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rainwater, and the dew water is in a reductive state. The concentration of the representative oxidant, hydrogen peroxide, was much lower in the dew water than in ..... Wagner, G. H., K. F. Steele, and M. E. Peden, Dew and frost chemistry at a.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D10, 4319, doi:10.1029/2002JD003058, 2003

Deposition of coarse soil particles and ambient gaseous components dominating dew water chemistry Masaki Takeuchi, Hiroshi Okochi, and Manabu Igawa Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, Yokohama, Japan Received 20 October 2002; revised 27 January 2003; accepted 6 March 2003; published 31 May 2003.

[1] Dew water characteristics are not as acidic as those of rainwater and fog water, and they

are abundant in weak acid components. The dew water components are dominated by the coarse-mode aerosol rather than the fine-mode aerosol, and calcium carbonate, the component of soil, is the important chemical species raising the pH of the dew water. Carbon dioxide is deposited as calcium carbonate into dew water and is the only major component released from dew water. The total concentration of carbon dioxide increased with an increase in the dew water pH, while the degassing rate increased with the decrease in dew water pH. In the low-pH region, the pe values in dew water are lower than those in rainwater, and the dew water is in a reductive state. The concentration of the representative oxidant, hydrogen peroxide, was much lower in the dew water than in the rainwater in Yokohama (dew water, 0.4 mM; rainwater, 13.1 mM in 1998). The representative conjugate base ions of a weak acid, sulfite and hydrogen sulfite ions, are easily oxidized by hydrogen peroxide, and the lower concentration of hydrogen peroxide in dew water may cause the higher concentration of sulfite ion in dew water than in rainwater. The dew water pH is higher than the rainwater and fog water pH, and a weak acid is easily dissolved into the dew water. Therefore these weak acid ions are more abundant in dew INDEX TERMS: 0345 Atmospheric Composition and water than in rainwater and fog water. Structure: Pollution—urban and regional (0305); 1854 Hydrology: Precipitation (3354); 2419 Ionosphere: Ion chemistry and composition (0335); KEYWORDS: carbon dioxide, dew water, hydrogen peroxide, oxidationreduction potential, pH Citation: Takeuchi, M., H. Okochi, and M. Igawa, Deposition of coarse soil particles and ambient gaseous components dominating dew water chemistry, J. Geophys. Res., 108(D10), 4319, doi:10.1029/2002JD003058, 2003.

1. Introduction [2] The chemical composition of dew water formed on various surfaces is strongly affected by the local atmospheric chemistry. The concentrations of chemical components are dominated by dew water amount and gases and aerosol concentrations in the atmosphere. The dew water amount on some artificial collectors have been measured, such as on gypsum plate [Duvdevani, 1947], glass plate [Janssen and Ro¨mer, 1991], nylon mesh [Zangvil, 1996], aluminum sheet [Baier, 1966], polyethylene sheet [Wagner et al., 1992], and Teflon sheet [Foster et al., 1990; Okochi et al., 1996; Pierson et al., 1986; Takeuchi et al., 2000, 2001, 2002a]. The reported pH values of dew water formed on the artificial collectors are quite variable, ranging from 3.04 to 8.20, and the pH is relatively high compared to rainwater and fog water [Okochi et al., 1996; Takeuchi et al., 2000, 2002a], although acid dew is sometimes formed in our sampling site and has been reported by other researchers [Mulawa et al., 1986; Pierson et al., 1986]. Since January 1993, we have collected dew water in an urban area, Yokohama, Japan, and have reported the factors controlling Copyright 2003 by the American Geophysical Union. 0148-0227/03/2002JD003058

