the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Jul 2023
Simulating the dust emissions and SOA formation over Northern Africa during the mid-Holocene Green Sahara period
Abstract. Paleo-proxy data indicates that a "Green Sahara" thrived in Northern Africa during the early- to mid-Holocene (MH; 11,000 to 5,000 years before present), characterized by more vegetation cover and reduced dust emission. Utilizing a state-of-the-art atmospheric chemical transport model TM5-MP, we assessed the changes in biogenic volatile organic compounds (BVOCs) emissions, dust emission and secondary organic aerosol (SOA) concentration in Northern Africa during this period relative to the pre-industrial (PI) period. Our simulations show that dust emissions reduced from 280.6 Tg a-1 in the PI to 26.8 Tg a-1 in the MH, agreeing with indications from eight marine sediment records in the Atlantic Ocean. The northward expansion in Northern Africa resulted in an increase in annual emissions of isoprene and monoterpenes during the MH, around 4.3 and 3.5 times higher than that in the PI period, respectively, causing 1.9 times increase in the SOA surface concentration. The enhanced SOA surface concentration and decreased sulfate surface concentration counteracted each other, leading to a 17 % increase in the cloud condensation nuclei at 0.2 % super saturation over Northern Africa. Our simulations provide consistent emission datasets of BVOCs, dust, and the SOA formation aligned with the northward shift of vegetation during the "Green Sahara" period, which could serve as a benchmark for MH aerosol input in future Earth system model simulation experiments.
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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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Supplement
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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.
- Preprint
(3654 KB) - Metadata XML
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Supplement
(1100 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1520', Yonggang Liu, 22 Aug 2023
Through model simulations, Zhou et al. found that although dust emission is greatly reduced during the mid-Holocene Green Sahara periods compared to PI, the emission of isoprene and monoterpenes increased substantially (~4 fold). The increased emission of these biogenic volatile organic compounds (BVOCs) enhanced the formation of secondary organic aerosol (SOA), eventually caused a 17% increase in cloud condensation nuclei. Such processes have been overlooked in almost all previous studies. The study is thus quite novel and the results have important implications to the climate impact of a green Sahara, and should be a useful contribution to the journal Climate of Past. The manuscript is well organized and written and the methods and results are reasonable, I suggest a minor revision.
Comments:
- In paragraph between lines 36-47, the influence of lakes on precipitation and west African monsoon should be mentioned. A few studies have worked on this, for example, Specht et al. (2022; https://doi.org/10.5194/cp-18-1035-2022) and Chandan and Peltier (2020; https://doi.org/10.1029/2020GL088728).
- L94: “formextremely” misses a space.
- The model was run for only two years, have the authors tested and checked that the results were stable already at the second year?
- L162: “a-1” -> a-1
- L191: I don’t understand what “the latter one” refers to.
- Figure 2, are the simulated emissions in PI reasonable compared to the observations? It would be useful if the authors can show the comparison between the pi_orig and the modern observations in the supplementary material.
- L270-282: Table 3 should cited in this paragraph
- L323: “significant” -> “significantly”
- In the captions of Figures 5&6, do the authors mean “same as Figure 4”?
- L125: It will be better if the authors directly state here that the present-day meteorological data were used in all 5 experiments. Although the sentence implies this but I assume it was only for the three PI experiments, and did not realize this until the end of the manuscript. This is a caveat of the study because impact of BVOC on CCN might be different if the MH meteorological data (some of the authors clearly have such data from model simulations) had been used.
Citation: https://doi.org/10.5194/egusphere-2023-1520-RC1 - AC1: 'Reply on RC1', Putian Zhou, 28 Sep 2023
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RC2: 'Comment on egusphere-2023-1520', Yong Sun, 25 Aug 2023
Dr. Zhang,
Thank you for assigning me to deal with this manuscript entitled” Simulating the dust emissions and SOA formation over Northern Africa during the mid-Holocene Green Sahara period”. This is an interesting work and is proposed to publish on the Climate of the Past after minor revisions.
Best,
Yong
Comments to the authors
Please delete the following texts in the section 2.2, as I do not think it is really relevant to the scientific issues discussed, but rather to the technical details of the model setup.
