Water Air Soil Pollut: Focus (2007) 7:85–92 DOI 10.1007/s11267-006-9081-y
Acid Rain in Downtown São Paulo City, Brazil Marcos A. dos Santos & Cynthia F. Illanes & Adalgiza Fornaro & Jairo J. Pedrotti
Received: 17 June 2005 / Revised: 15 December 2005 / Accepted: 12 February 2006 / Published online: 19 January 2007 # Springer Science + Business Media B.V. 2007
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 + 2+ 2+ − major ions (Na+, NHþ 4 , K , Ca , Mg , Cl , NO3 , 2 SO4 ) and carboxylic acids (acetic, formic and oxalic) using ion chromatography. The volume weighted mean, − 2 VWM, of the anions NO 3 , SO4 and Cl was, respec−1 tively, 20.3, 12.1 and 10.7 μmol l . 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), −1 determined as NHþ 4 (VWM=32.8 μmol l ), was the main acidity neutralizing agent. Considering that the H+ ion is the only counter ion produced from the non-seasalt fraction of the dissociated anions, the contribution of each anion to the free acidity potential has the following profile: SO2 (31.1%), NO 4 3 (26.0%), − − CH3COO (22.0%), Cl (13.7%), HCOO− (5.4%) and C2 O2 (1.8%). The precipitation chemistry showed 4 M. A. dos Santos : C. F. Illanes : J. J. Pedrotti (*) Departamento de Química, Universidade Presbiteriana Mackenzie, Rua da Consolação, 896, 01302-907 São Paulo, SP, Brazil e-mail:
[email protected] (J.J Pedrotti) A. Fornaro Departamento de Ciências Atmosféricas, Instituto de Astronomia, Geofísica e Ciências Atmosféricas – USP, Rua do Matão, 1226, 05508-900 São Paulo, SP, Brazil
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. Keywords acid rain . ionic composition . urban area . wet deposition . air pollution
1 Introduction In the last two decades, the study of the chemical composition of rainwater samples has increasingly been investigated in several parts of world. This type of research provides useful information on the atmospheric composition, helps identify the potential chemical sources of wet precipitation, and improves the understanding of regional and local air pollution and its effects on the ecosystems. The atmospheric composition is determined by natural sources – mainly oceans and continents – and anthropogenic sources, associated to human activities such as biomass burning (forests and plantations), industrial processes and fossil fuel burning. In several cities of the world, significant changes in atmospheric composition by anthropogenic sources have caused serious environmental problems, among then increasingly degraded air quality. In the São Paulo metropolitan area (MASP), atmospheric pollutants like SO2, CO, NOx, O3 and inhalable particulates (PM10) have been measured since 1973 and continuously monitored since 1981 by an automatic network (Martins et al., 2004; CETESB, 2005).
86
The results of these measurements have been indicated that the atmospheric emissions in the MASP are linked to pollutants released by industries and mainly, by the large fleet of vehicles. With reference to liquid phase, there are some studies of rainwater chemistry in the city of São Paulo. Most of this work is restricted to inorganic major ions in rainwater samples collected in the city’s west zone (Forti, Moreira-Nordemann, Andrade, & Orsini, 1990; Paiva et al., 1997). In the same sampling site, recent results showed that the organic acids provide a significant contribution to the acidity of rainwater samples (Fornaro & Gutz, 2003). In downtown São Paulo, the first results for major cations and inorganic and organic anions in rainwater samples collected during the end of the winter of 2002 up to the summer of 2003 were described more recently (Leal, Fontenele, Pedrotti, & Fornaro, 2004). This work presents the chemical composition of wet-only deposition samples collected in downtown São Paulo during a period of two years. The inorganic + 2+ 2+ − 2 ions (Na+, NHþ 4 , K , Ca , Mg , Cl , NO3 , SO4 ) − − and organic anions (CH3COO , HCOO , C2 O2 4 ) were measured by ion chromatography in wet precipitation in order to evaluate the main anthropogenic sources responsible for contamination in the area. The relative contribution of carboxylic acids to the potential free acidity of rainwater is also discussed.
