Liquid–liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors

Song, Mijung; Maclean, Adrian M.; Huang, Yuanzhou; Smith, Natalie R.; Blair, Sandra L.; Laskin, Julia; Laskin, Alexander; DeRieux, Wing-Sy Wong; Li, Ying; Shiraiwa, Manabu; Nizkorodov, Sergey A.; Bertram, Allan K.

doi:https://doi.org/10.5194/acp-19-12515-2019

Articles | Volume 19, issue 19

Atmos. Chem. Phys., 19, 12515–12529, 2019

https://doi.org/10.5194/acp-19-12515-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Atmos. Chem. Phys., 19, 12515–12529, 2019

https://doi.org/10.5194/acp-19-12515-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 19

Research article 08 Oct 2019

Research article | 08 Oct 2019

Liquid–liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors

Mijung Song et al.

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Final revised paper (published on 08 Oct 2019)
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Preprint (discussion started on 26 Apr 2019)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of "Liquid-liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors"', Anonymous Referee #1, 28 May 2019
- AC1: 'Response to Anonymous Referee #1', Mijung SONG, 25 Jul 2019
RC2: 'Referee comment on "Liquid-liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors"', Anonymous Referee #2, 14 Jun 2019
- AC2: 'Response to Anonymous Referee #2', Mijung SONG, 25 Jul 2019

Peer-review completion

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

AR by Mijung SONG on behalf of the Authors (25 Jul 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Aug 2019) by Annele Virtanen

RR by Anonymous Referee #2 (09 Aug 2019)

Suggestions for revision or reasons for rejection

To address my comment and the first comment by the referee 1 the authors have modified the manuscript by adding discussion on evaporation of compounds during conditioning. I have concerns regarding the calculations the authors present for this issue and the conclusions they draw from them. I recommend a revision before the manuscript is accepted.

Authors state: “In the poke-and-flow experiments (as well as the LLPS experiments), the particles are exposed to a constant flow of gas which can lead to a change in the composition of the particles by partitioning of semi-volatiles to the gas phase. For a 1 hr poke-and-flow experiment, the amount of gas exposed to the SOA is 30 L compared to 380 L collected from the environmental chamber. Exposing the SOA to this amount of gas can be considered equivalent to changing the mass loading used to generate the SOA from 550 μg m-3 to 510 μg m-3. Exposing the particles to a constant gas flow for 27 hours (maximum amount of time a sample was exposed to a constant gas flow) can be considered equivalent to changing the mass loading from 550 μg m-3 to 175 μg m-3. This should be considered a worse-case scenario since this estimation does not consider kinetic constraints to evaporation. Based on previous measurements, the viscosity of toluene SOA is independent of mass loadings ranging from 800 μg m-3 to 80 μg m-3 (Song et al., 2016a). Assuming that diesel fuel SOA behaves like toluene SOA, the viscosity of diesel fuel SOA should not be influenced by exposure to a constant flow of gas in our poke-and-flow experiments. Consistent with this discussion, we did not observe a relationship between particle viscosity and time the SOA was exposed to a constant flow of gas in our experiments.”

This does not seem valid reasoning. This would approximately hold if adding 30 L of clean air to a 380 L expansible chamber where mass loading where initially 550 μg m-3. However, according to my understanding, in this study the authors collect the particles on a glass slide and then clean air is flowing above this glass slide during conditioning. In this case the equilibration calculation presented above does not hold and the system to consider is the particle phase formed by the particles on the glass slide and the organic-free clean gas flowing above them. As the particles are surrounded by the clean air continuously, they are evaporating. Given enough time they should actually evaporate completely. Therefore the presented calculation is not a worse-case scenario like the authors stated. The observed lack of a relationship between viscosity and the time the particles were exposed to the gas flow could be due to the relatively long exposure time even in the fastest case: 1 hour is enough time for semi-volatiles to evaporate considerably. Therefore the particle composition could be changing during the conditioning in all of the experiments to the extent that it does not represent the viscosity of the original particles. The issue of evaporating compounds and the resulting uncertainty or systematic error in viscosities should be discussed in the text.

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ED: Publish subject to minor revisions (review by editor) (20 Aug 2019) by Annele Virtanen

AR by Mijung SONG on behalf of the Authors (03 Sep 2019) Author's response Manuscript

ED: Publish as is (11 Sep 2019) by Annele Virtanen