Hydrogen peroxide in the rainwater of Sao Paulo megacity: measurements and controlling factors

ARTICLE

Hydrogen peroxide in the rainwater of Sao Paulo megacity: measurements and controlling factors

Carolina Gonçalves^I; Marcos A. dos Santos^II; Adalgiza Fornaro^III; Jairo J. PedrottiI, ^*

^IDepartamento de Química, Centro de Ciências e Humanidades, Universidade Presbiteriana Mackenzie, Rua da Consolação, 896, 01302-907 São Paulo-SP, Brazil

^IIDepartamento de Química Fundamental, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000 São Paulo-SP, Brazil

^IIIDepartamento de Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226, 05508-090, São Paulo-SP, Brazil

ABSTRACT

Measurements of H₂O₂, major ions and meteorological parameters were made in rain events occurred in downtown Sao Paulo City, Brazil between July 2001 and January 2006. The H₂O₂ concentration ranged from 0.5 to 78 μmol L^-1 with an average of 14.5 μmol L^-1. The SO₄^2- and HCOO^- concentrations mean were of 13.5 and 9.62 μmol L^-1, respectively. The H₂O₂ concentrations showed a positive and significant correlation with HCOO^- (r = 0.70, n = 129, p < 0.0001) suggesting that the production of H₂O₂ in aqueous phase by consumption of formaldehyde by reaction with OH^• radical is a factor which may control the H₂O₂ levels in the rainwater samples. Estimation of rate of wet deposition of H₂O₂ shows that nearly 70% of hydrogen peroxide is annually removed from the atmosphere by wet deposition in spring and summer. Sequential rainstorm analyses indicated that lightning activity and rainfall rate can influence the H₂O₂ contents in rainwater in this area.

Keywords: hydrogen peroxide, rainwater, air pollution, megacity

RESUMO

Determinações de H₂O₂, íons majoritários e parâmetros meteorológicos foram realizados em eventos de chuva no centro de São Paulo entre julho de 2001 e janeiro de 2006. As concentrações de H₂O₂ variaram entre 0,5 e 78 μmol L^-1, sendo a média de 14,5 μmol L^-1. As concentrações médias de SO₄^2- e HCOO^- foram 13,5 e 9,62 μmol L^-1, respectivamente. Observou-se uma correlação significativa de H₂O₂ com íon HCOO^- (r = 0,70, n = 129, p < 0,0001) indicando que a oxidação de formaldeído por radicais OH^• pode ser uma rota de formação de H₂O₂ nas águas de chuva de São Paulo. A estimativa da deposição úmida de H₂O₂ mostra que aproximadamente 70% da deposição úmida anual de H₂O₂ ocorre na primavera e verão. A ocorrência de relâmpagos, assim como a intensidade da chuva, foi importante no controle da concentração de H₂O₂ na fase líquida da atmosfera.

Introduction

Photochemical oxidants like hydrogen peroxide (H₂O₂) and ozone (O₃) are air pollutants which cause adverse effects on plants and crops as well as on the human health. The hydrogen peroxide, H₂O₂ is one of the most important oxidants of the troposphere contributing to control the chemical composition of the atmosphere at trace gas levels.¹ In the gas phase, the H₂O₂ production results from the interactions among hydroperoxyl (HO₂^•) radicals and hydrated hydroperoxy (H₂O^•HO₂), which are produced by photolysis of ozone (O₃) in atmosphere containing sufficiently high humidity and gases such as CO and hydrocarbons, some of those provided naturally and others resulting from anthropogenic activities.^2-4

