Isotopic composition and secondary evaporation effects on precipitation from the urban centre of Bologna, Italy.

Forty-nine monthly precipitations were collected in the densely urbanized centre of the city of Bologna during the four years from March 1996 to March 2000. These samples were measured for their hydrogen and oxygen isotope composition, and the results were compared with literature data on monthly precipitations collected in Bologna in a suburban, green-belt, open-air site between August 1997 and August 2001. The ?2H-?18O meteoric lines in the two sites differ significantly in slope and intercept, both parameters being lower in the urban site by 12% and 9? delta-units, respectively. The amount weighted mean deuterium excess ("d") is also lower by 2? than the suburban value. These findings all support that the isotopic composition of the samples studied are appreciably influenced by secondary evaporation during falling through the atmosphere.