the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Nov 2025
Sources, concentrations, and seasonal variations of VOC and aerosol particles in downtown Munich in 2023/24
Abstract. Only little is known about molecular composition and sources of air pollution in Germanys third largest city, Munich. Therefore, we investigated sources, concentrations, and seasonal variations of volatile organic compounds (VOC), semi-volatile organic aerosol (SVOA), and organic aerosol (OA) in an urban street canyon in Munich utilizing online mass spectrometry and positive matrix factorization (PMF). Organic aerosol concentrations were higher in summer (4.3 ± 2.9 µg m-3) than late winter (3.3 ± 1.7 µg m-3) due to enhanced photochemical reactions, while nitrate exhibited the opposite trend with elevated concentrations in winter (4.5 ± 3.2 µg m-3) compared to summer (0.3 ± 0.2 µg m-3). During summer heat, photochemistry generates low-volatile oxygenated OA (33 ± 20 %), while aged biomass burning organic aerosol (BBOA) (25 ± 21 %) from barbecue activities and biogenic OA (22 ± 14 %) from nocturnal monoterpene chemistry further shape aerosol composition. The colder seasons are characterized by combustion-derived aerosols (Winter: fresh BBOA 13 ± 9 %, aged 36 ± 12 %; Spring: fresh 27 ± 17 %, aged 37 ± 19 %), whose dynamics are driven mainly by anthropogenic activity patterns. Traffic contributed at this urban kerbside surprisingly little to aerosol mass (5–9 %) but more to VOC (22–35 %). Our findings point to efficient ways to improve air quality e.g. by reducing monoterpene emissions by urban vegetation management as well as reducing biomass burning including barbecue emissions, a major source of aerosol particles and precursor gases of secondary organic aerosol throughout the seasons.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.-
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
- RC1: 'Comment on egusphere-2025-5191', Anonymous Referee #1, 22 Dec 2025
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RC2: 'Comment on egusphere-2025-5191', Anonymous Referee #2, 10 Feb 2026
General comments
Li et al. present results from measurements of organic compounds in the gas- and particle phase in a street canyon in downtown Munich, Germany, in late winter and summer. Positive matrix factorization is applied to mass spectra from PTR-MS, CHARON-PTR-TOF-MS and HR-TOF-AMS individually to find factors and identify sources of VOCs, SVOA, and OA. Based on this analysis, the authors find photochemical oxidation of biogenic emissions and biomass burning to be relevant sources for organic compounds in general, and traffic to be more relevant for VOC than OA concentrations.
Overall, this is a well written manuscript with interesting results, of relevance for air quality considerations for an urban area like Munich. As always with PMF studies, the choice of input matrices, as well as number of factors, requires detailed documentation to ensure minimum subjectivity, and this study is no exception. I have added comments where I think additional information is needed to justify certain choices made by the authors. I have several comments and questions regarding the interpretation of the BB-related factors, especially in summer. I wonder about the relevance of BBQ activities. Are there additional observations to corroborate its relevance?
I would also like to caution the authors on the summary of the paper (and this is now a potentially subjective note from this reviewer). Whereas I understand the conclusion, based on their results, that urban monoterpene emissions should be reduced, this in my opinion sends a potentially incorrect message to policy makers. The difference between winter and summer OA is within uncertainty, whereas e.g. for nitrate it is not, and PM concentrations overall are higher in winter (e.g. lines 279-284). Toxic aerosol components from traffic-related emissions such as tire and break wear, black carbon, etc. are not considered in this study. A message that urban vegetation should be controlled based on these results may potentially be misleading and could only be made after consideration of the entirety of PM2.5 or PM10 mass in Munich.
Overall, I am in favor of eventual publication of this manuscript once comments have been addressed.
Specific comments
L. 101-105: Please clarify what data/measurements/instruments were used for this comparison. Was it the Fidas? Did you get closure with AMS+BC measurements?
L. 114-116: Please revise wording, the two measurement periods in March are clearly not during “distinct seasons”. It would be good to clearly show with data how the separation between late winter and spring was determined. The separation into late winter and spring is not evident for me from Figure 1 (where I assume the horizontal dotted line indicates the separation, however this is not mentioned in the figure caption).
L. 148: Would you not expect a diel cycle of gas-phase background based on concentrations and temperature/RH? Please elaborate. Were the weekly background checks done during different times of the day? How variable was the background? Please add more information to the supplement.
L. 223-226: Please state clearly that PMF was run separately for the three different datasets and seasons (at least this is how I understand it). Please motivate this choice and motivate why e.g. PMF was not run on the entire combined dataset, or for one instrument and all seasons. Refer to diagnostics etc. in the supplement where needed.
