the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jul 2026
The effect of spruce bark beetle on BVOC, secondary organic aerosol and ozone: Integrating biotic stress in a chemical transport model
Abstract.
Droughts and other weather extremes affect the resilience of trees and facilitate herbivore insect infestation. In the last decade, the spruce bark beetle population expanded over mid to northern Europe, infesting large spruce forest areas. Spruce bark beetles bore holes into the stem of the trees, which activates a stress response and causes the release of large amounts of stored BVOC. Based on BVOC measurements from the stem of healthy and infested Norway spruces, a biotic stress parametrization was developed integrating stress-related BVOC emissions into the chemical transport model COSMO-MUSCAT (COnsortium for Small scale MOdelling - MUltiScale Chemistry Aerosol Transport). This study investigates a high impact scenario representing an intensive Europe wide spruce bark beetle infestation. With the stress parametrization, the BVOC emissions of spruces were increased substantially which affects atmospheric chemistry. The parametrization leads to higher nighttime emissions and concentrations of α-pinene and limonene. The altered diurnal cycle of the monoterpenes concentration is in closer agreement with measurements. The enhanced BVOC affect ozone concentration with local variations between −7 % and +10 % and significantly elevate secondary organic aerosol (SOA). On the regional scale, the SOA concentration increases in summer by 55 % and at spruce-abundant locations by ~150 %. The largest increases occur during night, with major contributions by the oxidation of lumped α-pinene by NO3 and O3. The higher SOA concentration during bark beetle infestation contributes to elevated fine particulate matter concentration. The study highlights the importance for integrating stress related BVOC emissions into chemical transport models.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4044', Tihomir Simin, 17 Aug 2026
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RC2: 'Comment on egusphere-2026-4044', Anonymous Referee #2, 26 Aug 2026
The paper makes a useful methodological contribution by incorporating species-specific, bark-beetle-induced BVOC emissions into COSMO-MUSCAT and explicitly separating trunk emissions from standard canopy/needle emissions. The simulations illustrate how an additional stress-related source can alter the diurnal cycle of monoterpenes and affect atmospheric composition. However, the highly idealized infestation scenario should not be interpreted as an estimate of the actual atmospheric effects of the 2021 European bark-beetle outbreak. The main quantitative conclusions depend on several substantial assumptions whose uncertainties have not been adequately evaluated through sensitivity experiments. My comments focus primarily on the emission calculations. I recommend major revision. I have a few concerns of the emission calculations
- The emission rates adopted from Jaakkola et al. (2023) represent total bark emissions measured from infested trees, rather than the additional emission above the healthy-bark baseline. Consequently, in Eqs. 3–4, adding the healthy-trunk term to the full infested-trunk emission rate appears to double count constitutive bark emissions for the infested fraction. Moreover, the formulation does not explicitly distinguish the trunk area associated with healthy and infested trees. f_trunk has been multiplied by both healthy and induced trunk emissions. I think Equation 4 is conceptually incorrect and should be revised. Although the authors may think that emissions from healthy trunks are much smaller, this does not justify double-counting emissions.
- In Eq 4, the derivation of f_trunk is insufficiently documented. As described by authors as the ratio between average lateral trunk surface area and the area covered by one tree; converting this ratio cannot get the emission rates to the per ground surface area?. A clarification is needed. Please specify the tree dimensions, geometric assumptions and also the area covered by one tree (crown area or stem area or stand area?)
- Table 2 reports the total monoterpene emission rate from Jaakkola et al. (2023), but provides emission rates and fractions for only selected compounds. It is therefore unclear how the complete measured monoterpene mixture is represented in the model. Are the unlisted compounds omitted or assigned to lumped chemical species? Please provide a complete mapping from the measured compounds to the model species, demonstrate conservation of total monoterpene emissions, and justify the surrogate reaction rates and SOA yields. This is particularly important because the conclusions concern oxidation pathways and SOA formation, which depend on emission composition as well as total emission magnitude.
- The reported monoterpene emission rates of 6600 +-6700 ug/m2/h exhibits substantial variability. Applying part of this single mean value to infested spruce throughout Europe does worry me about the representativeness of the outputs. As a minimum, the author should have performed sensitivity analyses covering plausible emission magnitudes and composition and present the results as a range of experiments with different emission magnitudes and clearly state the uncertainty when interpreting the quantitative atmospheric results.
- Although the author explained that the stem temperature is unavailable, I don’t think it is justifiable to assume that emissions are temperature-independent. Given that enhanced nighttime monoterpene concentrations and SOA formation are central findings, sensitivity tests using plausible temperature responses are needed to determine how strongly these results depend on the assumed temperature-independent source.
More generally, the equations are difficult to follow because the definitions and interpretations of some symbols are incomplete. Please review all equations and accompanying text to ensure that every symbol, unit, and parameter value is clearly defined when first introduced.
Abstract: Please briefly describe the stress parameterization and its key assumptions, particularly the prescribed infestation extent and the treatment of bark-emission temperature dependence. The conditional nature of the quantitative results should be clear.
L10: suggest changing to “The enhanced BVOC emissions”
L53-54, bark beetles often occur after storm as well.