Hydrogen peroxide is extremely soluble in water (K_H = 1.2×10⁵ mol L^-1 atm^-1 at 25 ºC), providing micromolar concentration levels even when the gaseous concentration is within ppbv range.^5-7 Gas-to-drop partitioning has been considered the predominant source of H₂O₂ in atmospheric water drops. On the other hand, some results have shown that solar UV radiation can initiate the photochemical reactions in aqueous phase for H₂O₂ formation.⁸ In the liquid phase, the hydrogen peroxide is considered the most effective oxidant for the conversion of dissolved sulfur dioxide (SO₂) to sulfuric acid (H₂SO₄) at pH range from 2 to 5, which is the main contributor to the acidification of cloud, fog and rainwater.^5-8 The H₂O₂ role over continents and oceans has been extensively studied in the atmospheric chemistry, showing that the H₂O₂ ambient concentration varies typically from 0.1 to 2.0 ppb in the gas phase and from < 10^-7 to 10^-4 mol L^-1 in fog, cloud, snow and rainwater. Higher concentrations have been observed in the summer and lower concentrations in the winter.^9-12

Field H₂O₂ measurements have been carried out by several groups in Southern Hemisphere.^13-16 In Brazil, the first H₂O₂ concentrations in rainwater were measured in the late summer of 1988 in three northeastern sampling sites of the Country (Mucugé, Itaparica Island and Salvador City) in Bahia State. The H₂O₂ concentrations showed values ranging from 19 to 200 μmol L^-1, with an average concentration of 66 μmol L^-1.¹³ More recently, two other studies concerning H₂O₂ field measurements were carried in urban areas in the Southeastern of the country. In Minas Gerais State, rains collected in Juiz de Fora City between November 2003 and January 2005 showed H₂O₂ concentrations ranging from 1.1 to 92.5 μmol L^-1, being the higher H₂O₂ levels measured during spring and summer.¹⁵ In urban area of Sao Paulo City, lower concentrations were recorded in rainwater collected between April 2003 and April 2004. The concentrations of H₂O₂ ranged from 2.29 to 48.6 μmol L^-1, with an average value of 13.1 μmol L^-1.¹⁶

In this work, the results of a 5-year study concerning the measurements of hydrogen peroxide, sulfate, formate, pH and electrical conductivity in Sao Paulo rainwater are presented and the factors controlling the levels of H₂O₂ are discussed. Sequential measurements of H₂O₂ in rainstorms were also carried out in order to evaluate the influence of meteorological factors on the H₂O₂ concentrations during the precipitation.

Site sampling

The Metropolitan Area of Sao Paulo (MASP) is located in the southeastern region of Sao Paulo State (46º43'W and 23º34'S), Brazil, around 45 km from the coast (Atlantic Ocean), at 780 m above the sea level. The MASP annual climatological data are: average temperature 19.3 ºC; relative humidity 78% and precipitation in 1455 mm. The rainy (689 ± 122 mm) periods between December and February and dry ones (135 ± 67 mm) between June and August are well-defined, summer and winter, respectively. Intermediary situations happen in the fall and in the spring.¹⁷ Sao Paulo City has, approximately, 11 million inhabitants and it is the largest part of the MASP, one of the most populated urban regions in the world, with more than 19 million inhabitants distributed in 1,747 km² of unplanned urban area, characterized by serious environmental problems.¹⁸

Sao Paulo State Environmental Agency (CETESB) has been monitoring atmospheric pollutants like SO₂, CO, NO_x, O₃ and inhalable particulates (PM₁₀) by an automatic network in the MASP (29 sites). In the last years, these results have indicated that the atmospheric emissions in the MASP are associated with pollutants released, mainly, by the large fleet of vehicles. The most common fuels used by almost seven million heavy and light vehicles are diesel, hydrated ethanol and gasohol (gasoline + 22% of ethanol). The vehicular fleet has been responsible for 97% of CO, 97% of HC, 96% of NO_x, 35% of SO₂ and 40% of PM₁₀ emissions.¹⁸

Although there is no monitoring net about rainwater chemical composition in Sao Paulo, recent results show that ammonium (NH₄⁺) is the dominant ion, followed by nitrate (NO₃^-), sodium (Na⁺), sulfate (SO₄^2-) and formate (HCOO^-). The average pH value was 5.1, indicating that the precipitation was slightly acidic.^19,20

