Impact of STress on uRban trEEs on ciTy air quality – sTREEt

A first experimental setup located in an urban site (Vitry/Seine, Fr) consisted in young plane trees (Platanus x hispanica), grown in pots to control water supply, from 2020 to 2023. The environmental parameters, gas exchanges, chlorophyll fluorescence and canopy temperature were measured. Half of the trees were subjected to drought treatments, in 2021 (moderate) and in 2022 (severe). BVOC emissions were measured at the branch scale with an online PTR-MS analyzer in 2021 and at the leaf scale with a portable GC-MS in 2022 (using an original method developed during the project). Biochemical markers of foliar stress (proline, malondialdehyde) were quantified and a metabolomic analysis was carried out in 2022.
The second experiment took place in summer 2022 in the “Jardin des combattants de la Nueve” in the centre of Paris, in a heavy traffic zone. In this garden, the BVOC emissions of 13 species of trees and shrubs were measured at the branch scale, together with the physiological monitoring of the trees studied. The composition of ambient air (VOC, NOx and aerosols including the 14C fraction in the particulate phase) was characterized in order to evaluate the impact of the different sources of BVOC on the gas and particulate phases.
The experimental data obtained were fed into the hydraulic functioning model of the urban tree. As urban trees are not taken into account in air quality models at both regional and street scales, the tree-related processes were parameterized and added. The city of Paris tree database was used to integrate BVOC emissions into the CHIMERE regional model. A method was developed to estimate the characteristics of trees that are used as input data for the different models (leaf area, dry biomass, crown size, etc.). The different effects of urban trees on air quality at the street scale have been added into the MUNICH street network model. The aerodynamic effect of trees in the streets was parameterized, using fluid mechanics simulations. The deposition on tree leaves was calculated, using a resistive approach adapted to the scale of the urban tree in the street. To represent the impacts of heterogeneities in the urban microclimate and the thermo-radiative effect of trees on gas and particle concentrations, a coupling between urban surface models (TEB), soil-plant-atmosphere continuum (SPAC) and air quality at street level (MUNICH) has been set up. The coupling between the MUNICH, TEB and SPAC models leads to more precise determination of BVOC emissions, thanks to the estimation of the surface temperature of the leaves and the solar radiation received by the leaves in the street.

During the first experiment in 2021, the physiological parameters and the variability (temporal, intra- and inter-tree) in BVOC emissions was characterized at the branch scale. Moderate drought did not have a significant effect on the isoprene emission factor, but the variability of emissions was very large. A 70-year-old plane tree close to the plot showed emissions similar to those of the young trees. In 2022, the method for measuring BVOCs at the leaf scale was validated and an intense drought was applied. The plane tree response was characterized by stomatal regulation of photosynthesis as long as water stress was moderate, then damage to the photosynthetic system and defoliation were observed with increasing stress severity. Isoprene emissions did not correlate with carbon assimilation but rather seemed linked to the reducing power produced by photochemistry. Thus, the consequences of water stress on the plane tree include a maintenance of isoprene emissions then a drop in emissions, mainly due to defoliation, when the stress becomes severe. Thus, no negative effect of isoprene emissions from urban plane trees on air quality is predicted during drought episodes.
The second experiment characterized a local biogenic VOC source dominated by isoprene, growing under stressful conditions, near very intense road traffic. Thus, urban ambient isoprene levels were higher than those measured at the peri-urban site (Saclay) apart from heatwave periods. However, it seems that the anthropogenic contribution to isoprene contents could be, in some cases, not negligible. Analyzes of 14C on filters will ultimately help to characterize and quantify the importance of the biogenic vs. anthropogenic sources on the gas and particle phases monitored in the ambient air.
Modeling in the Paris region has shown that on average over the months of June and July 2022 in Paris, the aerodynamic effect trapped compounds emitted in the street (NO2, carbon soot) and this phenomenon increased with the leaf surface and traffic intensity (+4.6% on average and up to +37% for NO2). NOx accumulation leads to a decrease in O3 contents (-2.3% on average and up to -23.2%). Dry deposition on leaves has a minimal effect on gas and particle concentrations (-0.6% on average and up to -2.5%). BVOC emissions mainly induce an increase in concentrations of organic particles (+4.6% on average and up to +11.5%) and to a lesser extent of O3 (+1.0% on average and up to +2.4%). The coupling of the different models shows that, during a drought, the trees no longer provide their cooling service and that BVOC emissions are stimulated by the increase in leaf temperature (up to +36% for isoprene).

The experimental data and the model obtained allow better consideration of BVOC emissions from trees in air quality models and by city managers. The recommendations that can be proposed would be to limit trees with large crowns in streets with heavy traffic, to ensure adequate water supply for the trees in summer and to limit the establishment of strong terpene-emitting species.