2 Experimental 2.1 Site Sampling São Paulo city is located in the southeastern region of São Paulo state, Brazil, around 45 km from the coast, at 780 m above the sea level, with an extension area over 1,528 km2. The city has 10.5 million inhabitants and it is the largest part of the São Paulo Metropolitan Area (MASP), one of the most populated urban regions in the world, with more than 18 million inhabitants. The main sources of pollution in the city are vehicular and industrial emissions. Diesel, hydrated ethanol and gasohol (gasoline + 25% of ethanol) are the most common fuels used by almost seven million heavy and light vehicles. According to estimates of CETESB (2005), the fluxes of the main pollutants emitted into the atmosphere in the last years have obeyed the following distribution: 1.7 million ton/year
Water Air Soil Pollut: Focus (2007) 7:85–92
of carbon monoxide (CO), 38,000 ton/year of sulfur oxides (SO2), 371,000 ton/year of nitrogen oxides (NOx), 404,000 ton/year of hydrocarbons (HC) and 63,000 ton/year of inhalable particles (PM10). The vehicular fleet has been responsible for 98% of CO, 97% of HC, 96% of NOx, 55% of SO2 and 40% of PM10 emissions. Even though studies on the ionic composition of these fuels used in the vehicular fleet of São Paulo city are scarce, Munoz et al (2004) published results of the quantitative determination of inorganic ions in ethanol used, both in the anhydrous (mixed with 75% gasoline) and in hydrated form (94% ethanol) by the car fleet. The analytical determinations carried out by capillary zone electrophoresis showed a significant presence of anions and cations usually found in rainwater samples. Among all the ions, the highest concen−1 + tration was for NO 3 (46.7 μmol l ), followed by Na −1 + −1 − −1 (34.4 μmol l ), K (14.3 μmol l ), Cl (9.0 μmol l ) −1 2+ þ and SO2 and Mg2+ 4 (7.9 μmol l ), while NH4 , Ca were not detected. 2.2 Rainwater Sampling The rainwater samples were collected within the campus of Mackenzie University, in downtown São Paulo, from July 2002 to June 2004. Wet-only samples were collected with an automatic rainwater collector, model G.K. Walter, installed on the top of the Education Faculty building (around 15 m high). High-density polyethylene flasks were used to store the rainwater during the period of the sampling. The precipitation samples were collected immediately after each rain event or in the early morning following night events. On arrival at the laboratory, each sample was weighed for volume determination. Afterwards, the sample was separated into two of 15 ml aliquots and transferred to high-density polyethylene flasks for different analytical determinations, usually made a week after the rain event. For pH and conductivity measurements, an unfiltered aliquot was preserved at 4°C in a refrigerator, while for chromatographic determination the remaining aliquot was filtered through a 0.22 μm cellulose acetate membrane and stored in a freezer at −18°C. 2.3 Reagents and Instruments All reagents were of analytical grade (>99% purity) and were used without further purification. Ultra-pure
Water Air Soil Pollut: Focus (2007) 7:85–92
water used to prepare the solution was obtained from a Barnstead Nanopure system (resistivity>18 MΩ cm−1). The stock standard solutions (100 mmol l−1), obtained by dissolution of their salts in deionized water, were stored in a refrigerator. The multi-element standard solutions of lower ion concentration were prepared just before their use. The pH measurements were made with a Digimed DM-20 potentiometer coupled to a glass electrode combined with a Ag/AgCl (saturated in KCl) reference electrode. For conductivity measurements, a Digimed model DM-31 conductivity meter, fitted with a constant 1.0 cm−1 cell was used. The conductivity meter was calibrated with 10, 1.0, and 0.1 mmol l−1 KCl standard solutions. All pH and conductivity measurements, made in triplicate, were carried out at 25°±0.1°C temperature. Major cations and anions were determined by a Metrohm chromatograph 761 Compact IC with conductivity detection. Anion and cations determinations were made using Metrohm accessories: a A-Supp 5 (250×4 mm) separator column with an anion micromembrane suppressor. For cations, a C2-250 (250× 4 mm) separator column protected by a C-2 guard column was used. The analytical determination of each major ion was made using a calibration plot with a concentration range from 5 to 50 μmol l−1.
