- Email: [email protected]
Influence of Gaseous Impurities on the Condensation of Water Vapor
Atmospheric Pollution 1978, Proceedings of the 13th International Colloquium, Paris, France, April 25-28,1978, M.M. Benarie (Ed.),Studiee in Environmental Science, Volume 1 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
267
INFLUENCE OF GASEOUS IMPURITIES ON THE CONDENSATION OF WATER VAPOR J . L . CMVELIN and P. MIRABEL I n s t i t u t de Chimie, Universitb Louis Pasteur, Strasbourg, France
ABSTRACT The nucleation of water i n presence of sulfuric acid and n i t r i c acid as pollutants
i s examined. I t i s concluded that only sulfuric acid can have an appreciable effect on the rate of nucleation of water under atmospheric conditions.
INTRODUCTICN Although many gaseous impurities are present i n the atmosphere, only those which exhibit large free energy of mixing with water (such as H2S04, HN03) may mix with water vapor and undergo binary (or heteranolecular) nucleation t o form binary solution droplets. Sulfuric acid formed by oxidation of SO2 i n the atmosphere is of special interest since it is known t o be a t the origin of a large part of sulfate aerosols. Although the detailed mechanism of SO2 oxidation remains unclear, the numerous results of smog chamber simulations leave l i t t l e doubt t h a t photochemistry plays an important role i n the formation of sulfuric acid and sulfate s a l t s . Sulfuric acid aerosol formation can be summarized as follows. 1. Oxidation of S 2 t o SO3 i n the gas phase followed by hydration of SO3 t o form gaseous sulfuric acid molecules. 2. Binary (or heteromolecular) nucleation of sulfuric acid and water vapor molecules t o form embryonic solution droplets. 3. Growth of the droplets by heteromolecular condensation and thermal coagulation. Based upon t h i s three step mechanism, several numerical kinetic models have been recently developped t o study the tine evolution of the number concentration, size distribution, e t c . . o f the newly formed liquid aerosols as a function of So2
.
concentration, W intensity, and other relevant atmospheric conditions. Many experimental efforts have been made t o verify these kinetic models using smog chambers. As these experiments generally measure the overall effect of this three step mechanism, it is d i f f i c u l t t o ascribe precisely t o one of these steps any disagremnt between the predictions of the kinetic model and the experimental results. I t seems thus desirable t o study each step independantly, i n particular
268
binary homogeneous nucleation, as it appears t o be the most crucial step. Although several theoretical predictions are available f o r the nucleation rates i n the sulfuric acid - water and nitric acid - water systems, there are only two experinaental studies of which we are aware, studies which allow only semi quantitative comparisons with theory [ 1 , 2 ] . Presented here are the measurements on the s u l f u r i c acid - water and n i t r i c acid - water systems obtained using an upward thermal diffusion cloud chamber. This chamber, which has been used extensively t o study uniary [3,41 as well as binary [ 51 homogeneous nucleation, possesses many advantages inherent i n i t s design and allows experimental determination around 2 5 O C i.e. the temperature where most of the thermodynamic data needed f o r the theory are available. EXPERIMENTAL Since a detailed description of the design and operating procedure of the chamber
has already been published [ 3 1, only the modifications necessary f o r the study of these two highly corrosive mixtures w i l l be given here.
The plates used were made out of copper. A l l parts of them which might come i n contact with the liquid were covered with a t h i n layer (0.2mm) of an enamel specially prepared f o r this study by the " I n s t i t u t de Mineralogie, Universite Louis Pasteur". The calibrated sensors f o r measuring the temperature of the evaporating pool are very small thermistors (diameter 0.5m) which have been sealed into a piece of glass tubing. Each thermistor passes through a conical rubber plug inserted i n t o holes d r i l l e d i n the b o t t m p l a t e and compressed by a screw device. The L shaped xubber gaskets used as s e a l s between the plates and the glass ring as w e l l as the rubber plugs were made specially for t h i s study by "Le Joint Franqais". They w i l l r e s i s t any s u l f u r i c or n i t r i c acid solution. Only pure and warm n i t r i c acid w i l l discolor and s l i g h t l y attack t h i s rubber. A special limitation a r i s e s with the s u l f u r i c acid
- water mixture which does
not allow measurements f o r water a c t i v i t i e s i n the gas phase below unity (or r e l a t i v e humidities l e s s than 100%). D u e t o the very l m vapor pressure of HZS04 compared t o t h a t of water, the liquid film on the top p l a t e i s nearly pure water. For an i n i t i a l acid concentration i n excess of X = 0.20 (58% by weight) and for the temperature needed t o achieve nucleation, i.e. approximately 78'C f o r the lower p l a t e and 1Ooc f o r the upper p l a t e , the p a r t i a l pressure of water on the top p l a t e becornes greater than t h a t on the bottom p l a t e leading t o chamber instability. This concentration (X = 0.20) corresponds t o a maximum r e l a t i v e humidity of 108%.
