the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Mar 2023
Explaining apparent particle shrinkage related to new particle formation events in western Saudi Arabia does not require evaporation
Abstract. The majority of new particle formation (NPF) events observed in Hada al Sham, western Saudi Arabia during 2013–2015, showed an unusual progression where the diameter of a newly formed particle mode clearly started to decrease after the growth phase. Many previous studies refer to this phenomenon as aerosol shrinkage. We will opt to use the term decreasing mode diameter (DMD) event, as shrinkage bears the connotation of reduction in the sizes of individual particles, which does not have to be the case. While several previous studies speculate that ambient DMD events are caused by evaporation of semivolatile species, no concrete evidence has been provided, partly due to the rarity of the DMD events. The frequent occurrence and large number of DMD events in our observations allow us to perform statistically significant comparisons between the DMD and the typical NPF events that undergo continuous growth. In our analysis, we find no clear connection between DMD events and factors that might trigger particle evaporation at the measurement site. Instead, examination of air mass source areas and the horizontal distribution of anthropogenic emissions in the study region leads us to believe that the observed DMD events could be caused by advection of smaller, less-grown, particles to the measurement site after the more-grown ones. Using a Lagrangian single-particle growth model, we confirm that the observed particle size development, including the DMD events, can be reproduced by non-volatile condensation, and thus without evaporation. In fact, when considering increasing contributions from a semivolatile compound, we find deteriorating agreement between the measurements and the model. Based on these results, it seems unlikely that evaporation of semivolatile compounds would play a significant role in the DMD events at our measurement site. In the proposed non-volatile explanation, the DMD events are a result of the observed particles having spent an increasing fraction of their lifetime in a lower growth environment, mainly enabled by the lower precursor vapor concentrations further away from the measurement site combined with decreasing photochemical production of condensable vapors in the afternoon. The correct identification of the cause of the DMD events is important as the fate and the climate-relevance of the newly formed particles heavily depends on it — if the particles evaporated, their net contribution towards larger and climatically active particle sizes would be greatly reduced. Our findings highlight the importance of considering transport-related effects in NPF event analysis, which is an often overlooked factor in such studies.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-333', Anonymous Referee #1, 08 Apr 2023
This study uses statistical methods and model simulations in an attempt to explain the decreasing mode diameter (DMD) events that were preceded by new particle formation (NPF) in western Saudi Arabia. The study area has frequent NPF and DMD events, and thus the collected data are of special interest in the study of aerosol formation and growth, as well as apparent particle shrinkage. The manuscript is well organized and written with clear hypotheses, but could be shortened and improved by focusing on the major findings. The presented methodology is reasonable and includes new analytical approaches to elucidating the causes of DMD events. The presented results are explained in detail and adequately discussed with existing work. The summary and conclusions, however, could be more specific and quantitative. My specific comments are given below:
- P4, L12, P11, L4-6, P14, L24: To put the observations in perspective, it would be helpful to provide the typical ground-level SO2 and PM concentrations at the study area and in Jeddah (or the region).
- P7, Table 1: Why is the T0 set at 278K? Please consider giving the exact numbers or steps of Psat, instead of “…” Also, please replace “…” with “to” or “–“ in the text.
- If applicable, please consider providing relevant references to the presented equations.
- P13, L5-6: Please be specific. Pearson correlation coeff.? Chi-square?
- Fig. 2: This is a nice and interesting illustration of the stated hypotheses. It would be more informative if the typical transport characteristics could also be considered in the graph, e.g., the average wind speed during the DMD events, and how far into the sea at the onset of NFP. I suggest adding this info in the discussion sections (e.g., P17, L13, and Fig. 6).
- P23, L9: Fig. 6 is informative. But what about including a plot of the probability of observing DMD particles? Would that be more straightforward for discussion?
- Section 4.3.1: The simulated outcomes of the model somewhat do not seem to capture the expected results or relationships, except wind speed. Please consider discussing the important variables that may be missing in the model. This is crucial for subsequent discussion about the model predictions in the following sections.
- Discussion: Based on the observed and model results, is it possible that the DMD phenomena are a result of measurements at the edge (i.e., boundary) of the NPF event? In other words, the NPF event is analogous to a large-scale “plume” event, in which condensable vapors and hence particle growth is strongest in the middle of the plume (i.e., source region), and decreases to the lowest at the edge (consider a plume traveling over a fixed measurement site).
