Review of Manuscript acp-2021-111:

Particle emissions from a modern heavy-duty diesel engine as ice-nuclei in immersion freezing mode: an experimental study on fossil and renewable fuels, by Korhonen et al.

General comments:

Korhonen et al. perform systematic experiments about the ice nucleation ability of diesel engine particulate emissions at high relative humidity and mixed phase cloud relevant conditions, with auxiliary measurements about particle size distribution, hygroscopicity and chemical composition. The ice nucleation activity data and conclusion presented, demonstrating the poor ice formation ability of diesel engine particles and limited effects from photochemical ageing processes on the particle ice nucleation activity, are of interests to the ice nucleation community. However, some part of discussion and data interpretation are not thoroughly or comprehensively presented. I would like to suggest further revisions before recommending acceptance.

The ice nucleation experiments performed cover a variety of diesel engine emissions generated by three kinds of fuels. Different engine exhaust treatment techniques are used to mimic diesel engine particulate emission atmospheric relevant ageing processes. The research interests are of significance of the atmospheric ice nucleating particles and thus climate. An interesting ice nucleation story is clear. Nevertheless, some improvements need to be made, regarding to data interpretation and results discussion. In general, I have five major comments on this manuscript.

The authors should clarify their samples and research focus clearly and construct an unambiguous approach about the research story, illustrating the relations among all the measurements. For instance, the manuscript title and abstract tell me the particulate emission from a diesel engine will be the object for this experimental study, but the authors directly introduce soot particles in the introduction and the following parts. Note, the particulate emissions from diesel engines are not only comprised of soot particles. Differentiating the concept of soot particles from diesel engine particulate emissions is necessary. The authors also need to explain why soot particles emitted by land transportation diesel engines are atmospherically relevant.

Second, I highly recommend to harmonise the ice nucleation terminology through the manuscript, see Vali et al. (2015) as a reference. The aim is to make dissemination more uniform and consistent within the community.

Third, the ice nucleation pathway, immersion mode freezing, is not well introduced. Only referring to a reference (Korhonen et al. 2020) without a brief introduction about how this can be achieved is not enough, in my opinion. Both the concept and the approach to achieve it should be explained even as a summary and then referring to the previous literature can be of more clarity and convincingness.

Forth, a new approach to present normalized ice activation fraction results should be explained more clearly in Sect. 2 and 3. Also, it should be applied carefully. Note that, the particle samples are polydisperse with different particle size distributions (see Fig. 2) and the proportion of large particles (e.g. > 100 nm ~ 10 %) is still comparable or even higher than the highest ice activation fraction measured. For instance, the highest ice activation fraction for unaged RME fuel engine particles is less than 0.35 % even at the lowest temperature addressed, as shown in Fig. 6. To figure out the contribution of large particles (e.g. 100, 200 or 300 nm) to the ice activation fraction, ice nucleation experiments for size selected (e.g. 100, 200 or 300 nm) large particles are expected to performed. In addition, the data processing or the calculation method needs to be formulated and then the specific equation can help the understanding.

Finally and most importantly, the results discussion is performed not thoroughly and reasonably. For example, comparing the ice nucleation results of polydisperse aerosol dominated by fine particles (< 100 nm) with the homogeneous freezing of larger (350 nm) ammonium sulfate (AS) particles is inappropriate. Instead of 350 nm AS particles, the comparison with the homogeneous freezing results of small (< = 100 or 150 nm) AS particles would be of more relevance to the results of diesel soot particles which has a small size distribution. Especially, the diesel engine particles already exhibit a homogeneous freezing depression event at temperatures lower than the homogeneous freezing temperature at such a high relative humidity (RH_w = 110 %).  In addition, the ice nucleation data is not well linked to the auxiliary measurement results. Similar findings in the literature are also helpful to support the conclusion (see detailed comments in next part). 

Specific comments:

Line 17: change ‘continuous-flow diffusion chamber’ to the same as it is in Line 87

Line 19: change to ‘-43 and -32ºC’. The same for Line 43, 92, 96, 175, 234 and 268. Please check through the mathematical notation and make it satisfy ACP terminology.