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the concentrations of the various components in dew water [Okochi et al., 1996; Takeuchi et al., 2000, 2001, 2002a]. The characteristics of dew water are high pH and high concentration of the conjugate base ions of weak acids compared to those in rainwater and fog water [Okochi et al., 1996; Takeuchi et al., 2000, 2002a]. The concentrations of the weak acid ions such as nitrite and sulfite are low and their oxidized state of nitrate and sulfate are abundant in rainwater and fog water, while these unstable ions are abundant in dew water. The abundant weak acid ions in dew water have been also reported in the samples collected in many other sites [Foster et al., 1990; Takenaka et al., 1999]. The reason for the high pH and high concentration of the weakly acidic ions of dew water has not been elucidated yet, although the high pH is one of the reasons for the high concentration of the conjugate base ions of weak acids. [3] We reported the controlling factors of the concentrations of weak acid and base ions in dew water using the resistance model, which is consisted of three steps, that is, aerodynamic, sublayer, and surface resistances [Okochi et al., 1996; Takeuchi et al., 2002a]. It is necessary to take into account the resistances when we discuss the absorption of gases into dew droplets because the dew is formed on surfaces under a light wind [Monteith, 1957]. The aerodynamic resistance is related to the rate of turbulent mixing of

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the species, and is mainly governed by the wind speed. This resistance increases exponentially with the decrease of the distance from the interface of the sublayer on the dew droplets. In the sublayer, the resistance is constant and is controlled by the rate of molecular and turbulent transport. The surface resistance is related to the concentration of species at the interface of the dew droplet, and is mostly dominated by the accumulation rate of dew water and the Henry’s law constant of the species. [4] The purpose of this study is to clarify the following two points. The first one is the factors controlling the dew water pH. The behavior of carbon dioxide in dew water will be discussed further by using the resistance model, because carbon dioxide is one of the key components dominating the pH of dew water. The second one is the reason of the stability of weak acid ions in dew water. An oxidationreduction potential dominates the form of species in natural waters [Stumm and Morgan, 1996]. Little attention, however, has so far been given to the oxidation-reduction potential of the precipitation because of the difficulties in determine the species that control the oxidation-reduction potential in the precipitation. In this study, the redox state of dew water will be compared to that of rainwater, because it is the dominating factor of the stability of weak acid ions in dew water.

2. Experiment [5] Four hundred and thirty six samples of dew water were collected from January 1993 to December 2001 on the roofs of the five- or six-story buildings on the campus of Kanagawa University in Yokohama, Japan (3528 0N, 139380E). Dew water collector is consisted of a 50 mm thick Teflon sheet mounted on a 10 cm thick slab of a Styrofoam. Three or four collectors were set up after sunset and dew water formed on the collectors was collected with Teflon scraper in the morning of the following day. Details of the sampling point and methods were described in previous papers [Okochi et al., 1996; Takeuchi et al., 2000, 2002a]. The collected samples were weighed and filtered by a 0.45-mm pore size membrane filter just after sampling. The electric conductivity, pH, and inorganic anions (Cl, NO2, NO3, SO32, SO42) in the dew water were measured using an electric-conductivity meter (Kyoto Electronics CM-117), a pH meter (Toa Electronics HM60S), and an ion chromatograph (Dionex DX-100 with a column of Dionex IonPac AS12 or Dionex 2000i/sp with a column of Dionex IonPac AS4A) in the day when the samples were collected. The residual samples were stored in a refrigerator, and were measured within three weeks. The cations except proton (NH4+, Na+, K+, Mg2+, and Ca2+,) and carboxylates (HCOO, CH3COO, and C2O42) were measured using an ion chromatograph (for cations, Dionex DX-100 with a column of Dionex IonPac CS12; for carboxylates, Dionex Bio-LC with a column of Dionex IonPac AS4A-SC or Dionex IonPac AS11). The oxidationreduction potential were measured with a pH meter (Toa Electronics HM-60S) equipped with a Pt electrode. The dissolved organic and inorganic carbons were measured with a TOC analyzer (Shimadzu TOC-5000) during two weeks after the sampling day. To prevent the degassing of carbon dioxide, each sample bottle was sealed with a cap

Figure 1. Dew event frequency, dew water amount, and the composition of the dew water as the function of the dew water pH in Yokohama from 1993 to 2001. and kept in a refrigerator as the case of the standard solution of dissolved inorganic carbons (sodium carbonate solution). During the dew water sampling period, aerosol samples were also collected with a low-volume aerosol sampler and the major ions were extracted from the filters into 25 mL of ultrapure water in a Teflon beaker by irradiating ultrasonic waves. The solutions were analyzed in a similar manner to that for the dew water. The data of wind speed, temperature, and relative humidity used in the calculation of the resistance model were obtained from the Yokohama Local Meteorological Observatory, which is located about 8 km southeast from our sampling site.