- Summary and conclusions: It would be helpful to provide the specific boundary conditions ([SO2], CS, WS, WD, T, BLH, Cvol, Cnon, etc.) that favor the occurrence of DMD in the study area. These could be used as a reference for other studies.
Citation: https://doi.org/10.5194/egusphere-2023-333-RC1 -
RC2: 'Comment on egusphere-2023-333', Anonymous Referee #2, 22 Apr 2023
The MS deals with decreasing mode diameter NPF (DMD) events in a special atmospheric environment. These events are important to formulate and check ideas on the volatility properties of the chemical species involved in the particle growth. The special environment is characterised by a very high relative occurrence frequency of NPF events of 73% without evident seasonal tendency. Of them 76% were identified as DMD events. These characteristics are related to the local properties at the site. The MS covers a complex study with a nice adaptation of physicochemical approaches. The findings are valuable and are discussed in detail. It is relevant and of interest for the scientific community. I can definitely propose its acceptance. The following comments can be advised to the authors for consideration.
1. The MS seems to be long and dense. The authors may want to reorganise it in a way that it focuses in the main text on the main relationships, on the conditions for their validity, on discussions of the bounding conditions and on their physical meaning, and on the findings, results and conclusions. The detailed derivation of the relationships could be in the Appendix. In this way, the authors would better communicate the messages. The Summary and conclusions section should be shortened to primary messages.
2. The MS could be reorganised also from structural point of view. The part P13, L26 – P14, L9 belongs more to the site description than to Hypotheses based on our previous results. Further parts (for instance P4, L6) are redundant in an article.
3. The actual results were obtained under very specific atmospheric and geographic conditions. Could the spatially localised (local) nucleating plume affect its dynamic development? In that sense, the MS could advantageous be complemented with a brief discussion on the representativity of its finding. Could they be valid for other, more ordinary locations?
4. Paragraph P37, L15-19 are not completely clear. Reformulation is requested.
5. Formal comments. Altitude is missing in P4, L10. Insert space after semicolons in many places (e.g. in references and P4, L16). Internal brackets in P4, L17 are redundant, while semicolon is needed.
Citation: https://doi.org/10.5194/egusphere-2023-333-RC2 -
CC1: 'Comment on egusphere-2023-333', Niku Kivekäs, 24 Apr 2023
After reading the manuscript for other reasons I agree with the comment brought up by both anynomous referees that the manuscript is too long.
The authors present their work well and in detail, enabling proper justification of the results and conclusions. They also present analysis starting from alternative assumptions to test whether the same results can be obtained this way. These both are very good from the ideal scientific perspective and would actually allow stronger conclusions than what the authors claim.
The issue, however, is in science communication. The same thoroughness that makes the manuscript scientifically solid makes it difficult and laborous to read and hides the main story line leading to the conclusions. I'm afraid that the long publication with extended what if-analysis will not be seen as solid as it is, and will therefore receive less citations and impact in the field. If the journal allows publication of supplementary material, I would strongly suggest moving a major fraction of the content in the supplementary side and leaving the stong main story line in the actual publication.
Citation: https://doi.org/10.5194/egusphere-2023-333-CC1 - AC1: 'Comment on egusphere-2023-333', Simo Hakala, 31 May 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-333', Anonymous Referee #1, 08 Apr 2023
This study uses statistical methods and model simulations in an attempt to explain the decreasing mode diameter (DMD) events that were preceded by new particle formation (NPF) in western Saudi Arabia. The study area has frequent NPF and DMD events, and thus the collected data are of special interest in the study of aerosol formation and growth, as well as apparent particle shrinkage. The manuscript is well organized and written with clear hypotheses, but could be shortened and improved by focusing on the major findings. The presented methodology is reasonable and includes new analytical approaches to elucidating the causes of DMD events. The presented results are explained in detail and adequately discussed with existing work. The summary and conclusions, however, could be more specific and quantitative. My specific comments are given below:
- P4, L12, P11, L4-6, P14, L24: To put the observations in perspective, it would be helpful to provide the typical ground-level SO2 and PM concentrations at the study area and in Jeddah (or the region).
- P7, Table 1: Why is the T0 set at 278K? Please consider giving the exact numbers or steps of Psat, instead of “…” Also, please replace “…” with “to” or “–“ in the text.
- If applicable, please consider providing relevant references to the presented equations.
- P13, L5-6: Please be specific. Pearson correlation coeff.? Chi-square?
- Fig. 2: This is a nice and interesting illustration of the stated hypotheses. It would be more informative if the typical transport characteristics could also be considered in the graph, e.g., the average wind speed during the DMD events, and how far into the sea at the onset of NFP. I suggest adding this info in the discussion sections (e.g., P17, L13, and Fig. 6).
