the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Sep 2026
Chemical composition of indoor and outdoor particles in a building near an international airport: Influence of local and long-range transported air pollution
Abstract. Atmospheric pollution poses a significant threat to human health. These effects on human health are well documented in outdoor environments. However, in the Western world, people currently spend most of their time indoors. Therefore, studying the relationship between outdoor and indoor pollutant concentrations is essential. In this study, outdoor and indoor concentrations were measured in an unoccupied, mechanically ventilated building in Finland equipped with air-filtration and heat-recovery systems. Sampling was performed using a switching valve system that alternated sampling location between the building's air intake before filtration and the exhaust air vent, enabling near-simultaneous measurement of indoor and outdoor pollutant concentrations. The measurement campaign was conducted between 13th and 27th of February 2025. During the measurement period, a strong long-range transport (LRT) episode was observed, providing a good opportunity to study the effects of LRT on indoor pollutant concentrations. The measured variables included the chemical composition of aerosol particles, black carbon concentration, particle mass and number concentrations, and particle size distributions. Also, the effect of the nearby airport on the pollutant concentrations was discussed. The airport was found to have a large effect on particle number concentrations but almost no effect on particle mass or black carbon concentrations. The most abundant chemical component of aerosol particles both indoors and outdoors was organic matter. However, outside, the second most abundant component was nitrate, while indoors, more sulfate was measured. For the measured variables, the indoor-outdoor (I/O) ratios were calculated under normal and LRT conditions. Most of the I/O ratios remained stable between normal and LRT conditions. Exceptions to this were particulate matter and black carbon concentrations, which had higher I/O ratios during normal conditions, while particle number concentration had notably higher I/O ratios during LRT. The notably higher IO ratio of particle number concentrations was most likely due to the increase in the geometric mean diameter of particles. Notably, the ratios varied between the different measured components. The lowest I/O ratio of 0.20 ± 0.08 was observed for nitrate during the LRT. The low I/O ratio of nitrate might be related to the increasing temperature of aerosol particles while being transported indoors and therefore to increased evaporation. Under LRT conditions, the indoor black carbon concentrations increased to levels similar to those at street canyon measurement sites in Finland. The nearby airport was found to have a notable effect on the particle number but not particle mass concentrations or BC concentrations.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Aerosol Research.
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.- Preprint
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Status: open (until 09 Nov 2026)
- RC1: 'Comment on ar-2026-32', Anonymous Referee #1, 07 Oct 2026 reply
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RC2: 'Referee comment on ar-2026-32', Anonymous Referee #1, 13 Oct 2026
reply
1. General assessment
This manuscript presents an interesting and potentially valuable field study on the relationship between outdoor and indoor aerosol concentrations and chemical composition in a mechanically ventilated building located close to Helsinki-Vantaa International Airport. The dataset is comprehensive and combines particle number and mass concentrations, particle-size distributions, aerosol chemical composition measured by SP-AMS, equivalent and refractory black carbon, wavelength-dependent light absorption, laboratory measurements of filter efficiency, ventilation variables, and air-mass back trajectories.
A particularly relevant aspect is the use of the same analytical instrumentation with an automated switching system between outdoor and indoor sampling locations. This reduces inter-instrument biases, although the alternating 15-min sampling strategy introduces a temporal limitation that should be quantified. The long-range transport (LRT) episode also provides a useful contrast for examining how changes in aerosol size and composition affect indoor penetration.
The strongest result is the contrasting behaviour of particle number concentration (PNC) and PM1 indoor/outdoor ratios during LRT conditions, which is plausibly related to the size-dependent filtration efficiency. The species-dependent I/O ratios, especially the comparatively low nitrate I/O ratio, are also scientifically relevant.
However, several interpretations are currently expressed with greater certainty than is supported by the measurements, particularly those concerning airport, traffic and biomass-burning contributions, wind-driven infiltration, nitrate volatilisation, and wavelength-dependent absorption. Methodological clarification is also needed regarding the I/O definition, representativeness of exhaust-duct sampling, unfiltered infiltration, airflow uncertainty, statistical treatment, size-range limitations, and uncertainty in filter-based predictions.
I consider the dataset sufficiently interesting to justify publication after substantial revision. The principal issues can largely be addressed through methodological clarification, additional analysis where feasible, explicit uncertainty treatment, and moderation of causal/source-attribution language; a new field campaign does not appear necessary.
