Chemical composition of nanoparticles from &alpha;-pinene nucleation and the influence of isoprene and relative humidity at low temperature

Caudillo, Lucía; Rörup, Birte; Heinritzi, Martin; Marie, Guillaume; Simon, Mario; Wagner, Andrea C.; Müller, Tatjana; Granzin, Manuel; Amorim, Antonio; Ataei, Farnoush; Baalbaki, Rima; Bertozzi, Barbara; Brasseur, Zoé; Chiu, Randall; Chu, Biwu; Dada, Lubna; Duplissy, Jonathan; Finkenzeller, Henning; Gonzalez Carracedo, Loïc; He, Xu-Cheng; Hofbauer, Victoria; Kong, Weimeng; Lamkaddam, Houssni; Lee, Chuan P.; Lopez, Brandon; Mahfouz, Naser G. A.; Makhmutov, Vladimir; Manninen, Hanna E.; Marten, Ruby; Massabò, Dario; Mauldin, Roy L.; Mentler, Bernhard; Molteni, Ugo; Onnela, Antti; Pfeifer, Joschka; Philippov, Maxim; Piedehierro, Ana A.; Schervish, Meredith; Scholz, Wiebke; Schulze, Benjamin; Shen, Jiali; Stolzenburg, Dominik; Stozhkov, Yuri; Surdu, Mihnea; Tauber, Christian; Tham, Yee Jun; Tian, Ping; Tomé, António; Vogt, Steffen; Wang, Mingyi; Wang, Dongyu S.; Weber, Stefan K.; Welti, André; Yonghong, Wang; Yusheng, Wu; Zauner-Wieczorek, Marcel; Baltensperger, Urs; El Haddad, Imad; Flagan, Richard C.; Hansel, Armin; Höhler, Kristina; Kirkby, Jasper; Kulmala, Markku; Lehtipalo, Katrianne; Möhler, Ottmar; Saathoff, Harald; Volkamer, Rainer; Winkler, Paul M.; Donahue, Neil M.; Kürten, Andreas; Curtius, Joachim

doi:https://doi.org/10.5194/acp-2021-512

Preprints

https://doi.org/10.5194/acp-2021-512

Preprints

07 Jul 2021

Chemical composition of nanoparticles from α-pinene nucleation and the influence of isoprene and relative humidity at low temperature

Lucía Caudillo¹, Birte Rörup², Martin Heinritzi¹, Guillaume Marie¹, Mario Simon¹, Andrea C. Wagner³, Tatjana Müller^1,4, Manuel Granzin¹, Antonio Amorim⁵, Farnoush Ataei⁶, Rima Baalbaki², Barbara Bertozzi⁷, Zoé Brasseur², Randall Chiu³, Biwu Chu², Lubna Dada⁸, Jonathan Duplissy^2,9, Henning Finkenzeller³, Loïc Gonzalez Carracedo¹⁰, Xu-Cheng He², Victoria Hofbauer¹¹, Weimeng Kong^12,13, Houssni Lamkaddam⁸, Chuan P. Lee⁸, Brandon Lopez¹¹, Naser G. A. Mahfouz¹¹, Vladimir Makhmutov^14,26, Hanna E. Manninen¹⁵, Ruby Marten⁸, Dario Massabò¹⁶, Roy L. Mauldin^17,11, Bernhard Mentler¹⁸, Ugo Molteni^8,20,21, Antti Onnela¹⁵, Joschka Pfeifer¹⁵, Maxim Philippov¹⁴, Ana A. Piedehierro²², Meredith Schervish¹¹, Wiebke Scholz¹⁸, Benjamin Schulze¹², Jiali Shen², Dominik Stolzenburg², Yuri Stozhkov¹⁴, Mihnea Surdu⁸, Christian Tauber¹⁰, Yee Jun Tham², Ping Tian²³, António Tomé²⁴, Steffen Vogt⁷, Mingyi Wang¹¹, Dongyu S. Wang⁸, Stefan K. Weber¹⁵, André Welti²², Wang Yonghong², Wu Yusheng², Marcel Zauner-Wieczorek¹, Urs Baltensperger⁸, Imad El Haddad⁸, Richard C. Flagan¹², Armin Hansel^18,19, Kristina Höhler⁷, Jasper Kirkby^1,15, Markku Kulmala^2,9,25, Katrianne Lehtipalo^2,22, Ottmar Möhler⁷, Harald Saathoff⁷, Rainer Volkamer³, Paul M. Winkler¹⁰, Neil M. Donahue¹¹, Andreas Kürten¹, and Joachim Curtius¹ Lucía Caudillo et al.

