The paper of Mijling et al. presents ground-based measurements of NO2 in a poorly sampled region in Africa with a low cost but reliable technique. The study is well conducted, the dataset and findings are relevant for the research as well as for the local community, and the paper is pleasant to read, so I highly recommend publication after the authors have considered the remarks below.  

My only major comment on the content is the strong statement in the abstract 'TROPOMI underestimates the tropospheric NO2 column densities by a factor of roughly 2.5...'. The parametrization method to reach this 2.5 number is clearly described and seems reasonable, but there s no validation of it. It seems to me that this could have been done with MaxDOAS at European sites. Did the authors consider doing that? I would simply add something like 'Using a parametrization method based on our ground-based measurements and TROPOMI' at the beginning of the sentence to improve this point. The formulation on the conclusion reads good on the other hand.    

Beside that, I have mainly minor comments. 

L.61: 'To the best of our knowledge, there are currently no operational AQMs openly reporting NO2 measurements in Sub-Saharan Africa (OpenAQ, 2025).'

What about South-Africa?  

https://saaqis.environment.gov.za/

In the intro, the authors could add that Palmes tube were used in citizen science projects in Europe, for instance in Antwerp with the curieuzeneuzen project, the data were used in scientific publication e.g.

D. Voordeckers, F.J.R. Meysman, P. Billen, T. Tytgat, M. Van Acker,

The impact of street canyon morphology and traffic volume on NO2 values in the street canyons of Antwerp,

Building and Environment, Volume 197, 2021, 107825, ISSN 0360-1323,

https://doi.org/10.1016/j.buildenv.2021.107825.

Also, the validation of TROPOMI in Kinshasa from Yombo et al. could be added as there are not many of such works in this region

Yombo Phaka, R., Merlaud, A., Pinardi, G., Friedrich, M. M., Van Roozendael, M., Müller, J.-F., Stavrakou, T., De Smedt, I., Hendrick, F., Dimitropoulou, E., Bopili Mbotia Lepiba, R., Phuku Phuati, E., Djibi, B. L., Jacobs, L., Fayt, C., Mbungu Tsumbu, J.-P., and Mahieu, E.: Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of NO2 and H2CO at Kinshasa and comparisons with TROPOMI observations, Atmos. Meas. Tech., 16, 5029–5050, https://doi.org/10.5194/amt-16-5029-2023, 2023.

On all the maps of Kumasi, I would add the North direction.  

L.91 and 94 Nitrate -> Nitrite? 

L.189 'true column density' -> I would remove 'true', reads tautological. 

Why is it 'campaign' in L.217 and 'campaigns' in L. 218, and again without s in L 237? 

L.239 'Mounting height is approximately 2 meters (see Fig. 4).'-> already stated above, L.228

L 296 'contribute less significantly to ambient NO2 levels in these areas' -> less than what? 

Fig 9: unclear what the map on the right adds. The roads could be added to the column map and the right map deleted. 

L.338 'Elevated NO2 levels in upwind regions are not readily apparent in surface measurements, although they

might be detectable with a more extend ground-based monitoring network that includes more urban background stations' -> I dont follow, did you mean 'downwind'? 

L. 381  'coarse a priori NO2 profiles from TM5-MP'-> what about the realism of the profile, even at low spatial resolution in Africa? Again it would be good to refer to Yombo et al here since they studied the profile effect. 

L 453 'we apply a factor of 0.69 is applied' -> I suggest the active voice 

 

The publication describes the setup of a low-cost measurement network of NO2 passive samplers in the city of Kumasi, Ghana. Air pollution is a growing concern in African cities, but observations are extremely scarce, almost non-existing. Even if this study is limited to one constituent (NO₂), it makes an important contribution to improve our knowledge of air pollution levels in this under-sampled part of the world. The study is also a great example of knowledge transfer, capacity building, and engagement with local scientists.