3 Results and Discussion 3.1 Rainwater Features From July 2002 to June 2004, 224 wet-only precipitation samples were collected. Figure 1 shows the monthly rainfall distribution during the period of study. The amount of rainfall recorded for the period was 2,197 mm, which is 20% below average for the last 30 years, for the same area and period. It shows that most rainfall occurs between October and March (spring and summer months). During this period, the wet precipitation amounted to 1,760 mm, corresponding to 80% of the total rainfall volume collected. The average daily rainfall for the wet period was 9.7 mm d−1, while in the dry period (usually between May and August) it was only 2.4 mm d−1. In order to verify if major components were measured in the rainwater samples, two quality control criteria were applied: (a) condition of electroneutrality
87
Fig. 1 Historical series (filled square) of monthly precipitation (1973–2003) and monthly profile of rainfall recorded during the study period, July 2002 up to June 2004 (columns). The wet period: October–March; dry period: April–September
(charge balance) and (b) the comparison between the measured conductivity and the calculated conductivity, the latter based on the ionic concentrations measured in each sample. Following these two criteria, 10 samples (4.5%) were rejected. The linear regression analysis P Pþ between the anions ( anions ) and cations ( anions ) measured showed a good correlation coefficient r= 0.97 and a slope of 0.97, indicating a slight excess of anions, as illustrated in Fig. 2a. Calculated conductance versus measured conductance (Fig. 2b) showed good concordance, too (r=0.96, slope=0.88). These results indicate that chromatography-measured ions and H+ concentration, obtained from pH values, comprise practically the major of ionic species in rainwater samples collected in downtown São Paulo. 3.2 Chemical Composition Figure 3 shows box-whisker plots for cations (Fig. 3a) and anions (Fig. 3b) concentrations in 214 rainwater samples. The data show a considerable variation of the concentration of all ions from sample to sample, which is a common characteristic in studies of rainwater chemistry. The average relative contribution of the ionic species concentration in rainwater follows this order: 2 þ NHþ 4 > NO3 > CH3 COO > Na > SO4 > Cl > 2þ 2 þ þ 2þ Ca > H > HCOO > K > Mg > C2 O4 > NO 2. The precipitation in downtown São Paulo is dominated by the NHþ 4 ion with an average concentration of 43.9 μmol l−1 representing 49% of all the cationic
88
Water Air Soil Pollut: Focus (2007) 7:85–92
Σ
μ
(NOX ¼ NO þ NO2 ) produced by combustion of fossil fuels used by the vehicular fleet (CETESB, 2005). The other abundant anions were CH3COO−, SO2 4 and Cl−, with arithmetic mean concentrations of 24.4, 17.0 and 15.3 μmol L−1, respectively. These three ions together contribute with 74% of total anion mass. The concentration of HCOO− was 6.8 μmol l−1, which is less than one-fourth of that of acetate. Fornaro and Gutz (2003) discussed acetic and formic acids ratios (A/F) in gas and aqueous phase in São Paulo. They considered that the A/F>1 ratio is a sign of the predominance of direct emissions (biogenic or/and anthropogenic). In this study, the A/F ratio in rainwater was approximately 3.5, evidencing the weight of direct emissions produced by the large vehicular fleet in this region.