L. 275: Suggest toning this down, as the number of particles <100 nm is not a good indicator for NPF.
L. 318: Please specify which ions were excluded (especially the large signal ones)
L. 350-355: What is a potential activity where biomass is being burnt in summer in Munich? Please clarify. Is there an option of wildfire influence? The time series shows a clear increase in late August
L. 362-365: Why would this not be a source in summer?
L. 371-373: Would cooking and outdoor grilling really be an activity in winter? Would this not rather be from residual heating? How do the two biomass burning factors for winter and summer compare in detail? Why is BBOA so high in late winter?
L. 399: Did you mean 171 SVOC compounds?
L. 429 -430: Do you see a difference between weekends and weekdays? I have a hard time imagining that this could be a relevant source during weekdays. Also, do you see compounds related to cooking? Would people not rather use coal for BBQing? Also here, this factor increases in late August – contribution from a wildfire episode?
L. 453-458: If it was cooking, should this factor not be around year-round? Oleic acid and palmitic acid can also be from the users handling the instrument, has this been investigated?
L. 500: In SVOC PMF, cooking was only identified in winter. Please elaborate.
L. 553-555: See previous comments on summer BBOA
Chapter 3.3: I am still not convinced the summer BBOA is not from wildfires, as in summer 2023 there were many wildfires in Europe. Please consider this as a source again.
L. 644: do you see influence from coal burning emissions?
Technical comments
L. 44: Put manufacturer in brackets after instrument name
L. 45: Commonly PM1. Specifically mention if PM2.5 lens was used.
L. 48: More correct: “PMF of AMS datasets” cannot specify sources
L. 56-57: What is meant by “qualitatively and quantitatively”? Please clarify.
L. 58 -59: How does CHARON-PTR-MS minimize thermal decomposition and ionization-induced fragmentation, compared to what technique? Please specify.
L. 63: Should read “in downtown Karlsruhe”
L. 70-72: Sentence structure needs revision
L. 96: Air pollution is very general here, I assume you mean organic aerosol
L. 175: How come you give as range for lens transmission 70-500 nm when you have a PM2.5 lens? Please clarify.
L. 198: Right bracket missing
L. 242: If “distinct seasons” are kept above, I would refer here rather to “three campaigns”
Citation: https://doi.org/10.5194/egusphere-2025-5191-RC2 -
AC1: 'Comment on egusphere-2025-5191', Yanxia Li, 10 Mar 2026
Dear Editor and Reviewers,
Please find attached our final author response to all referee comments (AC). We thank the reviewers for their constructive and helpful comments, which have helped us improve the manuscript.
Detailed responses to all comments are provided in the attached PDF. In addition, following the reviewers’ suggestions, we have included supplementary figures in the Supplement to further support our revisions.
On behalf of all co-authors.
Best regards,
Yanxia Li
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-5191', Anonymous Referee #1, 22 Dec 2025
This manuscript provides a clear and well-organized synopsis of the study, moving coherently from the motivation and methodology to the key seasonal contrasts, source apportionment results, and broader implications. The application of online mass spectrometry coupled with PMF is well suited to disentangling sources and seasonal variability of VOCs and OA in a street-canyon setting. The central findings—enhanced summertime OA consistent with photochemical production versus elevated wintertime nitrate, alongside distinct contributions from combustion, biogenic influences, and traffic—are communicated effectively and supported by quantitative evidence. The manuscript aligns well with the journal's scope, and I recommend its acceptance following minor revisions.
- Lines 18–32: Please correct phrasing such as “Only little is known…” → “Little is known…”. Also consider softening causal wording (“due to”, “from”) where attribution is inferred, and remove subjective wording such as “surprisingly little” (e.g., “relatively small”).
- Lines 24–32: Statements linking aged BBOA “from barbecue activities” and mitigation recommendations (urban vegetation management; BBQ emissions) would read more robustly if phrased as “attributed to / consistent with / likely influenced by…”, unless the manuscript later provides strong diagnostic evidence in the main text/SI.
- Lines 223–240: The PMF input preparation and software are described, but the manuscript would benefit from a concise summary of how the final factor numbers were selected and how rotational ambiguity was evaluated (e.g., Q/Qexp behavior, residual structure, FPEAK exploration, bootstrap/DISP if available). If these diagnostics are already in the SI, please cite them clearly in the main text.