2. Major scientific and methodological comments
- Definition and calculation of I/O.Location: Section 2.2, Table 1 and Figure 3. The manuscript defines I/O as indoor/outdoor, whereas the methodological description appears to refer to incoming-air/exhaust-air, which would describe the inverse ratio. The numerical values support an indoor/outdoor calculation; for example, the LRT PNC values give 1.4/4.3 ≈ 0.33, consistent with the reported value of 0.34. Please correct the formula and terminology consistently throughout the text, equations, captions, tables and conclusions. Suggested wording: “Please correct the description of the I/O calculation. The reported values appear to represent indoor concentration divided by outdoor concentration, whereas the current methodological wording describes the inverse ratio.”
- Representativeness of sampling from the exhaust duct.The indoor measurements are obtained from the exhaust duct rather than directly from the occupied zone. Please provide sufficient information on duct geometry, sampling location, distance from the building/return-air path, airflow, residence time, temperature and expected mixing. Discuss whether the sampled exhaust air can be considered representative of indoor air, and whether duct transport could modify aerosol composition or size, particularly for semi-volatile nitrate.
- Alternating 15-min sampling strategy.Please describe the switching valve/system, tubing dimensions and material, sample flow rates, line volume and residence time, stabilization procedure after each switch, and the treatment of transition periods. State whether data immediately after switching were discarded and, if so, for how long. This is particularly important for rapidly varying ultrafine particle concentrations and for interpreting indoor/outdoor ratios.
- Unfiltered infiltration versus mechanically supplied air.The building was overpressurised because the nominal supply flow exceeded the exhaust flow, but positive pressure does not demonstrate that every air parcel entering the building passed through the mechanical filter. Wind-induced pressure differences can drive local exchange through the envelope. Please distinguish clearly between filtered mechanical supply and uncontrolled infiltration. A suitable formulation is: “Most mechanically supplied air passed through the filter, while uncontrolled infiltration may also have contributed.”
- Airflow uncertainty and evidence for overpressure.The supply and exhaust airflow measurements have an uncertainty of approximately ±15%, while the nominal difference is 150 L/s. Please discuss how this uncertainty affects the inferred flow imbalance. The measured pressure difference, where available, is more direct evidence of the pressure state of the building and should be used alongside the flow balance rather than relying on the nominal difference alone.
- Airport contribution to PNC.The association between elevated PNC and winds from the airport sector is plausible, but the observations do not independently separate airport emissions from road traffic, residential combustion or meteorological effects. The manuscript should distinguish “consistent with an airport-related contribution” from “demonstrates an airport effect”. Suggested wording: “Elevated PNC was associated with winds from the airport sector, and the observed particle-size distribution was consistent with an airport-related contribution.”
- Lower size limit of ELPI+ measurements.The ELPI+ analysis begins at approximately 16 nm, while the discussion refers to aircraft-emitted particles below 23 nm. The manuscript should explicitly acknowledge that particles below the lower measurement limit cannot be quantified with this dataset. Please also explain why the sub-10 nm/lowest stage information was excluded because of interference and how this limitation affects the airport-related PNC interpretation.
- Interpretation of nitrate loss.The lower indoor nitrate concentration is consistent with loss of particulate ammonium nitrate, potentially through volatilisation during warming, but this mechanism was not directly measured. Please state this as a plausible hypothesis rather than a demonstrated process. If available, report indoor/outdoor/supply-air temperature and relative humidity because these variables are relevant to nitrate partitioning.
- Filter-only prediction of indoor concentration.The calculation linking outdoor size distributions with measured filter efficiency is useful, but its uncertainty should be quantified. The predicted indoor concentration depends on outdoor concentration, filter efficiency, instrumental uncertainty, aerosol properties, infiltration, deposition and possible transformation. Please propagate the relevant uncertainties or clearly identify the result as a “filter-only predicted indoor concentration”.
- Laboratory filter aerosol versus ambient aerosol.The filter-efficiency experiment uses DEHS and KCl test aerosols, which differ from ambient atmospheric aerosol in composition, morphology, density and possibly charge state. Therefore, differences between measured and filter-only predicted indoor concentrations cannot uniquely demonstrate infiltration or indoor sources. Please discuss the representativeness and limitations of transferring the laboratory efficiency curve to the ambient aerosol.
