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Further experiments on SO2 oxidation rate in monodisperse droplets grown on carbon nuclei in presence of O2 and NO2
J. Aerosol Sci., Vol. 24, No. 5, pp. 683 685, 1993 Printed in Great Britain.
0021 8502/93$6.00+0.00 ~: 1993PergamonPress Ltd
TECHNICAL NOTE F U R T H E R E X P E R I M E N T S O N SO 2 O X I D A T I O N RATE IN M O N O D I S P E R S E D R O P L E T S G R O W N O N C A R B O N N U C L E I IN P R E S E N C E OF 0 2 A N D NO2 G. SANTACHIARA,F. PRODI* and F. VIVARELLI Istituto FISBAT-CNR, Bologna, Italy (Received l0 July 1992; accepted 14 October 1992)
A h s t r a c ~ T h e oxidation of SO2 in monodisperse droplets grown on carbon nuclei in presence of 0 2 and N O 2 has been studied. Both N O 2 and 0 2 were determined to be effective oxidants for SO 2. W h e n the 0 2 concentration is ~< 2%, N O 2 influences S(IV) oxidation if the former's concentration is higher than about 0.1 ppm. At the Oz atmospheric concentration, NO2 influences oxidation at concentrations higher than 0.3 ppm.
INTRODUCTION Oxidation of sulfur dioxide in the atmosphere can occur through homogeneous gas-phase reaction or as a heterogeneous process either on solid particles or in liquid droplets with traces of catalyst (Fe 3 +, M n 2 +, CI , soot, etc.). The average daily rate of gas-phase homogeneous conversion is usually considered to be 1% h - 1 and its daily m a x i m u m value is about 4 - 5 % h - 1 at midday in the ~ummer with sunny sky (Calvert and Stockwell, 1984). Heterogeneous conversion in clouds or fog drople'~ and deliquescent salts (e.g. NaCI, MgC12), gives higher conversion rates, i.e. upward of 30% h - 1 (Clarke et al., 1982; Hegg and Hobbs, 1982). If we consider carbon-catalyzed oxidation, laboratory experiments can be classified schematically in two kinds: those done with "dry" particles in a h u m i d atmosphere and those in liquid phase, i.e. using aqueous suspensions containing various concentrations of sulfurous acid or droplets growing on carbon particles and absorbing gases (SO2, NO2, 03, 02, etc.). The latter experimental set-up better approximates an atmospheric situation. There is no general agreement a m o n g either dry or aqueous rate expressions. Several researchers conclude that oxidation of SO 2 on soot particles, even at high relative humidity (r.h.), is irrelevant in the formation of sulfate (Judeikis et al., 1978; M a m a n e and Gottlieb, 1989). Experiments run in liquid phase give different results depending on the experimental set-up, i.e. on use of aqueous suspension, m o n o or polydispersed droplets and so on (Chang et al., 1979; Benner et al., 1982; Santachiara et al., 1989). In addition, heterogeneous oxidation of SO 2 due to N O 2 and 0 2 in the presence of carbon particles has been studied in bulk solution (Rogowski et al., 1982) and in monodisperse droplets (Santachiara et al., 1990). Results of the latter experiments showed that carbon-catalyzed SO 2 oxidation in the presence of O z and N O 2 can contribute to atmospheric sulfate production. The aim of the present experiment is to complete the previous data by adding other results at different 0 2 and N O 2 concentrations. The experimental set-up is the same as described previously.
