Dear reviewers,

thank you for your valuable work and your comments. We see that they helped us to improve our manuscript. We hope that our responses below are sufficient. We have included your comments below as normal text and our comment in italics style.

On behalf of the all authors,

Topi Rönkkö

Referee 1:

General comment

This study by Mylläri et al.  investigates emissions from a large-scale (100 MW) wood pellet–fired heating plant in Helsinki, focusing on both primary particle emissions and the potential for secondary aerosol formation in the atmosphere. They conducted comprehensive measurements before and after the plant’s baghouse filter (BHF) system across different load conditions (30, 60, and 100 MW), and used an oxidation flow reactor to simulate atmospheric aging and evaluate secondary aerosol formation. They found that BHF filtration removed over 99.95% of particle number emissions, with BC reduced below detection limits, indicating very low direct particle emissions under normal operation. However, they also discovered that the potential for secondary particle formation was up to 1,000 times higher than the remaining primary particle emissions, primarily driven by nitrate formation from NOₓ. The authors state that while biomass combustion with efficient filtration can minimize primary emissions, secondary aerosol formation remains a significant concern and must be considered in future emission regulations and impact assessments.



The manuscript is written in appropriate English and the topic is within the scope of Aerosol Research. Although I do not share the stated general low level of information on power plant emissions (see for example Guevara 2024 ESSD), the study provides detailed information of physical particle properties. I am mainly criticizing the discussion of the results, i.e. the missing context, so it remains much as a presentation of results. I would suggest to compare emission factor from the 100 MW biomass power plant (1) to emissions from common coal- or gas-fired plants, also co-fired with biomass or peat, (2) to small-scale combustion appliances of similar operation mode and draw a quantitative conclusion about the apparent benefit from a central heating over residential heating in individual houses, (3) among different operation modes regarding the filtration efficiency of the BHF, and (4) the relevance of such this biomass power plants for the air quality of Helsinki. After addressing these aspects and my specific comments below, I would recommend this study for publication in Aerosol Research.

 Answer to the comment:

We thank reviewer for these comments which are very broad. We added Guevara et a. to the references and focused the lack of knowledge to be related to aerosol emissions. In the discussion of the manuscript, we are presenting the results with larger perspective, aiming to communicate to the potential readers the importance and relevance of results. We highlight the relatively low level of pollutants in direct atmospheric emissions, seeing also that low emissions should be described in the scientific literature to maintain the correctness of overall picture of atmospheric emissions. We discuss the efficiency of combustion and role of filtration in emission mitigation simultaneously referring other studies.

We see that the context of the our study is energy transition and atmospheric impacts of combustion emissions. This context led to the discussion where we put the emissions to the air quality context and highlight the low emissions of BC which is also climate forcer but simultaneously recommend to keep secondary aerosol (SecA) formation in mind when aiming to even more cleaner combustion. According to our knowledge, there are not SecA data available from other large scale power plant sources, which makes comparison unmeaningful.

Modifications to the text: Added Guevara 2024 ESSD

Specific comments

Line 44: I think this is to general, and not valid for e.g. H2 combustion. You may constraint to solid fuel combustion or carbon-based fuels?

Modified to “..combustion processes of carbon-based fuels…”

Line 47-48: I guess you refer to Sippula et al. (2009a).

Corrected

Lien 48: The upper limit of 92 mg/MJ is principally correct, but this emission factor results from plant operation without exhaust aftertreatment. The upper limit of PM1 emission factor is then 31 mg/MJ, but different from a filtration process of this study. Scrubbing may be less efficient than filtration, but also remove gases and increase the particle diameter, which might be beneficial in terms of deposition in the human lung. A fair comparison would be the two electrostatic precipitators used in Sippula et al. (2009a) with only 4 and 6 mg/MJ of PM1. Please have a look on the particle number emission factors accordingly.

We added “It should be noticed that the flue gas cleaning has remarkable effects on the emission factors; e.g. in Sippula et al. (2009a) the highest particle emission factors (92 mg/MJ) were obtained for the situation without exhaust cleaning, and the situation when the electrostatic precipitators were used led to more than one order of magnitude reduction in PM1 emissions.” to the end of that paragraph.