3. Results and Discussion 3.1. Factors Controlling the Dew Water pH and the Behavior of Carbon Dioxide in the Dew Water [6] The dew water components are dominated by the coarse-mode aerosol components rather than the fine-mode aerosol components [Takeuchi et al., 2001]. Figure 1 shows the dew event frequency and the average dew water amount for each dew event classified by a unit of pH, and the volume-weighted average composition of the dew water as the function of the dew water pH. The dew water amount and the concentrations of each component shown in Figure 1 were the average values for the samples collected at about 0600 JST, because these values change with time and the dew water amount becomes maximum at about 0600 JST in most cases. The acidic dew water is formed by the absorption of acidic gas, nitric acid [Takeuchi et al., 2000], but dew water is not so acidic in most of cases and sometimes becomes alkaline. The average pH of dew water was 6.08, while that of rainwater collected at the same place was 4.81.

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The acid components such as nitric acid have a relatively low concentration in dew water rather than in rainwater and fog water because the air at the surface of the dew droplets is stable and the transfer process in the air is the limiting step in the transport of gaseous components while the rain and fog droplets are suspended in the air and the air at the surface of the droplets is turbulent. The concentration of nitric acid in the dew water was estimated by the resistance model with the mass transfer in the air shown by the following equation [Okochi et al., 1996].  ½HNO3  ¼ CHNO3 ð zÞt= w  103  ðra þ rb þ rc Þ

ð1Þ

where CHNO3 (z) is the concentration of nitric acid (mol cm3) at the height (z) where the nitric acid concentration in the air was measured, 1 m, t is the dew water collection periods (s), w is the dew water amount (g cm2), and ra, rb, and rc are the aerodynamic, the sublayer, and the surface resistances (s cm1) [Chameides, 1987], respectively. Assuming the ambient concentration of nitric acid to be 0.5 ppb [Takeuchi et al., 2000], the estimated dissolved nitric acid concentration is 12 mM in dew water. In the case of fog, the nitric acid concentration in fog water is high and can be calculated on the basis of the assumption of a closed system [Seinfeld and Pandis, 1998]. The nitric acid concentration in fog water is estimated to be about 1 mM on the basis of the assumption that nitric acid in the ambient air is 0.5 ppb, and the liquid water is the possible low value of 0.02 g m3 [Igawa et al., 1998]. The concentrations of the weak acid components increased with the dew water pH as shown in Figure 1 and the major component which can increase the pH of the dew water is the carbonate salt, because the hydrogen carbonate ion is abundant in the alkaline dew water and much more basic than other conjugated bases shown in Figure 1. Therefore not only the limited transfer of nitric acid gas into dew droplets but also the deposition of the carbonate salt dominate its chemistry. That is to say, dew water is not acidified as frequently as rainwater and fog water, and furthermore, dew water becomes alkaline more readily than rainwater and fog water. [7] The concentration of total dissolved carbon dioxide (H2COT3 = CO2 H2O + HCO3 + CO32) in dew water equilibrated with ambient air can be described by the following equations for the open system: ½H2 CO3 T ¼ H *CO2 pCO2

ð2Þ

  2 H *CO2 ¼ HCO2 1 þ Ka1 =½Hþ þ Ka1 Ka2 =½Hþ 

ð3Þ

The total dissolved carbon dioxide concentration of the dew water can be calculated by using these equations and the pH of the dew water. The open circles in Figure 2 show the relationship between the measured concentrations of total dissolved carbon dioxide in the dew water and the concentrations calculated by these equations on the basis of Henry’ law using the partial pressure of CO2 and the measured pH. The concentration of CO2 in Yokohama is reported to be 398 ppm in 1996, 402 ppm in 1997, and 414 ppm in 1998 [Kanagawa Prefecture, 1999]), and then the value of 400 ppm was used in the calculation. These

Figure 2. Relationship between the estimated total H2CO3 concentration and the measured concentration in the dew water in Yokohama from 1996 to 2001. The open circle is estimated from the deposition of gaseous CO2 by Henry’s law and the concentration of CO2 in the ambient air at the sampling site and pH in the dew water, while the closed circle is estimated from the deposition of carbonate salt in the aerosol.

