the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Chemical characterization and source apportionment of fine particulate matter in Eastern Africa using aerosol mass spectrometry
Abstract. Ambient air pollution poses a significant threat to public health, particularly in low and middle-income countries, where detailed data on particulate matter (PM) mass and composition are scarce. We conducted a year-long study on PM composition and sources in Eastern Africa (Kigali, Rwanda). The annual mean concentration of PM1 was 31 μg/m3, with slightly higher concentrations during the dry season. Organic aerosols (OA) contributed 73 % of the observed PM1 mass, black carbon (BC) 16 %, nitrate 6 %, sulfate and ammonium 2 % each, and chlorine 1 %. BC is approximately 60 % due to fossil fuel and 40 % from biomass burning emissions. Tracer ions detected by the mass spectrometer suggest that photochemistry plays a significant role in the formation of secondary OA during the daytime (6:00 am to 6:00 pm), while primary OA dominates in the morning and evening due to increased anthropogenic activity and shallower boundary layer height. PM1 in Kigali is primarily composed of Oxygenated Organic Aerosols (OOA, 45 %), Hydrocarbon-like OA (HOA, 32 %), and Biomass Burning OA (BBOA, 23 %). Secondary organic aerosol (SOA) accounted for 47 % and 41 % of PM1 during the wet and dry seasons, respectively, while primary OA (POA: BBOA + HOA) contributed 53 % and 59 %. This suggests that seasonal changes in PM1 mass in Kigali are primarily driven by deposition rather than shifts in emissions, chemical processing, or source strengths.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1700', Anonymous Referee #1, 14 Jun 2025
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AC1: 'Response to reviewers(1 and 2) comments', Theobard Habineza, 12 Sep 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1700/egusphere-2025-1700-AC1-supplement.pdf
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AC1: 'Response to reviewers(1 and 2) comments', Theobard Habineza, 12 Sep 2025
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RC2: 'Comment on egusphere-2025-1700', Anonymous Referee #2, 17 Jun 2025
Review of Habineza et al
The authors present aerosol chemical composition data in Kigali Rwanda using a Q-ACSM and a BC aethalometer over a 12 month period. This is an impressive dataset in a critically understudied location. The scientific results are robust, though the writing, formatting, and proofreading could use some improvement. I recommend publication subject to what are largely minor revisions.
My only major comment is that the paper aims to be representative of all of East Africa; however, then spends a lot of time focusing in specifically on one location in Kigali, and attributing results to local behaviors in Kigali. East Africa is a large, heterogenous place, with larger cities than Kigali. The authors should spend some effort justifying why they think these results are representative of broader East Africa. A few sentences to a paragraph should be sufficient.
Minor comments:
There are a number of typos throughout the manuscript, be sure to proofread carefully.
Abstract: The abstract ends kind of abruptly after a detailed result about seasonality. Consider adding some kind of broader concluding statement. I also don’t quite follow how the pretty small seasonal variation in the SOA/POA split (47/53 wet and 59/41 dry) suggests that (wet) deposition is driving PM mass changes? The differences between dry and wet aren’t very large. Maybe this is clarified further in the paper but the abstract should be internally consistent as well.
Intro: There appear to be some citation issues (“n.d.” for no date after a date is given).
There is also some repetition in the intro. For instance, line 65 says “There is limited information on PM composition and sources in East Africa.” But line 48-49 already makes this point.
Methods: the total PM2.5 mass data being 10 km away from the ACSM and BC sounds like a significant uncertainty. Especially in a heterogeneous city like many growing cities in Africa. How does this impact the comparisons? Also, clarify in the text if the Rwanda Meto station is at a third location and how far that is from everything else?
Does the aethalometer AE-33 have a 1 µm cut point PM cyclone as well? If the NR-PM1 is simply added to the BC from the AE-33 without a PM1 cyclone, the assumption would be that all BC is 1 µm or less. Not necessarily a bad assumption, but it should be cited/documented if so.
Results: Line 217 and several other parts of section 3.2 mentions a PBL height impact. Is this a result? If so, provide the data that supports it. If it is a general assumption add a citation.
Line 245: Regarding the discussion of SO2 or sulfate sources. What about vehicle fuel? Any diesel being used in Kigali?
Line 385. Related to the final line of the abstract, I think the reasoning for why the lack of seasonal variation implies that rain (wet deposition) is driving composition changes. As written it is a bit unclear.
Citation: https://doi.org/10.5194/egusphere-2025-1700-RC2 -
AC1: 'Response to reviewers(1 and 2) comments', Theobard Habineza, 12 Sep 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1700/egusphere-2025-1700-AC1-supplement.pdf
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AC1: 'Response to reviewers(1 and 2) comments', Theobard Habineza, 12 Sep 2025
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Review of “Chemical characterization and source apportionment of fine particulate matter in Eastern Africa using aerosol mass spectrometry” by Habineza et al., doi:10.5194/egusphere-2025-1700.
This manuscript reports the chemical composition of particles in the city of Kigali, Rwanda, over the course of a year (April 2023 - May 2024). In addition to data from aerosol mass spectrometry (AMS) (mentioned in the title), black carbon (BC) from an aethalometer and sub-2.5 micron in diameter particulate matter (PM2.5) mass measurements are reported here. The main data analysis includes comparing seasonal variations in composition and variations of species including AMS tracer ions as a function of time of day. The AMS mass spectra were further processed by positive matrix factorization and BC data were attributed to fossil fuel or biomass burning using differences in absorption at two different wavelengths. These processed data were also compared as a function of time of day to determine source contributions to the particulate matter found in this city.
Kigali is the capital of Rwanda with a high population density and located only 2 degrees south of the equator at ~1500 m elevation. The measurements reported here appear to be the first detailed PM composition data for this city and represent a small but growing body of PM data for other locations in Africa.
Readers will find this manuscript useful and informative for understanding the air quality issues in this part of the world. I recommend publication in ACP after addressing the concern/questions noted below.
Main Concerns/Questions:
Minor Concerns/Questions: