the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jun 2025
Variability in BVOC emissions and air quality impacts among urban trees in Montreal and Helsinki
Abstract. Many cities attempt to mitigate poor air quality by increasing tree canopy cover. Trees can indeed capture pollutants and reduce their dispersion, but they can also negatively impact urban air quality. For example, trees emit biogenic volatile organic compounds (BVOCs) that participate in both ozone (O3) and secondary organic aerosol (SOA) formation, yet these emissions have been little studied in urban contexts.
We sampled BVOCs from the leaves of mature urban trees using lightweight enclosures and adsorbent tubes in two cities: Montreal, Canada and Helsinki, Finland. In both cities, we targeted five common broadleaved species, comparing their standardised BVOC emission potentials 1) between parks and streets and 2) to nonurban BVOC emission potential estimates from emission databases. Finally, we calculated the potential O3 and SOA formation by urban trees at the leaf scale and upscaled to the neighbourhood.
We found that the BVOC emission potentials differed slightly between park and street trees. Compared to park trees, street tree emissions were higher in Montreal (specifically isoprene and sesquiterpenoids) and lower in Helsinki (specifically green leaf volatiles). However, the measured BVOC emission potentials generally deviated little from the emission database estimates, supporting the use of database estimates for urban trees. In addition, we found that O3 formation from urban tree BVOC emissions was dominated by isoprene, while SOA formation was also affected by lower monoterpenoid and sesquiterpenoid emissions. These findings highlight the importance of species selection and management strategies that protect trees from BVOC-inducing stresses.
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Status: open (until 13 Aug 2025)
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RC1: 'Comment on egusphere-2025-2500', Anonymous Referee #1, 11 Jul 2025
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General Comments
The research addresses the important and timely topic of biogenic volatile organic compound (BVOC) emissions from urban trees and their potential impact on air quality. The selection of two representative high-latitude cities, Montreal and Helsinki, and the novel comparison between park and street habitats, provides a valuable contribution to the field. The methodology, based on direct, in-situ measurements, is commendable and yields useful data. However, despite these strengths, the manuscript in its current form suffers from several critical flaws in experimental design, data analysis, and interpretation that undermine the reliability and generality of its conclusions. These deficiencies must be thoroughly addressed before the manuscript can be considered for publication.
Specific Comments
- The most significant flaw is the disparity in sampling strategy between the two cities. BVOC emissions were sampled twice in Montreal (June and August 2022) but only once in Helsinki (July 2022). Given the strong seasonal dynamics of BVOC emissions, this temporal mismatch makes any direct comparison between the cities scientifically unsound. For example, the higher emissions observed in Montreal in August could be due to late-season phenology or heat stress, a dynamic that was not captured in the single mid-summer measurement in Helsinki. This makes the discussion of inter-city differences highly speculative.
- While small sample sizes may have been common in early BVOC research, a sample of n=3 is statistically insufficient for a variable as notoriously heterogeneous as biogenic emissions. This limitation results in low statistical power for analyses like ANOVA and prevents generalizable conclusions. The extremely large error bars in Figures 1, 2, and 4 are a direct reflection of this high variability. The authors' own caution to interpret the results "with care" does not absolve this fundamental design weakness, which undermines the validity of the study's findings.
- The manuscript attributes differences in emissions to the binary park vs. street classification. However, these habitats represent a complex amalgam of confounding environmental factors (e.g., temperature, light, soil moisture and compaction, pollutant concentrations, human disturbance). The study itself found higher temperatures and O3 concentrations on Montreal streets but not in Helsinki, suggesting the "street effect" is not uniform and is city-dependent. The failure to systematically disentangle these factors makes the causal link between the "street environment" and emissions weak.
- The authors repeatedly invoke "stress" as a key driver of BVOC emissions, particularly in street environments. Yet, the only stress metric measured was leaf water potential, which showed that most trees were not experiencing significant drought stress. There is a critical lack of direct physiological indicators for heat stress, oxidative stress, or mechanical damage. This leaves the entire discussion of stress-induced emissions in the realm of speculation.
- The method to upscale leaf-level measurements to the neighborhood scale is overly simplistic and introduces massive uncertainty. It relies on multiple unvalidated assumptions, including a fixed LAI, approximate canopy areas derived from Voronoi polygons, and a simplified shading correction factor. Most importantly, the calculation only includes the study species, which represent a small fraction of the total canopy area in the test sites (23% in Montreal, 36% in Helsinki). The resulting neighborhood-scale emission maps (Fig. 5) are therefore of low accuracy and potentially misleading.
- A central conclusion is that the measured emission potentials show little deviation from database values, thus supporting the use of these databases for urban trees. This conclusion paradoxically diminishes the novelty and necessity of the present study. Furthermore, the data show significant deviations for some species. Instead of a blanket statement that "database estimates are generally usable," the authors should provide a deeper analysis of why these deviations occur.
- The authors state that for a portion of the samples, the incoming replacement air was not scrubbed for ozone. Although a post-hoc correction was applied based on literature values, this introduces an unquantified source of uncertainty. The accuracy of this correction, without rigorous validation for this specific experimental setup, is questionable and could have systematically biased the measurements of highly reactive terpenes.
- The manuscript uses fixed MIR and FAC values to calculate ozone and SOA formation potentials. However, these coefficients are highly sensitive to the ambient chemical regime, especially NOx concentrations. The high-NOx environment typical of urban atmospheres can significantly alter BVOC oxidation pathways and product yields.
- The study is located in two cities with similar cool, humid continental climates. The observed patterns, such as higher street emissions in Montreal versus higher park emissions in Helsinki, are likely not transferable to cities in other climatic zones (e.g., Mediterranean, arid, or tropical) where the dominant environmental stressors are entirely different. The authors must be more explicit about these geographical and climatic limitations in their discussion and conclusions.
- The manuscript's narrative vacillates between two somewhat contradictory messages: 1) that urban environments have complex, city-specific effects on BVOC emissions, and 2) that existing non-urban databases are generally adequate for urban trees. The authors must clarify what their single most important and robust scientific finding is and rebuild the manuscript's narrative to unequivocally support it.
Technical Corrections
- In the graphical abstract, the '2.5' in PM2.5 should be a subscript.
- The text relies heavily on species abbreviations (QM, PC, AP, etc.). A list or table of abbreviations should be provided at the beginning of the manuscript to aid readability.
Citation: https://doi.org/10.5194/egusphere-2025-2500-RC1
Data sets
BVOC emission potentials for street and park trees of five common species is Montreal (Canada) and Helsinki (Finland) K. Rissanen et al. https://doi.org/10.5281/zenodo.15379393
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