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¹Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany
²Institute for Atmospheric and Earth System Research (INAR)/Physics, Faculty of Science, University of Helsinki, 00014 Helsinki, Finland
³Department of Chemistry & CIRES, University of Colorado Boulder, Boulder, CO, 80309-0215, USA
⁴Max Planck Institute for Chemistry, Mainz, 55128, Germany
⁵CENTRA and FCUL, University of Lisbon, 1749-016 Lisbon, Portugal
⁶Leibniz Institute for Tropospheric Research, Leipzig, 04318, Germany
⁷Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
⁸Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
⁹Helsinki Institute of Physics, University of Helsinki, 00014 Helsinki, Finland
¹⁰Faculty of Physics, University of Vienna, 1090 Vienna, Austria
¹¹Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, 15213, USA
¹²Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
¹³California Air Resources Board, Sacramento, CA 95814, USA
¹⁴Lebedev Physical Institute, Russian Academy of Sciences, 119991, Moscow, Russia
¹⁵CERN, 1211 Geneva, Switzerland
¹⁶Dipartimento di Fisica, Università di Genova and INFN, 16146 Genova, Italy
¹⁷Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, CO 80309, USA
¹⁸Institute for Ion and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria
¹⁹Ionicon Analytik GmbH, 6020 Innsbruck, Austria
²⁰Forest Dynamics, Swiss Federal Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf, Switzerland
²¹Department of Chemistry, University of California, Irvine, Irvine, CA 92697, USA
²²Finnish Meteorological Institute, 00560 Helsinki, Finland
²³Beijing Weather Modification Office, China
²⁴IDL, Universidade da Beira Interior, R. Marquês de Ávila e Bolama, Covilhã, 6201-001, Portugal
²⁵Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, P.R. China
²⁶Moscow Institute of Physics and Technology (National Research University), 117303, Moscow, Russia

¹Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany
²Institute for Atmospheric and Earth System Research (INAR)/Physics, Faculty of Science, University of Helsinki, 00014 Helsinki, Finland
³Department of Chemistry & CIRES, University of Colorado Boulder, Boulder, CO, 80309-0215, USA
⁴Max Planck Institute for Chemistry, Mainz, 55128, Germany
⁵CENTRA and FCUL, University of Lisbon, 1749-016 Lisbon, Portugal
⁶Leibniz Institute for Tropospheric Research, Leipzig, 04318, Germany
⁷Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
⁸Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
⁹Helsinki Institute of Physics, University of Helsinki, 00014 Helsinki, Finland
¹⁰Faculty of Physics, University of Vienna, 1090 Vienna, Austria
¹¹Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, 15213, USA
¹²Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
¹³California Air Resources Board, Sacramento, CA 95814, USA
¹⁴Lebedev Physical Institute, Russian Academy of Sciences, 119991, Moscow, Russia
¹⁵CERN, 1211 Geneva, Switzerland
¹⁶Dipartimento di Fisica, Università di Genova and INFN, 16146 Genova, Italy
¹⁷Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, CO 80309, USA
¹⁸Institute for Ion and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria
¹⁹Ionicon Analytik GmbH, 6020 Innsbruck, Austria
²⁰Forest Dynamics, Swiss Federal Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf, Switzerland
²¹Department of Chemistry, University of California, Irvine, Irvine, CA 92697, USA
²²Finnish Meteorological Institute, 00560 Helsinki, Finland
²³Beijing Weather Modification Office, China
²⁴IDL, Universidade da Beira Interior, R. Marquês de Ávila e Bolama, Covilhã, 6201-001, Portugal
²⁵Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, P.R. China
²⁶Moscow Institute of Physics and Technology (National Research University), 117303, Moscow, Russia

