General comments

the study by Song et al. combines real-time measurements of atmospheric volatile organic compound (VOC) and organic aerosol concentrations on a highly complex study site influenced by a biogas power plant, a mixed temperate forest stand, a nearby village and a clear-cut area. The deployed instruments, especially the PTR-TOF-MS coupled with a CHARON particle inlet and the VOCUS-PTR-TOF-MS, are state of the art and allowed the authors to investigate VOC concentrations both in the gas and particle phase during a three-week field campaign. Additional measurements of trace gases (methane, carbon dioxide, carbon monoxide, ozone), water vapor, particulate matter and black carbon concentrations in the atmosphere, as well as meteorological parameters (temperature, relative humidity, soil moisture, planetary boundary layer height, wind speed and wind direction) built a comprehensive data set that is generally well suited to achieve the objective of the study. This was, as indicated in the title, to characterize the concentrations of biogenic VOCs and their oxidation products at a stressed forest close to a biogas power plant. The sources of the measured VOCs were nicely disentangled on the basis of the wind direction. Investigating the impact of a biogas power plant on atmospheric VOC concentrations in direct contrast to a stressed conifer forest is quite a novelty and also the impact of insect outbreaks in atmospheric chemistry is not well understood yet. Thus, in my opinion, the content of this study fits well with the scope of ACP. However, I have some major concerns regarding the overall presentation of results, as described in detail below.

Briefly, the study is written rather descriptive and substantial conclusions regarding a broader context become not always clear. I would strongly recommend to present the implications of the findings in more detail. Further, there were some inconsistencies regarding the definition of the wind direction sectors with possible implications on the data interpretation. Also, the tree species composition of the investigated forest and the stress status of the same should be characterized with more detail, as no physiological parameters are given in the present version of the manuscript.

Overall, I think the study contributes to a highly relevant topic and should be considered for publication in ACP, but there is still quite some scope of improvement.

Specific comments

Title

1. In the title the authors state, that a “stressed pine forest” was investigated. The term “pine forest” is in this context a little confusing or even wrong, as the study was conducted next to a forest composed of Picea abies (Norway spruce) and Fagus sylvatica (European beech) (L. 86) with no reported occurrence of Pinus spp. (Pine). In my understanding, “Pine” should be replaced with “temperate” in L2. Depending on the species composition in the studied area (which should be described with more detail) the forest type could be further specified as “temperate mixed forest” or “temperate coniferous forest”.
2. In addition, the stress status of the forest is insufficiently documented. With no doubt, the forests in the Eifel were strongly affected by bark beetle outbreaks, heat waves and drought over the last years. However, this regional situation does not explain sufficiently the current status of the investigated forest. For this, further stress indicators like tree mortality, chlorophyll fluorescence, leaf/needle water potential or comparable stress parameters should be included in the study in any case.

Abstract

1. The abstract could be substantially improved by adding a few sentences at the beginning about the general topic of the study, the research gap and the specific research questions.
2. : “In the WD-forest group [...] biogenic emissions of isoprene, monoterpenes and sesquiterpenes [...] exceeded the photochemical consumption” – is this surprising? I think this is exactly what we would expect for a temperate forest, especially, when it is stressed.
   
   I would recommend the authors to have a closer look on studies, that were conducted at the “Stations for Measuring Ecosystem-Atmosphere Relations” (SMEAR) in Estonia and Finland (SMEARII), because there are quite some similarities between the experimental set-ups and ecosystems studied (eg. Bourtsoukidis, E., Bonn, B., & Noe, S. M. (2014). On-line field measurements of BVOC emissions from Norway spruce (Picea abies) at the hemiboreal SMEAR-Estonia site under autumn conditions. Boreal environment research, 19(3), 153.“
3. : Here, the authors limit the scope of their conclusions to their specific study site. I would strongly recommend to highlight aspects of the study that are relevant for a broader context and/or more generalizable.

