Chemistry and sulfur isotopic composition of precipitation at Bologna, Italy

Individual and monthly precipitation samples from the polluted atmosphere of Bologna (Emilia-Romagna province) were collected during March 1996 to May 1997 and analyzed for major ions in solution and S isotopes in dissolved SO4. Weighted mean enrichment factors relative to seawater are found to be 1.0 for Na, 15.2 for K, 105 for Ca, 3.3 for Mg, 17.3 for SO4 and 663 for HCO−3. Very good positive correlations are observed for the Ca2+–Mg2+–HCO−3–SO2−4–NO−3 system, indicating that dissolution of Ca (±Mg)-carbonate particles by H2SO4 and HNO3 from combustion of oil and gas is a major process controlling the chemical composition of rain and snow. Na+ and Cl− in monthly precipitation derive essentially from sea spray, but the contribution of Na+ from continental sources is appreciable in a number of individual rains. NH+4 appears to be on average more abundant in spring and summer precipitation, its main sources being microbial activity in soils and application of fertilizers. K+ is probably of continental origin from soil dust. The S isotopic composition of SO4 is systematically positive, with mean δ34S values of +3.2±1.6‰ (n=40) in individual precipitation and +2.8±1.4‰ (n=12) in monthly precipitation. These isotopic compositions are interpreted in terms of a dominant contribution of S from anthropogenic emissions and subordinate contributions from biogenic and marine sources. Pollutant SO4 is estimated to have a δ34S value in the range +2.5 to +4.5‰, whereas a distinctive δ34S of −4.5‰ or lower indicates SO4 from oxidation of biogenic gases. The isotopic and chemical compositions of SO4 do not depend on wind direction, thus testifying to a mostly local source for pollutant S in the Bologna atmosphere.