1. Kim Y., Lugon L., Maison A., Sarica T., Roustan Y., Valari M., Zhang Y., André M., Sartelet K. (2022) TI MUNICH v2.0: a street-network model coupled with SSH-aerosol (v1.2) for multi-pollutant modelling, Geoscientific Model Development, 15, 7371-7396. doi.org/10.5194/gmd-15-7371-2022
2. Maison A., Flageul C., Carissimo B., Tuzet A., Sartelet K. (2022a) Parametrization of Horizontal and Vertical Transfers for the Street-Network Model MUNICH Using the CFD Model Code_Saturne. Atmosphere 13, 527. doi.org/10.3390/atmos13040527
3. Maison A., Flageul C., Carissimo B., Wang Y., Tuzet A., Sartelet K. (2022b) Parameterizing the Aerodynamic Effect of Trees in Street Canyons for the Street Network Model MUNICH Using the CFD Model Code_Saturne. Atmospheric Chemistry and Physics 22, 9369-9388. doi.org/10.5194/acp-22-9369-2022
4. Wang, Y., Flageul, C., Maison, A., Carissimo, B., Sartelet, K. (2023). Impact of trees on gas concentrations and condensables in a 2-D street canyon using CFD coupled to chemistry modeling. Environmental Pollution, 121210. doi.org/10.1016/j.envpol.2023.121210
5. Maison, A., Lugon, L., Park, S.-J., Baudic, A., Cantrell, C., Couvidat, F., D'Anna, B., Di Biagio, C., Gratien, A., Gros, V., Kalalian, C., Kammer, J., Michoud, V., Petit, J.-E., Shahin, M., Simon, L., Valari, M., Vigneron, J., Tuzet, A., and Sartelet, K. (2023) Significant impact of urban-tree biogenic emissions on air quality estimated by a bottom-up inventory and chemistry-transport modeling, EGUsphere, accepted for publication in Atmos. Chem. Phys. doi.org/10.5194/egusphere-2023-2786

Urban vegetation is a source of ecosystem services, such as reduction of heat island phenomenon via evapotranspiration or support for pollution deposition. Urban trees emit a number Biogenic Volatile Organic Compounds (BVOC), including isoprene, which can react with atmospheric pollutants to form secondary pollutants, such as ozone (O3) and secondary organic aerosols (SOA). However, knowledge on the contribution of urban trees to air quality is scarce due to a blatant lack of studies on their BVOC emissions. Considering that BVOC emissions by plants are regulated by environmental conditions, further studies should be centered on the impact of urban stresses (limited water resource, pollutions, high temperature) on these emissions. The goals of the sTREEt project are i) to analyze the interactions between urban abiotic factors, tree physiology and their BVOC emissions, ii) to model the physiology of urban trees and iii) to integrate the aforementioned interactions into an air quality model through development of improved BVOC emission factors. The project will be based on a combination of experimental work and modeling, at tree and atmospheric levels.
A first experimental setup includes young plane trees (Platanus x hispanica, strong isoprene emitter) in pots to control water supply. Located in an urban site, they will be studied for two years, with two measurement campaigns per year (spring and summer). A number of climate parameters will be continuously monitored on site. Half of the trees will receive optimal water supply while the others will be submitted to drought. In-depth physiological analysis, including gas exchange, chlorophyll fluorescence parameters, leaf water potential, canopy temperature and spectral indices, will be carried out. In addition, BVOC emissions will be assessed at both leaf and branch scales, with an original leaf chamber system to be developed and a previously used dynamic chamber, respectively. BVOC analysis will be carried out, using on-line analysers, a PTMRS and a portable GC zNose®, connected to the chambers. Biochemical stress markers (proline, mannitol, protein carbonylation) will be studied on leaf samples collected at each measurement campaign. For further understanding of carbon fluxes between primary metabolites and precursors of BVOC, in relation with tree physiological conditions, metabolomic analyses will be conducted. In the third year of the project, a second experimental setup including “true” adult street trees (a plane tree and an emitter of monoterpenes, which are strong SOA precursors) will be deployed. Data of in situ BVOC emissions, gas exchange (at branch and leaf scales) and composition of surrounding air will be used to evaluate the impact of BVOC emissions on the gaseous and particulate phase. Online analysers, similar to those used in the first experiment, will be employed for gas phase characterization. Finally, the chemical composition of aerosols will be analysed using an ACSM monitor and 14C will be analysed in order to estimate the fraction of organic aerosols of biogenic origin (including organic nitrate).
The data obtained from the two experimental setups will feed the modeling part of the project. A soil-plant-atmosphere continuum model originally designed for crops and forest will here be adapted to the urban environment and to the prediction of urban tree BVOC emissions. Through this model pertinent BVOC emission parameters required for BVOC emission parameterization in air quality modeling (Polyphemus) will be provided. As a result, concentrations of O3, NOx, PM10, PM2.5, organic compounds will be modeled at the regional scale as well as in the streets of Paris. Predicted pollution data will be validated by comparison with actual pollution measurements. This is the first time that a management tool predicting the impact of vegetation and vegetation stress on O3 and particle concentrations in Paris and Greater Paris is developed.