Since low-cost air quality sensors continue to suffer from issues such as drifts and cross-sensitivity, passive sampling is a valid alternative. It doesn't rely on expensive instruments but nevertheless requires a reasonably well-equipped laboratory and trained staff. The study demonstrates that operating such a network is feasible with the limited resources available at a University in Africa.

The manuscript is overall very well written and the results are presented in a thorough and relevant manner. The analysis of duplo samples and the comparisons between remote and local lab indicate that the measurements are of reasonably good quality. The comparisons with TROPOMI satellite observations and the analysis of surface-to-column ratios are interesting and relevant.

I thus have only a few minor points.

Where will the local laboratory obtain the chemical reagents and solutions (HCl, TEA, NEDA, sulphanilamide, NaNO2) in the future? Are these materials all available in Ghana? Please comment.

While a simple design of tube holders is clearly a necessity, the current design doesn't seem ideal as it provides no protection against rain, radiation and wind. It is common practice to mount diffusion tubes with some sort of shelter. Although this makes the design slightly more complicated, it should probably be feasible and certainly recommended. This comment links to another comment below regarding potential measurement biases, which depend on the way the tubes are exposed.

I am also a bit worried about tube handling: The time differences between preparation and deployment and between deployment and analysis of the tubes was a few weeks in case of the remote-lab tubes. I was wondering where the tubes were stored during this time, e.g. whether they were put into a fridge, which is recommended e.g. in the practical guidelines for the UK " Diffusion Tubes for Ambient NO 2 Monitoring: Practical Guidance" (/https://uk-air.defra.gov.uk/reports/cat05/0802141004_NO2_WG_PracticalGuidance_Issue1a.pdf). They should be stored in a fridge or at least in a cool, dark place both pre-exposure and post-exposure to avoid degradation of the solvent. I understand that some of this can be accounted for by measuring blanks, but it is nevertheless recommended.

Comparison against an accredited lab is valuable but insufficient to guarantee a high quality of the measurements, because both the remote-lab and local-lab batches can be affected by the same biases caused by the way the tubes are exposed. An overview of potential sources of biases was given in Heal et al. (2019; https://www.mdpi.com/2073-4433/10/7/357). Another study on sources of biases is Vardoulakis et al. (2009; https://www.sciencedirect.com/science/article/pii/S1352231009001757). One important source of bias is ambient wind flow at the entrance of the tubes. In other tube designs, e.g. as described in a report for a Swiss network (https://www.ostluft.ch/fileadmin/ostluft/pdf/projekte/2021/BE_ReferenzbezugNO2-Passivsammler_GeK_20210527.pdf), there is therefore an additional wind protection element. Furthermore, remaining systematic differences to reference instruments were corrected for by a linear calibration function.

I fully acknowledge that there was no reference instrument available, but I recommend considering this option for the future. The main point I would like to make is that the comparison with the reference lab does not fully account for all uncertainties, which needs to be acknowledged in the manuscript.

It is stated several times that a longer exposure period of 4 weeks would improve precision. However, it should also be considered that a longer exposure might enhance biases, e.g. due to degradation of the solution.

Further small points:

• Line 43: NOx are not only precursors of ozone but also of PM.
• Equation 1: The factors p_ref/p x T/T_ref seem to convert to standard pressure and temperature. It should thus be stated explicitly that C is the average concentration during the measurement at standard pressure and temperature.
• Line 196: Averaging kernels presented in Figure 11 are much larger than one.
• Line 228: Mounting at 2 m altitude doesn't seem to be a good idea as the tubes can easily be reached by anyone passing the location. In other networks a typical altitude is 3-4 m above ground to prevent damage by vandalism.
• Line 241: Why only at 10 of the sites? Please comment.
• Line 479: As argued above, comparisons of duplo samples do not capture the full uncertainty. I would rather talk of precision here.
• Line 500: The average difference of 2 ug m-3 may not be significant. What is the standard error of this average?