## 4 Conclusions  
Total carbon fixation in _S. molle_ was similar to _P. \(\times\)acerifolia_ during the hot dry summer sampling days, but BVOC emissions were proportional to photosynthesis at a much greater rate in _P. \(\times\)acerifolia_ (2%) compared to _S. molle_ (0.24%). Methane emission fluxes were not significantly different from zero for both species and were therefore considered negligible. The total EF of _P. \(\times\)acerifolia_ was 20.72 \(\mu\)g g\({}_{\mathrm{ldw}}\)\({}^{-1}\) h\({}^{-1}\), being around 6.4 times greater than the EF of _Schinus molle_ (3.14 \(\mu\)g g\({}_{\mathrm{ldw}}\)\({}^{-1}\) h\({}^{-1}\)). For both species, isoprene was the dominant terpene with the contribution to the total EF being 99.4% and 86.3% for _P. \(\times\)acerifolia_ and _S. molle_, respectively. Moreover, isoprene emission rates increased throughout the day reaching its maximum at 5:00 p.m. The highest emission of monoterpenes in _P. \(\times\)acerifolia_ corresponds to linalool with 0.047 \(\mu\)g g\({}_{\mathrm{ldw}}\)\({}^{-1}\) h\({}^{-1}\) at 11:00, while in _S. molle_ the greater monoterpene emission was limonene with an EF of 0.32 \(\mu\)g g\({}_{\mathrm{ldw}}\)\({}^{-1}\) h\({}^{-1}\) at 5:00 p.m. No significant correlations were found between the environmental variables and the normalized concentrations and/or EF of terpen except for the terpionle-temperature relationship in _Schinus molle_. The  
Figure 5: Leaf methane (**a**) and photosynthesis (**b**) fluxes for _Platanus \(\times\)acerifolia_ and _Schinus molle_ (mean \(\pm\) 1 standard error). total POCI in _S. molle_ was 342 with about 92.1% accounted for by isoprene, while for _P. \(\times\) acerifolia_ the total POCI was 2359, being 6.9 times higher compared to the former, with an isoprene contribution of 99.6%. The 6.4 times greater BVOCs emission factor of _P. \(\times\) acerifolia_ compared to _S. molle_ may suggest that _P. \(\times\) acerifolia_, being an exotic species growing outside its ecological range, might have been subject to water, radiative and temperature stresses that triggered such a response. If it is true that stressed plants would exhibit higher BVOCs emission rates, then selecting plant species better adapted to particular environmental conditions may contribute to reducing a tropospheric ozone in highly polluted urban environments. _P. \(\times\) acerifolia_ is a species that loses its leaves in the autumn-winter period. On the contrary, _S. molle_ is a species with permanent leaves and therefore carries out the fixation process the whole year. These results should be taken into account by the authorities for urban reforestation plans. We are aware that only one tree of _P. \(\times\) acerifolia_ and one of _S. molle_ were measured repeatedly over five days and four times daily. This is a limiting factor of the study brought about by the infeasibility to measure more trees. Both trees selected were healthy adult individuals without irrigation during the summer period. Rainfall was the only source of water for these trees. There are uncertainties associated with the lack of replication, although repeated measurements for the same individuals may provide indications of the identification of the main chemical species of BVOCs and magnitude. This study should be considered as exploratory rather than statistically representative of the populations studied. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/atmos13020298/s1](https://www.mdpi.com/article/10.3390/atmos13020298/s1)]([https://www.mdpi.com/article/10.3390/atmos13020298/s1](https://www.mdpi.com/article/10.3390/atmos13020298/s1)), Table S1: Retention time and main terpene masses; Table S2: Detection (LOD) and quantification (LOQ) limits of the studied terpen. Table S3 Transformation procedure from emission factor to carbon flux of terpen emitted by _Platanus \(\times\) acerifolia_ and _Schinus mole_. Formal analysis, investigation, methodology and software, I.F.