Line 23: change ‘present’ to ‘presented

Line 24: make ‘different emission after-treatment systems’ specified

Line 27: change to ‘the radiative forcing of the Earth and thus climate in different ways’

Line 39: change to ‘homogeneous ice nucleation’

Line 41: change to ‘ice nucleating particles’ and specify its abbreviation ‘INPs’

Line 39 to 41: Please provide evidence or reference to show combustion emissions are relevant to the lower troposphere ice nucleation activities.

Line 44 and 45: If you write that soot particles are not active INPs, the relative humidity and temperature condition also need to be reported.

Line 50: the reference ‘Mahrt et al. 2018’ should be irrelevant to the atmospheric aging processes for INPs but Mahrt et al. (2020a) and (2020b) can be references.

Line 58 and 59: change to ‘the climate forcing due to anthropogenic soot particles immersion freezing’

Line 60: change to ‘ice nucleation abilities’

Line 72 to 74: The environmental pollution caused by diesel engine without DPF or DOC technique is not relevant to this research topic.

Line 129 to 131: was the Aerosol Instrument Manager (AIM) software used to log the SMPS data? If so, the SMPS scan size upper limit should be much larger than 500 nm with such a high sheath to aerosol sample flow ratio (10 : 1) and a 180 s scanning time. And if the size scan did not cover the whole range of the aerosol particle size distribution, the multiple charge correction is not finished and then the results are biased by the uncomplete correction calculation.

Line 130 to 142: Better to introduce the measurements work flow following the sample flow sequence depicted in Fig. 1.

Line 179 to 181: The authors need to make a more conceivable and clear statement for distinguishing water droplets from ice crystals. I understand that the basic idea is to let the OPC running in different size channels and then to differentiate the particle phase according to their survival abilities through the evaporation section, i.e. water droplet can be evaporated because of the relative humidity condition. The statement about CCN ability and immersion mode freezing make readers confused. In addition, referring to the study performed by Korhonen et al. (2020) as an example does not make sense for me. This is because the samples are different between the current study (i.e. diesel engine particulate emissions) and the previous study (i.e. particulate emissions from solid-biomass-fired cookstoves). The OPC channel size used to discriminate water droplets from ice crystals should be stated from the current study.

Line 190: change to ‘exiting the IN chamber’ or ‘exiting the SPIN’. Or, the authors can decide to use ‘SPIN’ or ‘the SPIN’ through the whole manuscript.

Line 215 to 225: In this paragraph is not well organised. In my point of view, the authors may need to explain how the freezing of a particle immersed in a water droplet could happen when the temperature decreases lower than the homogeneous freezing temperature (HNT), to illustrate the results presentation. A suggestion could be that sample particles might be activated as cloud droplets at RHw = 110 % for temperature conditions higher than the HNT, thus makes it possible to investigate the particles immersion mode freezing ability at T < HNT in the flowing temperature scan because a droplet would freeze homogeneously when T is lower than HNT. Here again, the ice crystal formation of droplet activated particles at T < HNT should be homogeneous freezing. If the authors claim this is immersion freezing, evidence of this should be presented. But if the freezing occurs at RH conditions above homogeneous RH condition at the same T, then it is unclear how the authors can conclude immersion freezing to be the relevant mechanism.

Line 223 to 225: A clear definition for the normalization of the ice activation fraction curves for each sample should be made. A formulation for this approach or an example may help.

Line 232 and 233: change ‘L/min’ to ‘L min^-1’. Please check the unit through the manuscript.

Line 235: The CCNC calibration curves should be provided in the following section or in an Appendix part.

Line 251: The calibration results should be provided in the following section or in an Appendix part.

Line 265: What is the ‘GMDs’?

Line 285: change to ‘Ice activation fraction curves for fossil diesel emissions are presented in Figs. 3 and 4’

Line 287: change to ‘Fig. 3a’. And the similar suggestion to that of Line 298, 310, 314, 324, 326 and 327. Please check the abbreviation for ‘Figure’ through the manuscript.

Line 291: change to ‘Fig. 3b’; Specify which two samples

Line 294: Here, what is the size range for the so-called ultrafine particles? It should be 100 nm if the number 90 % refers to the size distribution results mentioned in Sect. 3.1. Please make the ultrafine particle with a quantitative value for clear discussion.