RESULTS
AND
DISCUSSIONS
The experimental results are summarized in Fig. 1. We first consider the sulfate rate production vs 0 2 concentration in the presence of graphite nuclei ( < 2 p p m of impurity) and in the absence of N O 2 (the first three points in the vertical ordinate). Given the small droplet diameters (D ~ 4 gm), mass transfer of gases should not limit the kinetic reaction in fog droplets (Schwartz and Freiberg, 1981). Experiments run at 0 2 rates of 2 x 10-6, 2 x 10 -2 and 2 × 10-1 atm (i.e. at atmospheric concentration), which assume the reaction to be first order in both S(IV) and C, i.e. d[S(VI)]/dt = k~ [C] [S(IV)] [02(1)] ~
(1)
give~t=0.53andkl=9.6x102M-lSas l, w i t h [ C ] = 0 . 1 6 M a n d T = 2 5 ° C . C h a n g et al. (1979) found that oxidation is 0.7th order with respect to water dissolved oxygen in systems containing various concentrations of sulfurous acid and suspended carbonaceous particles. Y a m a m o t o et al. (1972), who studied the reaction kinetics on dry activated carbon in the presence of 0 2 and water vapor, obtained an oxidation rate proportional to 0 0.5. We can evaluate the contribution to atmospheric sulfate concentration using rate (1). After t = 1 h, at SO 2 = 10- 2 p p m and 0 2 = 2 x 10-1 atm, this gives [S(VI)] = 2.8 x 1 0 - , and 9.2 x 10- * M by assuming [Co] = 10- z M and 10- t M, respectively. These carbon concentrations are present in a polluted atmosphere. In fact, in Ljubljiana Bizjak et al. (1984) measured 120/~g m - 3 of particulate carbon which, in the presence of 0.1 g m - 3 of fog water,
* Also: Dipartimento di Fisica dell'Universita, Ferrara, Italy. 683
684
I~echnical Note
t0~ t
,
,4
o" r~
÷
i
J
lO11_41 It
0
I
I
I
I
0.5
1.0
1.5
2.0
NO 2 (ppm) Fig. 1. Sulfate concentration vs NO2 at various oxygen concentrations: 0 : 2 × 1 0 - t a t m ; Ill: 2× 10 -2 atm; A: 2 x 10 -1 atm; (8027g=0.65 ppm.
results in a liquid-phase carbon concentration of 10- 1 M. To evaluate the relative significance of various SO2 conversion processes in the liquid phase, we calculated the sulfate concentration produced in the presence of other catalysts, such as Mn and Fe. Given [Mn(II)] = [Fe(IlI)] = 10- 6 M and the rate expression suggested by Grgicet al. (1992), we get with SO2 = 10 -2 ppm [S(VI)] =9.9 x 10 -5 M. Using the SO 2 oxidation rate in the liquid phase proposed by Hoffman and Calvert (1985) for O3, we obtain after t = 1 h [S(VI)] =2.6E-5 M. The concentrations used in the computation are: SO2 --- 10- ~ ppm; 03 = 50 ppb. These results indicate that carbon catalysis can be an important process, especially in fog, when the concentration of soot is high, i.e. in a plume of fossil fuel-fired power plants or in a heavily polluted urban area. Sulfate concentrations can range from 2 to 30 mg 1-1 in clouds (Hegg and Hobbs, 1981; Waldman et al. 1985); 25-5000 mgl-1 in fog (Jacob et al., 1984; Fuzzi, 1986; Miller et al., 1987). Let us now turn to the other results in Fig. 1, at increasing NO2. We can see that when the O2 concentration is ~<2%, NO2 influences S(IV) oxidation if the former's concentration is higher than about 0.1 ppm. At the 02 atmospheric concentration of 2 x 10-1 atm, NO2 influences oxidation at concentrations higher than 0.3 ppm. Also evident is a higher sulfate conversion at 02 = 2% with respect to O2 = 20%, at the same SO2 concentration. A similar result was reported by Cofer et al. (1980) in experiments on carbon particles in air and nitrogen at 6 5 0 r.h. and in the presence of NO2. On the basis of the Hinshelwood-Langmuir kinetic model: this could be explained by assuming competition between NO 2 and 02 in the adsorption process on active sites and that NO2 is a more effective oxidant