the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Nov 2024
Strong influence of Black Carbon on aerosol optical properties in Central Amazonia during the fire season
Abstract. During the dry season, the Amazonian atmosphere is strongly impacted by fires, even in remote areas. However, there are still knowledge gaps regarding how each aerosol type affects the aerosol radiative forcing. This work characterizes the chemical composition of submicrometer aerosols and source apportionment of Organic Aerosols (OA) and Equivalent Black Carbon (eBC) to study their influence on light scattering and absorption at a remote site in central Amazonia during the dry season (August–December 2013). We applied Positive Matrix Factorization (PMF) and multi-linear regression models to estimate chemical-dependent mass scattering (MSE) and extinction (MEE) efficiencies. Mean PM1 aerosol mass loading was 6.3±3.3 μg m-3, with 77 % of organics, grouped into 3 factors: Biomass Burning OA (BBOA), Isoprene derived Epoxydiol-Secondary OA (IEPOX-SOA) and Oxygenated OA (OOA). The bulk scattering and absorption coefficients at 637 nm were 17±10 Mm-1 and 3±2 Mm-1, yielding a single scattering albedo of 0.87±0.03. Although eBC represented only 6 % of the PM1 mass loading, MSE was highest for the eBC (13.58–7.62 m2 g-1 at 450–700 nm), followed by BBOA (7.96–3.10 m2 g-1) and ammonium sulfate (AS, 4.79–4.58 m2 g-1). MEE was dominated by eBC (30.8 %), followed by the OOA (19.9 %) and AS (17.6 %). The dominance of eBC over light scattering, in addition to absorption, depicts a surprisingly high role of this important climate agent, indicating the need to further investigate the chemical processing and interaction between natural and anthropogenic aerosol sources over remote tropical forested areas.
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RC1: 'Comment on egusphere-2024-3339', Anonymous Referee #1, 09 Dec 2024
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General Comments
This manuscript presents a comprehensive study of submicrometer aerosols during the dry season in central Amazonia, focusing on their chemical composition and optical properties. The study employs state-of-the-art analytical techniques, including Positive Matrix Factorization (PMF) and Multi-linear Regression (MLR), to provide valuable insights into the contribution of different aerosol components to radiative forcing. The findings, particularly the high scattering efficiency of equivalent black carbon (eBC), are novel and carry significant implications for climate modeling. However, there are several areas where the manuscript can be improved, both in terms of scientific content and presentation. Below are specific comments.
Major Comments
- Introduction: Lack of Logical Flow and Clear Scientific Questions
The introduction provides a rich background on Amazonian aerosols, their sources, and their climatic impacts, but it lacks a clear logical progression that leads to the importance of this study. Specific issues include:
- While the first four paragraphs review broad background knowledge, they do not adequately justify the importance of studying eBC and organic aerosols in the Amazon. The scientific problem and research gap are not clearly articulated.
- The statement "the intrinsic optical properties of each aerosol species are still rare" is insufficient as a scientific rationale. The authors need to more explicitly identify why studying these properties is critical and how the study addresses unresolved questions in the field.
- Logical inconsistencies in content, such as between lines 71–74, need to be addressed. For example, the discussion of secondary oxidized aerosols (lines 71–73) and scattering efficiencies (line 74) appears disconnected and lacks a clear link to the study’s objectives. The author should ensure logical transitions between sentences and paragraphs and ensure all background information directly supports the scientific rationale of the study.
- Mechanisms of eBC Scattering Efficiency
The study identifies a surprisingly high mass scattering efficiency (MSE) for eBC, suggesting a potential role of coating in enhancing light scattering. However, the manuscript does not provide sufficient experimental or theoretical discussion to explain this phenomenon. It is recommended to include additional analyses or citations to studies focusing on eBC coatings and their optical effects. If possible, provide size distribution or coating thickness data to support the hypothesis of enhanced scattering due to coating effects.
- Comparisons with Other Studies
While the manuscript cites relevant literature, a more detailed comparison with similar studies would enhance its impact:
- Compare the reported MSE, MEE, and SSA values with those from other regions and seasons, particularly regions strongly impacted by urban or biomass burning pollution.
- Discuss how the findings align or contrast with previous studies on eBC optical properties, especially in terms of scattering contributions.
- Uncertainty in Optical Properties
The derived optical properties (e.g., MSE, MEE) rely on specific assumptions, such as absorption cross-section values for eBC. The manuscript should:
- Clearly state the sensitivity of the results to these assumptions.
- Discuss potential sources of uncertainty in the measurements and their implications for the reported efficiencies.
Minor Comments
- Abstract: The abstract effectively summarizes the main findings but could provide more emphasis on the broader implications of eBC's scattering role for climate modeling.
- Figures: Improve the clarity and readability of some figures. For example, in Figures 2 and 4, use consistent scales and labels to make comparisons across factors easier.
- Table 3: clearly indicate which values are derived from measurements vs. calculations.
- It will be better to add a schematic figure summarizing the contributions of different aerosol components to light scattering and absorption.
- Lines 289-291: The inference oversimplifies the source attribution of black carbon (eBC) by directly linking it to biomass burning. While biomass burning is indeed a significant source in the Amazon during the dry season, local and regional sources, such as emissions from Manaus (e.g., diesel generators and vehicular emissions), must also be considered. A more nuanced approach would be to discuss the dominant sources in the region, then use the correlation between eBC and biomass burning markers (if available) to strengthen the attribution to biomass burning.
- Language: Some sentences, particularly in the Introduction and Discussion, are overly long and could be simplified for better readability.
- Code and Data Availability: The manuscript mentions that data will be made available upon acceptance. It would be more transparent to provide a DOI or link to a repository containing at least some preliminary data.
- The discussion of diurnal variations could be expanded to include a more detailed analysis of potential drivers (e.g., boundary layer dynamics, local emissions).
- The manuscript needs to provide further explanation for why the MSE (mass scattering efficiency) calculated by PMF decreases with increasing wavelength. Is this related to the optical properties of the aerosols, changes in particle size, or their mixing state? Providing some theoretical background or relevant literature would help clarify this observation.
- eBC is known to be a light-absorbing aerosol, but why does it have the highest MSE in this study, and why is it higher than in other studies? The authors should provide possible explanations, such as the influence of coating, particle size changes, or other mixing effects (Li et al., npj, 2024). This is crucial for understanding eBC's scattering properties.
- The manuscript states that eBC does not show wavelength dependence, yet Figure 7a shows large fluctuations in MSE at shorter wavelengths. Can the authors explain these fluctuations? Additionally, while Figure 7a shows higher MSE for eBC at shorter wavelengths, its contribution to scattering is lower compared to other wavelengths. This seems contradictory. Please explain the relationship between these two observations.
- The SSA value for eBC in Figure 8 is 0.57, indicating that scattering dominates its extinction. However, theoretically, black carbon, being a strong absorber, typically has a low SSA, often below 0.3 for pure black carbon (Wang et al., 2021). How do the authors reconcile this higher SSA for eBC in the study? Can this be explained by coatings or other factors? Further clarification on this point is needed.
References:
Wang et al., (2021). Constructing Shapes and Mixing Structures of Black Carbon Particles With Applications to Optical Calculations. Journal of Geophysical Research: Atmospheres, 126(10), e2021JD034620. https://doi.org/10.1029/2021JD034620
Li et al., Microphysical properties of atmospheric soot and organic particles: measurements, modeling, and impacts. npj Climate and Atmospheric Science 2024, 7, (1), 65.
Citation: https://doi.org/10.5194/egusphere-2024-3339-RC1
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