the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jan 2025
The transport history of African biomass burning aerosols arriving in the remote Southeast Atlantic marine boundary layer and their impacts on cloud properties
Abstract. The transport of African biomass burning aerosols (BBAs) and their impacts on cloud formation and properties over the Southeast Atlantic (SEA) remain one of the largest sources of uncertainty in understanding climate effects across this region. In this study, vertical structures of thermodynamics, aerosol properties and cloud microphysics were characterized around Ascension Island during the CLARIFY-2017 (CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017; August–September 2017) aircraft campaign, providing insights into the relationship between transported African BBAs and clouds in the marine boundary layer (MBL) over the remote SEA. The biomass burning (BB)-impacted MBL exhibited substantially enhanced aerosol number concentration (Nₐ, 0.1 – 3 µm) compared to the clean MBL around Ascension Island, leading to generally increased cloud droplet number concentration (Nd) but smaller cloud effective radius (Rₑ) in BB-impacted clouds compared to clean clouds. The cloud-layer mean Nd values were observed to be strongly correlated with aerosols below the cloud (sub-Nₐ) but were more weakly associated with aerosols immediately above the cloud. The increase in the sub-Nₐ is caused by entrained BBAs from above-cloud to below-cloud regions along long-range transport pathways and/or at the place of observation. We also explored possible simplifications to establish relationships between Nd and sub-Nₐ or Rₑ from in-situ measurements. Similar droplet activation fractions were observed in the clean and moderately BB-impacted (sub-Nₐ < 700 cm–³) clouds, while a greater variability was noted in more polluted clouds. The relationship between Nd and Rₑ remained consistent regardless of the levels of BB influence. Backward simulations were conducted using UK Met Office’s Numerical Atmospheric Modelling Environment (NAME), to track the sources and pathways of air parcels reaching Ascension Island. NAME results indicate that air parcels arriving in the Ascension Island MBL can originate from both the boundary layer (BL) and free troposphere (FT) during long-range transport, and entrainment mixing from the FT into the MBL over the SEA is likely to occur. BB pollution in the Ascension Island MBL could occur, when FT air parcels, primarily originating from the African continent (20° S – 0° N), carry BB smoke. By coupling NAME simulations with SEVIRI (Spinning Enhanced Visible and Infrared Imager) retrievals (aerosol and cloud fields) along the simulated transport path, the study suggests that efficient entrainment of African air parcels from the FT into the MBL occurs multiple days over the SEA before reaching the Ascension Island MBL, mainly in the region to the west of 0° E for examined cases. This study provides important aerosol and cloud parameterizations for climate models, and also provides observational constraints for evaluating the effects of transported BBAs on clouds and their subsequent radiative forcing over the SEA. Furthermore, the identified BBAs entrainment region may provide additional constraints for refining the vertical transport processes of African BBAs in models, thereby improving the representation of their vertical structures over the remote SEA.
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RC1: 'Comment on egusphere-2024-3975', Anonymous Referee #1, 25 Mar 2025
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Review of Manuscript EGUSphere-2024-3975:
The transport history of African biomass burning aerosols arriving in the remote Southeast Atlantic marine boundary layer and their impacts on cloud properties, by Wu et al.
General comments:
Wu et al. presented aircraft observations on aerosol-cloud interactions over Ascension Island in the Southeast Atlantic, focusing on biomass burning aerosols from Africa. In-situ measurements of aerosol and cloud droplet properties, along with simulation experiments using the UK Met Office's Numerical Atmospheric Modelling Environment (NAME) and Unified Model, were conducted. This study also considered the effects of atmospheric conditions, such as in the marine boundary layer or free troposphere, on data analysis. Despite many language and presentation issues, the key findings are presented. In general, this study provides insights into cloud-aerosol interactions over remote oceans and shows the role of biomass burning aerosols under different atmospheric conditions. However, the manuscript requires substantial revision to improve the quality of data analysis and presentation. The main text should be better structured, and the results (see detailed comments below) need to be reanalyzed and better interpreted. Once these issues are addressed, the paper could be further considered for publication in Atmospheric Chemistry and Physics (ACP).
