Plants in the city and their gaseous exchanges with the atmosphere. A possible way to estimate the air pollutant removal by plants and the related biological cost.

The basic concepts on gas exchange between plants and the atmosphere, both at leaf and ecosystem level, are summarized and presented in this paper. After the illustration of the diffusional processes and the introduction of the electric analogy for the stomatal resistance, the turbulent processes are described and the Eddy Covariance technique is presented with all its assumptions and the applicability criteria for meaningful measurements. Then, the interpretation of measurement at ecosystem level is described for a paradigmatic phytotoxical pollutant, ozone. The big-leaf concept and the procedure of flux partition between stomatal and non-stomatal deposition are presented and discussed, as well as the metric used to estimate the phytotoxical ozone dose and the ozone risk assessment for vegetation. The non-stomatal deposition processes are also discussed and examples of measurements performed in peri-urban reserves and other ecosystems mainly under the TreeCity PRIN 2010-2011 project and the ECLAIRE FP7 Project, are presented