the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2024
Vertically resolved aerosol variability at the Amazon Tall Tower Observatory under wet season conditions
Abstract. The wet season atmosphere in the central Amazon resembles natural conditions with minimal anthropogenic influence, making it one of the rare pre-industrial-like continental areas worldwide. Previous long-term studies have analyzed the properties and sources of the natural Amazonian background aerosol. However, the vertical profile of the planetary boundary layer (PBL) has not been assessed systematically. Since 2017, such a profile assessment has been possible with the 325 m high tower at the Amazon Tall Tower Observatory (ATTO), located in a largely untouched primary forest in central Amazonia. This study investigates the variability of submicrometer aerosol concentration, size distribution, and optical properties at 60 and 325 m height in the Amazon PBL. The results show significant differences in aerosol volumes and scattering coefficients in the vertical gradient. The aerosol population was well-mixed throughout the boundary layer during the daytime but became separated upon stratification during nighttime. We also found a significant difference in the spectral dependence of the scattering coefficients between the two heights. The analysis of rainfall and related downdrafts revealed changes in the aerosol populations before and after rain events, with absorption and scattering coefficients decreasing as optically active particles are removed by wet deposition. The recovery of absorption and scattering coefficients is faster at 325 m than at 60 m. Convective events were concomitant with rapid increases in the concentration of sub-50 nm particles, likely associated with downdrafts. We found that the aerosol population near the canopy had a significantly higher mass scattering efficiency than at 325 m. It was also observed a clear spectral dependence, with values for λ = 450, 525 and 635 nm of 7.74±0.12, 5.49±0.11and 4.15±0.11 m2 g−1, respectively, at 60 m, while at 325 m, the values were 5.26±0.06, 3.76±0.05 and 2.46±0.04 m2 g−1, respectively. The equivalent aerosol refractive index results, which were obtained for the first time for the wet season in central Amazon, show a slightly higher scattering (real) component at 60 m compared to 325 m, of 1.33 and 1.27, respectively. In contrast, the refractive index’s absorptive (imaginary) component was identical for both heights, at 0.006. This study shows that the aerosol physical properties at 60 and 325 m height are different, likely due to aging processes, and strongly depend on the photochemistry, PBL dynamics, and aerosol sources. These findings provide valuable insights into the impact of aerosols on climate and radiative balance and can be used to improve the representation of aerosols in global climate models.
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RC1: 'Comment on egusphere-2023-2607', Anonymous Referee #1, 27 Mar 2024
General comments:
This article provides and analyzes very valuable information of the sub-micron aerosol in a remote Amazon site. It informs not only number concentration and size distribution (10 nm to 400 nm) but also extensive and intensive optical properties of the aerosols, at two different heights, thanks to the well known ATTO tower. This kind of data is very scarce in the region, since most of the (few) air quality monitoring stations in South America are placed in urban regions.
As the authors claim, comparing continuum measurements at different heights is very important, since it provides information regarding sources (new particle formation, biogenic emissions, long range transport) and helps validate and improve aerosol models and remote sensing retrievals. The location of the site allows to study a pristine Amazonian atmosphere, with impact of Africa long range biomass burning, and in addition provides insight regarding the effect of downdraft events.
For instance, they observe the higher relevance of BC at 325 m due to the effect of the African plume (and optical aging of BrC); they show new particle formation near the canopy (sub-50 nm) and growth during the day through aging (accumulation mode); they observe the rate of new particle formation after wet deposition; etc.
The article provides very relevant figures (both main article and supplement) that are adequate to derive the conclusions in the main text. The article is well written and supported with previous results from literature, adequately discussed.
I recommend publication after minor revision.
Specific comments:
- Section 2.3: if aerosol volume is estimated from SMPSs (as I imagine), it would be relevant to introduce in this section a comment on that, and maybe the equations that connect the measured size distributions with the aerosol volume (here or in the Supplement).
- Section 2.4: I believe it would be important to make clear which cut size (if any) have each optical instrument. For instance, usually the aethalometers use PM2.5 size cut, but not always. From the discussion it seems that nephelometer and MAAP let coarse aerosols in (effect of large biogenic aerosols is discussed), but it is not clear what sizes are allowed.
- Section 2.5: The adjustment of m and k with Mie's calculations is interesting. Do you think is reasonable to assume sphericity for these particle sizes? Do you have microscopy information for similar samples?
- Section 3.2: "Although differences were observed for the SAE and the vertical profile, indicating different aerosol coarse mode populations, the ω0,637nm indicates that the fine mode aerosol population in both highs is very efficient in scattering radiation." I would like the authors to comment a little further on this phrase (and other connected phrases below in the article). In the one side, I understand that you did not measure the coarse mode aerosols in this work, but that is plausible that there are more coarse biogenic particles at 60 m, which should be expressed as a lower a SAE. But here you see small differences in SAE in the vertical profile, and you assign it to the fact that "the fine mode aerosol population in both highs is very efficient in scattering radiation". Am I correct? Later in this section, you also suggest that the differences in the accumulation mode median diameter could influence the aerosol optical properties. Please comment a little bit more on your view regarding the impact of coarse and fine mode aerosols in scattering coefficient and SAE. Maybe comparing the effects that increase or decrease the scattering coefficient and SAE and the suggested balance among these factors/causes at the end of this section? It can also be connected with complementary results and discussion in Sections 3.5 and 3.6.