- Interpretation of the outdoor-to-indoor size shift.The shift toward smaller indoor particle sizes should not automatically be interpreted as physical particle shrinking. Size-dependent filtration, removal, evaporation, composition changes and the differing transmission efficiencies of the instruments may all contribute. Please moderate the language and discuss the alternatives.
- Wind speed correlations and inferred leakage.The reported relationships between wind speed and I/O ratios (R² = 0.59 for nitrate, 0.51 for ammonium and 0.45 for organic aerosol) are interesting, but correlation alone does not establish wind-driven leakage as the mechanism. Wind speed covaries with direction, temperature, stability and outdoor concentration. Please report regression details, sample size and uncertainty and, where possible, compare the relationships with the measured pressure difference.
- eBC and AAE source attribution.The interpretation of the two high eBC episodes as biomass-burning and traffic events should be made more cautiously. The reported biomass-burning fraction is approximately 42% for both periods, while AAE is not a unique tracer of biomass burning. These observations can support qualitative source hypotheses but do not constitute independent source apportionment. Please revise the wording accordingly.
- BrC/BC fractions and wavelength dependence.Please clarify whether quantitative BrC/BC fractions were actually derived or whether the discussion is based on wavelength-dependent absorption. Similar spectral behaviour indoors and outdoors does not by itself demonstrate identical filtration of BrC and BC. If no formal spectral source apportionment was performed, keep the interpretation qualitative.
- Short-term eBC peaks versus annual averages.The comparison of short-duration campaign peaks with annual average concentrations at traffic locations provides useful context but involves different averaging periods, site types and exposure conditions. Please present this comparison cautiously and avoid using it as direct evidence of exceptional exposure without qualification.
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- Alignment of objectives with the presented results.The introduction places substantial emphasis on local sources, whereas much of the results and discussion focus on the LRT event and outdoor-to-indoor transport. Please align the stated objectives more closely with the actual analyses, for example by explicitly framing the study around outdoor-to-indoor transfer under contrasting normal and LRT conditions and the role of local source sectors.
- Novelty claim concerning proximity to an airport.The statement that this topic has “never been studied” or is uniquely close to an airport is difficult to substantiate exhaustively. Please soften the claim unless a systematic literature review supports it. A safer formulation is: “Few studies have simultaneously characterised indoor and outdoor aerosol properties in buildings located in the immediate vicinity of major airports.”
- Scope, limitations and conclusions.The study concerns one building, one winter campaign of approximately two weeks, one ventilation/filter configuration and one major LRT episode, with alternating sampling. The conclusions should explicitly recognise these limitations and frame the results as a case study rather than a universally representative relationship. Suggested wording: “The results illustrate the importance of outdoor pollution levels for indoor exposure in mechanically ventilated buildings under the conditions investigated in this case study.”
- Santiago et al. (2022).The reported 0.56 ± 0.24 value should be identified precisely as an average indoor/outdoor ratio (if that is what the cited source reports), rather than described generically as a “relationship” or correlation.
- ePM1 60% terminology.Please check the wording against ISO 16890. “ePM1 60%” should not be described as simply removing 60% of every particle in a fixed 0.3–1 µm interval. Explain the efficiency metric correctly.
- Most penetrating particle size (MPPS).If MPPS is discussed, introduce the term briefly and explain its relevance to the minimum filter efficiency observed in the laboratory test.
- Meaning of ± values.For Table 1 and other reported averages, specify whether ± represents standard deviation, standard error, confidence interval or instrumental uncertainty.
- Filter history and condition.Provide the relevant information on the tested field filter: installation/use history, loading or pressure drop if available, handling, storage and whether the laboratory test was performed on the actual installed filter or a representative new filter.
- SP-AMS collection efficiency and upper size range.The manuscript indicates approximately 100% collection efficiency over roughly 60–600 nm. Please explain how this affects comparisons with ELPI+ and PM1, especially because some of the reported PM1 concentration maxima occur near or above 600 nm.
- Possible indoor sources.Although the building was described as unoccupied, briefly discuss possible contributions from HVAC components, heating, electrical equipment, sampling/instrumentation or other intermittent indoor sources. “Unoccupied” does not necessarily mean source-free.