Line 56: kWh is not a unit for power.

Corrected to kW.

Line 79-81: In Ortega et al. (2013) it is described that “FLAME-3 was a targeted investigation of emissions from burning of plant species that are relevant to North American local and regional air quality and often undergo prescribed and wildfire burning”. Burning experiments were done by igniting 0.1 to 1kg of biomass on a ceramic plate by heating coils, so combustion is not conducted “in a small-scale device”. For example, Czech et al (2017) presents secondary emissions from a small-scale pellet boiler in addition to Heringa et al (2011).

Modified to “Ortega et al. (2013) obtained relatively similar results for biomass which was burned openly on the top of a ceramic plate; the aging of biomass-burning smoke in an oxidation flow reactor resulted in a total organic aerosol (OA) average of 1.42±0.36 times the initial POA.” We added Czech et al (2017)  to the references and the sentence “In the study of Czeck et al (2017), the secondary aerosol formation potential of the emissions from pellet burner was also evaluated to be relatively low, when compared to log wood stoves, demonstrating pellet boilers as a relatively clean technologies for biomass combustion.” to the introduction.

Line 78: What does “full-scale” mean in this context? Do you mean rather large-scale?

Modified to “large-scale”.

Line 102-103: Please provide more information about the boiler configuration and combustion technology. Based on the Valmet Ltd. Website, pellets are pulverized before burning and a low-NOx combustion technology is used.

We added a sentence “More information about the boiler configuration, combustion technology and combustion process modelling can be found from Niemelä et al (2022).” to the first paragraph of experimental section. In Niemelä et al., detailed modelling of the combustion process was done for the same boiler, and we think that the study can provide the needed additional information for the reader.

Line 105: Please define what is “dust” in this context. To the best of my knowledge, Dusthunter P100 is not measuring gases, so please provide instrument information.

Dust is the regulated metric for particulate emissions. Modified to “Continuous regulatory measurements of gaseous emissions (CO, NO_x, SO₂) and regulated dust emissions (Sick Dusthunter SP100, EN 15267) of the power plant and flue gas volume flow, done by power plant operator, are conducted through probes located in the stack at the height of approximately 25 m.”

Lien 112-113: What was the raw material of the pellets? The difference in elemental composition is quite remarkably because the most different elements are determining largely the PM1 emissions (Kleinhans 2018 Progress Energy Combust Sci).

We have limited information on the raw material of pellets. It was wood as mentioned in the manuscript, but more detailed information cannot be provided. if available.

Line 128-130: If you fix the diluted emissions to 30°C, what does it refer to in terms of dilution ratio? How much is the primary emission temperature changing (with changing load for example)? What does “to mimic the atmospheric dilution of the aerosol” mean? After atmospheric dilution, i.e. atmospheric concentrations, further additional dilution for analysis is surprising.

Dilution air temperature was set to 30 °C. Text was modified according to that. In addition, we monitored the dilution ratios of the setup and set the primary PTD diluter dilution ratio to 12. This has been observed to mimic the real-world nanoparticle formation in cooling dilution of combustion process exhaust e.g. in Rönkkö et al. 2006 and Keskinen and Rönkkö (2010). We modified the text to “…(dilution air temperature set to 30 °C, dilution ratio set to 12), followed by a residence time tube to mimic the potential nanoparticle formation during atmospheric dilution of the aerosol (Rönkkö et al. 2006; Keskinen and Rönkkö 2010).” and added the references to reference list. Further dilution is in many experiments needed to reach particle concentrations suitable for aerosol instruments like CPCs. These further steps have not been observed to affect significantly e.g. nanoparticle concentrations. 

Line 142-143: Assuming dilution air of 20°C, the undiluted flue gas has only a temperature of 140°C. Is this correct and if yes, was any intense heat recovery involved? 

This is very relevant question. In our experiments the dilution air temperature was set to 30 °C. We have not glue gas temperature data available from sampling location but it can be expected to be relatively low if compared to e.g. combustion temperatures.

Lien 159: How was the TSAR operated? External or in situ generation of ozone, at which humidity and proximate OH reactivity? What about the influence of non-OH chemistry compared to OH-driven photooxidation?