I have some major comments as below:
- The abstract is overly redundant and needs revision for clarity and conciseness. It should be carefully restructured to avoid repetition and follow the guidelines provided by ACP, particularly regarding word limit and structure. Please refer to the ACP author guidelines for the required format (https://www.atmospheric-chemistry-and-physics.net/policies/guidelines_for_authors.html).
- The introduction is poorly developed and lacks a clear logical flow. In the first paragraph, the authors attempt to discuss the effects of biomass burning aerosols (BBAs) on cloud properties under different scenarios based on their vertical location relative to the cloud layer. However, the points are disorganized, and the lack of a clear leading sentence makes it difficult for readers, especially for those outside the field of cloud-aerosol interactions, to follow. The second and third paragraphs discuss the limitations of space-borne observations and model simulations respectively, but the authors fail to introduce specific research questions for the study. The statements in Lines 123-131 are too general and do not contribute enough detail. We would like to suggest the authors to streamline the introduction and clarify the research questions to improve clarity and readability.
- The Cloud Droplet Probe detects cloud droplets down to 2.0 µm. How does overlooking droplets smaller than 2.0 µm affect the investigation and conclusions?
- For Figure 3, the authors need to provide evidence or results to clearly classify biomass burning (BB)-impacted cases and those free of BB impacts. The manuscript mentions this classification, stating that "From 21st to 25th Aug, the MBL became cleaner, with BB pollution predominantly in the FT. From 26th to 31st Aug, the MBL was BB-impacted again, with BBAs observed in both the MBL and FT." However, more detailed results and clear classification criteria should be unambiguously provided.
- For the results in Figure 4, further discussion is needed to gain more insights. For example, some above-Na values (2000-2600 cm⁻³) are significantly higher than sub-Na Are these high above-Na data points due to Nd activation being limited by updraft velocity or supersaturation? For lower Na values, both above- and below-cloud cases may be limited by aerosol particle concentrations. Thus, the differing correlation coefficients between Nd and Na for sub-cloud and above-cloud cases may result from different Na ranges. What happens if the same Na range is compared for both cases? The coefficient difference may be smaller, and this should be further investigated.
- The results section lacks coherence and a clear storyline. A more organized presentation would improve readability. For example, first classify conditions with and without BBA influences, providing evidence of BBA presence. Air mass footprint and trajectory results should be presented earlier to highlight differences in aerosol sources. Then, present the properties (aerosol and cloud) of each classified case. Next, show the relationship between cloud-layer mean Re and the average LWC/Nd ratio, followed by the development of the parameterization and comparisons with the literature. This is just a suggestion; the authors may find other ways to better present the results.
- As shown in Figure 2, the flight altitude can reach 6 km, where ice crystals may be sampled, meaning that the aircraft could fly through mixed-phase clouds. What is the impact of sampling ice crystals? Were measures taken for the sampling inlet to account for this or to evaluate the influence? The authors refer to previous studies for details on instrumentation and experiments, but this may not provide enough information for new readers.
- For high-speed aircraft in-situ sampling, correcting the sampled volume to the ambient volume (or standard conditions) is challenging due to air compression, which can cause significant volume differences. Did the authors account for the effects of air compression at high speeds? If so, how was it corrected or calculated?
- This study used a water condensation particle counter (CPC, model: 3786-LP). It is reported that water CPC undercounts hydrophobic organic-rich soot particles (Keller et al., 2013), potentially a component of BBAs. How does this undercounting affect the results and analysis in the study?
- Relevant statements are needed to introduce and briefly discuss the results in the supplementary tables and figures. Simply presenting a figure or table without context is too abrupt.