269
RESULTS Results for the n i t r i c acid - water system with helium as the carrier gas are shown i n Fig. 1 . Each c i r c l e represents the experimental a c t i v i t i e s of each compound needed t o obtain a r a t e of nucleation of 2 - 3 nuclei un-3 sec-l. The results are given f o r two temperatures : 298.2 K (upper s e t of data) and 278.2 K (lower set of data). These data are cmpared with the corresponding predictions of the theory of binary homogeneous nucleation for a rate of 1 and -3 -1 100 nuclei cm sec , The predicted rates were calculated from equation (1): J
=
Cexp(-AG/kT)
where AG i s the free energy required t o form a c r i t i c a l nucleus and C i s a slowly varying frequency factor. (For detailed calculations of C and A G , see Reiss [ 6 1, Mirabel and Katz [ 71, Mirabel and Clavelin [ 8 ) ) .
i
298.2 K
I
5
1.0
15 2.0 23 WATER ACTIVITY
30
!
1
33
Fig. 1. Comparison of experiment and theory (J = 1 , solid line; J = 100, dashdotted line) f o r the homogeneous nucleation of the mixture HN03 - HZO.
As can be seen from Fig. 1 , the agreement between theory and experiment i s very g o d , especially f o r water a c t i v i t i e s between 0 and 2.5. Under atmospheric -2 conditions, n i t r i c acid w i l l nucleate water a t a c t i v i t i e s i n the range 10 t o 1. This corresponds t o p a r t i a l pressures of HN03 above 1 t o r r , pressure much too high t o be found i n the atmosphere. Experimental r e s u l t s for the sulfuric acid - water system w i t h helium or hydrogen a s a c a r r i e r gas are shown in Fig. 2 and are compared with theory. Two of the upper curves were calculated from equation (1) but, taking into
270
account hydrate formation i n the gas phase (see reference 9 ), and for a r a t e of nucleation J = 1 (solid line) and J = 100 (dotted line). The third upper l i n e was determined from the theory of S H U W e t al. [ 91, equally taking i n t o account hydrate formation, and for a r a t e of nucleation J = 1 (dashed line). The l m e r two curves were determined without taking i n t o account hydrate formation, and are given for a r a t e of nucleation of J = 1 (dash - dotted line) and J = 100 (dash - dash - dotted line).
298.2 K
-8 10
-
0.5
1.0
1.5 2.0 2.5 WATER ACTIVITY
3.0
33
Fig. 2. Comparison of experiment and theory for the homogeneous nucleation of the mixture H2S04 - HZO. As can be seen from F i g . 2 , agreement between theory and experiment i s very good i f one considers hydrate formation, while there are no water a c t i v i t i e s for which our results verify the “non hydrated” theory. Our experimental method does not allow measurements f o r r e l a t i v e humidities lower than 1008, but the r e s u l t s can be e a s i l y extrapolated t o atmospheric conditions. Under these conditions, s u l f u r i c acid w i l l nucleate water a t a c t i v i t i e s i n the range lo-’ t o i.e. a t -6 p a r t i a l pressures i n the range 3 10 t o 3 t o r r corresponding roughly t o concentrations of 4 lom4 t o 4 lov3 ppm f o r one atmosphere t o t a l pressure. Such concentrations occur frequently near heavily populated area.
CONCLUSION Among the mo pollutants examined here, only sulfuric acid w i l l have an appreciable e f f e c t on the nucleation of water under atmospheric conditions. O t h e r pollutants having a weaker f r e e energy of mixing with water (such as W03, SOz, NH3) w i l l have no e f f e c t on t h i s nucleation.
