General Comment

The paper reports on measurements of particulate and gaseous compounds in primary emissions and secondary mass formation in the exhaust plumes of transit buses that utilize different fuels and after-treatment systems. The primary focus of this study to explore the potential for secondary pollutant formation using an oxidative flow reactor and to conduct a detailed chemical characterization of both gas and particle phase compounds.

The study is interesting and presents comprehensive data on primary emissions of buses and composition of aged exhaust. With regards to the novelty of methods the study should be published in Atmospheric Chemistry and Physics. However, the employed methodologies for exhaust sampling and determining particle mass emission factors of fresh and aged emissions are difficult to evaluate based on the provided information. The paper lacks an explanation of the regulatory purpose of secondary particle mass (Delta-PM) and how such a regulation can be implemented across engines and fuels given the wide variation of oxidation conditions in the oxidative flow reactor.

Specific Comments:

1.) Introduction (P3, line 77-82): Mention that dicarboxylic acids have been found in the primary emissions of diesel vehicles (Arnold et al., 2012) and that they are possible candidates for nucleation of particles in diesel exhaust, please refer to Pirjola et al. (2015).

2.) Introduction (P3, line 85-92): Suggest a separate paragraph on oxidation flow reactors. Their use in connection with point sampling at street locations should be explained in more detail. For example, the cited study by Kuittinen et al. (2021) uses the OFR to assess secondary aerosol production during standard driving cycles and real-world driving cycles. The combination with point sampling seems to be relatively new and the in-plume measurement at a single point is not well characterized compared to the measurement in driving cycles where the diluted exhaust is directly introduced into an oxidation flow reactor.

3.) Methods (P4, line 104-106) It remains uncertain how well the sampling represents concentrations in the bus plume. Does the sampling integrate over the horizontal and vertical expansion of the plume? Please discuss how temperature, wind speed and direction affected the emission factor of fresh PM.

4.) The aged PM from different measurements seems to be hardly comparable because of a wide range of different OH exposure in the chamber. Were there any controls made for ageing PM to ensure that no desorption from the wall or losses on the walls affects it? Lubrication oils might substantially contribute to aged PM, which do not depend on the type of fuel.

5.) Page 7, line 209-210: The explanation for the less pronounced dependence of EF_PM:aged on OHexp for other buses is unclear. Potential large differences in emissions and dilution effects are already reflected in the fresh PM, how can that affect the dependence of aged PM on OHexp?

6.) Figure 3: The grouping of Delta-PM according OH exp in Fig. 3 should be explained. Where all buses measured at five different OHexp in the Go:PAM or do the groups represent different vehicle and fuel types? What is the explanation of the lower dPM at 1-5 OH days in the tunnel study by Tkacik et al., 2014?

7.) Table 2: a sulfur-containing compound with sum formula CH4SO3 is among the top 10 emission factors of fresh gaseous emissions. Is this methanesulfonic acid, CH4O3S? If yes, then elaborate more on it because it is usually known as a tracer of secondary biogenic organics in marine environments.

8.) Conclusion/ atmospheric implications (P17, line 389-392): It is obvious that the omission of the secondary formation of particulate matter in engine exhaust in current legislation is problematic for understanding the potential impacts of mobile sources. However, it is unclear how the secondary PM from mobile sources should be implemented in regulations, given that dPM measurements depend on a variety of environmental factors that cannot be controlled, such as OHexp and dilution. The authors should present recommendations how to standardize the comparison of dPM measured at different OHexp.

9.) Conclusion/ atmospheric implications (P17, line 396-400): Please elaborate more on the atmospheric implications of HNCO from city buses, e.g. chemical reactions in the atmosphere, relevance of urea-SCR exhaust systems in different buses and bus fleets, and expected concentration in street environments. For example, relate the results of this study on bus emissions of isocyanic acid to the study by Jathar et al. (2017) who investigate diesel engines as an atmospheric source of isocyanic acid in urban areas.

References:

Arnold, F., Pirjola, L., Rönkkö, T., Reichl, U., Schlager, H., Lähde, T., Heikkilä, J., and Keskinen, J.: First online measurements of sulfuric acid gas in modern heavy-duty diesel engine exhaust: Implications for nanoparticle formation, Environ. Sci. Technol. 46, 11227−11234, 2012, https://doi.org/10.1021/es302432s, 2012.

Pirjola, L., Karl, M., Rönkkö, T., Arnold, F.: Model studies of volatile diesel exhaust particle formation: are organic vapours involved in nucleation and growth?, Atmos. Chem. Phys., 15, 10435–10452, https://doi.org/10.5194/acp-15-10435-2015, 2015.

Jathar, S. H., Heppding, C., Link, M. F., Farmer, D. K., Akherati, A., Kleeman, M. J., de Gouw, J. A., Veres, P. R., and Roberts, J. M.: Investigating diesel engines as an atmospheric source of isocyanic acid in urban areas, Atmos. Chem. Phys., 17, 8959–8970, https://doi.org/10.5194/acp-17-8959-2017, 2017.

This paper studies the primary and secondary emissions of buses with a variety of different modern fuels, using a combination of gaseous, PM and CIMS measurements. This is potentially important as people switch from traditional fossil fuels to more sustainable fuels and buses often lead the way on this, mandated by local authorities.

Generally, ACP's remit would normally be concerned with atmospheric processes and implications rather than emissions profiling, however given the chemistry associated with the secondary production, I would see this an in-scope. That said, the paper is very much focused on the results rather than the atmospheric implications. But the data that is presented seems to have been collected and treated in an appropriate manner. It should be noted that the use of OFRs is not established as a perfect simulation for atmospheric processes (some of the shortcomings are referred to in the paper), but still should be taken as an indication that secondary aerosols are worthy of concern. The manuscript is generally well written, although it does rely too much on acronyms and symbols, making the paper unclear in places. I recommend publication after minor corrections.

General comments:

The paper's title and overview would give the impression that this is a more comprehensive measurement set than it is, but it must be stressed that the acetate-CIMS method is very selective towards polar molecules such as organic acids. I would suggest modifying the title to something like "... primary and secondary emissions of particulate matter and polar molecules ..." or similar to better reflect the content of the paper. Also, the part dealing with the CIMS measurements (3.2) seems based around what the CIMS is capable of seeing, rather than what is important for the atmosphere. It would help the paper to remain in-scope if the discussion could be based around the importance of the subset of molecules observed in atmospheric chemistry, rather than merely observing what was seen by the CIMS.

Presentation, wise, I found parts of the paper over-reliant on acronyms and symbols, particularly in the figure captions, where I found myself having to jump back and forward several times to understand what was being talked about. It would make the manuscript much clearer if the descriptions could be given verbally more.

Specific comments:

Section 3.3.2: It is worth noting that many of the gaseous emissions measured can also directly form secondary particulate matter in the atmosphere in the presence of water vapour and a base (e.g. ammonia), so these could also contribute to the overall secondary PM yield (in theory).

Line 401: The phrase " nitrogen-containing compounds were significantly reduced" is an unfortunate choice of words, because it is not clear whether "reduced" is in the magnitude or chemical sense. Please rephrase.