values did not correlate well, and most of the measured values were much larger than the calculated values. [8] The carbonate ion in the aerosol deposited primarily as the chemical forms of CaCO3 on the dew water. Calcium carbonate is the representative component of the carbonate salt, which originated from the soil and is dispersed in ambient air as coarse-mode aerosol. The dry deposition velocities of the aerosol components onto dew water were estimated by a regression analysis between their concentrations in the dew water and those in the aerosol and they depend on the size of the aerosol. The dry deposition velocity of Ca2+ was determined to be 0.626 cm s1, which can be regarded as that of coarse-mode aerosol, because calcium ion is the representative coarse-mode aerosol component [Takeuchi et al., 2000]. The carbonate ion concentration in the aerosol during the dew water sampling period was measured; it ranged from 142 to 564 nmol m3 with a mean of 312 nmol m3 at the sampling site. The closed circles in Figure 2 show the estimated values calculated by the deposition velocity and the measured concentration in the aerosol of the carbonate ion. The values also did not correlate well, and contrary to the former case shown by the open circles, most of the estimated values were much larger than the measured values. The solubility of calcium carbonate is low, and this low solubility may cause the low concentration of carbonate ion in the dew water. However, this was confirmed not to be the case, because the carbonate ion changes to hydrogen carbonate ion at the dew water pH, the carbonate ion concentration becomes very low in the dew water, and the concentration product of the calcium ion and the carbonate ion for most of the samples was less than 1% of the solubility product of calcium carbonate. Therefore the degassing of the carbon dioxide may result in these

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on the dew water can be calculated by using equation (4) and is shown in Figure 3a. The dew water is the field of the transformation of calcium carbonate to carbon dioxide and the calcium salt. Figure 3b shows the total concentration of carbon dioxide in the dew water as a function of pH. The total concentration of carbon dioxide increased with pH, which was already suggested in Figure 1, although the degassing flux did not depend on the dew water pH. Figure 3b also shows the ratio of the degassing flux to the total concentration of carbon dioxide as the function of pH. The ratio is the degassing rate of carbon dioxide from the dew water and it increased with the decrease of dew water pH. The concentration of hydroxide ion is increased by degassing  based on the following equilibrium, HCO 3 = OH + CO2, but it is partly neutralized by the absorption of the ambient acidic gaseous components into the dew water. Figure 3. Concentration profile of CO2 in the air on the surface of the dew water (a) and the relationship between the total concentration of carbon dioxide and the ratio of the degassing flux to the total concentration and pH (b) in Yokohama from 1996 to 2001. These values were estimated by a resistance model using the average values obtained at the sampling site (CO2 concentration = 400 ppm; wind speed = 2.30 m s1).

3.2. Comparison of Oxidation-Reduction Potential in Dew Water and Rainwater [10] The oxidation-reduction potential was measured to clarify the abundance and the stability of conjugate base ions of weak acids such as sulfurous and nitrous acids in the dew water. It ranged from 416 to 552 mV and volume-weighted average was 492 mV in the dew water. Figure 4 shows the relationship between the pe and the pH in the dew water and rainwater in Yokohama. The pe value is defined as follows. pe ¼ F Eh=ð2:3 RT Þ

situations. It has been already reported that the deposition fluxes of the chemical components originated from aerosol onto dew water were several times higher than those onto dry surface except for carbonate salt [Takeuchi et al., 2002b], and the exception also suggests the degassing of carbon dioxide from the dew water. [9] The degassing flux, F (mol cm2 s1), can be described as follows in the gaseous phase at the surface of the dew water.  F ¼ Cs  400  106 = ðra þrb Þ  RT  103

ð6Þ

where F is Faraday’s constant (9.6485  104 C mol1), and Eh is the oxidation-reduction potential (V). The open and closed circles are the values of dew water and rainwater, respectively. The pe values increased with the decrease in the pH values in both cases, and hence these precipitations become more reductive as the solution pH rises. The

ð4Þ

where Cs is the CO2 concentration (atm) in the gas phase at the interface of the dew droplet, ra and rb are the aerodynamic resistance and the sublayer resistance, respectively, and R and T are the gas constant (0.082 L atm K1 mol1) and absolute temperature (K), respectively. The excess carbonate salt deposition from the ambient air over the deposition flux calculated from the measured carbonate salt concentration in the dew water may correspond to the degassing of carbon dioxide. The degassing flux, F 0, in the liquid phase of the dew water can be described as follows, and it is equal to F. F ¼ F 0 ¼ Caerosol  v  D