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Received: 16 Jun 2021 – Discussion started: 07 Jul 2021

Abstract. New Particle Formation (NPF) from biogenic organic precursors is an important atmospheric process. One of the major species is α-pinene, which upon oxidation, can form a suite of products covering a wide range of volatilities. A fraction of the oxidation products is termed Highly Oxygenated Organic Molecules (HOM). These play a crucial role for nucleation and the formation of Secondary Organic Aerosol (SOA). However, measuring the composition of newly formed particles is challenging due to their very small mass. Here, we present results on the gas and particle phase chemical composition for a system where α-pinene was oxidized by ozone, and for a mixed system of α-pinene and isoprene, respectively. The measurements took place at the CERN Cosmics Leaving Outdoor Droplets (CLOUD) chamber at temperatures between −50 °C and −30 °C and at low and high relative humidity (20 % and 60 to 100 % RH). These conditions were chosen to simulate pure biogenic new particle formation in the upper free troposphere. The particle chemical composition was analyzed by the Thermal Desorption-Differential Mobility Analyzer (TD-DMA) coupled to a nitrate chemical ionization time-of-flight mass spectrometer. This instrument can be used for particle and gas phase measurements using the same ionization and detection scheme. Our measurements revealed the presence of C_8-10 monomers and C_18-20 dimers as the major compounds in the particles (diameter up to ~ 100 nm). Particularly, for the system with isoprene added, C₅ (C₅H₁₀O_5-7) and C₁₅ compounds (C₁₅H₂₄O_5-10) are detected. This observation is consistent with the previously observed formation of such compounds in the gas phase. However, although the C₅ and C₁₅ compounds do not easily nucleate, our measurements indicate that they can still contribute to the particle growth at free tropospheric conditions. For the experiments reported here, most likely isoprene might enhance growth at particle sizes larger than 15 nm. Besides the chemical information regarding the HOM formation for the α-pinene (plus isoprene) system, we report on the nucleation rates measured at 1.7 nm and found that the lower J_1.7nm values compared with previous studies are very likely due to the higher α-pinene and ozone mixing ratios used in the present study

How to cite. Caudillo, L., Rörup, B., Heinritzi, M., Marie, G., Simon, M., Wagner, A. C., Müller, T., Granzin, M., Amorim, A., Ataei, F., Baalbaki, R., Bertozzi, B., Brasseur, Z., Chiu, R., Chu, B., Dada, L., Duplissy, J., Finkenzeller, H., Gonzalez Carracedo, L., He, X.-C., Hofbauer, V., Kong, W., Lamkaddam, H., Lee, C. P., Lopez, B., Mahfouz, N. G. A., Makhmutov, V., Manninen, H. E., Marten, R., Massabò, D., Mauldin, R. L., Mentler, B., Molteni, U., Onnela, A., Pfeifer, J., Philippov, M., Piedehierro, A. A., Schervish, M., Scholz, W., Schulze, B., Shen, J., Stolzenburg, D., Stozhkov, Y., Surdu, M., Tauber, C., Tham, Y. J., Tian, P., Tomé, A., Vogt, S., Wang, M., Wang, D. S., Weber, S. K., Welti, A., Yonghong, W., Yusheng, W., Zauner-Wieczorek, M., Baltensperger, U., El Haddad, I., Flagan, R. C., Hansel, A., Höhler, K., Kirkby, J., Kulmala, M., Lehtipalo, K., Möhler, O., Saathoff, H., Volkamer, R., Winkler, P. M., Donahue, N. M., Kürten, A., and Curtius, J.: Chemical composition of nanoparticles from α-pinene nucleation and the influence of isoprene and relative humidity at low temperature, Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2021-512, in review, 2021.