μ
μ
μ
Σ
μ
content of the rainwater samples. Similar supremacy was obtained in two other studies carried out in the west region of the city. The next most abundant cations are Na+ and Ca2+ with arithmetic mean concentration of 19.3 and 12.7 μmol l−1, respectively. The lower concentrations were measured for free H+ (7.8 μmol l−1), K+ (5.6 μmol l−1) and Mg2+ (3.9 μmol l−1) ions. These three ions together contribute with approximately 20% of the total cation mass. The Ca2+, K+ and Mg2+ cations in rainwater from São Paulo are usually associated with the ressuspension of the dust from the soil and the intensive activities of the construction industry involving the use of cement and gypsum. Among the anions, nitrate showed the highest arithmetic mean concentration, 27.5 μmol l−1. The main anthropogenic source of nitrate in rainwater in urban areas like São Paulo is the oxidation of nitrogen oxides
μ
Fig. 2 Ionic balance in rainwater samples (n=207): a electroneutrality (μeq l−1); b Correlation between measured and calculated conductance. The continuous line has been drawn considering a unitary slope
Fig. 3 Box and whisker plots for concentrations of cations a and anions b in rainwater samples for the period of July (winter) 2002 up to June 2004 (end of the autumn), in São Paulo city. Horizontal box lines: 25, 50 and 75th percentile values; error bars, 5 and 95th percentile values; (x symbol) 1st and 99th percentile; (- sign) minimum and maximum values. The arithmetic mean corresponds to square inside the box
Water Air Soil Pollut: Focus (2007) 7:85–92
In areas under influence of sea breeze, it is usual to discriminate the marine and continental and/or anthropogenic sources from concentration of major ions. This is frequently made considering Na+ as the reference element, assuming that all Na content in rainwater is of marine origin. This assumption is usually adopted in studies of rainwater of urban areas due to difficulties to identify sodium sources and the absence of other tracer elements of marine origin. In large urban areas like São Paulo City, this assumption may be susceptible to errors, as it disregards the possible contribution of sodium from crust and anthropogenic emissions. In order to illustrate this fact, studies about inorganic ions of ethanol fuel (consumed by 25% of the vehicular fleet), indicated a Cl/Na ratio of 0.26 (Munoz et al., 2004). Another factor to be remarked in the MASP is related to the contribution of the biomass burning from commercial establishments like pizzerias and bakeries, which use wood as fuel, emitting particles (PM2.5) containing inorganic ions (CETESB, 2005; Ynoue & Andrade, 2004). Considering these difficulties to characterize the sodium sources, the Na content in rainwater samples collected in downtown São Paulo was not evaluated as exclusively from − marine origin. Based on this assumption, SO2 4 , Cl , 2+ 2+ Ca and Mg were also predominantly considered coming from continental/urban sources. The C2 O2 was determined in 64% of the 4 rainwater samples, with an arithmetic mean concentration of 0.98 μmol l−1, while the NO 2 was the ion with lowest concentration, 0.90 μmol l−1. Table 1 shows seasonal differences in VWM concentrations for all species. The concentrations of − − acidic ions NO 3 , CH3COO and Cl do not show significant differences between the dry and wet period. In the dry period, the VWM concentrations 2+ of alkaline ions NHþ increased 11 and 43%, 4 and Ca which can explain the decrease of the free H+ concentration in rainwater samples.
89 Table 1 Data of the VWM concentrations of the ionic components in different seasons Ions
Dry period Wet period VWM (μmol l−1)
Annual
CH3COO− HCOO− Cl− NO 2 NO 3 SO2 4 C2 O2 4
16.4 6.31 10.9 0.74 19.7 13.4 1.00 10.8 36.3 4.55 10.6 3.04 4.94
17.1 4.21 10.7 0.55 20.2 12.1 0.70 13.5 32.7 3.81 7.39 3.16 6.29
Na+ NHþ 4 K+ Ca2− Mg2+ H+
17.4 3.51 10.7 0.48 20.4 11.6 0.60 14.4 31.5 3.55 6.31 3.20 6.74
precipitation in downtown São Paulo is slightly acidic. On the other hand, around 4% of rainwater samples had pH values higher than 7.0, suggesting the significant contribution of alkaline species to wet precipitation in this region. The pH results obtained in this study are slightly higher than those of other studies of rainwater carried out in the west region of São Paulo, where the pH average values ranged from 4.5–5.0 (Paiva et al., 1997). In comparison with others large cities around the world, the average pH from this study was similar to Rio de Janeiro, 5.12 (de Mello, 2001); higher than the average pH of Mexico City, 4.65 (Baez, Belmont, & Padilla, 1996), Los Angeles, 4.67 (Kawamura, Steinberg, & Kaplan, 1996) and Seoul, 4.7 (B. K. Lee, Hong, & D. S. Lee, 2000) but much lower than Madrid, 6.6 (Hontoria et al., 2003). The relative contribution of each anion to the potential free acidity, PFA, of a rainwater sample was determined by using the following equation: PFA ¼ P
½X anions
3.3 Profile of Rainwater Acidity Figure 4 illustrates the frequency distribution of pH. These values range from 4.0 to 7.3, presenting an average of 5.1 and VWM of 5.2. More than 55% of the rainwater samples had pH values
