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

A first experimental setup located in an urban site includes young plane trees (Platanus x hispanica), in pots to control water supply. The first year, the trees will be well-watered and the second year, half of the trees will be subjected to drought. In-depth physiological analysis, including gas exchange, chlorophyll fluorescence parameters and canopy temperature, will be carried out. BVOC emissions will be assessed at both leaf and branch scales. And analyzed using on-line analyzers, a PTMRS and a portable GC-MS. Biochemical stress markers (proline, protein carbonylation) will be studied. For further understanding of carbon fluxes between primary metabolites and precursors of BVOC, in relation with tree physiological conditions, metabolomic analyses will be conducted. A second experimental setup including “true” adult street trees, a plane tree (second year) and an emitter of monoterpenes (third year), which are strong SOA precursors will be deployed. Data of in situ BVOC emissions, gas exchange and composition of surrounding air will be used to evaluate the impact of BVOC emissions on the gaseous and particulate phase. Finally, the chemical composition of aerosols will be analyzed using an ACSM monitor and 14C will be analyzed in order to estimate the fraction of organic aerosols of biogenic origin (including organic nitrate).
The experimental data obtained will feed the model of hydraulic functioning of the urban tree, according to the radiative conditions, the stomatal regulation and the radial diffusion of water from the soil to the roots. It will allow the prediction of urban tree BVOC emissions required for BVOC emission parameterization in air quality modeling (Polyphemus). With parametrizations of the aerodynamic effect of street trees, this model will simulate the concentrations of O3, NOx, PM10, PM2.5 at the regional scale as well as in the streets of Paris. Predicted pollution data will be validated by comparison with actual pollution measurements.

The 2020 measurement campaign on young plane trees in an urban area made it possible to measure the following parameters under well-watering conditions: BVOC emissions, gas exchange (net photosynthesis, stomatal conductance), allometric measurements (leaf areas and biomasses, tree height), leaf pigment content. Tree-to-tree variability was estimated by measurements on 8 trees and intra-tree variability by measurements on several branches of the same tree.
The model of the urban tree functionning is under development.
To improve the representation of urban trees in air quality models, the aerodynamic effect of trees in canyon streets has been parameterized. The parameterization was built from simulations with a fluid dynamics model. The parameterization will be used subsequently in the street network model that will be set up in Paris for the modeling of the 2022 campaign.

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. It will be useful to optimize the managements of vegetation in cities

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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.