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Lucía Caudillo et al.

Interactive discussion

Status: closed

RC1:
'Comment on acp-2021-512', Anonymous Referee #1, 28 Jul 2021

Caudillo and co-authors present and discuss the gas and particle phase composition of pure biogenic nucleation events measured with a nitrate chemical ionization atmospheric pressure interface time of flight mass spectrometer (coupled with a thermal desorption-differential mobility analyzer for the particle phase) in the CLOUD chamber at a range of conditions representing free tropospheric conditions. Specifically, alpha-pinene and a mix of alpha-pinene and isoprene were oxidized at -30 deg C and - 50 deg C, and at 20 % or 60 - 100 % relative humidity. The authors find C8-10 monomers and C18-20 dimers as major compounds, and C5 and C15 compounds contributing to particle growth when isoprene is present in the system. I very much appreciate the systematic analysis. The manuscript is well-written and the experimental results are thoroughly discussed. In my opinion, this is an original and valuable contribution to the field. Therefore, it should be published in ACP after minor revisions.

Specific comments:

In the abstract, the last sentence ("Besides the chemical information...", lines 64-66) was confusing to me. After reading section 3.4.1, I suggest to be more specific in the abstract, e.g. "Compared with previous studies, we found lower nucleation rates measured at 1.7 nm, very likely due to higher alpha-pinene and ozone mixing ratios used in the present study."

In section 2.1, there is no information about GCR conditions during the experiments, please add.

In section 2.2, please add some more information about the heating procedure of the filament. Is the temperature slowly ramped up, or do you apply high temperature directly to instantaneously vaporize the sample? This is also relevant for the discussion of potential thermal decomposition of molecules in section 3.2.2.

Regarding the non-size selective mode of operation of the TD-DMA, it would be helpful to get an idea about the contribution of freshly nucleated particles vs. grown particles to the sampled mass. From the measured particle size distributions and the PSM and CPC total number concentrations, could you calculate a rough estimate of the volume/mass fraction of particles < 15 nm in the samples collected in the periods shown in Figure 1 as shaded areas? Please add this information to Table 1.

In Figure 3f, to me it is not obvious that specifically C4-5 and C13-16 compounds are enhanced as stated in lines 237/238. Please clarify.

In section 3.3, looking at Figures 5 and S4 I agree with the statement that mainly LVOC and ELVOC compounds were detected in the particle phase, however, the ULVOC compounds appear to be a minor fraction.

Section 3.4.2: Regarding the discussion of isoprene suppression of new particle formation, please consider adding a reference to Lee et al. (2016), doi:10.1002/2016JD024844.

In Figures 6 and 8, please explain the meaning of "1α run-to-run uncertainty".

Technical comments:

lines 225, 301, 322 : Make sure that there is no line break between sign and number in "- 30" and "- 50".

line 226: "While the gas..." is not a full sentence.

Section 3.2, first paragraph: When presenting and discussing Figure 2, add a reference to supplementary material Figures S1 and S2 for other systems.

line 248: "While GR..." is not a full sentence.

line 285: Change "autooxidation" to "autoxidation" to be consistent throughout the manuscript.

lines 204, 312, 336, 375: Here, "new particle formation rate" is used, while J_1.7nm is introduced as "nucleation rate" in the manuscript. For consistency, I recommend using "nucleation rate" throughout the manuscript.

In Figures 6 and 8: "GRC" should read "GCR" in the figure legend. Also, in the last sentence of the figure caption, remove "and" before "is not shown".