Introduction

1. L40-42: There are several earlier publications that should be cited here as a primary source, eg. Rasmussen & Went 1964 (10.1073/pnas.53.1.215) or Trainer et al. 1987 (https://doi.org/10.1038/329705a0)

Methods

1. Subsection “2.1 Sampling Site”: Information about the species composition, as well as about the stress status of the forest stand should be added to this subsection (see comment above). Furthermore, the clear-cutting areas mentioned in L112 seem to cover large areas around measurement location (Fig. 1a). There also seem to be some afforested areas in the close vicinity of the measurement location, which potentially influenced the measurements. The authors should indicate clearly in figure 1a which areas are covered with intact forest, and which areas are affected by clear-cutting or afforestation. Adding a colored layer to the satellite image might be suitable for this purpose.
2. L138-142: Here, different temperature settings of the drift tube of the CHARON-PTR-TOF-MS are described. In L161-167 is stated, that measurements with the drift tube temperature set to 80°C were discarded – please join this two paragraphs.
3. L170: Please explain, why another time period than 2020/06/05-2020/06/30 was chosen for the
   
   measurements with the Vocus-PTR-TOF-MS. The reasons are currently not clear.
4. L205: Soil moisture has an extremely high local variability. In this study only one soil moisture probe was used – the authors should be aware that the soil moisture data are not very reliable and should be transparent about this in the manuscript.

Results and discussion

1. The authors make intensive use of the supplement and present in total 23 (!) Figures and Tables (10 in the main manuscript and 13 in the supplement). This makes it sometimes difficult to follow the overall line of argumentation throughout the manuscript. I would highly recommend to opt for fewer Figures and make a selection based on relevance to support the main conclusions.
2. Throughout the manuscript many abbreviations are used. Some of them are common and make the text easier to understand (eg. PTR-TOF-MS, VOC, OA, SOA, LOD, PM2.5, PM10). However, some abbreviations are introduced, but never used again and should, in my opinion, be removed from the text (e.g. TDU in L.124 or FIMR in L171). Further, there are some abbreviations for short terms, like black carbon and planetary boundary layer, where the terms could be written out in full in order to reduce the total number of abbreviations and make the text easier to follow. In any case, the entire manuscript should be carefully checked to ensure that all abbreviations are introduced the first time they are used in the text (see technical comments). Further, nested abbreviations (explaining one abbreviation with another one) like in “semi-volatile oxygenated OA” (L293) should be avoided. Instead, please write out e.g. “semi-volatile oxygenated organic aerosols”
3. The authors should reconsider, whether giving averages over the entire campaign is the best way to present their data. Especially, for parameters with diurnal variations, like ambient temperature (see L.249) or BVOC concentrations, it would be more informative to report day and night averages, and for temperature, additionally, daily maximum and minimum values. This might not be necessary for more constant parameters, like soil moisture.
4. L252 ff.: Please, add some kind of systematic definition of the two episodes.
   
   A systematic definition could be: If the temperature of single days was > the 50% quantile of the temperature of the entire measurement campaign for a number of x consecutive days, then these days were defined as high-T episodes.
5. Please, explain and quantify what “good agreement” means in L. 266.
6. Throughout the entire section “Results and discussion” the French Landes forest is used as one of the main references to compare the results of this study with (eg. L 268, L269, L271, L280, and more). I have some doubts, whether the Landes forest, which is (other than the forest investigated in the present study) a pine forest with oceanic climate, the best choice to compare the results with to this extend. I would recommend to check the literature carefully for studies that were conducted in forests dominated by Norway Spruce and incorporate them into the discussion. One relevant study might be Petersen et al. 2023 (10.5194/acp-23-7839-2023) published in this same journal.
7. L284: Without direct calibration the measured sesquiterpene concentrations are probably not only lower than the actual concentrations due to fragmentation, but also due to the typically relatively low transmission rate of sesquiterpenes during the proton transfer reaction in the PTR-TOF-MS.
8. For me, it doesn´t always become clear whether a statement refers to results of the authors, or rather to a cited study. As an example in L.324: “The fragmentation pattern of oxidized organic compounds in the CHARON-PTR-TOF-MS varied depending on the instrument settings (Leglise et al. 2019). What is the meaning of the reference in this case?
9. In my opinion a drawback of the study is, that the wind sectors are not defined uniformly and, that the clear-cut sites are not represented adequately in the sector definition. While in L334ff there are three wind sectors defined (0-240° forest, 240-300° biogas power plant, 300-330° village), there are only two sectors defined in L372 (0-240°forest, 240-330° biogas power plant). Based on Figure 1a it seems like there was intact forest from ~0-90°, afforested or clear-cut areas from ~90-240°, the biogas power plant from ~240-300 and a zone influenced by forest emissions and anthropogenic emissions of the village from ~300°-360°. I would kindly ask the authors to check the definition of the sectors and indicate the land use with a colored layer in figure 1 (see comment above).
10. L401: Keep in mind, that plants also emit less VOCs during nighttime (see eg. Holzke et al. 2006 for European Beech, doi.org/10.1007/s10874-006-9027-9, Fig. 3a; and Ghirardo et al. 2010 doi.org/10.1111/j.1365-3040.2009.02104.x Fig. 1b; Meischner et al. 2024 doi.org/10.1093/treephys/tpae059, Fig. 4 for Norway Spruce).
11. L564: I would avoid statements about the concentration of sesquiterpenes, since measurements were not calibrated, as described in L. 282ff.
12. L577: Formulas and calculations should be defined and explained in the material and method section.
13. L598: I have some difficulties to follow the argumentation why European beech should have emitted mainly α-pinene and β-pinene. For Norway spruce this might be correct, however, there is strong evidence, that European beech emits mainly sabinene (>30 % of total monoterpene emissions) and only <10% α-pinene and β-pinene (Holzke et al. 2006, Table 2, 10.1007/s10874-006-9027-9)