- P23, L9: Fig. 6 is informative. But what about including a plot of the probability of observing DMD particles? Would that be more straightforward for discussion?
- Section 4.3.1: The simulated outcomes of the model somewhat do not seem to capture the expected results or relationships, except wind speed. Please consider discussing the important variables that may be missing in the model. This is crucial for subsequent discussion about the model predictions in the following sections.
- Discussion: Based on the observed and model results, is it possible that the DMD phenomena are a result of measurements at the edge (i.e., boundary) of the NPF event? In other words, the NPF event is analogous to a large-scale “plume” event, in which condensable vapors and hence particle growth is strongest in the middle of the plume (i.e., source region), and decreases to the lowest at the edge (consider a plume traveling over a fixed measurement site).
- Summary and conclusions: It would be helpful to provide the specific boundary conditions ([SO2], CS, WS, WD, T, BLH, Cvol, Cnon, etc.) that favor the occurrence of DMD in the study area. These could be used as a reference for other studies.
Citation: https://doi.org/10.5194/egusphere-2023-333-RC1 -
RC2: 'Comment on egusphere-2023-333', Anonymous Referee #2, 22 Apr 2023
The MS deals with decreasing mode diameter NPF (DMD) events in a special atmospheric environment. These events are important to formulate and check ideas on the volatility properties of the chemical species involved in the particle growth. The special environment is characterised by a very high relative occurrence frequency of NPF events of 73% without evident seasonal tendency. Of them 76% were identified as DMD events. These characteristics are related to the local properties at the site. The MS covers a complex study with a nice adaptation of physicochemical approaches. The findings are valuable and are discussed in detail. It is relevant and of interest for the scientific community. I can definitely propose its acceptance. The following comments can be advised to the authors for consideration.
1. The MS seems to be long and dense. The authors may want to reorganise it in a way that it focuses in the main text on the main relationships, on the conditions for their validity, on discussions of the bounding conditions and on their physical meaning, and on the findings, results and conclusions. The detailed derivation of the relationships could be in the Appendix. In this way, the authors would better communicate the messages. The Summary and conclusions section should be shortened to primary messages.
2. The MS could be reorganised also from structural point of view. The part P13, L26 – P14, L9 belongs more to the site description than to Hypotheses based on our previous results. Further parts (for instance P4, L6) are redundant in an article.
3. The actual results were obtained under very specific atmospheric and geographic conditions. Could the spatially localised (local) nucleating plume affect its dynamic development? In that sense, the MS could advantageous be complemented with a brief discussion on the representativity of its finding. Could they be valid for other, more ordinary locations?
4. Paragraph P37, L15-19 are not completely clear. Reformulation is requested.
5. Formal comments. Altitude is missing in P4, L10. Insert space after semicolons in many places (e.g. in references and P4, L16). Internal brackets in P4, L17 are redundant, while semicolon is needed.
Citation: https://doi.org/10.5194/egusphere-2023-333-RC2 -
CC1: 'Comment on egusphere-2023-333', Niku Kivekäs, 24 Apr 2023
After reading the manuscript for other reasons I agree with the comment brought up by both anynomous referees that the manuscript is too long.
The authors present their work well and in detail, enabling proper justification of the results and conclusions. They also present analysis starting from alternative assumptions to test whether the same results can be obtained this way. These both are very good from the ideal scientific perspective and would actually allow stronger conclusions than what the authors claim.
The issue, however, is in science communication. The same thoroughness that makes the manuscript scientifically solid makes it difficult and laborous to read and hides the main story line leading to the conclusions. I'm afraid that the long publication with extended what if-analysis will not be seen as solid as it is, and will therefore receive less citations and impact in the field. If the journal allows publication of supplementary material, I would strongly suggest moving a major fraction of the content in the supplementary side and leaving the stong main story line in the actual publication.
Citation: https://doi.org/10.5194/egusphere-2023-333-CC1 - AC1: 'Comment on egusphere-2023-333', Simo Hakala, 31 May 2023
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Simo Kalervo Hakala
Ville Vakkari
Heikki Lihavainen
Antti-Pekka Hyvärinen
Kimmo Neitola
Jenni Kontkanen
Veli-Matti Kerminen
Markku Kulmala
Tuukka Petäjä
Tareq Hussein
Mamdouh I. Khoder
Mansour ِA. Alghamdi
Pauli Paasonen
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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