with respect to 02. At an 02 concentration of 2%, we observe a saturation phenomenon at NO2 I> 1 ppm. Rogowski et al. (1982) observed no saturation effect in liquid bulk experiments, whereas just such an effect was reported for SO2 on dry particles (Novakov et al., 1974; Judeikis et al., 19787. Britton and Clarke (19807, indry experiments on carbon particles, found that high NO2 levels suppress SO 2 oxidation. This could depend on a competitive oxidation mechanism producing nitrate from the reaction between NO2 and O: adsorbed on active carbon sites. The maximum nitrate concentration in our experimental results is 12 ppm when the sulfate concentration is about 800 ppm. The present experiment, even though performed with SO2 and NO 2 higher than in the free atmosphere, confirms the conclusion reached in the previous paper that carbon-catalyzed SO: oxidation in presence of NOT and 0 2 can contribute to atmospheric sulfate production, especially in fog. Acknowledoements--The cooperation of Mrs L. Santoli and Mr G. Tigli (PMP-USL 28, Sezione Chimica, Bologna 7 in performing the chemical analyses and the assistance of M. Tercon are gratefully acknowledged. This study Was supported by CNR-ENEL Project--Interactions of Energy Systems with Human Health and Environment-Rome, Italy. REFERENCES Benner, W. H., Brodzinsky, R. and Novakov, T. (19827 Atmos. Envir. 16, 1333-1339. Bizjak, M., Hudnik, V. Gomiscek, S., Hansen, A. D. A. and Novakov, T. (1984) Sci. Total Envir. 36, 377-382. Britton, L. G. and Clarke, A. G. (1980) Atmos. Envir. 14, 829-839. Calvert, J. G. and Stockweil, W, R. (1984) Acid Precipitation (Edited by Calvert, J. G.), Vol. 3. Butterworth, Boston. Chang, S. G., Brodzinsky, R., Toossi, R., Markowitz, S. S. and Novakov, T. (1979) Prec. Conference on Carbonaceous Particles in the Atmosphere. pp. 122-130. Lawrence Berkeley Laboratory, Report LBL-9037. Clarke, A. G., Williams, P. T. and Radojevic, M. (1982) Prec. o f the 2nd European Symp. on Physico-Chemical Behaviour of Atmospheric Pollutants, Varese, Italy, pp. 203-211. D. Reidel, Dordrecht. Cofer, W. R., Schryer, D. R. and Rogowski, R, S. (t980) Atmos. Envir. 14, 571-575. Fuzzi, S. (1986) Heterogeneous Atmospheric Chemistry Project (Edited by Fuzzi, S.). Istituto FISBAT CNR, Bologna, Italy. Grgic, I., Hudnik, V., Bizjak, M. and Levee, J. (19927 Atmos. Envir. 26A, 571-577. Hegg, D. A. and Hobbs, P. V. (1981) Atmos. Envir. 15, 1597-1601. Hegg, D. A. and Hobbs, P. V. (19827 Atmos. Envir. 16, 2663-2668.
Technical Note
685
Hoffman, M. R. and Calvert, J. G. (1985) Chemical Transformation for Eulerian Acid Deposition Models. Vol. 2. The Aqueous-Phase Chemistry. National Center for Atmospheric Research, Boulder, CO. Jacob, D. J., Waldman, J. M.~ Munger, J. W. and Hoffman, M. R. (1984) Tellus 36B, 272-285. Judeikis, H. S., Stewart, T. B. and Wren, A. G. (1978) Atmos. Envir. 12, 1633-1641. Mamane, Y. and Gottlieb, J. (1989) J. Aerosol Sci. 20, 575-584. Miller, D. R., Byrd, J. E. and Perona, M. J. (1987) Water, Air Soil Pollut. 32, 329-340. Novakov, T., Chang, S. G. and Harker, A. B. (1974) Tellus 186, 259-261. Rogowski, R. S., Schryer, D. R., Cofer, W. R. and Edahl, R. A. (1982) In Heterogeneous Atmospheric Chemistry (Edited by Schryer, D.R.), pp. 174-177. Geophysical Monograph 26. Santachiara, G., Prodi, F. and Vivarelli, F. (1989) Atmos. Envir. 23, 1775-1782. Santachiara, G., Prodi, F. and Vivarelli, F. (1990) J. Aerosol Sci. 21, Suppl. 1, $221-$224. Schwartz, S. E. and Freiberg, J. E. (1981) Atmos. Envir. 15, 1129-1144. Waldman, J. M., Munger, J. W., Jacob, D. J. and Hoffman, O. R. (1985) Tellus 35, 91-108. Yamamoto, K., Seki, M. and Kawazoe, K. (1972) Nippon Kagaku Kaishi 6, 1046-1052.