Conclusion

1. 618: This is inconsistent with L.282 where it says, that sesquiterpenes could not be quantified due to missing calibration standards. Hence, sesquiterpene measurements should only be used to calculate correlations with other parameters or interpretation of temporal variations.

Figures

1. Figure 1b - RH: Please, change the color of the x-axis to black. The pink color could be interpreted as constantly low precipitation rates.
2. Figure 4: I really like the highlighted areas that indicate the wind direction from the biogas power plant. Why not adding shaded areas for the other sectors, too?

Technical corrections

• L26: Please, introduce the abbreviation for wind direction (WD)
• L50: Please, change “forests” to “forest ecosystems”
• L55: Please, change “showed” to “shows”
• L57: Please, change “sunlight” to “sunlight intensity”
• L58-59: Please, assign cited studies to specific stress types, since not all of the cited studies in L59 addressed the effect of high temperature, drought AND herbivory attack on BVOC emissions from trees
• L60: This is optional, but may be the sentence becomes clearer if “significantly” is exchanged with “especially”. In this way the role of terpenoids in the stress response of trees is highlighted and the sentence is less redundant with the previous one.
• L64: Please, change “showed” to “shows”
• L64ff: “…lower values during daytime”, compared to what?
• L83/L84: A connecting sentence would make the text easier to follow
• L86: Please, complete common species with Latin species names eg. “Norway spruce (Picea abies (L.) H. Karst.)”
• L92: Please, quantify the increase in BPPs, if possible

• L124: The term “thermo-desorption unit” appears only once in the entire text, so the abbreviation can be deleted (same for PEEK in L135)
• L133: Please, introduce the abbreviation “PFA”
• L145: Please be consistent with the abbreviation “TOF” or “ToF” throughout the whole text
• L201: Is “malfunction” instead of “multifunction” meant? Please, check.
• L202: Please, introduce abbreviations for “relative humidity” and “planetary boundary layer”. Check consistency with other parts of the manuscript, eg. in L 202 is says “boundary layer” and in L976 “planetary boundary layer”
• L208: Please, avoid introducing abbreviations in the title.
• L226: Please, change “into” to “from”
• L252-254: Please, change punctuation. E.g. Two characteristic episodes, [..], were observed [...].
• L350: In my opinion, it is not ideal to start a new paragraph with a reference to the supplement.
• L395: Please, add “,respectively,” after “<0.01 ppb”

General Comments

This study presents a detailed investigation into the real-time measurements of biogenic volatile organic compounds (BVOCs) and their oxidation products in both gas and particle phases in a stressed pine forest near a biogas power plant. The authors performed comprehensive measurements using two advanced mass spectrometers and analyzed the influence of various factors including meteorology, local emissions, and chemical transformation processes. This study provides valuable information, but it has a more limited scope than typical research articles. Several major and minor comments need to be addressed before the manuscript can be considered for publication.