Citation: https://doi.org/10.5194/acp-2021-512-RC1
- AC1: 'Interactive discussion_Response to both referees_Caudillo et al', Lucía Caudillo Murillo, 27 Sep 2021
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-512/acp-2021-512-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2021-512-AC1
RC2:
'Review of Caudillo et al.', Anonymous Referee #2, 11 Aug 2021

General comments

Caudillo et al. present chamber measurements of new-particle formation from α-pinene (AP) oxidation products at low temperatures, and study the effects of added isoprene (IP) and increased relative humidity (RH). The main focus is on the chemical composition of gas-phase species and (non-size-resolved) ultrafine particles, determined with a nitrate chemical ionization mass spectrometer and a thermal desorption-differential mobility analyzer, and in addition also nucleation rates are reported. Isoprene is observed to affect the chemical composition through an increase in e.g. C₅ and C₁₅ compounds, and to suppress new-particle formation as also reported in other studies.

Simultaneous measurements of gas- and particle-phase composition are essential for improving the understanding of biogenic secondary aerosol formation. The manuscript is generally well written and the results are clearly presented. I can recommend the work to be published in ACP after the authors have addressed the following comments:

Specific comments

1. Regarding the discussion on the effects of isoprene on the elemental composition, especially C₅ and C₁₅ compounds are stated to be increased in intensity. This seems clearer in the case of gas phase, whereas for particle phase the effects seem more diverse and e.g. Figure 3 shows similar increases in the signal intensity for various compounds with carbon content of up to ca. C₁₈, C₁₉. I cannot clearly distinguish a stronger increase specifically at C₁₅ in Figs. 3 or S1; can this be further clarified?

2. It may not be obvious that higher particle growth rates (GR) at larger particle sizes are due to isoprene (Section 3.2.1, last paragraph: “Reaching the same mass with a lower number of particles for the experiment with isoprene (αIP-30,20) compared to α-30,20, means that the growth rates at larger sizes (> 15 nm) are higher in the presence of isoprene”).

Particle GRs can generally be higher at lower particle number concentrations, as the amount of available condensable vapor per particle is higher. Can it be concluded that the enhanced growth at larger sizes is specifically related to isoprene, and not to such dynamic effects?

3. Effect of RH (Section 3.2.2): the particle mass concentration is observed to increase at elevated RH at otherwise similar conditions. However, Fig. S3 shows that the α-pinene level is somewhat higher for the experiments with higher RH. Can the higher AP level contribute to the increased particle mass?

Also, the RH range of 60-100% for the high-RH experiments is rather broad. What is the reasoning for lumping together these different RH values, and is it possible that the RH effects vary within this 60-100% range?

4. Similarly to the RH experiments, it seems that the AP level during the particle formation event and sample collection of the isoprene experiment is not exactly similar to the experiments without IP; it seems to be lower for the AP-IP set-up (Figure 1, third row). Can this affect the AP vs. AP-IP comparison?

5. Section 3.3: It would be helpful to list the actual fractions of the different VBS bins instead of only stating that the particle-phase species are mainly LVOC, ELVOC and ULVOC (it also seems that ULVOC is only a minor fraction). This could be a table with the bin fractions given for the different experiments.

6. The O₃ level of 100 ppbv seems rather high. How does it compare with typical tropospheric values? This is relevant considering the discussion on the effects of mixing ratios on the composition and nucleation of HOM (Section 3.4.1).

7. On P5L149-150, it is stated that particle evaporation before analysis should not be substantial; can this be assessed in a quantitative manner? Are there other uncertainty sources such as different charging efficiencies or transmission of the compounds?

I also agree with Referee #1 that an assessment of the relative contributions of the smallest and the larger particles to the particle-phase mass samples would be very useful.

8. P11L350: The meaning of “GCR conditions” is not explained; please clarify.

Technical corrections

P6L174: The particle formation rate is said to be defined as the flux of particles of a certain size as a function of time, but presumably the reported rates are not actually time-dependent; “as a function of time” should thus be removed, for clarity.

P6L202: It may be more appropriate to write “this is in line with the results of Kiendler-Scharr et al….” instead of “this confirms the results of …”

P7L226-227: Please reformulate the expression “the gas and particle of α-pinene” and similar occurrences.