There is a reference Simonen et al 2017 to fully describe the TSAR. In Simonen et al., the layout of the OFR is presented in Figure 1. TSAR is an OFR254-type oxidation flow reactor (OH radicals are produced from the photolysis of the ozone at 254 nm UV radiation). It consists of a residence time chamber, an oxidation reactor, an ozone generator, three mass flow controllers and an expansion tube that connects the residence time chamber and oxidation reactor. The residence time chamber ensures the mixing of the sample and makes the sample flow laminar. The ozone needed for OH radical production is mixed with the sample prior to the residence time chamber. We operated the device so that the OH exposure of the flue gas sample was between 0.46 and 0.82 days as described in the result section.

Line 248: Please provide an upper limit of the EF for THC based on your LOD.

We decided not to provide that value due to the potential effect of this particular setup on the value. This is included in the result section. We recommend to use more sensitive methods for future investigations of gaseous organic compounds so that reliable data can be added to the literature.

Line 273-276: In line 167 you refer to Li et al. (2015) for the determination of OH exposure by CO, but disregarding that it is recommended for OH exposures of 5e11 to 1e13 s/cm3. CO reacts slowly with OH having an atmospheric half-life of 22 days. To determine an OH exposure equivalent to 0.5 days, you have to detect a different in CO_in and CO_out of about 1.5%, to differentiate between 0.46 and 0.82 days of about 1%, to give significant digits of 0.01 days even 0.3%. Additionally, in using the concept of photochemical clock, also the rate constant has an error of typically 25-30% which needs to be considered. Therefore, I am curious how you performed the measurements with the CO12M instrument to obtain results with the precision required.

We used CO addition in TSAR measurements and CO measurements. Furthermore, stable loads in power plants and relatively long measurement period for each load made the OH exposure evaluation possible. We modified the manuscript to include “We used controlled CO addition upstream the TSAR and measured the CO concentration (CO12M, Environnement S.A)…”

Line 290: Often, OFR aging leads to intense formation of nucleation mode particles from oxidized vapors. How significant was this fraction compared to the particle size range accessible by the AMS?

We agree with this, compounds can for new particles in the OFR and those can be in the particle size range below the lowest sizes measured with AMS. However, when analyzing the particulate mass instead of number, this is not a problem when the limitation is taken into account when interpreting the results. We did not have analyzed this in the study, due to the lack of SMPS after the OFR, but now we added following sentence to the article: “In addition, when interpreting particle composition results, it should be kept in mind that the instruments have different particle size ranges covered; e.g. particles in small particle sizes can be outside the range of composition measurement.”

Line 303-305: How does the aging affect the composition of the PM1 emissions from fresh to aged?

We did not measure reliably the composition of fresh flue gas particles due to their relatively small concentrations

Line 314: I would not call your analysis “comprehensive” if the only information about the aerosol chemical composition is derived from AMS, see also line 381.

We removed the word “comprehensive”.

Line 320-321: To draw such a conclusion, you need to provide emission data from fossil fuel burning. Moreover, impacts on air quality were not investigated in this study and almost only particle emissions are presented. What about the air quality impact of gaseous emission constituents?

Correct. Sentence was removed. In addition, we added to the end of that paragraph that “Importantly, the total impact of emitted flue gases requires even more detailed characterization of pollutants. E.g. gaseous emission constituents should be investigated in future studies more comprehensively, e.g. from secondary aerosol precursor point of view.”

Line 322-326: Several studies (e.g. Lepistö 2023 Environ Int) pointed out that ultrafine particles should receive more attention explaining health effects. If the particle size is shifted towards smaller ones, how does this fit to the conclusion about positive population health?

Good comment again. We added to the discussion that “ In general, the size of particles is important factor e.g. in lung deposition, and recent studies emphasize the role of smaller particles in potential health impacts caused by particulate pollutants (e.g. Lepistö et al. 2023).

Line 337-342: This paragraph is correct but lacks of novelty. Bag-house filters became popular in the 70s and can be considered as standard technology for power plant emission mitigation since the 90s. Therefore, I encourage the authors to compare the filtration efficiency to previous similar studies in a quantitative manner or discuss filtration efficiency for different loads as the flue gas velocity has an impact.

