General Comment

This technical note deals with the spatial distribution of particles emitted from road traffic and from cooking sources in different vertical levels inside a street canyon. The PALM model coupled with the sectional aerosol dynamics model SALSA is used to determine the relevance of aerosol processes in a similar configuration as in Kurppa et al. (2019). From the analysis of time scales it is concluded that deposition mainly affects particles in the air close to the canyon surfaces, while the relevance of coagulation is related to the mean tracer age. Compared to Kurppa et al. (2019) the novelty of the present work appears to be the consideration of particles emission spectra from kitchen exhaust ducts, which have a higher fraction of small particles than emission spectra from traffic.

The presentation of the Methodology part should be better organized, in particular with a separate section for the emission scenarios. The emission scenarios need to be in one place early in the method section because all the result sections are referring to the scenario abbreviations. The validation section is confusing.

My main concern is that the time scale of mean circulation, i.e. residence time in the street canyon, of 380 s is very long in comparison to other published studies on street canyons. For example, Nikolova et al. (2014) report CFD simulations of aerosol particles for a real street canyon in Antwerp having unit aspect ratio and a dilution time scale of 110 s for low wind. Ketzel and Berkowicz (2004) give dilution time scales within a range of 45−120 s. The long recirculation cycle does not seem realistic even at low winds, hence leading to an overestimation of the contribution of coagulation to the reduction of mean particle number concentrations compared to the case with no aerosol processes.

The formation of a stable vortex holds for neutral conditions, but it needs to be tested what consequences unstable conditions with thermal convection have on the concentration distribution in the street canyon. Another aspect to consider is that when the wind is parallel with the street, the recirculating structure within the cavity disappears completely, and the concentration field becomes very different. The authors should state such important limitations of the presented CFD simulations early in the text. Therefore, I suggest emphasizing that an idealized configuration of the street canyon was chosen, for the purpose of the study of particle emissions from different pollutant sources inside the street canyon. In conclusion, I believe that the paper should have a careful revision before publication.

Specific Comments

1.) Section 2.2.1: Provide more details on the configuration of the street canyon and mention which aspects are different to the street canyon simulated in the work of Kurppa et al. (2019).

2.) Section 2.2.2 has to be divided in a section on configuration of SALSA in this study and a section on emission scenarios.

3.) More details on the coupling of SALSA with PALM need to be provided. For example, are the particle emissions entering into the SALSA model or first into the PALM model? Deposition of particles: only to street surface or also and wall surfaces; in which distance from the surface are particles affected by deposition? Condensation of which gases?

4.) Section 2.3: the presentation of the validation is unclear. Several references to figures are missing. Maybe first mention what kind of validations were performed, then describe each test in a separate paragraph.

5.) P. 7 lines 148 - 153: explain the difference of the simplified computational domain used in the validation and the computational domain in K19. Is the simplified computational domain intended to mimic the real street canyon in Cambridge? I think Figure 5 belongs to this validation, but it is not referenced here.

6.) P. 13 line 2: Figure 8 shows only boiling. Where is the figure panel for isolated kitchens, deep frying? Figure parts fig. 8a and 8d are not referenced in the text.

7.) P. 14 lines 219 - 222: Time scale analysis for a street canyon in Cambridge by Kumar et al. (2008) reveals that dry deposition to road surface is much faster than deposition to wall surfaces. Can such a differentiation be made in this study as well?

8.) P.17 lines 254 – 256: The "plume-like structure" for case CO-D cannot be inferred from Figure 11. Should this refer back to Figure 7? It should be better indicated in the plot, how the plume like structure from column kitchen emissions develops.

9.) Section 3.4: it is not immediately clear where in the street canyon the aerosol number distribution were taken. If it is the canyon average distribution, then the standard deviations should be included in Figure 12. Where in the street canyon should measurements be done to be most sensitive to emissions of each of the different source types?

10.) Section 5.1: the description of the background particles needs to be improved. How is the background aerosol mixed into the street canyon - is the spatial distribution the same as for the emissions or is the background entering from the boundaries? Did the simulations take into account heterogeneous coagulation between the emitted particles and the background particles, or were they assumed to be of the same population?