Experimental

Rainwater samples were collected in high-density polyethylene bottles at the Mackenzie University campus located in downtown Sao Paulo City. The automatic wet-only sample collector (G.K. Walter Germany) is positioned at the balcony of the Education Building, 15 m above the ground. Preliminary experiments carried out with rainwater samples showed that H₂O₂ concentration suffer a decrease 30-50% within a period of 2 h after their collection at ambient temperature. The conditioning of the samples carried out at -18ºC or at -80 ºC in order to preserve the H₂O_-2 content did not show good results. A significant H₂O-₂ decomposition was observed in both temperatures, although the fastest process occurred at -18ºC. So, the H₂O₂ determination was made only in samples collected during or immediately after the end of the rain events. Usually within 15 min after their collection, the rainwater samples were filtered through a 0.22 μm pore size cellulose acetate membrane and analyzed for H₂O₂. The remaining sample volume were separated into aliquots and stored in a freezer (at -18 ºC) for additional analytical determinations, which were made within 2 weeks after their collection. Meteorological data were obtained from the Meteorological Station (IAG/USP).

Reagents and chemicals

All chemicals were reagents of analytical grade and were used without further purification. Hydrogen peroxide, 4-aminoantipyrine, mono and di-hydrogen phosphate were obtained from Merck (Darmstadt, Germany). Peroxyacetic acid was purchased from Aldrich (Milwaukee, WI, USA). Phenol, commercial catalase and peroxidase were obtained from Sigma (St. Louis, MO, USA). Solutions were prepared with deionized water (Nanopure system, resistivity > 18 MΩ cm). The stock solution containing 0.10 mol L^-1 H₂O₂ was stored at 4 ºC and standardized with KMnO₄, which had previously been standardized with sodium oxalate. The reference solutions of H₂O₂ were prepared just before their use by dilution of the stock solution.

Instrumentation

Flow spectrophotometric measurements were made by using a model 600 Femto visible spectrophotometer (Sao Paulo, Brazil) provided with a 180 μL glass flow cell with a 1.0 cm light path. The solutions were propelled with an Ismatec pump model REGLO. Teflon tubing of 0.8 mm i.d. was used for manifold construction except for pump tubing, where short sections of Tygon tubing were employed. Sample and colorimetric reagent solutions were injected in the FIA system with a homemade, manually operated, rotatory valve provided with 500 μL sampling loops.

The pH values were measured using a Digimed DM-20 pH-meter provided with a glass electrode combined with an Ag/AgCl (KCl sat.) reference electrode. The anions analyses were made by ion chromatography Metrohm model 761 with an electrical conductivity detector. The mobile phase was a 4.0 mmol L^-1 Na₂CO₃ /1.2 mmol L^-1 NaHCO₃ solution and the suppressor column was regenerated with 50 mmol L^-1 H₂SO₄. Acetaldehyde and formaldehyde were measured as 2,4 dinitrophenylhydrazones (DNPH) derivatives by using a Dionex P680 liquid chromatograph with a 20 μL sampling loop and 340U UV detector operating at 360 nm. The separation of the hydrazones was carried out with a C₁₈ reversed phase Supersil ODS column (5 μm, 4.6 × 150 mm) with isocratic elution by using a 30/70% (v/v) water-acetonitrile solution at 0.7 mL min^-1 flow rate as mobile phase.²¹

Hydrogen peroxide analysis

A flow injection method with spectrophotometric detection was used for hydrogen peroxide determination.²² The analytical method is based on the one-electron oxidation of phenol by hydrogen peroxide catalyzed by peroxidase. The phenoxy radicals react with 4-aminoantipyrine and hydrogen peroxide to generate an antipyrilquinoimine red product, which exhibits the maximum absorption at 505 nm. The absorbance signal is proportional to the rate of H₂O₂ consumption, which is also dependent on the concentration of active enzyme in the solution. A schematic diagram of the FI analytical method is shown in Figure 1.