0021 8502/93$6.00+0.00 ~: 1993PergamonPress Ltd
TECHNICAL NOTE F U R T H E R E X P E R I M E N T S O N SO 2 O X I D A T I O N RATE IN M O N O D I S P E R S E D R O P L E T S G R O W N O N C A R B O N N U C L E I IN P R E S E N C E OF 0 2 A N D NO2 G. SANTACHIARA,F. PRODI* and F. VIVARELLI Istituto FISBAT-CNR, Bologna, Italy (Received l0 July 1992; accepted 14 October 1992)
A h s t r a c ~ T h e oxidation of SO2 in monodisperse droplets grown on carbon nuclei in presence of 0 2 and N O 2 has been studied. Both N O 2 and 0 2 were determined to be effective oxidants for SO 2. W h e n the 0 2 concentration is ~< 2%, N O 2 influences S(IV) oxidation if the former's concentration is higher than about 0.1 ppm. At the Oz atmospheric concentration, NO2 influences oxidation at concentrations higher than 0.3 ppm.
INTRODUCTION Oxidation of sulfur dioxide in the atmosphere can occur through homogeneous gas-phase reaction or as a heterogeneous process either on solid particles or in liquid droplets with traces of catalyst (Fe 3 +, M n 2 +, CI , soot, etc.). The average daily rate of gas-phase homogeneous conversion is usually considered to be 1% h - 1 and its daily m a x i m u m value is about 4 - 5 % h - 1 at midday in the ~ummer with sunny sky (Calvert and Stockwell, 1984). Heterogeneous conversion in clouds or fog drople'~ and deliquescent salts (e.g. NaCI, MgC12), gives higher conversion rates, i.e. upward of 30% h - 1 (Clarke et al., 1982; Hegg and Hobbs, 1982). If we consider carbon-catalyzed oxidation, laboratory experiments can be classified schematically in two kinds: those done with "dry" particles in a h u m i d atmosphere and those in liquid phase, i.e. using aqueous suspensions containing various concentrations of sulfurous acid or droplets growing on carbon particles and absorbing gases (SO2, NO2, 03, 02, etc.). The latter experimental set-up better approximates an atmospheric situation. There is no general agreement a m o n g either dry or aqueous rate expressions. Several researchers conclude that oxidation of SO 2 on soot particles, even at high relative humidity (r.h.), is irrelevant in the formation of sulfate (Judeikis et al., 1978; M a m a n e and Gottlieb, 1989). Experiments run in liquid phase give different results depending on the experimental set-up, i.e. on use of aqueous suspension, m o n o or polydispersed droplets and so on (Chang et al., 1979; Benner et al., 1982; Santachiara et al., 1989). In addition, heterogeneous oxidation of SO 2 due to N O 2 and 0 2 in the presence of carbon particles has been studied in bulk solution (Rogowski et al., 1982) and in monodisperse droplets (Santachiara et al., 1990). Results of the latter experiments showed that carbon-catalyzed SO 2 oxidation in the presence of O z and N O 2 can contribute to atmospheric sulfate production. The aim of the present experiment is to complete the previous data by adding other results at different 0 2 and N O 2 concentrations. The experimental set-up is the same as described previously.