Technical Corrections:

P. 7 lines 139: correct “ine Appendix B”.

P. 14 line 229: deep frying?

26 line 431: delete "happens".

P. 27 line 440: delete "Jacobson and"

P. 27, equation (A.5): c is the particle mole concentration and thus should have index of particle-phase, not gas "g".

References

Ketzel, M. and Berkowicz, R.: Modelling the fate of ultrafine particles from exhaust pipe to rural background: An analysis of time scales for dilution, coagulation and deposition. Atmos. Environ., 38, 2639-2652, doi:10.1016/j.atmosenv.2004.02.020, 2004.

Kumar, P., Fennell, P., Langley, D., and Britter, R.: Pseudo-simultaneous measurements for the vertical variation of coarse, fine and ultrafine particles in an urban street canyon, Atmos. Environ., 42, 4304-4319, doi:10.1016/j.atmosenv.2008.01.010, 2008.

Nikolova, I., Janssen, S., Vos, P., and Berghmans, P.: Modelling the mixing of size resolved traffic induced and background ultrafine particles from an urban street canyon to adjacent backyards, Aerosol and Air Quality Research, 14, 145-155, doi:10.4209/aaqr.2013.06.0221, 2014.

Review of the manuscript “Technical note: Dispersion of cooking-generated aerosols from an urban street canyon” by Shang Gao et al.

The manuscript deals with the interesting and atmospheric relevant topic of the dispersion and processing of cooking and road traffic generated aerosol particles in urban street canyons. The authors address this topic using the building-resolving computational fluid dynamics model PALM, which also include the sectional aerosol dynamics module SALSA. The model was setup for an hypothetical and simplified street canyon with road traffic emissions or cooking generated aerosol emissions from the surrounding buildings. The authors address how the type, location and aerosol dynamics of the emissions influence the concentration in the street canyon.  

Apart from a few typos and minor grammatical errors I think the manuscript is generally well written. I agree with reviewer 2 that it could benefit from some restructuring. Also consider if you need all 16 figures. After careful revision I think the manuscript has the potential to be publishing as an atmospheric relevant “technical note” in atmospheric chemistry and physics.   

General comment:

What I mainly miss with the manuscript is a more careful motivation to the choice of the simplified (idealized) street canyon, the primary particle emission size distributions from the different emission sources, the meteorological conditions and the location of the cooking emissions, especially since the model is compared against real observations of wind velocity profiles from a wind-tunnel study (Figure 4), and observations from a specific street canyon in Cambridge (Figure 5).

No very much specific information is given about how the aerosol dynamics is represented in the model in the current study. Only the primary particle emissions are described, with some details. Especially I miss information about how the condensation of different vapors were treated in the model. E.g. what properties were assigned to the semi-volatile condensable vapors HNO₃, NH₃ and SVOCs.and how do you calculate their volatility with respect to the aerosol particle phase? For HNO₃, NH₃ is should depend on the aerosol liquid water content and acidity.  

Specific comments:

Abstract, I miss one sentence which motivate why this study is important from an atmospheric chemistry and physics perspective.

L24-26, “Deposition is usually the only aerosol process included in urban CFD models as it is the most important for traffic emissions within street canyons (Kumar et al., 2011).” The reference to this statement is a bit old, is this statement still true?  Also consider to replace “as it is the most important” with as it is often assumed to be the most important loss process of ultrafine particles. If you do not consider other process you cannot judge their importance. Hear you also only refer to loss processes and not formation processes such as atmospheric new particle formation which can be a major source of ultrafine particles also in urban environments.

Line 35-36, “There are strong reasons for expecting the dispersion of traffic-generated and cooking-generated aerosols to differ qualitatively.”  Consider to reformulate this sentence.

L37, “diameter of O(10 nm)” What do you mean with O?

L44 “The effects on the aerosol dynamic processes are highlighted” Do you mean he effects of the aerosol dynamic processes are highlighted ?