L112: specify which anthropogenic emissions
L189: Explain what biomass density is. Aboveground or leaf biomass? If it is aboveground biomass, the author needs to explain why use the aboveground biomass instead of leaf biomass and provide further supports
L220-224, Please state explicitly that the bark-emission rates standardized to 30 °C are applied without an actual-temperature correction, if this is the intended implementation. The current explanation leaves this unclear.
L235, figure 2. we cannot call this function as “exponential population growth” as it is rather bell-shaped seasonal function.
Section 4.1
Norunda is a mixed forest, and I think it is mainly dominated by Scots pine. It will be interesting to show contributions from spruce, pine, healthy trunks, and stress separately. The shortcomings in capturing baseline emissions might be confounded with missing stress emissions. Then, the improved nighttime pattern could be influenced by the non-temperature-dependent sources rather than accurately representing infestation. A sensitivity experiment is clearly needed to justify that the improvements come from infestation.
Citation: https://doi.org/10.5194/egusphere-2026-4044-RC2
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Data for Publication "The effect of spruce bark beetle on BVOC, secondary organic aerosol and ozone: Integrating biotic stress in a chemical transport model" Jana Wackermann and Hanna Wiedenhaus https://doi.org/10.5281/zenodo.21221331
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- 1
The study "The effect of spruce bark beetle on BVOC, secondary organic aerosol and ozone: Integrating biotic stress in a chemical transport model" investigates the effect of herbivore feeding on Spruce VOC blends and integrates them into a global VOC model. Therefore, this is a very important study to improve our general understanding of the complexities of how biogenic VOCS are produced and how they affect other important pieces of the atmospheric system, such as secondary organic aerosols, clouds and ozone concentration.
General comments:
I am not an expert in modelling, but I think that the study is impressive in its ambitions to parametrize not only the emissions from spruce bark, but more importantly adjust them to the biotic stress of beetle herbivory. Truly a great addition to large VOC models, such as COSMO-MUSCAT.Maybe it can be adjusted for other models, one day, such as MEGAN for example?
I find that the manuscript is very well written and mostly ready for publication in Atmospheric Chemistry and Physics.
I have only minor comments for revision.
Specific comments:
L17: I would recommend saying "an increasing strain" rather than "increasingly strain".
L46: I think you mean "threat", instead of "thread". Please correct.
L68: Hydroperoxyl radicals or hydroxyperoxy radicals? In L 134 it's spelled differently.
L80: I don't disagree with you here, but please check the new article by Yu et al. 2024 (https://doi.org/10.1126/science.ado6779). It might provide some important context here in regards to SOAs formed from BVOCs from evergreen needle trees induced by bark beetle herbivory.
L125: Does hv in R2 stand for solar radiation? If so, maybe you can spell it out in full? There should be enough space. What does M stand for?
Equation 4: Have you seen the study by Ghimre et al. (2016; https://doi.org/10.1016/j.atmosenv.2015.11.049)? They have measured the emissions of bark VOCs from spruce directly. Does your model match the emissions from that study? I feel like it would be an important study to at least mention in your work, seeing as closely related they are in scope and intention.
L216: Please provide any citation on the claim that isoprene cannot be emitted from or stored in the bark.
Figure 2: Please spell out the meaning of δSIE in the figure description for an interpretation that is more indpenedent from the main text.
Table 2: Similarly, please spell out the SIE.
L260: Please replace "Scandinavia" with "Fennoscandia", as Finland is clearly included in the regional D0 domain. Also applies to rest of the text, for example line 341.
Figure 3: It's a nice figure, but if you are presenting % in the right-hand Y-axis, I think it sould be actual percentages, rather than the decimal numbers. Because technically it's showing now between 0 and 1%, rather than from 0 - 100% (which I assume was your intention, juding by the density of evergreen forests in northern Europe). Please correct this.
Figure 3, 4, etc: Please redraw all maps in your manuscript without country borders. This is a clear instruction from the journal guidelines - "Please adhere to United Nations naming conventions for maps used in your manuscript. In order to depoliticize scientific articles, authors should avoid the drawing of borders or use of contested topographical names."
L300: All at the same time or not all at the same time? I'm a little bit confused here.
L323: Heights? Do you mean elevation?
Figure 5: Please spell out MTPs here as well.
L339: You might be interested in how the bark-beetle induced VOC emission from evergreen trees actually responds to outdoor environments with elevated levels of NOx or O3. Does it compare to your predictions? (https://doi.org/10.1038/s43247-025-03175-3)
Figure 7: Could you please explain again here what do N0 and D0 domain meaan, so that the reader does not need to go back to the main text to see?
L398: Can you please provide a reference for different SOA formation from BVOCs and AVOCs?
L434: Recent findings show that SOAs are also an effective communication tool for plants, especially for evergreen trees (https://doi.org/10.1126/science.ado6779). I feel like this is important to consider and acknowledge.
L463: One thing to consider here is that SQTs can act as repellents for bark beetles, it affects bark palatibility (https://doi.org/10.1038/s43247-025-03175-3). So maybe this is an important aspect that is not being modelled in COSMO-MUSCAT and that's why you see the difference. SQT emission has been shown to be negatively correlated with beetle damage. I think that SQT emission is intrinsically tied to the biotic stress factor.