- Wind regressions.For the wind/I/O regressions, provide the fitted equation, sample size, uncertainty and significance where formal regression is retained. Consider whether pressure difference provides a more direct mechanistic variable.
- BC terminology.Keep eBC, rBC and EC distinct. Avoid using them interchangeably unless the conversion/relationship is explicitly justified.
- eBC versus rBC/EC conversion.Please check the cited literature before stating a single eBC = 2.7–3.1 × rBC relationship. The cited studies distinguish eBC, EC and rBC, and the numerical factors should not be combined unless they are explicitly applicable to the same comparison.
- Health-effects statements.Where health effects are mentioned, use an authoritative assessment or review and avoid extending a general health statement beyond what the cited source supports.
- Grey-literature references.For institutional sources such as DigiTraffic or HSY, provide a stable institutional author/title/year/access formulation consistent with the journal reference style.
- Reference formatting.Perform a complete reference audit for DOI/URL presentation, journal titles, author formatting, capitalisation and typographical errors. Correct “abosrption” and similar typographical issues.
- Independent literature on airport effects.The Finnish literature cited is relevant, but because airport attribution is a central interpretation, include independent studies where possible rather than relying predominantly on one research group.
- Figure 2 particle modes.Where modal diameters are discussed, report the approximate numerical size ranges/locations directly in the text rather than relying only on visual interpretation of the figure.
- Figures 3–5.Ensure that indoor/outdoor and chemical-species curves remain distinguishable in grayscale or for readers with colour-vision deficiencies, and make the captions sufficiently self-contained.
- Figure 6 I/O values above unity.When eBC I/O exceeds one, show or discuss the corresponding absolute indoor and outdoor concentrations and uncertainty because the ratio becomes unstable when the outdoor denominator approaches zero.
- Figure 7 source interpretation.Unless a formal source-apportionment method is applied, present wavelength-dependent absorption as evidence supporting qualitative source hypotheses rather than definitive source identification.
- Language and units.A final English-language edit is recommended, including terminology, unit consistency and typographical errors such as “airpost”.
- Indoor aerosol concentrations were strongly influenced by outdoor concentrations despite mechanical ventilation and filtration.
- Outdoor-to-indoor transport depended on particle size and chemical composition.
- During LRT conditions, the particle-size distribution shifted toward a range where the installed filter showed lower efficiency, which plausibly contributed to the increase in PNC I/O together with the decrease in PM1 I/O.
- Nitrate showed lower indoor penetration than sulfate and organic aerosol, indicating additional loss or transformation beyond simple size-dependent filtration; volatilisation should be presented as a plausible mechanism rather than a demonstrated one.
- Elevated PNC was associated with winds from the airport sector, but the available measurements do not independently quantify the airport contribution. No comparable airport-sector enhancement was evident for BC mass.
- The conclusions should explicitly state that the findings are specific to the investigated building, ventilation/filter configuration, winter campaign and meteorological conditions.
Major Revision
The manuscript contains a valuable dataset and addresses an important topic at the intersection of outdoor air pollution, indoor exposure, aerosol size-dependent filtration and long-range transport. I do not recommend rejection. However, substantial revision is required, particularly for the I/O definition, representativeness of exhaust-duct sampling, separation of filtered mechanical supply from uncontrolled infiltration, airflow and instrumental uncertainty, statistical treatment, transfer of laboratory filter efficiency to ambient aerosol, and moderation of airport/source-attribution and mechanistic conclusions. If these issues are addressed carefully, the manuscript could make a useful contribution.
6. Editorial checklist for the revision- I/O formula and terminology corrected consistently throughout the manuscript.
- Exhaust-duct sampling representativeness and possible duct transformations discussed.
- 15-min switching procedure, residence times and transition-data treatment documented.
- Filtered mechanical supply distinguished from uncontrolled envelope infiltration.
- Airflow uncertainty and measured pressure difference discussed.
- Filter, instrument and filter-only prediction uncertainties reported or bounded.
- Statistical methods, n, tests, significance level and temporal dependence clarified.
- Airport-associated wording moderated unless independent attribution is demonstrated.
- Nitrate volatilisation presented as plausible rather than directly demonstrated.
- eBC/AAE and biomass-burning/traffic interpretations made explicitly qualitative where appropriate.
- ELPI+ lower size limit and SP-AMS collection-efficiency range acknowledged.