Our aim was not to highlight novelty of the observation but provide the viewpoint via discussion. We think the viewpoint is important, and we did not find similar observations to be quantitatively compared.

Line 343-350: In lines 319-323, there are only positive aspects discussed with respect to particle emissions, but here suddenly negative impacts on air quality by BC are addressed despite being irrelevant from the data presented in this study.

Good comment. However, we think that BC should be one quantity to be discussed. E.g. WHO have underlined its role and need to get more BC data.

Line 355-357: Whenever fresh particle emissions are filtered and subsequently compared to the secondary aerosol formation potential, the enhancement ratio turns out to be extraordinarily high suggesting air quality problems. However, the secondary aerosol formation has to be discussed on an EF-basis for a meaningful comparison. Following this, my conclusion would be that biomass combustion in a power plant generates orders of magnitude lower (primary and secondary) aerosol emission than small-scale (Ozgen 2014 Atmos Environ; Shen 2012 Environ Sci Tehcnol; Syc 2011 Environ Sci Technol; Aurell 2012 Environ Sci Technol), turning the enhancement ratio irrelevant.

We wrote that “In our experiment, the absence of semi-volatile compounds before the BHF and low total hydrocarbon concentrations after the BHF were in line with the observation that the flue gas potential to cause SOA in the atmosphere was relatively low, when compared with the primary particle concentrations before the BHF, the SOA potential of small-scale wood combustion (Ihalainen et al. 2019; Hartikainen et al. 2020), or SOA potential of vehicle exhaust (Gordon et al. 2014; Timonen et al. 2017).” In that sense we are thinking similarly. However, we do not think that the enhancement factor is irrelevant: if somebody wants to find further potential emission reductions or air quality improvements, the enhancement factor indicates that the target could be the compounds that contributes to secondary aerosol.

Lien 363: Which secondary aerosol precursors may be formed during NOx removal? In biomass burning, air staging or flue gas recirculation (Steiner 2024 Renew Energy) is much more common compared to exhaust aftreatment like SCR.

SCR can contribute to ammonia emissions. Yes, SCR may be future technology in power plants, e.g. combined with other emission reduction techniques. We modified the sentence in discussion: “However, removal of NO_x can generate other compounds that affect secondary aerosol emissions, such as ammonia.”

Line 375-377: If there are only two orders of magnitude between the emission and the ambient concentration of Hg, I would conclude that the power plant is well below average contributor. Dilution ratios of emissions from a point source to ambient air rather ranges from three to five orders of magnitude (Robinson 2007 Science).

Thank you, good comment again. We added following sentence to the end of discussion: “However, due to the dilution of flue gases in the atmosphere, the observed flue gas concentrations of Hg may not contribute significantly to urban air quality.”

Lien 380: I know small- and large-scale appliances, but what is real-scale in this context? I would expect that you design a lab scale experiment and set it to real-scale in the second setup.

Corrected to “large-scale”.

Language

Line 41-34: I suggest to remove the brackets. Done

Line 309: Replace “societies’” by “anthropogenic”. Done

Line 358: Two times “especially”. Solved

References

Aurell, J., Gullett, B. K., Tabor, D., Touati, A., & Oudejans, L. (2012). Semivolatile and volatile organic compound emissions from wood-fired hydronic heaters. Environmental Science & Technology, 46, 7898–7904.

Czech, H., Pieber, S. M., Tiitta, P., Sippula, O., Kortelainen, M., Lamberg, H., Grigonyte, J., Streibel, T., Prévôt, A. S. H., Jokiniemi, J., & Zimmermann, R. (2017). Time-resolved analysis of primary volatile emissions and secondary aerosol formation potential from a small-scale pellet boiler. Atmospheric Environment, 158, 236–245.

