Acid Rain in Downtown São Paulo City, Brazil

Abstract

During the period from July 2002 to June 2004, the chemical characteristics of the rainwater samples collected in downtown São Paulo were investigated. The analysis of 224 wet-only precipitation samples included pH and electrical conductivity, as well as major ions (Na⁺, $\rm NH^{ + }_{4}$, K⁺, Ca²⁺, Mg²⁺, Cl⁻, $\rm NO^{ - }_{3}$, $\rm SO^{{2 - }}_{4}$) and carboxylic acids (acetic, formic and oxalic) using ion chromatography. The volume weighted mean, VWM, of the anions $\rm NO^{ - }_{3}$, $\rm SO^{{2 - }}_{4}$ and Cl⁻ was, respectively, 20.3, 12.1 and 10.7 μmol l⁻¹. Rainwater in São Paulo was acidic, with 55% of the samples exhibiting a pH below 5.6. The VWM of the free H⁺ was 6.27 μmol l⁻¹), corresponding to a pH of 5.20. Ammonia (NH₃), determined as $\rm NH^{ + }_{4}$ (VWM = 32.8 μmol l⁻¹), was the main acidity neutralizing agent. Considering that the H⁺ ion is the only counter ion produced from the non-sea-salt fraction of the dissociated anions, the contribution of each anion to the free acidity potential has the following profile: $SO^{{2 - }}_{4}$ (31.1%), $NO^{ - }_{3}$ (26.0%), CH₃COO⁻ (22.0%), Cl⁻ (13.7%), HCOO⁻ (5.4%) and $C_{2} O^{{2 - }}_{4}$ (1.8%). The precipitation chemistry showed seasonal differences, with higher concentrations of ammonium and calcium during autumn and winter (dry period). The marine contribution was not significant, while the direct vehicular emission showed to be relevant in the ionic composition of precipitation.