Unheralded contributions of biogenic volatile organic compounds from urban greening to ozone pollution: a high-resolution modeling study

Abstract. Urban Green Spaces (UGS) are widely advocated for mitigating urban atmospheric environment. However, this study reveals that it can exacerbate urban ozone (O₃) levels under certain conditions, as demonstrated by a September 2017 study in Guangzhou, China. Utilizing the Weather Research and Forecasting Model with the Model of Emissions of Gases and Aerosols from Nature (WRF-MEGAN) and the Community Multiscale Air Quality (CMAQ) model with a high horizontal resolution (1 km), we assessed the impact of UGS-related biogenic volatile organic compound (BVOC) emissions on urban O₃. Our findings indicate that UGS-BVOC emissions in Guangzhou amounted to 666.49 Gg, primarily from isoprene (ISOP) and terpenes (TERP). These emissions contribute ~30 % of urban ISOP concentrations and their incorporations to the model significantly reduce the underestimation against observations. The study shows improvements in simulation biases for NO₂, from 7.01 µg/m³ to 6.03 µg/m³, and for O₃, from 7.77 µg/m³ to -1.60 µg/m³. UGS-BVOC and UGS-LUCC (land use cover changes) integration in air quality models notably enhances surface monthly mean O₃ predictions by 3.6–8.0 µg/m³ (+3.8–8.5 %) and contributes up to 18.7 µg/m³ (+10.0 %) to MDA8 O₃ during O₃ pollution episodes. Additionally, UGS-BVOC emissions alone increase the monthly mean O₃ levels by 2.2–3.0 µg/m³ (+2.3–3.2 %) in urban areas and contribute up to 6.3 µg/m³ (+3.3 %) to MDA8 O₃ levels during O₃ pollution episodes. These impacts can extend to surrounding suburban and rural areas through regional transport, highlighting the need for selecting low-emission vegetation and refining vegetation classification in urban planning.