P8L245: Please change “nSEMS” to “nSMPS” (?).

P8L254: Change “growth at higher sizes” to “growth at larger sizes”.

P9L268: The term “mass distribution” (here referring to particle mass size distribution?) may be a bit misleading as it might be confused with elemental composition or volatility distribution; please reformulate.

Figure 2 and similar plots: also the particle-phase fractions should preferably be written as positive instead of negative numbers (even if they are presented on the “negative” axis).

Caption of Figure 2: For clarity, “each system” could be changed to “each system and phase”.

Caption of Figure 3 and similar occurrences: the expression “mass defect plots of gas and particle phase and the intensity difference between them” is misleading; this sounds like the intensity difference between the gas and particle phases instead of the difference between the experiments. Please reformulate.

Legend of Figure 6 and similar occurrences: Please change “GRC” to “GCR”.

Caption of Figure 8 and similar occurrences: Please change “galactic comic rays” to “galactic cosmic rays”. :-)

Caption of Figure S4: Please state that this is Figure 5 in linear scale.

Figure S5: Why are the orange shades triangle-shaped?

Caption of Figure S6: Please explain the meaning of “overflow bin” and why the values of the first bin are both negative and positive.

Citation: https://doi.org/10.5194/acp-2021-512-RC2
- AC2: 'Interactive discussion_Response to both referees_Caudillo et al', Lucía Caudillo Murillo, 27 Sep 2021
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-512/acp-2021-512-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2021-512-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Lucía Caudillo Murillo on behalf of the Authors (27 Sep 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (08 Oct 2021) by Kelley Barsanti

AR by Lucía Caudillo Murillo on behalf of the Authors (15 Oct 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (17 Oct 2021) by Kelley Barsanti

Interactive discussion

Status: closed

RC1:
'Comment on acp-2021-512', Anonymous Referee #1, 28 Jul 2021

Caudillo and co-authors present and discuss the gas and particle phase composition of pure biogenic nucleation events measured with a nitrate chemical ionization atmospheric pressure interface time of flight mass spectrometer (coupled with a thermal desorption-differential mobility analyzer for the particle phase) in the CLOUD chamber at a range of conditions representing free tropospheric conditions. Specifically, alpha-pinene and a mix of alpha-pinene and isoprene were oxidized at -30 deg C and - 50 deg C, and at 20 % or 60 - 100 % relative humidity. The authors find C8-10 monomers and C18-20 dimers as major compounds, and C5 and C15 compounds contributing to particle growth when isoprene is present in the system. I very much appreciate the systematic analysis. The manuscript is well-written and the experimental results are thoroughly discussed. In my opinion, this is an original and valuable contribution to the field. Therefore, it should be published in ACP after minor revisions.

Specific comments:

In the abstract, the last sentence ("Besides the chemical information...", lines 64-66) was confusing to me. After reading section 3.4.1, I suggest to be more specific in the abstract, e.g. "Compared with previous studies, we found lower nucleation rates measured at 1.7 nm, very likely due to higher alpha-pinene and ozone mixing ratios used in the present study."

In section 2.1, there is no information about GCR conditions during the experiments, please add.

In section 2.2, please add some more information about the heating procedure of the filament. Is the temperature slowly ramped up, or do you apply high temperature directly to instantaneously vaporize the sample? This is also relevant for the discussion of potential thermal decomposition of molecules in section 3.2.2.

Regarding the non-size selective mode of operation of the TD-DMA, it would be helpful to get an idea about the contribution of freshly nucleated particles vs. grown particles to the sampled mass. From the measured particle size distributions and the PSM and CPC total number concentrations, could you calculate a rough estimate of the volume/mass fraction of particles < 15 nm in the samples collected in the periods shown in Figure 1 as shaded areas? Please add this information to Table 1.

In Figure 3f, to me it is not obvious that specifically C4-5 and C13-16 compounds are enhanced as stated in lines 237/238. Please clarify.