- Santiago et al., Sharma et al., ISO 16890 and grey-literature references checked.
- Conclusions include study-specific limitations and avoid over-generalisation.
Citation: https://doi.org/10.5194/ar-2026-32-RC2
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- 1
General comments
This manuscript presents a comprehensive field investigation of indoor and outdoor aerosol concentrations and chemical composition in a mechanically ventilated building located near Helsinki-Vantaa International Airport. The study combines particle number and mass size distributions, aerosol chemical composition, equivalent and refractory black carbon, wavelength-dependent light absorption, ventilation parameters, pressure measurements, and laboratory measurements of filter efficiency. The comparison between normal conditions and a long-range transport (LRT) episode is particularly valuable for examining how changes in aerosol size and composition affect the transfer of outdoor particles into the indoor environment.
Overall, the dataset is valuable and potentially suitable for publication. The contrasting behaviour of particle number concentration and PM1 indoor/outdoor ratios during the LRT period is particularly interesting, as is the species-dependent indoor penetration. However, several methodological descriptions and interpretations require clarification before the conclusions can be considered fully supported by the measurements.
The main issues concern the definition of the indoor/outdoor ratio, the representativeness of sampling from the exhaust duct, the implications of the 15 min alternating sampling strategy, and the distinction between filtered mechanically supplied air and possible unfiltered infiltration through the building envelope. The uncertainty associated with ventilation flows and the application of laboratory filter-efficiency measurements to ambient aerosol should also be discussed more explicitly.
Several source-attribution and mechanistic interpretations should be moderated. In particular, the evidence for an airport contribution to particle number concentration, the attribution of individual black-carbon peaks to traffic or biomass burning, the interpretation of wind-speed correlations as evidence for infiltration, and the proposed volatilisation of nitrate are plausible interpretations, but they are not demonstrated directly by the available measurements.
I therefore recommend major revision. The principal issues appear addressable through methodological clarification, additional analysis of the existing data, a clearer treatment of uncertainties, and more cautious wording of some conclusions; a new field campaign does not appear necessary.
Specific comments
The manuscript should explicitly provide the equation used for the I/O ratio and ensure that the terminology is consistent throughout the text, figures and tables. The reported values appear to correspond to indoor concentration divided by outdoor concentration, whereas parts of the methodological description appear to describe the inverse ratio.
Indoor aerosol was sampled from the exhaust duct rather than directly from the occupied space. Please clarify the representativeness of this location, including duct geometry, airflow, residence time, mixing and temperature where available. This is particularly relevant for semi-volatile species such as nitrate.
Please describe the switching procedure, sampling-line dimensions and materials, flow rate, residence time, stabilization period and treatment of transition periods. This is especially relevant for rapidly varying concentrations and ultrafine particles.
The discussion should distinguish mechanically supplied air that passes through the ePM1 filter from uncontrolled air exchange through the building envelope. Positive pressure does not necessarily imply that every air parcel entering the building has passed through the filter.
The stated ±15 % uncertainty of the supply and exhaust flows should be considered when interpreting the nominal 150 L s−1 flow imbalance. Measured pressure differences, where available, provide complementary evidence of building pressurisation.
Elevated PNC associated with winds from the airport sector is physically plausible, but the wind sector is not unique to the airport and may include roads and other local sources. The wording should therefore distinguish between evidence consistent with an airport contribution and evidence demonstrating such a contribution.
Because the ELPI+ analysis does not fully cover the smallest particle sizes and the lowest stage was excluded because of interference, the possible contribution of particles below the measurement range should be acknowledged when discussing aircraft-related ultrafine particles.
The smaller indoor particle-size distribution is consistent with size-dependent filtration, but does not by itself demonstrate particle shrinking. Filtration, deposition, evaporation and changes in composition may all contribute.
The conclusions should explicitly recognise the case-study nature of the work: one building, one winter campaign, one ventilation/filter configuration and one principal LRT episode. The findings should not be presented as universally representative of buildings near airports.
Technical corrections
Overall recommendation
Major Revision. The manuscript contains a valuable dataset and addresses an important topic for indoor air quality and aerosol exposure. The study has clear potential for publication if the methodological issues, uncertainty treatment and statistical interpretation are clarified and the source-attribution and mechanistic conclusions are appropriately moderated.