the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Feb 2025
Comprehensive Non-targeted Molecular Characterization of Organic Aerosols in the Amazon Rainforest
Abstract. The Amazon rainforest plays a crucial role in the global climate system, hydrological cycle, and earth's energy balance. As one of the planet's least industrialized regions, it allows investigation of organic aerosol formation and constituents under almost pristine conditions. Nevertheless, human activities are known to affect this ecosystem – especially during the dry season. In this study, ambient aerosol samples collected at the Amazon Tall Tower Observatory (ATTO) during two dry and two wet seasons were characterized by high-resolution mass spectrometry (HR-MS). Comprehensive non-targeted data evaluation was applied to identify thousands of molecular formulae. Most were found to be associated with oxidation products of isoprene and monoterpenes, highlighting the predominance of biogenic secondary organic aerosols (SOA) at ATTO. The chemical composition exhibited distinct seasonal patterns with more processed organic compounds during the dry season, which can be explained by an increase of later-generation oxidation products due to reduced wet deposition and enhanced long-range transport. Mono- and polycyclic heteroaromatic components from biomass burning (BB) sources were enhanced during the dry seasons and the second wet season. The wet season was generally characterized by less oxidized compounds, associated with freshly formed SOA particles. Height-resolved measurements showed the forest canopy to be the main source for biogenic emissions with higher concentrations of early terpene oxidation products lower down. Overall, our results provide new insights into the molecular characteristics and seasonality of organic particulate matter at ATTO, helping to constrain the sources and interactions of aerosols, clouds, and precipitation in the Amazon rainforest.
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Status: open (until 11 Apr 2025)
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RC1: 'Comment on egusphere-2025-141', Anonymous Referee #1, 18 Mar 2025
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The manuscript by Leppla et al. investigates the chemical composition and potential sources and chemistry of organic aerosols in two different wet and dry seasons in the Amazon rainforest with the deployment of an UHPLC-HR-Orbitrap mass spectrometer. It also compares the molecular composition and volatility of background compounds with relatively low variability and compounds with higher variability. The topic of this manuscript is very interesting. However, some revisions are needed before its possible publication on ACP. Please see my comments and questions below.
Major:
More discussions on the comparison of the results at different heights are needed as the abstract underlined the height-resolved measurements. If the chemical composition is similar at different altitudes (Line 366-367, 390-392), then it seems not so important to include all heights in the main text (?). Is there any nocturnal differences in chemical composition below and above forest canopy? Could the authors comment on this?
Specific:
Line 29-30. The reviewer didn’t find the results in the main text on the forest canopy height being the main source of biogenic emissions or early terpene oxidation products. Wet season 2018 seems to have the filters collected at the closest height (40m) to the canopy height (35m); however, Wet season 2018 had less compounds (both background and variable ones) in Figure 2 and 3.
Line 105-106. Could the authors explain the purpose/reason of sampling at different heights in wet and dry seasons (e.g. wet season 2018 at 42m and 150m but dry season at 0m and 80m)? How would this affect the result interpretation and comparison between different seasons? Could the authors comment on this?
Line 111. How many filters were collected at each height and each season? Please add this information.
Line 132-134. Why did the authors exclude fluorine in the data evaluation if recent studies have found fluorine-containing species in Amazon? Are they present in your dataset? What’s their signal intensity contribution? Considering the high resolution of Orbitrap, it should be possible to identify fluorine-containing species, unless they are not present in this study.
Line 186-188. What did the authors mean in terms of “Only compounds that were observed in more than 75 % of all samples were defined as background compounds”? Is it based on the presence of the compound in the samples or based on some concentration criteria?
Line 188-190. Are the remaining compounds unidentified compounds since the variable compounds are the remaining species of the total identified compounds from the background compounds? Would be nice to add total numbers of identified compounds for all groups (background compounds, variable compounds, and remaining compounds), and more importantly their signal intensity fraction.
