Wet deposition and related atmospheric chemistry in the São Paulo metropolis, Brazil: Part 1. Major inorganic ions in rainwater as evaluated by capillary electrophoresis with contactless conductivity detection

The metropolitan region of São Paulo (17.8 million inhabitants) presents serious air quality problems. An official network monitors key air pollutants, however, there is no regular program of evaluation of the wet deposition and data about rainwater composition is scarce. Opening a series of articles on this subject, capillary zone electrophoresis with contactless conductivity detection (CZE-CCD) is proposed and applied as a quick and inexpensive alternative to ion chromatography for the determination of the ionic composition of rainwater. Excellent resolution of the peaks and sufficient sensitivity were obtained for major ions. Switching from anion to cation determination is fast (30 min) and as simple as inverting the polarity of the voltage supply and changing the modifier added to the buffer solution. CZE-CCD was applied to the study of wet-only deposition collected in São Paulo during the period from May l997 to March 1998. The volume weighted means of the anions, sulfate, nitrate and chloride, were, respectively, 17, 22 and 29 μmol l−1. Among the cations, ammonium was the dominating one, with 28 μmol l−1, followed by calcium, 23 μmol l−1, sodium, 12 μmol l−1, and potassium, 5.8 μmol l−1. The wet flux of these anions and cations were, respectively, 2.5, 2.2, 1.6, 0.78, 1.4, 0.43 and 0.35 g m−2 yr−1. By attributing all sodium to marine origin, half of the chloride and more than 90% of all other ions are ascribable to continental/anthropogenic sources. Literature data for rainwater from inland regions ( 200 km apart from São Paulo) reveals lower deposition of all ions but H+. Absorption of NH3 and incorporation of calcium carbonate, mainly in the metropolitan region itself, accounts for decreased acidity. The enrichment in all other ions during the studied period indicates the prevalence of the anthropogenic emissions from the metropolis over continental sources and explains the high correlation between the ions NO3−, SO42−, and NH4+; the same ions responsible for a factor that, alone, explains 42% of the variability in the PCA.