. I think my only scientific comment (and I do not think it is required to be addressed for publication) is that PBL height was mentioned in passing in Section 3.7.2 e.g. "generally shallower boundary layers". If it's not too much trouble, and if the data exists, it may be good to show some PBL height observations from an urban and rural location nearby to strengthen this argument. It could be purely vegetation based in terms of the diurnal variation, but could it also be due to urban/rural PBL variations too? Just something to consider, overall very nice work!