- Section 3.4: at the end of the section, the authors provide a rate of biogenic emissions after wet deposition events. Are there previous results in literature that could be discussed? Or maybe biogenic emissions rates from models like MEGAN?
- Section 3.5, p.18:
"The results suggest that aerosols at 325 m, as observed in Figure S6, are likely to be more processed and are less efficient in scattering radiation. In contrast, smaller aerosols in direct contact with fresh VOC emissions from vegetation are more likely to scatter radiation more efficiently. Another possibility that might explain the results is that the higher apparent MSE at 60 m is likely due to the presence of a coarse mode, which is not detected by the SMPS. In fact, Prass et al. (2021) observed that bioaerosols account for about 70% of the aerosol coarse mode at ATTO, with higher concentrations at 60 m, which decreases with height. "
Are there previous results that suggest that "smaller aerosols in direct contact with fresh VOC emissions from vegetation are more likely to scatter radiation more efficiently"? Or do you believe that the second hypothesis is more likely?
Technical corrections:
P.18, "Shorter wavelengths have higher scattering efficiencies:"
It seems to me that the correct expression would be "Scattering efficiencies are higher for shorter wavelenghts"
(to my mind, aerosols HAVE higher or lower scattering efficiencies, not wavelenghts).Citation: https://doi.org/10.5194/egusphere-2023-2607-RC1 -
RC2: 'Comment on egusphere-2023-2607', Anonymous Referee #2, 02 Apr 2024
General comment:
The study discussed vertical variations in size distribution and numerical concentration of submicron aerosols, in addition to intensive and extensive properties of aerosols. These measurements carried out at ATTO are in themselves a great scientific contribution, in addition, the authors discuss these variations satisfactorily with a vast current bibliography, which supports the justification of the study and favors explanations of the observed phenomena.
The authors found significant differences in concentrations and optical properties for different heights and explained these differences using consistent methods, such as analysis of downdraft events and analysis of the aerosol refractive index. Throughout the discussion, several processes of formation, removal, and contributions arising from aerosol transport are highlighted.
The study makes an important scientific contribution that elucidates the processes of emission, formation, and transport of aerosol along the vertical profile. The text is well structured and written, and the figures (main and complementary text) and analysis allow us to reach important conclusions that, in addition to explaining the vertical variations, quantify important parameters such as the absorption of BC, BrC, and optical properties at different heights, which contributes for future model development and tuning in inversion algorithms for satellite products.
I recommend publication after minor revision.
Specific comments:
- About the Introduction: The text is very well written. The information is clear and objective, and the citations are appropriate and contextualize what has already been done and what is new about the work. In my opinion, the only point that needs to be adjusted is the textual term "vertical distribution of aerosols" which is mentioned in the last paragraph, when highlighting the objective of the work. As this study analyzed measurements (optics, size distribution, and numerical concentration) at two specific heights, the authors should refer to vertical variation and avoid the terms distribution or vertical profile, unless this terminology is introduced in the text for the specific case.
- Section 2.1: Although the authors cite Andreae et al. (2015) for additional information from the study site, I encourage the authors to provide a figure with the location of the site, preferably showing a schematic with the arrangement of instruments for different heights, as well as the average canopy height. The scientific contribution of the study justifies this increase, as the work will serve as a reference for many future studies.
- Section 2.2: The authors justify not analyzing particles larger than 400 nm due to the limitations of SMPS. However, instrumentation definitions only appear in the next topic. I suggest an inversion of these topics.
- Section 2.4: What was the cut section, 2.5? the same question for the aethalometer and MAAP. I believe it is important to add this information to the text.
- "As both MAAP and aethalometers measure at 637 nm, a comparison between the two was performed to ensure that there were no calibration issues." It would be interesting to add this comparison in the supplementary material.
- Section 3.2: Wilcoxon rank-sum test, the authors find a significant difference using the Wilcoxon test. Were other tests also evaluated? The authors need to discuss the maximum and minimum variations for these coefficients, as visually there does not appear to be a significant difference for the different heights. I encourage the use of more robust tests using parametric statistics. Although these data do not present a normal distribution, I believe that the sample size justifies the application of parametric statistics.
- Section 3.6: Although the study focuses on vertical differences for the real and imaginary components of the aerosol refractive index, and therefore has precise estimates for each height. I ask the authors: is there a possibility of comparing the average value of m with the AERONET inversion estimates? This would be an interesting topic for future work.
Technical corrections:
P1, P11, P16, P21, and P22: Avoid the term "slightly".
Example P11: "the absorption coefficients are slightly higher at 325 than at 60 m" replace with the absorption coefficients are greater in magnitude at 325 than at 60 m. Avoid "slightly higher".
Citation: https://doi.org/10.5194/egusphere-2023-2607-RC2
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