RESULTS
AND
DISCUSSIONS
The experimental results are summarized in Fig. 1. We first consider the sulfate rate production vs 0 2 concentration in the presence of graphite nuclei ( < 2 p p m of impurity) and in the absence of N O 2 (the first three points in the vertical ordinate). Given the small droplet diameters (D ~ 4 gm), mass transfer of gases should not limit the kinetic reaction in fog droplets (Schwartz and Freiberg, 1981). Experiments run at 0 2 rates of 2 x 10-6, 2 x 10 -2 and 2 × 10-1 atm (i.e. at atmospheric concentration), which assume the reaction to be first order in both S(IV) and C, i.e. d[S(VI)]/dt = k~ [C] [S(IV)] [02(1)] ~
(1)
give~t=0.53andkl=9.6x102M-lSas l, w i t h [ C ] = 0 . 1 6 M a n d T = 2 5 ° C . C h a n g et al. (1979) found that oxidation is 0.7th order with respect to water dissolved oxygen in systems containing various concentrations of sulfurous acid and suspended carbonaceous particles. Y a m a m o t o et al. (1972), who studied the reaction kinetics on dry activated carbon in the presence of 0 2 and water vapor, obtained an oxidation rate proportional to 0 0.5. We can evaluate the contribution to atmospheric sulfate concentration using rate (1). After t = 1 h, at SO 2 = 10- 2 p p m and 0 2 = 2 x 10-1 atm, this gives [S(VI)] = 2.8 x 1 0 - , and 9.2 x 10- * M by assuming [Co] = 10- z M and 10- t M, respectively. These carbon concentrations are present in a polluted atmosphere. In fact, in Ljubljiana Bizjak et al. (1984) measured 120/~g m - 3 of particulate carbon which, in the presence of 0.1 g m - 3 of fog water,
* Also: Dipartimento di Fisica dell'Universita, Ferrara, Italy. 683
684
I~echnical Note
t0~ t
,
,4
o" r~
÷
i
J
lO11_41 It
0
I
I
I
I
0.5
1.0
1.5
2.0
NO 2 (ppm) Fig. 1. Sulfate concentration vs NO2 at various oxygen concentrations: 0 : 2 × 1 0 - t a t m ; Ill: 2× 10 -2 atm; A: 2 x 10 -1 atm; (8027g=0.65 ppm.
results in a liquid-phase carbon concentration of 10- 1 M. To evaluate the relative significance of various SO2 conversion processes in the liquid phase, we calculated the sulfate concentration produced in the presence of other catalysts, such as Mn and Fe. Given [Mn(II)] = [Fe(IlI)] = 10- 6 M and the rate expression suggested by Grgicet al. (1992), we get with SO2 = 10 -2 ppm [S(VI)] =9.9 x 10 -5 M. Using the SO 2 oxidation rate in the liquid phase proposed by Hoffman and Calvert (1985) for O3, we obtain after t = 1 h [S(VI)] =2.6E-5 M. The concentrations used in the computation are: SO2 --- 10- ~ ppm; 03 = 50 ppb. These results indicate that carbon catalysis can be an important process, especially in fog, when the concentration of soot is high, i.e. in a plume of fossil fuel-fired power plants or in a heavily polluted urban area. Sulfate concentrations can range from 2 to 30 mg 1-1 in clouds (Hegg and Hobbs, 1981; Waldman et al. 1985); 25-5000 mgl-1 in fog (Jacob et al., 1984; Fuzzi, 1986; Miller et al., 1987). Let us now turn to the other results in Fig. 1, at increasing NO2. We can see that when the O2 concentration is ~<2%, NO2 influences S(IV) oxidation if the former's concentration is higher than about 0.1 ppm. At the 02 atmospheric concentration of 2 x 10-1 atm, NO2 influences oxidation at concentrations higher than 0.3 ppm. Also evident is a higher sulfate conversion at 02 = 2% with respect to O2 = 20%, at the same SO2 concentration. A similar result was reported by Cofer et al. (1980) in experiments on carbon particles in air and nitrogen at 6 5 0 r.h. and in the presence of NO2. On the basis of the Hinshelwood-Langmuir kinetic model: this could be explained by assuming competition between NO 2 and 02 in the adsorption process on active sites and that NO2 is a more effective oxidant