L56-57, “the inclusion of transient dynamics allows for nonlinear aerosol processes to be represented more accurately (see Sec. 5.2).” What do you mean with this statement?

L60-61, “Nucleation, which is computationally expensive to simulate, is not considered in this work.” In which way do you mean that nucleation is computationally expensive to simulate? Usually nucleation is parameterized as a rate only depending on e.g. the H₂SO₄ concentration, or H₂SO₄ and NH₃. The concentrations of these vapors you anyway have to calculate in the model for the condensation growth.

L71-72, “semi-volatile (NVOCs) and non-volatile organics (SVOCs)”. It should be semi-volatile (SVOCs) and non-volatile organics (NVOCs)

L73, “however, chemical transformations are excluded.” What exactly do you mean with this statement? Did you not consider any gas-phase chemistry at all? If this is the case, please state this clearly.

L83, “The flow is driven by an external pressure gradient, dp/dx = -0.0006 Pa m⁻¹.” I cannot judge if this is a reasonable value. Can you add some information about typical values and a reference?

L109-110, “The emission factor for the number of particles emitted by a vehicle per unit distance travelled is 3.0×10¹⁴ km⁻¹ veh⁻¹ (Fujitani et al., 2020)” This, cannot always be a fixed value. At least replace “is” with e.g. “was estimated to be”.

L115-116, “The emission factors for the number of particles emitted per unit time by a kitchen of unit volume are 3.75×10¹⁰m⁻³ s⁻¹ and 4.31×10⁹m⁻³ s⁻¹, for deep frying and boiling, respectively.”  Replace “are” with e.g. “were estimated to be”.

Figure 3. The selected traffic emission spectrum from Janhäll et al., 2004 is relatively old. Is this still representative for the more modern car fleet today? I imagine that the fraction of nucleation mode particles may have gone up while the soot mode may have decreased with more modern cars? But, I may be wrong. Can you find any more recent references to at least compare with? Quite a lot of your conclusions are based on the selected size distributions of traffic, deep-frying and boiling emission size distributions.  

Line 148-149, “Following K19, the coupled PALM-SALSA model is validated against evening measurements of the aerosol number concentration within a real street canyon in Cambridge, UK (Kumar et al., 2008). Can you really evaluate your model results against these observations? How similar are the Cambridge street canyon compared to your idealized street canyon. How does the meteorological conditions during the measurements agree with the neutral conditions with the temperature fixed at 300 K?

L151, “only are considered.” Change to are only considered

Figure 5, I miss a describing text and reference to Fig. 5 in the manuscript.

L169-170 “Deep frying (NG-D) and boiling (NG-B) yield identical concentrations in the absence of aerosol dynamic processes. Please explain why this is the case. E.g. Deep frying (NG-D) and boiling (NG-B) yield identical concentrations in the absence of aerosol dynamic processes because the location of the emission sources are identical.

L175-176 “One possible explanation for this discrepancy is that the emission spectra differ:

the mean particle size is larger in the current study, i.e. 47.9 nm rather than 32.7 nm.” This again makes me wonder about how representative the selected traffic emission spectrum is.

L176-177 “This is significant because smaller particles may have a larger deposition velocity” When you refer to small particles I think you mean submircon particles < 1000 nm in diameter. In this, case are not small particles (e.g. ultrafine particles) always having greater deposition velocities than larger >100 nm diameter particles?  

L216, “Condensation has a negligible effect …” Does this not also depend on the model assumptions/limitations? Also evaporation of semi-volatile species from the fresh exhaust particles could potentially have large influence on the particle number size distribution, especially at the selected high temperature of 300 K. Some recent studies claim that particles can grow very rapidly by nitric acid an ammonia condensation, see e.g:

Wang, M., Kong, W., Marten, R. et al. Rapid growth of new atmospheric particles by nitric acid and ammonia condensation. Nature 581, 184–189 (2020). https://doi.org/10.1038/s41586-020-2270-4.

Could the importance of such claimed rapid growth phenomenon be studied and verified or dismissed using PALM-SALSA?

L275 “O” What do you mean?