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Many plants are well known to emit biogenic volatile organic compounds (BVOCs). Under certain conditions BVOCs strongly enhance the photochemical formation of ozone (O3) and impact the levels of atmospheric photo-oxidants. Urban environments under the influence of climate change may face an increasing risk of elevated ozone formation potentials, because abiotic stressors such as heat and drought can stimulate BVOC emissions. However, it is largely uncertain how a combination of heat episodes and reduced soil water potentials affects air quality in cities. The effect of abiotic stress on BVOC emissions and urban O3 formation was assessed for the coastal metropolitan area of Hamburg in Germany during the vegetation period of 2018, characterized by remarkable drought and heat periods. BVOC emissions were modelled using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 3 that accounts for several abiotic stresses. Isoprene is the single VOC with the highest share (∼60%) in the BVOC emissions of the study area. Drought stress was identified as the most important abiotic stressor that modulates BVOC emissions in this area. Modelled biogenic emissions calculcated with MEGAN3 were included together with emissions of relevant anthropogenic sectors in simulations with the chemistry transport model EPISODE-CityChem to calculate ozone concentrations under a scenario of prolonged drought stress. As a major result we identified that isoprene concentrations in Hamburg were reduced by 65% (range 6% to 95%) under drought stress during the growing period compared to non-stress conditions. Reduction of isoprene concentrations due to drought stress spatially coincided with a reduction of ozone concentrations. To asses the importance of chemical reactions involved in the formation of ozone, concentrations of isoprene, methacryloyl peroxy nitrate (MPAN) and methacrolein (MACR) have been analysed. The drought stress effect on isoprene emissions led to reductions of MACR and MPAN by approximately 80% and 20%, respectively. Since a VOC limited regime is found presently for Hamburg, it is likely that further reductions in anthropogenic NOx emissions and/or increased BVOC emissions driven by extended green infrastructure and long-term temperature increases may lead to an enhanced photochemical production of ozone in Hamburg in the future.

Climate change, urbanisation and demographic change affect urban areas and pose a range of health-related challenges to urban residents, including heat waves, drought periods, air pollution and densification processes. Urban green spaces provide ecosystem services that can help to mitigate the effects of these challenges. Urban green spaces such as parks, urban gardens and street trees regulate the microclimate and buffer noise as well as a variety of air pollutants. Parks promote physical activity, relaxation and social interaction. The potential to provide these services might be limited during extreme weather events such as heat waves and drought periods. With this experience-based perspective paper, we introduce an interdisciplinary project that consists of multi-method field campaigns to assess the potential of urban parks to provide regulating and recreational ecosystem services in the context of the 2018 and 2019 heat and drought periods in Germany. We highlight that multi-method field campaigns that combine sensor-based environmental measurements with social science approaches, including visitor observations, counts, and questionnaire surveys, are highly useful when urbanisation and climate change-related challenges must be effectively addressed in the context of the complex socio-ecological systems of a city. Based on our hands-on experiences, we provide recommendations for local urban green space planning and outline prospects for future research.

Relationships between vegetation and air quality are intricate and still not fully understood. For regional air quality assessments, a better understanding of the diverse feedback mechanisms is crucial. The present article investigates the impact of land use data set detailedness on air quality predictions. Therefore, two different land use data sets were applied for simulations with COSMO‐MUSCAT for Germany in May 2014. One data set includes detailed information about tree species while the second one obtains generalized widely applied land use classes including mixed and coniferous forests. Moreover, we examined the role of agricultural NO soil emissions, agricultural biomass density enhancements, and model resolution. For a more comprehensive implementation of the secondary organic aerosol (SOA) formation, the SOA module was extended considering additional biogenic volatile organic compound (BVOC) precursor groups from isoprene, α‐pinene, limonene, and sesquiterpene oxidations. The model studies showed substantial differences in BVOC emission patterns between the two land use data sets. The application of detailed tree species information leads to complex BVOC emission patterns with high emission spots. In contrast, coarser forest information lead to standardized comprehensive emissions which result in 50% higher BVOC emissions. These differences affect both the atmospheric oxidizing potential and the production rates of SOA precursors. Land use induced regional differences (tree species minus forest information) in NOx (±2.5%), ozone (−2.5%), OH (±50%), NO3 radical (+70%) concentrations, and SOA (−60%) mass are modeled. Overall, the simulations demonstrate that detailed land use information, extended organic chemistry treatment, and high spatial resolution are mandatory for air quality assessments., Plain Language Summary: Trees are associated with being the lungs of the atmosphere as they filter out harmful substances from the air, they store CO2, and produce oxygen via photosynthesis. Other by‐products of photosynthesis are biogenic volatile organic compounds (BVOCs). BVOCs are chemical substances with a high vapor pressure already at room temperatures, so they quickly evaporate from the leaves into the surrounding air and are responsible for the characteristic forest smell. The amount and composition of BVOC emissions strongly depend on the tree species. Every plant has its own distinct emission properties. The chemical degradation of BVOCs impacts the chemical composition of the troposphere and is connected to ground level ozone production and the formation of secondary organic aerosols (SOA), contributing substantially to particulate matter (PM). On a global scale, standardized BVOC emission information on forest levels are often used, but for regional air quality assessments detailed plant specific information is crucial, but still often lacking. Therefore, two different land use data sets were applied in the present study to investigate the impact of standardized forest versus detailed tree‐species information for Germany in May 2014. The study reveals changes in NOx (±2.5%), ozone (−2.5%), OH (±50%), NO3 radical (+70%), and SOA (−60%) concentrations., Key Points: Detail of land use data sets crucial for biogenic volatile organic compound emission strength and composition. Composition and concentration variation of these organic compounds induce changes in regional air quality predictions. Detailed land use information, extended organic matter treatment, and high‐resolution simulations are mandatory for air quality assessments., Deutsche Bundesstiftung Umwelt (DBU) http://dx.doi.org/10.13039/100007636, https://doi.org/10.5281/zenodo.4783106, http://ebas.nilu.no/