Major Comments

1. The authors emphasize in the title and discussion that this forest is stressed. However, there are no details about the nature of this stress (e.g., when it occurred, to what extent, any dead trees, etc.). A discussion about potential changes in emissions due to this stress would be beneficial.
2. The authors identified two organic acid factors using PMF based on VOCUS-PTR data. However, these could be fragments of larger parent ions and not necessarily acids. Additionally, the choice of identifying 6 factors instead of 5, 7 or more needs justification. Figure S6d suggests that 6 factors may not fully explain the measured signals. For source apportionment, the correlation analysis between the factor and its dominating species seems unnecessary and does not support source identification convincingly.
3. The conclusion about the impact of relative humidity (RH) on gas-to-particle phase partitioning should consider the influence of temperature changes (about 10-15°C difference), which could not be excluded here.

Minor Comments

More details about the tree species are needed.

Lines 54-55 & 390-394: Discuss the temperature and light dependency of monoterpene emissions. Are they emitted in higher amounts during the daytime? The authors should discuss the different synthesis, storage, and emission mechanisms of isoprene (de-novo) and monoterpenes (mainly pool emissions from boreal pines).

Line 133: Add the diameter of the sampling tube (also for other relevant sections).

Lines 227 & 302-304: Clarify how many compounds were detected and identified by both instruments, and how many were excluded from further analysis due to low signal. Explain what is meant by "missing".

Line 229: The explanation for excluding C4H9+ is unconvincing. Figure S5 shows a high contribution of C4H9+ from 6/11 to 6/14, but a low contribution outside this period despite relatively stable TVOCs signal (by CHARON-PTR). Was this due to instrument performance?

Lines 292-300: Discuss the potential impacts of fragmentation on the mass and O:C and H:C ratios detected by CHARON-PTR.

Lines 310-312: It is also common to see C5-C8 compounds in boreal forest environments as oxidation products from monoterpenes. Compare these results with more field observations using CIMS with other reagent ions that cause less fragmentation and provide some conclusions.

Line 316: Have you done any correlation analysis between the parent ions and their potential fragment ions? How strong are these correlations?

Lines 598-599: Are there also spruce and beech trees at the sampling site?

Technical Corrections

Line 391: Add a space between "values" and "during".

SI

Figure S9, panel (b): Confirm if the isoprene signal was also multiplied by 10, as in panel (a). Double-check the legend.

Line 155: Correct to "pink and grey" instead of "grey and pink".

General comment

The study by Song et al. presents atmospheric observations of VOCs and aerosol composition at an interesting site in Germany, influenced by a nearby biogas power plant (BPP), a temperate forest, and a local village. This setting offers a unique opportunity to disentangle the contributions of these sources to ambient VOC composition and to assess their impact on local organic aerosol loading and atmospheric chemistry processes. The authors employed state-of-the-art analytical instrumentation for VOCs (VOCUS-PTR-ToF-MS) and aerosol composition (Ionicon PTR-ToF-MS coupled with CHARON), alongside a comprehensive array of gaseous, particulate, and meteorological measurements. Additionally, they conducted PMF analysis on 157 VOCs, creating a robust framework for both source identification and atmospheric impact assessment. Despite the evident efforts behind this study, its scientific conclusions are hindered by generalized statements that fall short in communicating a clear and novel message. However, the technical aspects of the paper are exceptionally well-articulated and valuable for future users of the equipment. Given the technical strengths and the uniqueness of the site, this study is valuable for the literature, but the following comments should be addressed before it is considered for publication.

Major Comments

1. The scientific focus of the study needs substantial improvement, and a clear conclusion should be articulated. While the results section is rich in information, it lacks flow and a cohesive scientific message that ties the observations together.
2. The PMF analysis requires further consideration. The study appears to be designed to distinguish the chemical fingerprints of the BPP from the forest and other sources, such as anthropogenic emissions from the nearby village. However, the identification of a factor labeled 'terpenes' suggests that the two dominant sources at this location were not successfully separated. Figure S6a-b indicates that alternative solutions were possible, and the choice of the 6-factor solution may not be as robust as implied. Moreover, additional analyses should be conducted; it is common practice to correlate factors with external variables, yet only vague correlation values (r) are provided here. I recommend that the authors reconsider their PMF solution and potentially re-run the analysis with a more robust setup, such as applying stricter criteria for data inclusion (e.g., considering thresholds greater than 20% for missing values (L227)).