- Figure 1 caption + Lines252 onward: BLH is taken from ERA5. Please add a brief caveat that ERA5 BLH may not fully represent street-canyon ventilation, and clarify how you expect this limitation to affect the interpretation of local NO₂ and mixing (especially in winter stable conditions).
- Lines257–265: The explanation combines heating emissions, reduced photolysis, shallow BLH, and then argues that higher summer BLH increases spatial differences in mean NO2 while improving temporal correlation via synchronized photochemical cycles. This is plausible, but currently reads somewhat qualitative. Please consider adding one small quantitative check (e.g., NO–NO2–O3 diurnal relationships, wind-sector stratification, or a correlation/regression between BLH and inter-site NO2 differences/correlation).
- Lines 273-284：Is the attribution of elevated ultrafine particle fractions to new particle formation (NPF) adequately demonstrated?The high summertime fraction of ultrafine particles (<100 nm) is interpreted as evidence of enhanced NPF under intense photochemical conditions, but size distributions alone may also reflect primary ultrafine emissions (e.g., traffic/combustion). Please clarify whether classic NPF events were identified (e.g., emergence of a new mode, growth rates, condensation sink, event frequency). If available, include supporting information on relevant gaseous precursors (e.g., SO2, proxies for H2SO4, low-volatility organic products/monoterpene oxidation chemistry, ion/cluster measurements). Alternatively, analyses stratified by weekday/weekend, wind sector, or traffic intensity indicators could help separate primary emissions from NPF and strengthen the seasonal interpretation.
- Lines495–512: You state that five OA factors are the “optimal interpretable solutions.” Please briefly summarize (main text or SI reference) what alternative solutions were tested and what criteria rejected them (e.g., splitting/merging behavior, tracer correlations, residuals).
- Lines585–594: The correlations with BBQ charcoal combustion tracers are compelling. Please add one sentence addressing tracer specificity (BBQ/charcoal vs other combustion) and whether time patterns (evening/weekend/holiday) are consistent with grilling activity (or point to an SI figure if already done).
- Lines618–623: The text implies fresh BBOA could originate from residential heating or barbecue charcoal combustion in late winter/early spring. Please clarify how you distinguish these two sources (or adjust wording to reflect uncertainty), since this affects the seasonal source narrative.
- Lines688–690: The implication that monoterpene emissions should be considered in urban vegetation management is interesting; please ensure the wording is balanced (urban green has benefits and trade-offs) and consider adding a concrete but non-prescriptive example (e.g., species selection/maintenance) or cite supporting literature more explicitly.
Citation: https://doi.org/10.5194/egusphere-2025-5191-RC1 -
RC2: 'Comment on egusphere-2025-5191', Anonymous Referee #2, 10 Feb 2026
General comments
Li et al. present results from measurements of organic compounds in the gas- and particle phase in a street canyon in downtown Munich, Germany, in late winter and summer. Positive matrix factorization is applied to mass spectra from PTR-MS, CHARON-PTR-TOF-MS and HR-TOF-AMS individually to find factors and identify sources of VOCs, SVOA, and OA. Based on this analysis, the authors find photochemical oxidation of biogenic emissions and biomass burning to be relevant sources for organic compounds in general, and traffic to be more relevant for VOC than OA concentrations.
Overall, this is a well written manuscript with interesting results, of relevance for air quality considerations for an urban area like Munich. As always with PMF studies, the choice of input matrices, as well as number of factors, requires detailed documentation to ensure minimum subjectivity, and this study is no exception. I have added comments where I think additional information is needed to justify certain choices made by the authors. I have several comments and questions regarding the interpretation of the BB-related factors, especially in summer. I wonder about the relevance of BBQ activities. Are there additional observations to corroborate its relevance?
I would also like to caution the authors on the summary of the paper (and this is now a potentially subjective note from this reviewer). Whereas I understand the conclusion, based on their results, that urban monoterpene emissions should be reduced, this in my opinion sends a potentially incorrect message to policy makers. The difference between winter and summer OA is within uncertainty, whereas e.g. for nitrate it is not, and PM concentrations overall are higher in winter (e.g. lines 279-284). Toxic aerosol components from traffic-related emissions such as tire and break wear, black carbon, etc. are not considered in this study. A message that urban vegetation should be controlled based on these results may potentially be misleading and could only be made after consideration of the entirety of PM2.5 or PM10 mass in Munich.
Overall, I am in favor of eventual publication of this manuscript once comments have been addressed.
Specific comments
L. 101-105: Please clarify what data/measurements/instruments were used for this comparison. Was it the Fidas? Did you get closure with AMS+BC measurements?