ð5Þ

where Caerosol is the ambient air concentration of the particulate carbonate salt (mol cm3), v is the deposition velocity, which may be approximately equal to the deposition velocity of the calcium ion, 0.626 cm s1 [Takeuchi et al., 2000], and D is the deposition flux of the carbonate salt into the dew water calculated from the measured carbonate salt concentration in the dew water. When the measured ambient air concentration was substituted into equation (5), the degassing flux can be calculated. The concentration profile

Figure 4. Relationship between the pe and the pH in the dew water and rainwater in Yokohama. The open and closed circles are the dew water and rainwater, respectively. The dew water values are the samples collected from March 2000 to December 2001 and rainwater values are the samples collected from August 2001 to June 2002.

TAKEUCHI ET AL.: DEPOSITION DOMINATING DEW WATER CHEMISTRY

intercept of the regression line of dew water was lower than that of rainwater, although the slopes of regression lines were similar to each other (dew water, 0.451; rainwater, 0.455). This means that the dew water is more reductive than the rainwater at the same pH region and the reductive dew water may be caused by the deposition of reductants or the limited transfer of oxidants into the dew droplets. The composition of the dew water is strongly affected by the coarse-mode aerosol such as soil dust because the dew is formed on the surfaces of some objects placed near the ground. The strong effect of the coarse-mode aerosol is revealed by the high pH of the dew water as already explained. Humic acid in soil dust can act as a reductant in dew water. The redox potential was preliminary measured as a function of the humic acid concentration and it decreased with the concentration. It has been reported that the humic acid-metal complex functions as a reductant in natural water [Ma et al., 2001; O’Loughlin et al., 1999]. Contrary to the humic acid, an oxidant such as hydrogen peroxide is scarcely absorbed in dew water because the transfer process in air is the limiting process in the transport of the gaseous components to the dew water. The concentration of hydrogen peroxide in the dew water was estimated by the resistance model with mass transfer in the air [Okochi et al., 1996]. Assuming the ambient concentration of hydrogen peroxide to be 1 ppb [Finlayson-Pitts and Pitts, 2000], the value estimated as the case of nitric acid is 24 mM in the dew water. The small amount of absorbed hydrogen peroxide may be consumed by the redox reaction in the dew water with the humic acids and others to lower the concentration further. The hydrogen peroxide concentration was observed to be much lower in dew water than in rainwater in Yokohama (dew water, 0.4 mM; rainwater, 13.1 mM in 1998), while all of the major ion concentrations were several times higher in dew water than in rainwater [Takeuchi et al., 2000]. Ortiz et al. [2000] reported that the volume-weighted average concentration for hydrogen peroxide was 1.6 mM in dew water and 6.6 mM in rainwater in Santiago, Chile. The other reason of low concentration in dew water is that dew water is formed at nighttime when the hydrogen peroxide concentration in the atmosphere is low. Sulfite ions are easily oxidized by hydrogen peroxide [Lagrange et al., 1996; Nair and Peters, 1989; Zuo and Hoigne´, 1993], and the lower concentration of hydrogen peroxide in dew water than in rainwater may cause the higher concentration of sulfite ion in the dew water than in the rainwater. The nitrite ion is also abundant in dew water and it is readily oxidized to nitrate ion. However, the oxidation process is more complex than that of sulfite and the cause of the abundant nitrite ion will be discussed in further study. The more reductive condition of the dew water than of rainwater as shown in Figure 4 is caused by not only the large deposition of reductive humic acid but also the limited transfer of an oxidant into the dew water. Furthermore, the pH of the dew water is higher than that of the rainwater and fog water, and weak acids are easily dissolved into the dew water. Therefore these weak acid ions are more abundant in the dew water than in rainwater and fog water.

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Acknowledgments. The present work was partly supported by the Japan Science & Technology Corporation, CREST program 1996 – 2001.

M. Igawa, H. Okochi, and M. Takeuchi, Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan. (igawam01@ kanagawa-u.ac.jp)