“The model was installed on Puhti supercomputer at CSC (IT Center for Science, Finland), and 90 CPU (Central Processing Unit) cores were utilized for each parallel simulation run. One simulation year cost about 10 hours in real life.”
Here The authors conduct two years run of TM5-MP：one year is used for spin up and the other is used for analysis. I am not sure if it is too short for one year simulation to calculate the mean state of variables.
Citation: https://doi.org/10.5194/egusphere-2023-1520-RC2 - AC2: 'Reply on RC2', Putian Zhou, 28 Sep 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1520', Yonggang Liu, 22 Aug 2023
Through model simulations, Zhou et al. found that although dust emission is greatly reduced during the mid-Holocene Green Sahara periods compared to PI, the emission of isoprene and monoterpenes increased substantially (~4 fold). The increased emission of these biogenic volatile organic compounds (BVOCs) enhanced the formation of secondary organic aerosol (SOA), eventually caused a 17% increase in cloud condensation nuclei. Such processes have been overlooked in almost all previous studies. The study is thus quite novel and the results have important implications to the climate impact of a green Sahara, and should be a useful contribution to the journal Climate of Past. The manuscript is well organized and written and the methods and results are reasonable, I suggest a minor revision.
Comments:
- In paragraph between lines 36-47, the influence of lakes on precipitation and west African monsoon should be mentioned. A few studies have worked on this, for example, Specht et al. (2022; https://doi.org/10.5194/cp-18-1035-2022) and Chandan and Peltier (2020; https://doi.org/10.1029/2020GL088728).
- L94: “formextremely” misses a space.
- The model was run for only two years, have the authors tested and checked that the results were stable already at the second year?
- L162: “a-1” -> a-1
- L191: I don’t understand what “the latter one” refers to.
- Figure 2, are the simulated emissions in PI reasonable compared to the observations? It would be useful if the authors can show the comparison between the pi_orig and the modern observations in the supplementary material.
- L270-282: Table 3 should cited in this paragraph
- L323: “significant” -> “significantly”
- In the captions of Figures 5&6, do the authors mean “same as Figure 4”?
- L125: It will be better if the authors directly state here that the present-day meteorological data were used in all 5 experiments. Although the sentence implies this but I assume it was only for the three PI experiments, and did not realize this until the end of the manuscript. This is a caveat of the study because impact of BVOC on CCN might be different if the MH meteorological data (some of the authors clearly have such data from model simulations) had been used.
Citation: https://doi.org/10.5194/egusphere-2023-1520-RC1 - AC1: 'Reply on RC1', Putian Zhou, 28 Sep 2023
-
RC2: 'Comment on egusphere-2023-1520', Yong Sun, 25 Aug 2023
Dr. Zhang,
Thank you for assigning me to deal with this manuscript entitled” Simulating the dust emissions and SOA formation over Northern Africa during the mid-Holocene Green Sahara period”. This is an interesting work and is proposed to publish on the Climate of the Past after minor revisions.
Best,
Yong
Comments to the authors
Please delete the following texts in the section 2.2, as I do not think it is really relevant to the scientific issues discussed, but rather to the technical details of the model setup.
“The model was installed on Puhti supercomputer at CSC (IT Center for Science, Finland), and 90 CPU (Central Processing Unit) cores were utilized for each parallel simulation run. One simulation year cost about 10 hours in real life.”
Here The authors conduct two years run of TM5-MP：one year is used for spin up and the other is used for analysis. I am not sure if it is too short for one year simulation to calculate the mean state of variables.
Citation: https://doi.org/10.5194/egusphere-2023-1520-RC2 - AC2: 'Reply on RC2', Putian Zhou, 28 Sep 2023
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Cited
Zhengyao Lu
Jukka-Pekka Keskinen
Qiong Zhang
Juha Lento
Jianpu Bian
Twan van Noije
Philippe Le Sager
Veli-Matti Kerminen
Markku Kulmala
Michael Boy
Risto Makkonen
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(3654 KB) - Metadata XML
-
Supplement
(1100 KB) - BibTeX
- EndNote
- Final revised paper