Guevara, M.; Enciso, S.; Tena, C.; Jorba, O.; Dellaert, S.; Dernier van der Gon, H.; Perez Garcia-Pando, C. (2024) A global catalogue of CO2 emissions and co-emitted species from power plants, including high-resolution vertical and temporal profiles. Earth System Science Data, 16(1), 337-373

Heringa, M. F., DeCarlo, P. F., Chirico, R., Tritscher, T., Dommen, J., Weingartner, E., Richter, R., Wehrle, G., Prévôt, A. S. H., & Baltensperger, U. (2011). Investigations of primary and secondary particulate matter of different wood combustion appliances with a high-resolution time-of-flight aerosol mass spectrometer. Atmospheric Chemistry and Physics, 11, 5945–5957.

Inman, M. (2008) Carbon is forever. Nature Climate Change, 1, 156-158

Khodaei, H., Guzzomi, F., Patiño, D., Rashidian, B., & Yeoh, G. H. (2017). Air staging strategies in biomass combustion-gaseous and particulate emission reduction potentials. Fuel Processing Technology, 157, 29–41.

Kleinhans, U., Wieland, C., Frandsen, F. J., & Spliethoff, H. (2018). Ash formation and deposition in coal and biomass fired combustion systems: Progress and challenges in the field of ash particle sticking and rebound behavior. Progress in Energy and Combustion Science, 68, 65–168.

Lepistö, T., Lintusaari, H., Oudin, A., Barreira, L. M. F., Niemi, J. V., Karjalainen, P., Salo, L., Silvonen, V., Markkula, L., Hoivala, J., Marjanen, P., Martikainen, S., Aurela, M., Reyes, F. R., Oyola, P., Kuuluvainen, H., Manninen, H. E., Schins, R. P. F., Vojtisek-Lom, M., Ondracek, J., Topinka, J., Timonen, H., Jalava, P., Saarikoski, S., & Rönkkö, T. (2023). Particle lung deposited surface area (LDSAal) size distributions in different urban environments and geographical regions: Towards understanding of the PM2.5 dose-response. Environment International, 180, 108224.

Ortega, A. M., Day, D. A., Cubison, M. J., Brune, W. H., Bon, D., Gouw, J. A. de, & Jimenez, J. L. (2013). Secondary organic aerosol formation and primary organic aerosol oxidation from biomass-burning smoke in a flow reactor during FLAME-3. Atmospheric Chemistry and Physics, 13, 11551–11571.

Ozgen, S., Caserini, S., Galante, S., Giugliano, M., Angelino, E., Marongiu, A., Hugony, F., Migliavacca, G., & Morreale, C. (2014). Emission factors from small scale appliances burning wood and pellets. Atmospheric Environment, 94, 144–153.

Robinson, A. L., Donahue, N. M., Shrivastava, M. K., Weitkamp, E. A., Sage, A. M., Grieshop, A. P., Lane, T. E., Pierce, J. R., & Pandis, S. N. (2007). Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical Aging. Science, 315, 1259–1262.

Shen, G., Tao, S., Wei, S., Zhang, Y., Wang, R., Wang, B., Li, W., Shen, H., Huang, Y., Chen, Y., Chen, H., Yang, Y., Wang, W., Wei, W., Wang, X., Liu, W., Wang, X., & Masse Simonich, S. L. y. (2012). Reductions in emissions of carbonaceous particulate matter and polycyclic aromatic hydrocarbons from combustion of biomass pellets in comparison with raw fuel burning. Environmental Science & Technology, 46, 6409–6416.

Sippula, O.; Hokkinen, J.; Puustinen, H.; Yli-Pirila, P.; Jokiniemi, J. (2009) Particle Emissions from Small Wood-fired District Heating Units, Energy and Fuels, 23, 2974–2982

Steiner, M.; Scharler, R.; Buchmayr, M.; Hochenauer, C.; Anca.Couce, A. (2024). Benchmarking of primary measures to achieve lowest NOx emissions in small-scale biomass grate furnaces. Renewable Energy, 234, 121226

Šyc, M., Horák, J., Hopan, F., Krpec, K., Tomšej, T., Ocelka, T., & Pekárek, V. (2011). Effect of fuels and domestic heating appliance types on emission factors of selected organic pollutants. Environmental Science & Technology, 45, 9427–9434

Citation: https://doi.org/10.5194/ar-2025-14-RC1

Referee 2:

The study by Mylläri et al. focuses on characterizing emissions of primary and secondary aerosols from a large-scale wood pellet fired heating plants. The problem of particulate matter (PM) emission by large boilers in power plants has been virtually solved, and the removal efficiency of these devices higher than 99% has been achieved, see for example thhttps://www.sciencedirect.com/science/article/abs/pii/S1364032120307334, the potential for the formation of Secondary aerosols is highlighted in this work.