In section 3.3, looking at Figures 5 and S4 I agree with the statement that mainly LVOC and ELVOC compounds were detected in the particle phase, however, the ULVOC compounds appear to be a minor fraction.

Section 3.4.2: Regarding the discussion of isoprene suppression of new particle formation, please consider adding a reference to Lee et al. (2016), doi:10.1002/2016JD024844.

In Figures 6 and 8, please explain the meaning of "1α run-to-run uncertainty".

Technical comments:

lines 225, 301, 322 : Make sure that there is no line break between sign and number in "- 30" and "- 50".

line 226: "While the gas..." is not a full sentence.

Section 3.2, first paragraph: When presenting and discussing Figure 2, add a reference to supplementary material Figures S1 and S2 for other systems.

line 248: "While GR..." is not a full sentence.

line 285: Change "autooxidation" to "autoxidation" to be consistent throughout the manuscript.

lines 204, 312, 336, 375: Here, "new particle formation rate" is used, while J_1.7nm is introduced as "nucleation rate" in the manuscript. For consistency, I recommend using "nucleation rate" throughout the manuscript.

In Figures 6 and 8: "GRC" should read "GCR" in the figure legend. Also, in the last sentence of the figure caption, remove "and" before "is not shown".

Citation: https://doi.org/10.5194/acp-2021-512-RC1
- AC1: 'Interactive discussion_Response to both referees_Caudillo et al', Lucía Caudillo Murillo, 27 Sep 2021
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-512/acp-2021-512-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2021-512-AC1
RC2:
'Review of Caudillo et al.', Anonymous Referee #2, 11 Aug 2021

General comments

Caudillo et al. present chamber measurements of new-particle formation from α-pinene (AP) oxidation products at low temperatures, and study the effects of added isoprene (IP) and increased relative humidity (RH). The main focus is on the chemical composition of gas-phase species and (non-size-resolved) ultrafine particles, determined with a nitrate chemical ionization mass spectrometer and a thermal desorption-differential mobility analyzer, and in addition also nucleation rates are reported. Isoprene is observed to affect the chemical composition through an increase in e.g. C₅ and C₁₅ compounds, and to suppress new-particle formation as also reported in other studies.

Simultaneous measurements of gas- and particle-phase composition are essential for improving the understanding of biogenic secondary aerosol formation. The manuscript is generally well written and the results are clearly presented. I can recommend the work to be published in ACP after the authors have addressed the following comments:

Specific comments

1. Regarding the discussion on the effects of isoprene on the elemental composition, especially C₅ and C₁₅ compounds are stated to be increased in intensity. This seems clearer in the case of gas phase, whereas for particle phase the effects seem more diverse and e.g. Figure 3 shows similar increases in the signal intensity for various compounds with carbon content of up to ca. C₁₈, C₁₉. I cannot clearly distinguish a stronger increase specifically at C₁₅ in Figs. 3 or S1; can this be further clarified?

2. It may not be obvious that higher particle growth rates (GR) at larger particle sizes are due to isoprene (Section 3.2.1, last paragraph: “Reaching the same mass with a lower number of particles for the experiment with isoprene (αIP-30,20) compared to α-30,20, means that the growth rates at larger sizes (> 15 nm) are higher in the presence of isoprene”).

Particle GRs can generally be higher at lower particle number concentrations, as the amount of available condensable vapor per particle is higher. Can it be concluded that the enhanced growth at larger sizes is specifically related to isoprene, and not to such dynamic effects?

3. Effect of RH (Section 3.2.2): the particle mass concentration is observed to increase at elevated RH at otherwise similar conditions. However, Fig. S3 shows that the α-pinene level is somewhat higher for the experiments with higher RH. Can the higher AP level contribute to the increased particle mass?

Also, the RH range of 60-100% for the high-RH experiments is rather broad. What is the reasoning for lumping together these different RH values, and is it possible that the RH effects vary within this 60-100% range?