The analytical method obeys the Beer's law in a concentration range of 0.5-100 μmol L^-1 of hydrogen peroxide with an excellent correlation coefficient (r = 0.9998) and a limit of detection of 0.3 μmol L^-1 (10.2 μg L^-1) at pH 7.0. At a flow rate of 1.2 mL min^-1, a sampling rate of 50 injections per hour can be reached and the relative standard deviation, RSD of the peak signals are better than 2%. The accuracy of the analytical method was evaluated from recovery tests made by fortification of rainwater samples with hydrogen peroxide concentration in 4 up to 20 μmol L^-1 range. The recovery results carried out with five different rainwater samples showed values ranging from 97 to 101% (n = 3). Organic hydroperoxides such as methyl hydroperoxide [CH₃O₂H] and peroxyacetic acid [CH₃C(O)O₂H], which are often present in rainwater samples, do not interfere in the determination of hydrogen peroxide. All measurements were made at room temperature (25 ± 1 ºC).

Results and Discussion

Between July 2001 and January 2006 the rainwater was collected in the central area of Sao Paulo City and analyzed for H₂O₂, SO₄^2-, HCOO^-, pH and electrical conductivity. The rainfall sampled was 1,067 mm, corresponding to 25% of the total precipitation in the period in this area. Most rainfall occurred between September and February months (spring and summer seasons), corresponding to 786 mm precipitation amount.

Table 1 shows the statistical data of the hydrogen peroxide, sulfate and formate concentrations, pH and conductivity values for rainwater samples. The H₂O₂ concentration ranged from 0.5 to 78.1 μmol L^-1, with an average value of 14.5 μmol L^-1. The hydrogen peroxide concentrations measured in Sao Paulo megacity showed higher variation than those measured in rainwater sampled in Salvador, Bahia (Brazil) and Miami, Florida (EUA).^13,23 The higher seasonal variation in sunlight and the more polluted air in Sao Paulo, when compared with the two other cities, may explain the difference in variation of H₂O₂ concentrations.

The higher concentrations (> 50 μmol L^-1) of H₂O₂ were observed in four rain events in 9/20/2002, 9/13/2003, 1/15/04 and 10/26/2005. All these rain showers occurred at early afternoon (between 1:00 and 3:00 pm) and were preceded by mornings with intense solar radiation and high temperature, which favors the photochemical process into the atmosphere and accelerates the formation of HO₂ radicals and the production of H₂O₂.

The volume-weighted mean H₂O₂ concentration was 12.4 μmol L^-1. This value was much lower than those concentrations measured in Brazilian urban areas of Salvador,¹³ Bahia, 124 μmol L^-1 and Juiz de Fora,¹⁵ Minas Gerais, 23.4 μmol L^-1 but almost two times higher than that in Santiago,¹⁰ Chile, 6.6 μmol L^-1 and Miami,²³ Florida, 6.9 μmol L^-1 and similar to that in Mexico City,¹² 11.2 μmol L^-1.

The concentrations of H₂O₂ exhibited a significant seasonal variability (Figure 2a). The higher H₂O₂ concentrations were measured in the spring rains (September-November) (average = 20.7 μmol L^-1, n = 61) while the lower concentrations were obtained in rainwater samples collected in winter (June-August) (mean = 8.6 μmol L^-1, n = 10). The spring season is characterized by strong solar radiation and higher biogenic emission of volatile organic compounds (VOCs) to the atmosphere, which favors the H₂O₂ production in the gas phase and the aqueous phase. Results obtained from air quality monitoring in MASP indicate that maximum ozone concentrations have also been observed in spring (Figure 2b), emphasizing the increase of the photochemical processes into the atmosphere in this season.¹⁸

The pH values ranged from 4.13 to 7.18, presenting a mean value of 5.10 and VWM of 5.14 (Table 1). More than 55% of the rainwater had a pH value between 5.0 and 5.5 and almost 23% of the samples presented pH values lower than 5.0. The average pH determined in this six-year study was higher than that measured in two other wet-only studies conducted in the West side of Sao Paulo City during the late 1980s and mid-1990s, which obtained mean pH values of 4.5 (n = 450) and 4.5 (n = 27), respectively.¹⁷

