## 1 Introduction  
Vegetation provides a wide range of ecosystem services in urban environments such as generating O\({}_{2}\) and absorbing CO\({}_{2}\), reducing soil erosion and wind speed, generating shade that reduces air temperature and capturing particulate matter (PM), among others. Hence, vegetation provides valuable services that contribute to improve air quality in cities [1]. However, trees can also emit biogenic volatile organic compounds (BVOC), which are precursors of tropospheric ozone (O\({}_{3}\)) negatively affecting human health and the environment. Volatile organic compounds (VOC) can also be of anthropic origin (AVOC) such as those from industrial processes and the combustion of motorized vehicles [2]. All plants emit BVOCs at a rate that depends on species, age, and external factors such as temperature, relative humidity, solar radiation, and stress, among others. The release of BVOC by plants may play different roles such as attracting pollinators or repelling agents of pest and diseases, acting as a countermeasure to external stimuli and a form of communication between different individuals [3].  
BVOCs are common in the environment and many of them are recognized by their fragences. These scented compounds are known as terpenes and their base unit is isoprene (C\({}_{5}\)Hs), a reactive molecule with five carbon atoms and two double bonds. They react in the atmosphere with the hydroxyl radical (\(\bullet\)OH), nitrate radical (\(\bullet\)NO\({}_{3}\)) especially during the night, ozone and other chemical species, leading to reactions between nitrogen oxides (NO\(\times\)) and organic reactive intermediates that ultimately enhance tropospheric ozone production and secondary organic aerosols (SOA), both related to different human and plant health problems [4; 5; 6; 7]). Nowadays, high concentrations of O\({}_{3}\) can be found in many cities due to the oxidation reactions of VOCs in the presence of NOx from combustion processes, especially motor vehicles. High concentrations carry risks such as cardio-respiratory diseases, oxidation of materials and damage to flora and fauna [6]. tropospheric ozone moving inside the leaf through the stomata brings about the production of reactive oxygen species damaging the photosynthetic machinery [7] despite the plant's increased production of compounds with antioxidative capacity [8]. The city of Santiago, Chile, exhibits serious problems with O\({}_{3}\) during the spring-summer period [9; 10; 11]. The threshold for ozone is 120 ug m\({}^{-3}\), considering an 8 h mobile average, being frequently exceeded during spring-summer, especially in the northeast of the city [10]. A recent work monitoring ozone concentration in eastern Santiago during the period 2010-2018 showed that on average 43 days per year were above the recommended levels and that the ozone formation regime is VOC-limited [12]. Although anthropogenic VOCs are the dominant chemical species [13], it becomes relevant to identify the sources of BVOC emissions and their potential as precursors of urban pollutants. Methane, not considered a BVOC, is only rarely measured but it might be worth comparing such fluxes against those of BVOCs. Besides, knowing the fraction that BVOCs represents in terms of carbon fixed through photosynthesis across plant species may shed light on the metabolic plant responses to stresses.  
The aim of this work was to assess CH\({}_{4}\), CO\({}_{2}\) and BVOCs fluxes by two relatively abundant tree species in the city of Santiago, _Platanus \(\times\) acerifolia_ and _Schinus mole_ (2.3% and 1.84% in abundance, respectively), measured during a hot dry period. In addition to emission factors (EF), the potential tropospheric O\({}_{3}\) formation index (POCI) was also estimated. This study may contribute towards a better understanding of the trade-off between carbon sequestration and BVOCs emissions in urban trees for the central region of Chile.