Line 196-202. Please add the signal fractions of compounds with MW below 250 Da, 300-450 Da, and above 450 Da correspondingly. Also a typical mass spectra would be very informative.
Line 208-211. Would be nice to add the compound subgroup contributions from the mentioned remote/suburban/urban environments in the literature. Also the elemental ratios obtained in this study in Line 216 could be added.
Line 297. Which figure or table showed this? Please specify. Also considering important contributions of biogenic emissions during the wet season, could the authors explain the reason why the intensities of a-pinene oxidation products were higher in the dry than in the wet seasons?
Line 343-344. Why did 2019 dry season have fewer HOM? Is it related to the higher NO levels or more fires during this period?
Line 378-386. Would be nice to have a table for variable compounds similar to Table S2 (for background compounds) in SI. Also since the authors separated daytime vs nighttime for the variable compounds, why didn’t you do the same for background compounds as well? Or is there no difference for day vs night for background compounds?
Line 393-395.
- If the authors would focus only on compounds with high intensities in the five areas, the reviewer would suggest to have a table for them similar to Table 2, and label them in Figure 3 similar to Figure 1.
-Also C5H12O7S was already discussed in the background compound in Table 1. The reviewer would assume it should not be present in the variable compound group here as well. Same for C5H6O4 in Line 427 which was also listed in Table 1.
Line 459-463. Do you mean the dry season 2018 (in Line 459) had higher levels of CHON species at night than day? Is it a typo? It doesn’t seem to be the case for wet season since there were very few CHON species both day and night.
Line 468-473. What’s the dominant species for these combustion-related highly unsaturated organic compounds?
Line 520-525. Would be nice to have similar plots for the year 2019 data in SI. Also for background compounds from section 3.2.
Line 526-529. The authors know the molecular formulae of the compounds with MW between 500-600, and therefore it's possible compare the dominant species to the sesquiterpene oxidation products from Gao et al., 2022.
Line 547-548. Would be nice to have similar plots for the year 2019 data in SI as those in Figure 6 and 7. Also for background compounds from section 3.2.
Line 581. Based on the discussions e.g. in Line 480-481 that the wet season 2019 was significantly influenced by biomass burning and combustion activities (Figure S11), the reviewer is wondering whether the wet season 2019 can still be classified as “clean” periods. Also the 7-d HYSPLIT backward trajectories in Figure S3 also shows the wet season 2019 had contact/source from the African continent but not the case for the wet season 2018.
Figure 5. Please change the y axis of lower panels from contribution of number of compounds” to “contribution of signal intensity” (Volatility Basis Set VBS; Donahue et al., 2006), since histogram of number of compounds doesn’t equal to their role in volatility.
Technical:
Line 46. Would change “their nucleation” to “their oxidation products’ nucleation”.
Line 83-84. Seems repetition with the sentences in the previous paragraph.
Line 84-86. Also seem a bit repetition with the sentences in line 80-81. Please consider combine them.
Line 201. Change “ions” to “molecules with MW”.
Line 236. Change “particle phase” to “particles”.
Line 262-322. Please consider removing the bold headlines (e.g. “General”, “Isoprene SOA”) in each of the paragraphs. Also the case for the bold headlines in section 3.3.1 for the five classes.
Line 336. Change “nuclei” to “clusters”.
Line 358. Typo for LV-OOA. Same for Line 452: typo for SV-OOA. Also Figure 4.
Line 477-478. Seems repetition with the previous sentence.
Line 599. Change “what” to “which”.
Table 1 and S2. Change “Signals” to e.g. “Number of compounds detected”.
Reference:
Donahue, N. M., Robinson, A. L., Stanier, C. O., and Pandis, S. N.: Coupled partitioning, dilution, and chemical aging of semivolatile organics, Environ. Sci. Technol., 40, 2635–2643, 2006.
Citation: https://doi.org/10.5194/egusphere-2025-141-RC1
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