Classically, the results registered in literature indicate that H₂O₂ is the main oxidant for SO₂ in the atmospheric liquid phase at pH values < 5, which can result in negative correlation between H₂O₂ and SO₄^2- concentrations in rainwater.²⁴ Nevertheless, a low positive and significant correlation (r = 0.37, n = 129, p < 0.0001) between H₂O₂ and SO₄^2- was obtained in this study, as it can be seen in Figure 3A. Additionally, Figure 3B shows that higher H₂O₂ concentrations were observed in samples with pH values < 5, indicating that the destruction of hydrogen peroxide by sulfur dioxide is not the predominant factor controlling H₂O₂ in the rainwater of this area. The positive correlation could be a result of higher concentration of H₂O₂ precursors in liquid phase with other pollutants that generate SO₄^2- in rain, such as the dissolution of particulate matter as well as the oxidation of SO₂ in gaseous phase with transfer to the liquid phase.^19,24

A significant correlation (r = 0.70, p < 0.0001) was observed between H₂O₂ and HCOO^- concentration (Figure 3C) suggesting the production of HCOO^- and H₂O₂ by reactions in aqueous phase through formaldehyde oxidation by OH^• radical.²⁵ The reactions begin with the HCHO dissolution in the atmospheric aqueous phase (intrinsic Henry's constant, H = 1.3 mol L^-1 atm^-1), followed by its hydration which increases its solubility (effective Henry's constant H* = 3.0×10³ mol L^-1 atm ^-1) and favors the gemdiol (geminal diol) production, reaction 1.²⁵ In parallel, there is the radicals dissolution, OH^• and HO₂^•, Henry's constant, H = 30 and 1-3×10³ mol L^-1 atm^-1, respectively,²⁶ which favors the oxidation processes (reactions 2-3), producing more radicals and formic acid. The reactions sequences finish with the H₂O₂ yielding (reaction 4):

In attempt to interpret these reactions in Sao Paulo rainwater, formaldehyde (HCHO) and acetaldehyde (H₃CCHO) were also measured. Results of this experiment (Table 2) indicate the predominance of acetaldehyde. On the other hand, likewise data shown in Figure 3C, a positive correlation was observed between H₂O₂ and HCOO^- for this set, even considering the small number of samples. Concentrations of aldehydes reported in gas phase of Sao Paulo City are at tens of ppbv levels and shows HCHO/H₃CCHO ratio close to one.^27,28 If these aldehydes concentrations in gas phase are similar, higher [HCHO] should be expected in aqueous phase, talking in account that the HCHO effective Henry's constant, H* = 3.0×10³ mol L^-1 atm^-1 is higher than the H₃CCHO, H* = 1.14×10 mol L^-1 atm^-1.²⁶ However, the results on Table 2 show H₃CCHO concentrations higher than those of HCHO, suggesting the consumption of gemdiol by reactions with radicals (reactions 2 and 3) and the production of H₂O₂ and HCOO^-.

The air temperature ranged from 9 to up 31.3 ºC, with and mean value of 22.5ºC. The higher temperature values were measured in the summer and lower in the winter. The relationship between H₂O₂ concentration in rainwater and air temperature showed a temperature range between 18 and 26 ºC in which H₂O₂ maximum concentrations were observed. This result suggest that reactions of H₂O₂ decomposition are predominant in higher temperatures while lower temperatures do not provide favorable conditions for photochemical reactions to production of H₂O₂, as previously suggested.¹⁶

The relationship between the precipitation volume and the concentrations of H₂O₂ was investigated using a regression analysis in order to evaluate the influence of rainfall on H₂O₂ content in rainwater. Most rainwater samples showed volume < 10 mm. The evaluation of H₂O₂ concentrations data versus the rainfall shows a weak correlation (r = -0.15, n = 180, p < 0.07), indicating that the H₂O₂ concentrations were not controlled by dilution during these measurements. On the other hand, the regression analysis of H₂O₂ concentrations as a function of the precipitation rate showed a significant linear correlation, denoting that lower rainfall rate lead to higher H₂O₂ content in several rainstorms, which agrees with the results obtained in H₂O₂ rainwater measurements carried out in Salvador area, Bahia.¹³