4. Similarly to the RH experiments, it seems that the AP level during the particle formation event and sample collection of the isoprene experiment is not exactly similar to the experiments without IP; it seems to be lower for the AP-IP set-up (Figure 1, third row). Can this affect the AP vs. AP-IP comparison?

5. Section 3.3: It would be helpful to list the actual fractions of the different VBS bins instead of only stating that the particle-phase species are mainly LVOC, ELVOC and ULVOC (it also seems that ULVOC is only a minor fraction). This could be a table with the bin fractions given for the different experiments.

6. The O₃ level of 100 ppbv seems rather high. How does it compare with typical tropospheric values? This is relevant considering the discussion on the effects of mixing ratios on the composition and nucleation of HOM (Section 3.4.1).

7. On P5L149-150, it is stated that particle evaporation before analysis should not be substantial; can this be assessed in a quantitative manner? Are there other uncertainty sources such as different charging efficiencies or transmission of the compounds?

I also agree with Referee #1 that an assessment of the relative contributions of the smallest and the larger particles to the particle-phase mass samples would be very useful.

8. P11L350: The meaning of “GCR conditions” is not explained; please clarify.

Technical corrections

P6L174: The particle formation rate is said to be defined as the flux of particles of a certain size as a function of time, but presumably the reported rates are not actually time-dependent; “as a function of time” should thus be removed, for clarity.

P6L202: It may be more appropriate to write “this is in line with the results of Kiendler-Scharr et al….” instead of “this confirms the results of …”

P7L226-227: Please reformulate the expression “the gas and particle of α-pinene” and similar occurrences.

P8L245: Please change “nSEMS” to “nSMPS” (?).

P8L254: Change “growth at higher sizes” to “growth at larger sizes”.

P9L268: The term “mass distribution” (here referring to particle mass size distribution?) may be a bit misleading as it might be confused with elemental composition or volatility distribution; please reformulate.

Figure 2 and similar plots: also the particle-phase fractions should preferably be written as positive instead of negative numbers (even if they are presented on the “negative” axis).

Caption of Figure 2: For clarity, “each system” could be changed to “each system and phase”.

Caption of Figure 3 and similar occurrences: the expression “mass defect plots of gas and particle phase and the intensity difference between them” is misleading; this sounds like the intensity difference between the gas and particle phases instead of the difference between the experiments. Please reformulate.

Legend of Figure 6 and similar occurrences: Please change “GRC” to “GCR”.

Caption of Figure 8 and similar occurrences: Please change “galactic comic rays” to “galactic cosmic rays”. :-)

Caption of Figure S4: Please state that this is Figure 5 in linear scale.

Figure S5: Why are the orange shades triangle-shaped?

Caption of Figure S6: Please explain the meaning of “overflow bin” and why the values of the first bin are both negative and positive.

Citation: https://doi.org/10.5194/acp-2021-512-RC2
- AC2: 'Interactive discussion_Response to both referees_Caudillo et al', Lucía Caudillo Murillo, 27 Sep 2021
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2021-512/acp-2021-512-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2021-512-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Lucía Caudillo Murillo on behalf of the Authors (27 Sep 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (08 Oct 2021) by Kelley Barsanti

AR by Lucía Caudillo Murillo on behalf of the Authors (15 Oct 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (17 Oct 2021) by Kelley Barsanti

Journal article(s) based on this preprint

Lucía Caudillo et al.

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https://doi.org/10.5194/acp-2021-512-supplement

Lucía Caudillo et al.

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Short summary

We performed experiments in the CLOUD chamber at CERN at low temperatures to simulate new particle formation in the upper free troposphere (at −30 °C and −50 °C). We measured particle and gas phase and found that most of the compounds that are present in the gas phase are detected as well in the particle phase. The major compounds in the particles are C_8-10 and C_18-20. Specifically, we showed that C₅ and C₁₅ compounds are detected in a mixed system with isoprene and α-pinene at −30 °C, 20 % RH.


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