The introduction and most of the paper seems to overlook most of the work on this subject since 2019. All the references are old (over 10 years old).  A more comprehensive Literature review that also compares small scale and large-scale heaters is needed. Comparisons with laboratory scale measurements will help validate the work.  Some recent work for example some listed below and many more need to be cited.

https://aaqr.org/articles/aaqr-21-11-oa-0301

https://www.sciencedirect.com/science/article/abs/pii/S0048969719318601

https://www.mdpi.com/1996-1073/17/13/3087

https://www.mdpi.com/2073-4433/10/9/536

https://www.cetjournal.it/cet/22/92/077.pdf

Thank you for this comment. We found those articles valuable for our work and added some of those to the references. Especially the observations on PAH emissions were interesting and motivate the OA measurements, not only in this study but also in future studies focusing on pellet burning emissions.

Our experiments were conducted at large-scale power plant, aiming to provide understanding of large-scale power plant emissions. Our experiments focused on detailed particle emission characterizations so that also the atmospherically relevant processes were taken into account. We included some of small-scale pellet boiler studies into the introduction, but decided not to focus more on those. In general, according to our knowledge, the data gap was especially in the emissions of large-scale power plants, and we know that their operation significantly differs from small-scale devices.

The experimental section is not clearly written and assumes everyone reading this paper knows the details. For example, no description is given on how the chemical composition of the Pellets was determined or how the results in Figure 5 were generated. The experimental section needs more details, describing the conditions of the measurements, assumptions etc. so what is done can be reproduced. As it appears it is very vague and lacks details.

We added more details to the experimental section. Probably most important addition was the addition of citation to Niemelä et al. which was conducted for same power plant (see also the fuel analyses part). Furthermore, we added references regarding the sampling and ageing of flue gas, and hope that the supplementary information help reader to find needed information. In general, together with references cited there in the experimental part, we think that the experiment can be reproduced.

Specific comments

Line 43: Is biomass renewable source? How about the climate impacts of Biomass burning? Biomass burning has significant climate impacts, primarily through the release of greenhouse gases and particulate matter.  See also https://www.sciencedirect.com/science/article/abs/pii/S1674200123001761

Yes, we think that the biomass is renewable energy source. Its use can have climate impacts, due to the effects on land-use (not investigated by us) and emissions of BC and secondary aerosols investigated in our study. Thank you for the reference, it was valuable addition to our work.

Line 44: other gaseous compounds? What are they?

We added “compounds such as Nox and CO” as examples and typically measure compounds.

Line 46. Lack of scientific literature on emission of large power plants utilizing biomass fuels is wrong. See for example https://docs.nrel.gov/docs/fy23osti/83308.pdf

We removed the sentence. It was meant to point out the lack of aerosol emission information from large scale power plants, meaning more detailed information than PM mass.

Line 112-113: Need to cite source for Pellet chemical composition for example https://pubs.acs.org/doi/10.1021/ef300884k

Cited now.

Line 248-49: How is the Hg content measured?

As described in the experimental section, “…mercury (Model 2537A Hg analyzer, Tekran) next to the SMPS and the CPC. For mercury, the analyzer setup removed the particles in the sample stream with a polytetrafluoroethylene (PTFE) membrane filter (47 mm, 0.2 µm); thus, we measured the total gaseous mercury that consists of both elemental and gaseous oxidized mercury (Kyllönen et al. 2012)” I hope this is the information needed to be involved in manuscript

Line 272-276. Need more details on the aging experiments and the SOA formation process. Hard to follow as written.

It is difficult to see what kind of additional information is needed. I think the Simonen et al. 2017 is good reference for that purpose. In addition, also Czech et al. and Heringa et al. in the introduction can help in that sense, and the supplementary information.

Citation: https://doi.org/10.5194/ar-2025-14-RC2