By using the volume-weighted mean H₂O₂ concentration of 12.4 μmol L^-1 and the annual average precipitation amount of 1.0 m y^-1 in this site sampling, the annual rate of H₂O₂ wet deposition for this 5-year study was 12 mmol m^-2 y^-1 . Considering the lack of other data for H₂O₂ wet deposition in Brazil, the annual wet deposition of H₂O₂ was compared with studies conducted in U.S.A. and in Japan. In Sao Paulo, it was slightly higher than that obtained in Kyoto in 1999 and in 2000, 11.7 and 9.11 mmol m^-2 y^-1, respectively,²⁹ and exactly equal to the wet deposition value obtained in Wilmington, North Carolina between 1992 and 1994.³⁰ However, it was much higher than the value of 1.5 mmol m^-2 y^-1 found in Los Angeles during 1985-1991.³¹ The lower anual average rain amount (0.34 m y^-1) and the greater decomposition of H₂O₂ by reactions with other pollutants in the Los Angeles area may explain this significant difference.

The H₂O₂ wet deposition showed significant seasonal variation. Considering that both concentrations and precipitation volume are higher in the spring and summer, nearly 70% of annual H₂O₂ wet deposition occurs between October and February in Sao Paulo City.

Profile of variations of H₂O₂ concentrations during some rain events

In order to investigate the variability patterns of H₂O₂ concentrations during the rainstorms in Sao Paulo and the meteorological factors that can influence its concentration, several continuous measurements of H₂O₂ were conducted. Most of these measurements were carried out in summer and spring (rainy period) and just one sequential rain sample was collected in fall, which corresponds to dry season. Usually, the rain events in Sao Paulo occur in the afternoon and are preceded by periods of intense solar radiation in spring and summer. Short lasting light showers are frequently observed in fall and winter. The hydrogen peroxide concentration was measured in thirteen rain events and showed three patterns of temporal variation. During the first 10 mm of rain, the H₂O₂ content increased in six events while its concentration declined in five others. An irregular variation in H₂O₂ concentration was observed in remaining storms.

Figure 4 shows some examples of temporal variation of H₂O₂ concentrations. The rainstorm of October 09, 2003 (Figure 4a) occurred at the middle of the afternoon and was preceded by a sunny morning. The temperature ranged between 22.8 ºC at early morning to 30.1 ºC at midday. From 1:00 pm, the air temperature was slowly decreasing and attained 28.3 ºC at 3:40 pm, when a heavy rain with thunder started. The H₂O₂ concentration measured in the initial rain sample (collected at 4:00 pm) was high (21.9 μmol L^-1). The H₂O₂ content increased continuously during the rainfall, reaching 42.6 μmol L^-1 in the last sample collected at 6:25 pm. The high H₂O₂ concentration in the first sample can be attributed to the photochemical reactions in the atmosphere before the precipitation. The increase of H₂O₂ concentration in the following samples could be due to the lightning during thunderstorms in the area producing H₂O₂ in clouds. Evidence of the hydrogen peroxide production in rainwater by lightning during thunderstorms was previously suggested.^32,33 Additionally, the decrease of rainfall rate from the second sample may also have contributed to the increase of H₂O₂ concentration in the rainwater.

On November 28, 2002 (Figure 4b), the H₂O₂ concentrations were measured in a rainstorm sampled between 2:00 and 5:45 pm. Likewise in October 2003, the precipitation occurred after an intense solar radiation in the morning and in the early afternoon. At 1:00 pm the temperature value attained 32.4 ºC, which indicated favorable conditions for photochemical H₂O₂ production in the atmosphere. The concentration of H₂O₂ in the first sample collected was high, 27.3 μmol L^-1 and it was virtually constant (27.6 μmol L^-1) in the second sample collected between 3:25 and 5:00 pm. Afterwards, the content of H₂O₂ started decreasing until the end of the rainstorm, probably due to significant increase in rainfall rate which minimizes the scavenging process of H₂O₂ in the atmosphere below clouds.

