Abstract

SANCHEZ, Ligbel et al. Ion composition and acidity levels of the Maracaibo rains, Venezuela, between 1989 and 2001. Rev. Int. Contam. Ambient [online]. 2009, vol.25, n.3, pp.169-179. ISSN 0188-4999.

The atmospheric gases and aerosols coming from various natural and anthropogenic emissions are incorporated into raindrops through various physicochemical processes during rain events. Knowledge of the chemical composition of precipitation is critical to understand regional and local air pollution and its effects on the ecosystems. Sulfur, nitrogen and chlorine compounds are mainly involvedin rain acidity production as acidic or potentially acidic species. An important factor in the acid-base characteristics of rain is the emission of alkaline materials such as calcium-magnesium carbonates and ammonia, generally found as a vapour responsible to sulfuric and nitric acids neutralizing. Rain chemistry reported in this work originates from several sites of the city of Maracaibo and the rainwater samples were gathered in three periods between 1989 and 2001, using manual and automatic samplers. The different analyses were made using a flame atomic absorption spectrometer, Perkin-Elmer 3110 and an ion chromatography equipment with a conductivity detector, Dionex 2000i/SP. The pH-PPV in Maracaibo rainwater was 4.8, indicating an atmosphere lightly acid and influenced by SO₄^-2, Cl^- and NO₃- in 86 %. NH₃ is the most important neutralizing species, though the concentrations of Ca⁺² and Mg⁺² are additional contributions to atmospheric neutralizing process. The major flow of wet deposition was exhibited by NH₄⁺, SO₄^-2, Cl^- and NO₃- with 67.5 %. Statistically, a high correlation was found between SO₄*, Cl* and NO₃- ions with the H⁺ ion (p<0.05), ratifying its influence in the acidity of Maracaibo's rain. It is concluded that a small fraction of the acid potentially ions contributes to free acidity of the rains, due to the neutralizing capacity of NH₃, Ca* and Mg*.

Keywords : acid rain; atmospheric pollution; ionic balance of rains; pH; wet deposition.