Line 300 to 307: The discussion in this paragraph can be better. First, the ice formation enhancement by lowering the ice onset temperature values should be clearly connected to the sample to make it easier for readability. Second, some evidence form auxiliary measurements should be provided to interpret the results. Also, relevant studies in the literature can be referred to for comparison, e.g. Zhang et al. (2020) also investigated the photochemical aging effects on soot particles ice nucleation activities at T < HNT.

Line 313: change to ‘the lowest temperature’

Line 316 and 317: Arguing that the -36.1 ºC is outside of the instrument uncertainty should refer to the homogeneous freezing temperate detection ability of SPIN.

Line 327 and 328: Why use the size distribution results about fossil fuel in Fig. 2a (‘left-hand panel of Fig. 2’ in text) to interpret the ice activation results of RME emissions?

Line 335 to 347: The discussion in this paragraph is too general and not specific enough. For example, the auxiliary measurement results for each sample should be connected to the sample directly, instead of making a general statement or a conclusion (e.g. Line 336 to 338 about CCNC results) for the overall study. The statements also should be clearly related to the quantitative values obtained from the supportive measurements. In addition, explanation or definition about each measurement result, e.g. OA, C₁₁/C₃, should be made in the main text. Necessary references in the literature also need to be referred.

Line 348: change to ‘Summary and conclusion’. Because the discussion is largely presented in previous sections and this part is more about conclusions.

Line 349 to 368: I disagree with the logicality in this part. On the one hand, the authors conclude that small diesel engine particles have no contribution to ice nucleation activities (Line 361). On the other hand, they are comparing the ice nucleation ability of the particles produced by different fuels. The authors need to firstly demonstrate ice nucleation activity via the immersion mode really occurs then they can make statement about the efficiency of the soot particles as potential ice nucleating particles (INPs). The reference about the study presented by Kanji et al. (2020) in Line 354 is inappropriate, which states that their findings are in complete agreement with Kanji et al. (2020).

Figures and Tables:

Figure 1: I cannot find where the ‘FPA-fast particle analyser’ is in the figure. It is not mentioned in the main text, either.

Figures 2: The size distribution measurement for ‘engine-out + BP’ sample presented in Fig. 5 is missed in Fig. 2b and should be provided. And there is no SPIN experiment corresponding to the sample ‘DOC + PAM’ in Fig. 2b. In addition, it would be helpful if the figure grids are on to guide reader’s eyes.

Figure 3: Is the ice activation curve for ‘’Engine-out + BP’ sample normalized by the sample approach as those of other samples? The highest ice activation fraction should be the unity. It looks in corrected.

References:

Kanji, Z. A., Welti, A., Corbin, J. C., and Mensah, A. A.: Black Carbon Particles Do Not Matter for Immersion Mode Ice Nucleation, Geophys Res. Lett., 47, https://10.1029/2019gl086764, 2020.

Korhonen, K., Kristensen, T. B., Falk, J., Lindgren, R., Andersen, C., Carvalho, R. L., Malmborg, V., Eriksson, A., Boman, C., Pagels, J., Svenningsson, B., Komppula, M., Lehtinen, K. E. J., and Virtanen, A.: Ice-nucleating ability of particulate emissions from solid-biomass-fired cookstoves: an experimental study, Atmos. Chem. Phys., 20, 4951-4968, http://10.5194/acp-20-4951-2020, 2020.

Mahrt, F., Alpert, P. A., Dou, J., Gronquist, P., Arroyo, P. C., Ammann, M., Lohmann, U., and Kanji, Z. A.: Aging induced changes in ice nucleation activity of combustion aerosol as determined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, Environ. Sci.: Processes Impacts, https://10.1039/c9em00525k, 2020b.

Mahrt, F., Kilchhofer, K., Marcolli, C., Grönquist, P., David, R. O., Rösch, M., Lohmann, U., and Kanji, Z. A.: The Impact of Cloud Processing on the Ice Nucleation Abilities of Soot Particles at Cirrus Temperatures, J. Geophys. Res. Atmos., 125, 1-23, https://10.1029/2019jd030922, 2020a.