Figure 4c shows the H₂O₂ concentrations for a light shower collected on November 16, 2004 after a rainy night. The diurnal temperature ranged from 19.7 to 21 ºC. The precipitation started again in the morning of November 16 and it was preceded by cloudy weather conditions. The H₂O₂ concentration was low in the first sample collected at 11:15 am, 2.8 μmol L^-1 and increased up to 9.3 μmol L^-1, measured in the forth sample collected when the rain was over at 3:40 pm. This is an indicative that even in rainy days the photochemical production of hydrogen peroxide in the atmosphere is enough to increase the H₂O₂ levels in rainwater. A similar pattern of variation in H₂O₂ concentrations was found in precipitation collected in warm season of North Carolina.³⁰

The rainstorm of September 20, 2002 (Figure 4d) started at 1:30 pm after a partial cloudy morning. At the beginning, H₂O₂ concentration was 18.7 μmol L^-1, rising to 51.8 and to 78.1 μmol L^-1 up to the first 5 mm of rain. The significant increase in H₂O₂ content may be attributed to the occurrence of intense activity of lightning in the area observed in the first 30 min of rain.³³ In subsequent rain fractions (Figure 4d), the H₂O₂ concentration declined continuously until 18 mm of rainfall. The raise of H₂O₂ concentration at the end of rain may have been a result of the reduction precipitation rate, which usually favours the removal processes of atmospheric hydrogen peroxide.

These sequential analyses in the rainstorm show that the previous meteorological conditions, lightning and rainfall rate can influence the H₂O₂ contents in rainwater in Sao Paulo.

Conclusions

The concentrations of H₂O₂, SO₄^2-, HCOO^- and H⁺ were measured in rainwater collected in the center of Sao Paulo City between July 2001 and January 2006. The H₂O₂ concentrations in rainwater ranged from 0.5 to 78.1 μmol L^-1 with an average value of 14.5 μmol L^-1. The concentrations of H₂O₂ exhibited a seasonal variation in rainwater with higher values obtained in the spring and lower in the winter. The pH values ranged from 4.13 to 7.18, presenting a mean value of 5.10 and VWM of 5.14, suggesting that the rainfall was slightly acidic. The hydrogen peroxide concentrations showed a positive correlation with sulfate and hydrogen ion concentrations, which suggests that the decomposition of hydrogen peroxide by reaction with SO₂ is not predominant in the rainwater in Sao Paulo City.

A positive and significant correlation was observed between H₂O₂ and HCOO^- concentrations, showing that the production of H₂O₂ in aqueous phase by consumption of formaldehyde by reaction with OH^• radical can be a factor controlling the H₂O₂ levels in the rainwater in this area. The main source of aldehydes in rainwater is the gas-phase aldehydes which are emitted by combustion of the ethanol and gasohol (a mix of gasoline and 22% of ethanol), the most common fuels used in almost six million light vehicles in MASP. Additionally, the dissolution of gaseous H₂O₂ and/or its production by lightning activities during thunderstorms may also contribute to the hydrogen peroxide concentrations in the rainwater of Sao Paulo City.

The levels of H₂O₂ were not controlled by dilution. However, there was a significant inverse correlation between the concentrations of H₂O₂ and the precipitation rate, indicating that lower rainfall rates conduct to higher H₂O₂ contents in several rainstorms. The estimated annual rate of H₂O₂ wet deposition was 12 mmol m^-2 y^-1 , and 70% of this amount is deposited in the spring and in the summer, approximately. The temporal measurements of the H₂O₂ concentrations monitored in thirteen rainstorms show a diurnal variation in rainwater H₂O₂ concentrations being the higher contents of H₂O₂ measured in rains occurying in the early afternoons after sunny mornings. Lightning activities and the rainfall rate were important factors which affected the H₂O₂ contents during the rainstorms.

Acknowledgments

The authors acknowledge Fundação de Amparo à Pesquisa do Estado de Sao Paulo (FAPESP) and Fundo Mackenzie de Pesquisa (MACKPESQUISA) for the financial support of this work.

Received: May 20, 2009

Web Release Date: November 26, 2009

FAPESP helped in meeting the publication costs of this article.

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Publication Dates

• Publication in this collection
  16 Mar 2010
• Date of issue
  2010

History

• Received
  20 May 2009
• Accepted
  26 Nov 2009