BVOC Emissions from Mediterranean Urban Shrubs: Implications for Ozone Formation and Air Quality

Urban greening strategies are increasingly implemented to mitigate urban heat island, restore biodiversity, improve urban carbon balance and improve air quality. Yet vegetation can emit biogenic volatile organic compounds (BVOCs) that influence atmospheric chemistry. BVOCs react with oxidants such as ^·OH and NO₃^· radicals and O₃, contributing to secondary air pollutants such as tropospheric ozone and secondary organic aerosol (SOA). While emissions from trees in forest ecosystems are well documented, urban environments feature diverse plant types, with shrubs being the most planted type of plants in greening strategies but rarely studied in terms of VOC emissions. The few studies on mediterranean shrubs mostly focused on rosemary and thyme, and were limited to terpene emissions (isoprene, monoterpenes and sesquiterpenes, (Malik et al., 2023; Duan et al., 2023; Bourtsoukidis et al., 2024; Pei et al., 2025)) while their volatilome is probably more complex as for other plant species (Gonzaga Gomez et al., 2019; Furnell et al., 2024).

This study is aimed at providing a complete characterization of BVOC emissions from six common Mediterranean shrub species in Marseille (France), and at evaluating their potential contribution to secondary air pollution. Six species were investigated, and selected based on their abundance and their potential use in vegetation initiative : Rosmarinus officinalis, Thymus vulgaris, Photinia fraseri, Euphorbia characias, Viburnum tinus and Ligustrum vulgare. Dynamic enclosure measurements were used to collect BVOCs for 3 different commercial plants. A non-target approach combining online and offline measurement techniques was used to characterise BVOC emissions. Proton Transfer Reaction Time of Flight Mass Spectrometry (PTR-ToF-MS) was used for online measurements, complemented by VOC sampling on TENAX tubes, analyzed by GC-MS, mainly for monoterpene speciation.

Emission profiles revealed high chemical diversity, ranging from 45 compounds in Ligustrum vulgare to 91 VOCs in Thymus vulgaris emissions. Oxygenated VOCs (OVOCs) dominated the emissions of all species (60–66%) including Thymus vulgaris and Rosmarinus officinalis, which have previously only been considered monoterpene emitters (Gros et al., 2022; Francolino et al., 2023). The emissions of OVOCs such as methanol, acetone, and acetic acid often exceeded those of monoterpenes. The non-target approach used here enabled the detection of unexpected compounds, such as dimethyl sulfide (DMS, C₂H₆S), typically associated with marine sources, suggesting specific processes that will be discussed.

Ozone Formation Potential (OFP) varied over several orders of magnitude among the 6 species: Rosmarinus officinalis (43.7 g _O₃ g _VOC⁻¹), Thymus vulgaris (13.1 g _O₃ g _VOC⁻¹), Photinia fraseri (6.8 g _O₃ g _VOC⁻¹), Euphorbia characias (3.5 g _O₃ g _VOC⁻¹), Viburnum tinus (1.7 g _O₃ g _VOC⁻¹), and Ligustrum vulgare (0.2 g _O₃ g _VOC⁻¹). The SOA formation potential of these emissions also warrants further investigation to fully assess their contribution to urban atmospheric reactivity. These results indicate that species selection in urban greening projects can strongly influence air quality outcomes. Our findings provide emission factors for Mediterranean shrubs and highlight the need to integrate BVOC data into urban vegetation planning to minimize the formation of secondary pollutant.