- There are way too many abbreviations, which does not help the presentation but makes the reading more difficult. Please check through the manuscript and reduce the use of abbreviations.
- The language needs revision to improve readability. Many statements are too long, increasing the likelihood of errors and making them hard to follow.
Specific comments:
Line 17: ‘vertical structures of thermodynamics, aerosol properties and cloud microphysics’ sounds awkward
Line 20-23: This sentence is too long and it is redundant.
Line 23-24: The last part of the sentence can be more concise. And ‘immediately’ is not an appropriate adverb here.
Line 47: It is stated that ‘stratocumulus cloud (Sc) has been extensively studied’. However, the authors later on tried to bring out the research question about the impacts of biomass burning aerosols (BBAs) from African wildfires on the Sc deck over the southeast Atlantic (SEA) Ocean. Isn’t it self-contradictory?
Line 47-48: The statement in ‘These clouds are climatically important, as they reflect a significant amount of solar radiation and exert only a small radiative effect in the longwave, leading to a net cooling effect.’ Is not very clear. Is the small longwave radiative effect positive or negative? And relevant references are missing.
Line 68: should it be ‘BB’ or ‘BBA layers’? Is it really necessary to abbreviate both BB and BBA?
Line 69: What is ‘sub-cloud relative humidity (RH)’? Please make a clear definition since it is not a standard or regular terminology and it is used quite often through the manuscript.
Line 69-70: The statement in ‘In the smoky MBL, BBA heating tends to reduce the sub-cloud relative humidity (RH) and liquid water content, thereby decreasing the cloud cover Zhang and Zuidema (2019) may lead to misunderstanding that BBAs heat the cloud and thus decrease liquid water path and cloud cover. It would be better to specify it as BBA induced solar radiation absorption.
Line 74: The authors have defined the semi-direct effect in Line 67. It would be good to also define the indirect effect and shortly explain what is the difference between semi-direct effect and indirect effect.
Line 79: how could the indirect effect of BBAs affect the cloud lifetime mentioned here? Is it a decrease or increase?
Line 83: Specify the region since it is at the beginning of the paragraph.
Line 98-99: ‘Recent models tend to show BBAs layers descending rapidly when off the western coast of the African continent, resulting in BBAs layers that are too low in altitude over the SEA (Das et al., 2017; Gordon et al., 2018).’ needs to be revised.
Line 100: ‘the models’. Which models? They are not introduced beforehand.
Line 122-123: Is it only because aircraft observations over SEA region is lacking? What is advantages or advancements for aircraft observations in this study in comparison to those conducted before?
Line 129-130: It should be Spinning 130 Enhanced Visible and Infrared Imager (SEVIRI), instead of ‘SEVIRI (Spinning 130 Enhanced Visible and Infrared Imager)’.
Line 136: It should be 7 September 2017 (British English) or September 7, 2017 (American English).
Line 137: What is the difference of ‘temperature (T)’ compared to ‘potential temperature (θ)’? Is it ambient temperature?
Line 137: What is ‘total water mixing ratio’? Definition is needed.
Line 138: Does it really help to abbreviate straight and level runs as SLRs? Maybe not very helpful for readers but one more abbreviation to keep in mind.
Line 142: Again, is the abbreviation of ‘POC’ helpful and necessary?
Line 151: Better delete ‘accumulation mode’ because some studies also define particles between 0.1 and 2.5 µm or between 0.1 and 1.0 µm as accumulation mode particles.
Line 155: Manufacturer for the water CPC is missing.
Line 161: Will the abbreviated ‘STP’ frequently used below? Please check through the manuscript for abbreviations and make sure only use it when it is really necessary.
Line 166: Is the period the same for all observations on different days during the campaign?
Line 176: To ‘STP’ or to the condition? Please use more scientific and formal language.
Line 176-184: The introduction statements of Nd, Re, and liquid water content (LWC) using the three equations should be better organized. Simply putting all variables and parameters together seems too cursory.
