Trends in chemical composition of precipitation in Nanjing, China, during 1992–2003

In this study, variations of the chemical composition of precipitation in Nanjing, China, over a 12-year period (1992–2003) are presented. The average annual concentration of pH value was 5.15, ranging from 4.93 to 5.36, and there was no significant trend in the acidity of precipitation. SO₄²⁻, Cl⁻ and NO₃⁻ were the main anions, while Ca²⁺, NH₄⁺ and Mg²⁺ were the main cations. The concentrations of these main ions were very high compared to those reported in many other areas around the world. Most of the ions came from anthropogenic and crustal sources. High correlations were found among dust-derived cations Ca²⁺, Mg²⁺and K⁺, between Cl⁻ and SO₄²⁻, between Cl⁻ and NH₄⁺ and between acidic anions and dust-derived cations, such as SO₄²⁻ and Ca²⁺, SO₄²⁻ and K⁺, Cl⁻ and Ca²⁺, Cl⁻ and K⁺, F⁻ and Mg²⁺ and F⁻ and K⁺. A significant decreasing trend was observed in concentration of SO₄²⁻ because of the abatement strategies for SO₂ emissions and energy policy change, while a significant increasing trend was found in the contribution of NO₃⁻ to acidification due to the rapidly growing number of motor vehicles. A significant decreasing trend was found in dust-derived cation Ca²⁺ due to more stringent controls of industrial dust emissions and rapid urbanization reducing the amount of open land, while the contribution of NH₄⁺ to neutralization increased relatively.