Vali, G., DeMott, P. J., Möhler, O., and Whale, T. F.: Technical Note: A proposal for ice nucleation terminology, Atmos. Chem. Phys., 15, 10263-10270, https://10.5194/acp-15-10263-2015, 2015.

Zhang, C., Zhang, Y., Wolf, M. J., Nichman, L., Shen, C., Onasch, T. B., Chen, L., and Cziczo, D. J.: The effects of morphology, mobility size, and secondary organic aerosol (SOA) material coating on the ice nucleation activity of black carbon in the cirrus regime, Atmos. Chem. Phys., 20, 13957-13984, https://10.5194/acp-20-13957-2020, 2020.

The authors have submitted an article titled: Particle emissions from a modern heavy-duty diesel engine as ice-nuclei in immersion freezing mode: an experimental study on fossil and renewable fuels. The article describes an ice nucleation study with combustion exhaust aerosol ice nuclei. The authors used 3 types of fuel and a diesel engine for aerosol generation. The polydisperse aerosol was introduced to a continuous-flow diffusion chamber at a constant RH while ramping the temperature between -43 ⁰C and -32 ⁰C.  Reference aerosol and 2 processing steps of the engine exhaust after-treatment system were intercompared. In addition, the impacts of different atmospheric processing and ageing steps on ice nucleation activity were evaluated. A range of complementary measurements were taken to characterize the aerosol and explain the IN observations. The authors report overall a poor ice-nucleation performance for all the different aerosol types, after-treatment systems, and photochemical aging with minor differences between the 15 experiments. Overall, the article fits within the scope of the journal and has the potential to have environmental relevance. The authors used state of the art instrumentation and decent experimental planning however, there are few points that would need to be addressed before this manuscript is published.

General comments

The connection between the selected diesel engine and the atmosphere is not established, not in the introduction, nor in the discussions of the results. The relevance of the specific diesel engine and these fuel types globally is further diminished in the paper. This undermines the justification of the selected MPC conditions for the experiments and the global relevance of this study. Consequently causing vagueness in the conclusions.

The manuscript lacking substantial information, some of which is split between several other publications e.g. setup details, characterization of the aerosol, size distributions, laminar flow uncertainty etc. (more details in minor comments).

The selected threshold of 6 micron for ice classification and calculation of activated fraction needs further clarification. This selection and the new approach for calculations of activated fraction are not sufficiently discussed (see minor comments).

Given the comprehensive list of instruments used, one would expect to see a deeper analysis. For example, shape factor, effective density, composition accompanied with figures or histograms, intercomparing aerosol properties in this study with other studies or IN activity of similar particles in previous studies that were mentioned here. This additional information would help to expand the discussion on the observations.

Grammar: in particular sentence structure, fluency, and connection between sentences require significant revisions.

Specific comments:

LN2: “an experimental study” I suggest to change to a laboratory study for clarity

Ln18: “and” change to “while the…”

LN19: The tested fuels i.e., EN 590 compliant low-sulfur fossil diesel, hydrotreated vegetable oil (HVO), and rapeseed methyl ester (RME), all were used without blending.

LN23: "….emitted particles for their physicochemical properties".

LN23: "We found that the studied particles were poor ice nuclei. The substitution…."

LN27:  First sentence: reword, don’t use “/”, what are “impacts on cloud properties” - the sentence is not clear

LN28:  change to “Direct effects can be monitored….”, also provide some examples of direct effects

LN29: “instruments” – provide examples and/or references

LN30: “due to complexity of the processes that contribute to their final effects” – sentence not clear, reword.

LN33: “Furthermore, most precipitation events…” - I couldnt find support for this definitive statement in the reference you provided.

LN38: “than those required”

LN39: “Particles that are active ice nuclei…”

LN41: “hydrocarbon fuels have the potential to nucleate ice at temperatures higher..”

LN41: “than that” – specify what “that” refers to

LN43: you haven’t defined “INP”

LN44: “on the contrary” – on the other hand?

LN50: add Zhang et al., 2020 to the reference list

LN50: “these studies taken altogether…” – sentence needs rewording e.g. “The studies mentioned above demonstrate the challenge in associating soot INP properties to the ambient soot particle population”.

LN53: abilities change to activity

LN54: “Multiple studies such as” – you listed only one study

LN54: “In addition to that” - this sentence doesn’t add to the previous sentence. Wrong conjunction.