Line 190: How could there be two ‘COT’s? ‘cloud optical thickness (COT)’ and ‘cloud-top height (COT)’
Line 195-196: This sentence is repeating the one in Lines 165-166.
Section 2.2: A clear definition for sub-cloud and above-cloud regions may be necessary.
Line 206: How can people understand the ‘reasonably well’ performance of NWP if people do noy go through the referred paper? It is too cursory to make such a statement.
Line 210-211: Isn’t this sentence a repetition of the content introduced in the previous paragraph?
Line 225: Didn’t the stratocumulus cloud defined as Sc in Line 46-47?
Line 230: What are the FT and BL products?
Section 2.3 can be shortened and more concise. It can be better structured. The introduction for 3D NAME experiments with Numerical Weather Prediction (NWP) model and the Unified Model (MetUM) is not clear. Which model is for what products? What is the difference between the two models? This is not easy for readers not from the field.
Line 235-236: It is not clear enough by just mentioning the classification was introduced in a previous study. How? and what are the criteria?
Line 236: It should be ‘Figure 2’ but not ‘Fig. 2’ at the starting of a sentence. This is clearly specified in the journal guidelines. Please follow the guideline carefully and check through the manuscript carefully.
Line 236: Does the number concentration of aerosol particles have a structure? Or it means the vertical distribution?
Line 238: How could field observations be consistent with the dates? Such a statement does not make sense.
Line 248: An equation is needed to show how ‘the θl was calculated from the T, θ and water mixing ratios’.
Line 248: ‘Fig. 3’, the same comment as for Line 236.
Line 251: What is lifting condensation level (LCL)? A clear definition is necessary since it shows up in the following discussions. It can also be clearly indicated in Fig.3a or 3b. What does it mean a surface layer? Whose surface? Or if it is a language issue, maybe better a flat layer or a layer with rather constant z/zi.
Line 254: ‘relatively’ means the comparison to what?
Line 256: How can the decoupling parameters αq and αθ be derived? It would be good to provide the equations and explain it with more details somewhere (e.g., in the supplement but properly referred in the main text).
Line 269: ‘become’ can be better replaced by ‘act as’
Line 274: What is the definition of ‘highly aged African BBAs in this study’ and what are their property differences compared to fresh ones?
Line 275: ‘CN20’ was not defined in the main text and only appeared once.
Line 280: ‘the majority of particles were in the size below 0.1 μm’ refer to which kind of particles? Aerosol particles larger than 3 nm or CCN particles?
Line 284: ‘more sulfate’. Yes, it was stated before that ‘The submicron aerosols from marine emissions in the clean MBL were previously reported to be dominated by sulfate (~ 60%) and organic (~ 24%).’ However, there is no evidence provided to show that aerosols in the clean MBL contain more sulfate.
Section 3.1: It seems the second paragraph is mainly citing a previous study from Wu et al. (2020) but not presenting the results in Figure 3 in this study. Figure 3d clearly shows that CCN/CN3 under clean MBL conditions is lower than that under BB-impacted MBL conditions and such a difference becomes smaller when it is in the FT. However, the CCN/Na in Fig. 3e did not show such a dependence on the atmospheric conditions (in the MBL or FT). Discussions on this finding is missing. The role of particles between 3 and 100 nm can also be investigated, which is also overlooked. In addition, why some data around z/zi in Figs. 3c, 3d and 3e is missing? Moreover, frequently using ‘some days’ (Line 253 and 255) or ‘sometimes’ (Line 292) makes the statement less convincible. Please use solid (clearly presented and referred) results to support the argument.
Line 288-290: ‘In Flight C032 with deep cumulus clouds, stepped profiles and SLRs were carried out and the average vertical profiles of cloud properties are provided.’ Where was the result provided? Please refer to a figure or table.