271
REFERENCES
1 H. Reiss, D . I . Margolese and F.J. Schelling, J. Colloid Interface Sci., 56
(1976) 511 - 526. 2 D. Boulaud, G. Madelaine and D. Vigla, J . Chen. Phys., 66 (1977) 4854 -4860. 3 J . L . Katz, C . J . Scoppa, N.G. Kumar and P. Mirabel, J. Chm. Phys., 62 (1975) 448 465. 4 J . L . Katz, P. Mirabel, C . J . Scoppa and T.L. Virkler, J. Chm. Phys., 65 (1976) 382 392. 1704. 5 P. Mirabel and J . L . Katz, J. Chan. Phys., 67 (1977) 1697 6 H. Reiss, J. Chan. Phys., 18 (1950) 840 - 848, 7 P. Mirabel and J . L . Katz, J . Chem. Phys., 60 (1974) 1138 - 1144. 8 P. Mirabel and J.L. Clavelin, J. Aerosol Sci., i n press. 9 W.J. Shugard, R.H. Heist and H. Reiss, J. Chen. Phys., 61 (1974) 5298 5305.
-
-
-
-
267
INFLUENCE OF GASEOUS IMPURITIES ON THE CONDENSATION OF WATER VAPOR J . L . CMVELIN and P. MIRABEL I n s t i t u t de Chimie, Universitb Louis Pasteur, Strasbourg, France
ABSTRACT The nucleation of water i n presence of sulfuric acid and n i t r i c acid as pollutants
i s examined. I t i s concluded that only sulfuric acid can have an appreciable effect on the rate of nucleation of water under atmospheric conditions.
INTRODUCTICN Although many gaseous impurities are present i n the atmosphere, only those which exhibit large free energy of mixing with water (such as H2S04, HN03) may mix with water vapor and undergo binary (or heteranolecular) nucleation t o form binary solution droplets. Sulfuric acid formed by oxidation of SO2 i n the atmosphere is of special interest since it is known t o be a t the origin of a large part of sulfate aerosols. Although the detailed mechanism of SO2 oxidation remains unclear, the numerous results of smog chamber simulations leave l i t t l e doubt t h a t photochemistry plays an important role i n the formation of sulfuric acid and sulfate s a l t s . Sulfuric acid aerosol formation can be summarized as follows. 1. Oxidation of S 2 t o SO3 i n the gas phase followed by hydration of SO3 t o form gaseous sulfuric acid molecules. 2. Binary (or heteromolecular) nucleation of sulfuric acid and water vapor molecules t o form embryonic solution droplets. 3. Growth of the droplets by heteromolecular condensation and thermal coagulation. Based upon t h i s three step mechanism, several numerical kinetic models have been recently developped t o study the tine evolution of the number concentration, size distribution, e t c . . o f the newly formed liquid aerosols as a function of So2
.
concentration, W intensity, and other relevant atmospheric conditions. Many experimental efforts have been made t o verify these kinetic models using smog chambers. As these experiments generally measure the overall effect of this three step mechanism, it is d i f f i c u l t t o ascribe precisely t o one of these steps any disagremnt between the predictions of the kinetic model and the experimental results. I t seems thus desirable t o study each step independantly, i n particular
268
binary homogeneous nucleation, as it appears t o be the most crucial step. Although several theoretical predictions are available f o r the nucleation rates i n the sulfuric acid - water and nitric acid - water systems, there are only two experinaental studies of which we are aware, studies which allow only semi quantitative comparisons with theory [ 1 , 2 ] . Presented here are the measurements on the s u l f u r i c acid - water and n i t r i c acid - water systems obtained using an upward thermal diffusion cloud chamber. This chamber, which has been used extensively t o study uniary [3,41 as well as binary [ 51 homogeneous nucleation, possesses many advantages inherent i n i t s design and allows experimental determination around 2 5 O C i.e. the temperature where most of the thermodynamic data needed f o r the theory are available. EXPERIMENTAL Since a detailed description of the design and operating procedure of the chamber
has already been published [ 3 1, only the modifications necessary f o r the study of these two highly corrosive mixtures w i l l be given here.