with respect to 02. At an 02 concentration of 2%, we observe a saturation phenomenon at NO2 I> 1 ppm. Rogowski et al. (1982) observed no saturation effect in liquid bulk experiments, whereas just such an effect was reported for SO2 on dry particles (Novakov et al., 1974; Judeikis et al., 19787. Britton and Clarke (19807, indry experiments on carbon particles, found that high NO2 levels suppress SO 2 oxidation. This could depend on a competitive oxidation mechanism producing nitrate from the reaction between NO2 and O: adsorbed on active carbon sites. The maximum nitrate concentration in our experimental results is 12 ppm when the sulfate concentration is about 800 ppm. The present experiment, even though performed with SO2 and NO 2 higher than in the free atmosphere, confirms the conclusion reached in the previous paper that carbon-catalyzed SO: oxidation in presence of NOT and 0 2 can contribute to atmospheric sulfate production, especially in fog. Acknowledoements--The cooperation of Mrs L. Santoli and Mr G. Tigli (PMP-USL 28, Sezione Chimica, Bologna 7 in performing the chemical analyses and the assistance of M. Tercon are gratefully acknowledged. This study Was supported by CNR-ENEL Project--Interactions of Energy Systems with Human Health and Environment-Rome, Italy. REFERENCES Benner, W. H., Brodzinsky, R. and Novakov, T. (19827 Atmos. Envir. 16, 1333-1339. Bizjak, M., Hudnik, V. Gomiscek, S., Hansen, A. D. A. and Novakov, T. (1984) Sci. Total Envir. 36, 377-382. Britton, L. G. and Clarke, A. G. (1980) Atmos. Envir. 14, 829-839. Calvert, J. G. and Stockweil, W, R. (1984) Acid Precipitation (Edited by Calvert, J. G.), Vol. 3. Butterworth, Boston. Chang, S. G., Brodzinsky, R., Toossi, R., Markowitz, S. S. and Novakov, T. (1979) Prec. Conference on Carbonaceous Particles in the Atmosphere. pp. 122-130. Lawrence Berkeley Laboratory, Report LBL-9037. Clarke, A. G., Williams, P. T. and Radojevic, M. (1982) Prec. o f the 2nd European Symp. on Physico-Chemical Behaviour of Atmospheric Pollutants, Varese, Italy, pp. 203-211. D. Reidel, Dordrecht. Cofer, W. R., Schryer, D. R. and Rogowski, R, S. (t980) Atmos. Envir. 14, 571-575. Fuzzi, S. (1986) Heterogeneous Atmospheric Chemistry Project (Edited by Fuzzi, S.). Istituto FISBAT CNR, Bologna, Italy. Grgic, I., Hudnik, V., Bizjak, M. and Levee, J. (19927 Atmos. Envir. 26A, 571-577. Hegg, D. A. and Hobbs, P. V. (1981) Atmos. Envir. 15, 1597-1601. Hegg, D. A. and Hobbs, P. V. (19827 Atmos. Envir. 16, 2663-2668.
Technical Note
685
Hoffman, M. R. and Calvert, J. G. (1985) Chemical Transformation for Eulerian Acid Deposition Models. Vol. 2. The Aqueous-Phase Chemistry. National Center for Atmospheric Research, Boulder, CO. Jacob, D. J., Waldman, J. M.~ Munger, J. W. and Hoffman, M. R. (1984) Tellus 36B, 272-285. Judeikis, H. S., Stewart, T. B. and Wren, A. G. (1978) Atmos. Envir. 12, 1633-1641. Mamane, Y. and Gottlieb, J. (1989) J. Aerosol Sci. 20, 575-584. Miller, D. R., Byrd, J. E. and Perona, M. J. (1987) Water, Air Soil Pollut. 32, 329-340. Novakov, T., Chang, S. G. and Harker, A. B. (1974) Tellus 186, 259-261. Rogowski, R. S., Schryer, D. R., Cofer, W. R. and Edahl, R. A. (1982) In Heterogeneous Atmospheric Chemistry (Edited by Schryer, D.R.), pp. 174-177. Geophysical Monograph 26. Santachiara, G., Prodi, F. and Vivarelli, F. (1989) Atmos. Envir. 23, 1775-1782. Santachiara, G., Prodi, F. and Vivarelli, F. (1990) J. Aerosol Sci. 21, Suppl. 1, $221-$224. Schwartz, S. E. and Freiberg, J. E. (1981) Atmos. Envir. 15, 1129-1144. Waldman, J. M., Munger, J. W., Jacob, D. J. and Hoffman, O. R. (1985) Tellus 35, 91-108. Yamamoto, K., Seki, M. and Kawazoe, K. (1972) Nippon Kagaku Kaishi 6, 1046-1052.