LN55: “ (IPCC) have identified gaps in our knowledge of ice nucleation activity of soot in their…”

LN56: This uncertainty? You haven’t mentioned uncertainty, specify what uncertainty you mean

LN56: “..uncertainty reflects to available parameterizations estimating the IN ability of the soot, causing them to range several orders of magnitude” – the context of this sentence is not clear, reword.

LN57: “Consequently, it leads” – what “it” refers to? clarify in the text

LN58: “challenges when the potential…” – rephrase e.g. “high uncertainty in estimation of the radiative forcing via modeling”

LN69-71: reword the sentence

LN72: globally still widely used – reference? Can you provide global estimates that will support the relevance of your study?

LN73: “long time” - please support this qualitative projection with a reference

LN74: “still about to maintain its global popularity for decades” – reference for this forecast?

LN75: I think there is a missing paragraph here that connects between diesel combustion emissions at ground level and how they reach and interact with atmospheric humidity and temperature to form clouds, what’s their known fraction at different altitudes etc. In what clouds they are most predominant to support your choice of temperature range. Perhaps connection to airborne measurements of combustion emissions and their ability to nucleate ice at altitude detected in flight e.g. Brown, 2018.

LN76: the sentence in not clear and is it relevant to this study?

LN78: diesel combustion emissions

LN79: “among other factors, remains less studied” – what other factors and why it’s less studied?

LN84: “as well as they impact” – reword

LN91: “temperatures of -35 ⁰C and -30 ⁰C. No immersion freezing…”

LN93:  remove ice-nucleating potential

LN104: alternatives in the near future

LN104: “it is likely that heavy-duty diesel engines will be in use further than that” – here you say likely in LN73 you were much more decisive

LN106: “We investigate…” – reword this sentence

LN111: did you monitor the temperature and humidity in the sampling line, if so, where? Please add to figure 1. Did you monitor pressure and airflow in the different sections of your setup? Would the high concentration cause sedimentation and narrowing of inner tubing diameter? Did that affect your measurements during the experiments?

LN112: “The test engine used was a six-cylinder inline Scania D13 heavy-duty diesel engine” - why this engine was selected? How representative it is of diesel engines in the world? You should provide few more details to establish how this experiment will provide conclusions relevant to real world (outside the laboratory).

LN115: “approach” – perhaps setup or configuration?

LN116: what is one full temperature scan? How many repetitions did you do to test repeatability?

LN122: “were cooled down” – did you monitor the temperature? Where? How?

LN124: PAM – reference for the instrument?

LN127: RH<10 , where and how it was measured?

LN129: “The latter scanned continuously” - you mentioned only SMPS so not clear what is the "latter".

LN130: “The size range of SMPS…” - The mobility diameter sampling range of the SMPS was set between 11 and 500 nm with automatic multiple charge correction in the software.

LN131: CCNc-100 ? one column?

LN138: remove Moreover

LN142: RH<5, in line127 you said RH<10, was it measured?

LN148: “A low level of exhaust gas recirculation (EGR) setting was used, 18% oxygen on intake air to the combustion cylinder” - why these settings were used? Are they common settings?

LN171: remove in the orders of

LN184: “Thus, we consider observed particles larger than 6 μm ice crystals” - what about ice smaller than 6um? For example, if you have pore filling happening as described. in Mahrt et al. 2018, where they chose 1 µm size threshold to detect ice crystals in their chamber. How would such shift in this threshold affect your results?

LN199: “such as particle losses at the laminar flow are discussed in detail by Korhonen et al. (2020)” - are discussed in Korhonen(2020) but how do you address these deviations from laminarity in your study? or any of the other issues discussed in the conclusions of Garimella et al 2017. If you dont, how much uncertainty it introduces to your results? are they still valid despite those known issues of SPIN?

LN201: 15 experiments – did you do any repetitions?

LN202: “SPIN, and polydisperse” – split into 2 sentences

LN220: switching back and forth between CFDC and SPIN, stick with one naming for the instrument

LN220: “We expect this particle fraction to be dominated by the larger particles, since they are more likely to act as CCN” – what about the doubly charged particles selected with DMA, how they affect this measurement?