L. 114-116: Please revise wording, the two measurement periods in March are clearly not during “distinct seasons”. It would be good to clearly show with data how the separation between late winter and spring was determined. The separation into late winter and spring is not evident for me from Figure 1 (where I assume the horizontal dotted line indicates the separation, however this is not mentioned in the figure caption).
L. 148: Would you not expect a diel cycle of gas-phase background based on concentrations and temperature/RH? Please elaborate. Were the weekly background checks done during different times of the day? How variable was the background? Please add more information to the supplement.
L. 223-226: Please state clearly that PMF was run separately for the three different datasets and seasons (at least this is how I understand it). Please motivate this choice and motivate why e.g. PMF was not run on the entire combined dataset, or for one instrument and all seasons. Refer to diagnostics etc. in the supplement where needed.
L. 275: Suggest toning this down, as the number of particles <100 nm is not a good indicator for NPF.
L. 318: Please specify which ions were excluded (especially the large signal ones)
L. 350-355: What is a potential activity where biomass is being burnt in summer in Munich? Please clarify. Is there an option of wildfire influence? The time series shows a clear increase in late August
L. 362-365: Why would this not be a source in summer?
L. 371-373: Would cooking and outdoor grilling really be an activity in winter? Would this not rather be from residual heating? How do the two biomass burning factors for winter and summer compare in detail? Why is BBOA so high in late winter?
L. 399: Did you mean 171 SVOC compounds?
L. 429 -430: Do you see a difference between weekends and weekdays? I have a hard time imagining that this could be a relevant source during weekdays. Also, do you see compounds related to cooking? Would people not rather use coal for BBQing? Also here, this factor increases in late August – contribution from a wildfire episode?
L. 453-458: If it was cooking, should this factor not be around year-round? Oleic acid and palmitic acid can also be from the users handling the instrument, has this been investigated?
L. 500: In SVOC PMF, cooking was only identified in winter. Please elaborate.
L. 553-555: See previous comments on summer BBOA
Chapter 3.3: I am still not convinced the summer BBOA is not from wildfires, as in summer 2023 there were many wildfires in Europe. Please consider this as a source again.
L. 644: do you see influence from coal burning emissions?
Technical comments
L. 44: Put manufacturer in brackets after instrument name
L. 45: Commonly PM1. Specifically mention if PM2.5 lens was used.
L. 48: More correct: “PMF of AMS datasets” cannot specify sources
L. 56-57: What is meant by “qualitatively and quantitatively”? Please clarify.
L. 58 -59: How does CHARON-PTR-MS minimize thermal decomposition and ionization-induced fragmentation, compared to what technique? Please specify.
L. 63: Should read “in downtown Karlsruhe”
L. 70-72: Sentence structure needs revision
L. 96: Air pollution is very general here, I assume you mean organic aerosol
L. 175: How come you give as range for lens transmission 70-500 nm when you have a PM2.5 lens? Please clarify.
L. 198: Right bracket missing
L. 242: If “distinct seasons” are kept above, I would refer here rather to “three campaigns”
Citation: https://doi.org/10.5194/egusphere-2025-5191-RC2 -
AC1: 'Comment on egusphere-2025-5191', Yanxia Li, 10 Mar 2026
Dear Editor and Reviewers,
Please find attached our final author response to all referee comments (AC). We thank the reviewers for their constructive and helpful comments, which have helped us improve the manuscript.
Detailed responses to all comments are provided in the attached PDF. In addition, following the reviewers’ suggestions, we have included supplementary figures in the Supplement to further support our revisions.
On behalf of all co-authors.
Best regards,
Yanxia Li
Peer review completion
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Hengheng Zhang
Xuefeng Shi
Yaowei Li
Sophie Abou-Rizk
Jessica Smith
Zhaojin An
Adrian Wenzel
Junwei Song
Thomas Leisner
Frank Keutsch
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(3449 KB) - Metadata XML
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Supplement
(4127 KB) - BibTeX
- EndNote
- Final revised paper
This manuscript provides a clear and well-organized synopsis of the study, moving coherently from the motivation and methodology to the key seasonal contrasts, source apportionment results, and broader implications. The application of online mass spectrometry coupled with PMF is well suited to disentangling sources and seasonal variability of VOCs and OA in a street-canyon setting. The central findings—enhanced summertime OA consistent with photochemical production versus elevated wintertime nitrate, alongside distinct contributions from combustion, biogenic influences, and traffic—are communicated effectively and supported by quantitative evidence. The manuscript aligns well with the journal's scope, and I recommend its acceptance following minor revisions.