Line 299-301: Cloud condensation nuclei concentrations depend on cloud supersaturations. For the focus of this study, it is the contribution of Na to cloud condensation nuclei numbers that impacts cloud microphysics.
Line 302: Should be subsection 3.2.1
Line 310-311: What is the p value for the calculated Pearson correlation coefficients? It would be good to indicate the value in Figure 4 accordingly.
Line 312: Regarding to the statement ‘especially for the flights with clean FT but high Na in the BB-impacted MBL (dashed black box in Fig. 4b)’, the data point with Nd close to 400 cm-3 in the black box looks like an outliner. There are only three data points in the box. It is far-fetched to treat it as a special case to investigate the correlations.
Line 318-321: What is the relative position of FT, BB layer and the cloud? What is the cloud vertical extent? Without a clear statement or a schematic to show their relative altitudes, one can image different scenarios and can misunderstand the results.
Line 345-346: Please refer to the results in which Figure and which panel.
Line 377-379: why there is a difference? Please shortly explain it. Is it because of limitations in this study?
Line 381-382: What aerosol properties are responsible for the differences? Please clarify it. Simply leaving a very general (well-known) statement does not contribute to the advances of the field.
Line 387: Should be subsection 3.2.2
Line 394-396: References are needed. And corresponding figures for the results should be referred.
Line 426-427: Why select these three cases but not others? Is there a reason? Or was it randomly picked up?
Line 431: The journal does not allow the use of sub-panels like a1, a2, and a3. Instead, all panels should be listed following the alphabetic table.
Line 435: Why start with case 2 first but not case 1?
Line 439-440: the results in which previous figure supports the statement ‘and the MBL was observed to be BB-impacted over 440 Ascension Island’ ? Please refer to it. If not, please provide relevant results to support it (for case 1 and 3).
Line 511-512: Why present the results in Fig. S9 but not in the main text? Since it is discussed in the main text.
Line 575: Better not to use ‘primary activation’ and ‘secondary activation’. Instead, please directly state-out the activation pathway. This helps the readability.
Technical corrections:
- The comments on language and statements in the above specific comments are not exhausted. There are also many long sentences which generally have more grammatic errors and make it hard to digest. Please carefully go through the manuscript and improve the presentation.
- Figure 1: It will be helpful to show the grid lines to guide naked eyes for reading flight paths.
- Equations should be numbered. This is missing.
- English for dates need to be checked through the manuscript. Please use either British or American style.
- Table S1: Full spelling for MBL and FT is necessary.
- Figure 2: Axis ticks should be indicated for the ease of reading. Please also show the height for MBL as a function of time, which will help the discussions in Line 239-242.
- Figure 3: Line 857 ‘shades represent 10% and 90% value’. The shading area represents a range.
- Figure 6: Revise the panel label and specify which panel corresponds to which case.
- Figure 7: The legend for cases should come earlier in panel (c). The two AOT for y-axis in panel (c) and (d) should be differentiated (further specified).
- Figure 8: X-axis label is missing.
References:
Keller, A., Tritscher, T., and Burtscher, H.: Performance of water-based CPC 3788 for particles from a propane-flame soot-generator operated with rich fuel/air mixtures, J. Aerosol Sci, 60, 67-72, https://doi.org.10.1016/j.jaerosci.2013.02.005, 2013.
Zhang, J. and Zuidema, P.: The diurnal cycle of the smoky marine boundary layer observed during August in the remote southeast Atlantic, Atmos. Chem. Phys., 19, 14493-14516, https://doi.org.10.5194/acp-19-14493-2019, 2019.
Citation: https://doi.org/10.5194/egusphere-2024-3975-RC1
Data sets
CLARIFY: in-situ airborne observations by the FAAM BAE-146 aircraft Facility for Airborne Atmospheric Measurements (FAAM), Natural Environment Research Council (NERC), Met Office https://catalogue.ceda.ac.uk/uuid/38ab7089781a4560b067dd6c20af3769/
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