Ozone and nitrogen dioxide concentrations in a Holm oak urban park and an adjacent open area in Siena

Urban parks play important role in providing social and ecosystem services; the improvement of air quality through the absorption of gaseous pollutants and the interception of airborne particles is one the most important services of urban trees within those parks. Surveys performed in Italian urban environments show that leaves of Holm oak (Quercus ilex L.) are efficient accumulators of airborne trace elements and polycyclic aromatic hydrocarbons, but few data are available on the uptake of gaseous pollutants such as O-3, NO2. This study aimed at determining, with passive samplers, seasonal variations in O-3 and NO2 air concentrations under the canopy of Q. ilex and in an adjacent open area at increasing distances (1, 5, 10m) from a busy road in Siena. The survey was performed in July, November 2011, January, April 2012; the results showed a similar seasonal distribution of average O-3 concentrations along the two transects, with much higher values (up to 69 mu g m(-3)) in July. On the contrary, the highest NO2 concentrations were recorded in January and in all seasons their distribution pattern was characterized by a decrease of average values at increasing distances from the road, especially under the Q. ilex canopy. Although preliminary, results indicate that in Siena Q. ilex trees attenuate NO2 concentrations but not O-3.