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

Abstract

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.

Introduction

Wet deposition is a very important route of pollutants removal from the atmosphere. Robert Boyle has noticed the presence of sulfur compounds and acids in the air and in rainwater as early as the 17th century; in the 19th century, the increase in the acidity of the rain was associated with anthropogenic emissions by Robert Smith. In the last decades, the extension and impacts of the so-called “acid rain” have been thoroughly investigated, specially in the northern hemisphere; harmful consequences attributable to acid deposition span from terrestrial and aquatic ecosystems impairment to corrosion and degradation of buildings and monuments. Considering exclusively the equilibrium with gaseous CO₂, rainwater should present a pH value around 5.6. Lower values are caused mainly by the presence of acids like sulfuric, nitric and carboxylic. Main precursors are sulfur and nitrogen oxides, hydrocarbons and carbonyl compounds, involved in complex homogeneous and heterogeneous phase reactions of the atmosphere (Cowling, 1982; Gatz, 1991; Losno et al., 1991; Mage et al., 1996). To subsidize research in atmospheric chemistry, to gather long series of data on pollutant emission by wet deposition and to study its correlation with observable impacts, monitoring networks have been established, mainly (but not exclusively) in developed countries of the northern hemisphere. These networks have proved, for example, the improvements reached with the reduction programs for sulfur oxides emissions, as conducted in USA and part of Europe, leading to lower sulfuric acid concentrations in rain, besides persistence of nitric acid and other acidic (and basic) components.

Although long distance transport, conversion and deposition of pollutants—including in-cloud scavenging—are important, bellow-cloud scavenging (washout) usually dominates in regions with high emissions. In Brazil, the main economic center is the metropolitan region of São Paulo, one of the biggest human conglomerates of the world (17.8 million inhabitants, about 10% of the nation's population). It is estimated that 90% of the anthropogenic emissions elapse from the enormous fleet of about 5.5 million automotive vehicles. The remaining fraction being emitted by stationary sources spread over the most densely industrialized region in Latin America, causing severe air pollution problems, such as formation of photochemical smog—with high concentrations of ozone and NO_x—as well as excess of particulate matter (CETESB, 2000; Mage et al., 1996). Although key air pollutants are monitored on a regular basis since 1973 (an automatic network runs since 1983), there is no similar counterpart for wet deposition. Possible reasons are: chemical characterization of rainwater is more labor intensive and expensive; precipitation events do not endanger human health directly and, in fact, alleviate the air pollution by washout process (bellow-cloud scavenging). Thus, very few studies dealing with the chemical composition of the rainwater in São Paulo can be found in the international literature (Forti et al., 1990; Fornaro et al., 1993; Paiva et al., 1997).

Conversion of the vehicular emissions in the photochemical smog and/or in the liquid phase are, probably, responsible for most of the sulfate, nitrate and acidity found in the rain water, but a more detailed investigation of local wet deposition and related atmospheric chemistry is required. In the series of articles headed by the present one, new methods for easier and less expensive determination of chemical constituents in the liquid phase (as well as in the gas phase for selected species like H₂O₂ and aldehydes) will be introduced, new systematic measurements will be presented and the scarce data from previous studies will be compiled and discussed jointly, looking for trends. The final goals are to evaluate the extension and improve understanding of the wet deposition and related atmospheric chemistry in the São Paulo Metropolis.

The site is approximately at 46°43′W and 23°34′S, 750 m of altitude and about 45 km from the coast, well inside the São Paulo city (10.5 million inhabitants), main part of the metropolitan region (17.8 million inhabitants). The climate of the metropolitan region of São Paulo is classified as cold and dry winter and hot and wet summer. The precipitation is around 1500 mm yr⁻¹, being more intense from October to the end of summer in March. As a rule, the meteorological conditions are more favorable to pollutants dispersion from September to April, when the weather shows great instability in the whole southeast region of Brazil due to frontal systems coming from the south of the continent, continental heating and tropical convection; these convective activities lead to wet precipitation on the São Paulo metropolitan region. During the colder months (from May to August) the region presents less rain and more stability because of the formation of high pressures (anticyclone), generating thermal inversions on the lower levels of the atmosphere, which renders the pollutant dispersion more difficult.

The standard method of analysis for ions in rainwater samples is ion chromatography (IC) (Valsecchi et al., 1997). Recently, attention has been extended for capillary zone electrophoresis (CZE) as a powerful tool for ion analysis (Kaniansky et al., 1999; Pacáková et al., 1999; Fukushi et al., 1999). In CZE, rapid separations can be performed with good sensitivity and selectivity, high efficiency, and low consumption of sample and reagents. Consequently, CZE is a potential alternative to IC for the analysis of rainwater and it has been demonstrated that even single drops of rain or fog-clouds can be discriminated using this technique (Bächmann and Tenberken (1996), Bächmann and Tenberken (1997); Bächmann et al., 1997). Most ionic constituents of wet deposition do not absorb radiation in the UV or visible region of the spectrum. The usual scheme to render these ions measurable is indirect UV or Vis detection, comprising addition of a chromophore to the buffer solution in order to originate negative peaks (decrease of absorbance) during passage of non-absorbing species (Deng, 1998; Fung and Lau, 1998). Although effective, this scheme results in less favorable detection limits than direct detection. Sometimes, it is worthwhile to derivatize the analytes, e.g. hidrazones of carbonylic compounds absorb in the UV–visible region (Bächmann et al., 1997). When applicable, very low detection limits can be reached with laser-induced fluorescence detection (Bächmann et al., 1996).

Electrochemical detection has not yet been extensively investigated in combination with CZE of environmental samples. For some anions, e.g. nitrite and sulfite, high sensitivity and selectivity has been reported for amperometric detection (Bächmann et al., 1996). Conductivity detection—used universally in combination with IC—has been considered for CZE of rainwater at least once (Valsecchi et al., 1997). In a comparative study of the separation of anions in rainwater by IC and CZE by using conductivity detection for both techniques, CZE was found suitable for this purpose and the results were validated with the NIST 2694-I and -II standard reference materials (Valsecchi et al., 1997). The construction of conductivity detectors for CZE, its coupling with the capillary and maintenance is, however, more laborious than for IC, due to the reduced dimensions. Interference of the high separation electric field and fouling of the small electrodes exposed to the samples can cause additional problems. Fortunately, by increasing the frequency of the excitation signal during the conductivity measurements (oscillometry), the electrodes can be installed outside of the capillary, overcoming all these problems. Fracassi da Silva and Lago (1998) and Zemann et al. (1998) have independently proposed and demonstrated this contactless conductivity detection (CCD) mode (the first authors with much better S/N ratio) by fitting two ring electrodes around the external polyimide coating of the fused silica capillary used in CZE. The response of this “contactless” detector is essentially due to the conductivity differences between the background electrolyte (BGE) and the analyte; therefore, it is very suitable for the ionic analysis.

In this work, it will be shown that CZE-CCD can be advantageously applied to the analysis of major cations like Na⁺, Ca²⁺, K⁺ and NH₄⁺, and anions like SO₄²⁻, NO₃⁻ and Cl⁻ in rainwater, accelerating, simplifying and reducing the cost of systematic studies of wet deposition, as proven during the research undertaken in the São Paulo metropolis.