Specific Comments

L1. The current title does not accurately reflect the content of the paper. The authors assume a stressed forest (L90), but there is no evidence provided to support that the forest ecosystem was under stress during the measurement period. While droughts, heatwaves, and beetle infestations are known to occur in most temperate forests, claiming 'stress' in this context is an unsupported assumption. This claim should be corrected throughout the paper.

L35-37. This conclusion is rather weak, especially considering the extensive use of highly sophisticated analytical equipment in this study. Please strengthen this conclusion in line with the major comments provided above.

L53. You may consider citing two recently published, relevant papers: Weber et al., 2023 (https://www.nature.com/articles/s41467-022-34944-9) and Bourtsoukidis et al., 2024 (https://www.nature.com/articles/s43247-023-01175-9).

L74-75.  The study by Penuelas and Staudt (2010; https://doi.org/10.1016/j.tplants.2009.12.005) is more appropriate for citation at this point.

L90, L100, etc. Please remove all comments on stress.

L213. You may consider adding the following relevant studies that deal with PMF analysis on PTR data: Desservetazz et al. (2023; https://doi.org/10.1016/j.scitotenv.2023.166592) and Jain et al. (2023; https://doi.org/10.5194/acp-23-3383-2023).

L263. How do the PMF factors relate to these distinct events?

L268, L280. Please also compare the findings to other German forests.

L282-286. Consider moving this part to the Methods section.

L340. The term "PBL" appears for the first time here, so it needs to be defined. Additionally, the connection between wind direction and boundary layer height is unclear, leading to a weakly supported statement in L341-342.

L343-349. This entire section needs to be rewritten for improved clarity. Several statements are vague and weakly supported by the data. For example, while it is suggested that temperature is not the main driver, there is no discussion of other environmental factors within the forest that could be influencing the results. A more thorough examination of these potential drivers is necessary.

L368-369. How can isoprene, monoterpenes and sesquiterpenes originate from chemical transformations?

L389. Please compare with temperate forests.

L411 and in general. It appears that you are using O₃ mixing ratios to attribute the atmospheric degradation of terpenes. What about the opposite, i.e., O₃ formation? Local terpene emissions contribute to O₃ formation, which is particularly relevant for emissions upwind of the measurement site and in relation to some of the PMF factors. This aspect should be addressed.

L426 & L436. A factor named ‘terpenes’ indicates that the entire scope of the PMF did not achieve its purpose. This may actually be considered the weakest point of the study.

L448. An R² value of 0.46 does not truly indicate ‘well-correlated’ parameters.

L570. Please provide a more detailed explanation of why you calculated the OA/ΔCO ratios.

L599. Needs citations.

L616-617. As mentioned earlier, you are not dealing with a stressed forest here. However, it is worth noting that June 2020 coincided with the COVID-19 lockdowns in Germany. Can you provide any insights into the potential influence of this on your dataset? For example, was the BPP operating as usual or at a reduced capacity?

L628-631. This statement is confusing and seems to imply that BVOC emissions are larger than their chemical sinks, which is a rather generalized comment. Please consider removing it.

L640-642. This is another generalized sentence that simply states, "high temperatures and radiation will enhance BVOC emissions, which will be oxidized in the atmosphere." While this is accurate, it represents textbook knowledge and highlights the need for clearer, more specific scientific conclusions rather than relying on well-known statements.

L649. Please specify this ‘minor role’.

L665. While no relationship between soil moisture and BVOC emissions was demonstrated here, the data were still used for analysis. Therefore, unless there are other reasons behind this decision, I would recommend including Heye Bogena in the author list.

Technical Comments

L23 and in numerous other parts. Technically, the values reported in ppb are volume mixing ratios, not concentrations. Volume mixing ratios (such as ppb) indicate the number of molecules of a substance relative to the total number of air molecules and are independent of temperature and pressure. In contrast, concentrations refer to the mass or number of molecules per unit volume of air, which can vary with changes in temperature and pressure. Please correct this accordingly.

L115. Please homogenize the temperature units.

L212. The term "non-methane VOCs" is uncommon and might cause confusion. The term "non-methane hydrocarbons (NMHC)" is typically used to describe lighter compounds so in this context, it’s clearer and more appropriate to simply refer to them as VOCs. I recommend removing "non-methane" and referring to these compounds as VOCs throughout the paper.