LN223: “The normalization was calculated via averaging the five highest observed ice-activation…” – do you expect to measure only one nucleation mechanism in these temperatures e.g. Marcolli 2014?

LN237: “In many cases, the supersaturation did not suffice to activate the studied particles, and results are presented only when a full CCN spectrum could be identified” - do these "many cases" have impact on the setting of 6um threshold for ice in SPIN, which was set mostly because of droplets?

LN243: “to elemental carbon (EC) concentrations” – how was EC measured?

LN253: “was calculated from SP-AMS and aethalometer data” – what is the propagated uncertainty combining the measurements for calculation of this ratio?

LN262: “Gren et al. (2021) present a more comprehensive description of the particle size distributions” - There are references to at least 3 other papers that contain substantial information about this study and the reviewer/reader needs to read these to understand this paper e.g. Gren et al. 2021, Kristensen et al. 2020 Korhonen et al. 2020. Some of the information specific to this experiment is missing and needs to be added either as main text, as appendix, or supplementary material.

LN268: “significant fraction” – how would you quantify this statement?

LN272: what is reasonable detection sensitivity?

LN273: “negligible, in the order of 10^-4 from total sampled concentration” - how this number compares to activated fraction, could all INP be larger than 250nm?

LN273: “Due to this” - Here you claim that most particles were small thus this approach for activated fraction is valid. If the IN are all large, would this approach still hold?

LN274: remove “calculating”

LN284: In addition, we estimate…

LN334: This short section should include deeper analysis, providing more than 1-2 sentences per instrument. For example you describe APM and effective density in the experimental section but I dont see effective density mention in the results, tables, or in the discussions.

LN354: “Besides, our results are in complete agreement with Kanji et al. (2020) who studied…” - how propane combustion aerosol and commercial black carbon particles relevant to this study?

LN357: “was found on the other fuels, HVO and RME” - where is the discussion part of the results? what is the possible explanation for this difference?

LN370-376: “It is worth mentioning that all experiments in this study were conducted under well-controlled laboratory…” – The authors start the article by suggesting there is an importance for such study in MPC conditions. From this paragraph, if I understand correctly, the authors conclude that their results can’t contribute much to our understanding of interactions of diesel combustion emissions in MPC and real atmospheric environment?

LN545-547: switch places

LN595: Fossil diesel has twice more 400nm particles, would you expect it to affect the comparison to HVO and RME?

LN598: “see Gren et al. (2021) for a more detailed analysis on emission particle properties of this study” - this sentence is not clear to me. Why more detailed analysis of this study is in another paper? is this an accompanying paper (part1)?

LN651 Table2: For the values presented with accuracy to 3 decimal places, what is the estimated error on these?

References:

• Brown, A.P. "Contrail Flight Data for a Variety of Jet Fuels," AIAA 2018-3188. 2018 Atmospheric and Space Environments Conference. June 2018.
• Garimella, S., Rothenberg, D. A., Wolf, M. J., David, R. O., Kanji, Z. A., Wang, C., Rösch, M., and Cziczo, D. J.: Uncertainty in counting ice nucleating particles with continuous flow diffusion chambers, Atmos. Chem. Phys., 17, 10855–10864, doi:10.5194/acp-17-10855-2017, 2017.
• Mahrt, F., Marcolli, C., David, R. O., Grönquist, P., Barthazy Meier, E. J., Lohmann, U., and Kanji, Z. A.: Ice nucleation abilities of soot particles determined with the Horizontal Ice Nucleation Chamber, Atmos. Chem. Phys., 18, 13363–13392, https://doi.org/10.5194/acp-18-13363-2018, 2018.
• Marcolli, C.: Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities, Atmos. Chem. Phys., 14, 2071–2104, https://doi.org/10.5194/acp-14-2071-2014, 2014.
• Zhang, C., Zhang, Y., Wolf, M. J., Nichman, L., Shen, C., Onasch, T. B., Chen, L., and Cziczo, D. J.: The effects of morphology, mobility size, and secondary organic aerosol (SOA) material coating on the ice nucleation activity of black carbon in the cirrus regime, Atmos. Chem. Phys., 20, 13957–13984, https://doi.org/10.5194/acp-20-13957-2020, 2020.