The plates used were made out of copper. A l l parts of them which might come i n contact with the liquid were covered with a t h i n layer (0.2mm) of an enamel specially prepared f o r this study by the " I n s t i t u t de Mineralogie, Universite Louis Pasteur". The calibrated sensors f o r measuring the temperature of the evaporating pool are very small thermistors (diameter 0.5m) which have been sealed into a piece of glass tubing. Each thermistor passes through a conical rubber plug inserted i n t o holes d r i l l e d i n the b o t t m p l a t e and compressed by a screw device. The L shaped xubber gaskets used as s e a l s between the plates and the glass ring as w e l l as the rubber plugs were made specially for t h i s study by "Le Joint Franqais". They w i l l r e s i s t any s u l f u r i c or n i t r i c acid solution. Only pure and warm n i t r i c acid w i l l discolor and s l i g h t l y attack t h i s rubber. A special limitation a r i s e s with the s u l f u r i c acid
- water mixture which does
not allow measurements f o r water a c t i v i t i e s i n the gas phase below unity (or r e l a t i v e humidities l e s s than 100%). D u e t o the very l m vapor pressure of HZS04 compared t o t h a t of water, the liquid film on the top p l a t e i s nearly pure water. For an i n i t i a l acid concentration i n excess of X = 0.20 (58% by weight) and for the temperature needed t o achieve nucleation, i.e. approximately 78'C f o r the lower p l a t e and 1Ooc f o r the upper p l a t e , the p a r t i a l pressure of water on the top p l a t e becornes greater than t h a t on the bottom p l a t e leading t o chamber instability. This concentration (X = 0.20) corresponds t o a maximum r e l a t i v e humidity of 108%.
269
RESULTS Results for the n i t r i c acid - water system with helium as the carrier gas are shown i n Fig. 1 . Each c i r c l e represents the experimental a c t i v i t i e s of each compound needed t o obtain a r a t e of nucleation of 2 - 3 nuclei un-3 sec-l. The results are given f o r two temperatures : 298.2 K (upper s e t of data) and 278.2 K (lower set of data). These data are cmpared with the corresponding predictions of the theory of binary homogeneous nucleation for a rate of 1 and -3 -1 100 nuclei cm sec , The predicted rates were calculated from equation (1): J
=
Cexp(-AG/kT)
where AG i s the free energy required t o form a c r i t i c a l nucleus and C i s a slowly varying frequency factor. (For detailed calculations of C and A G , see Reiss [ 6 1, Mirabel and Katz [ 71, Mirabel and Clavelin [ 8 ) ) .
i
298.2 K
I
5
1.0
15 2.0 23 WATER ACTIVITY
30
!
1
33
Fig. 1. Comparison of experiment and theory (J = 1 , solid line; J = 100, dashdotted line) f o r the homogeneous nucleation of the mixture HN03 - HZO.
As can be seen from Fig. 1 , the agreement between theory and experiment i s very g o d , especially f o r water a c t i v i t i e s between 0 and 2.5. Under atmospheric -2 conditions, n i t r i c acid w i l l nucleate water a t a c t i v i t i e s i n the range 10 t o 1. This corresponds t o p a r t i a l pressures of HN03 above 1 t o r r , pressure much too high t o be found i n the atmosphere. Experimental r e s u l t s for the sulfuric acid - water system w i t h helium or hydrogen a s a c a r r i e r gas are shown in Fig. 2 and are compared with theory. Two of the upper curves were calculated from equation (1) but, taking into
270
account hydrate formation i n the gas phase (see reference 9 ), and for a r a t e of nucleation J = 1 (solid line) and J = 100 (dotted line). The third upper l i n e was determined from the theory of S H U W e t al. [ 91, equally taking i n t o account hydrate formation, and for a r a t e of nucleation J = 1 (dashed line). The l m e r two curves were determined without taking i n t o account hydrate formation, and are given for a r a t e of nucleation of J = 1 (dash - dotted line) and J = 100 (dash - dash - dotted line).
298.2 K
-8 10
-
0.5
1.0
1.5 2.0 2.5 WATER ACTIVITY
3.0
33
Fig. 2. Comparison of experiment and theory for the homogeneous nucleation of the mixture H2S04 - HZO. As can be seen from F i g . 2 , agreement between theory and experiment i s very good i f one considers hydrate formation, while there are no water a c t i v i t i e s for which our results verify the “non hydrated” theory. Our experimental method does not allow measurements f o r r e l a t i v e humidities lower than 1008, but the r e s u l t s can be e a s i l y extrapolated t o atmospheric conditions. Under these conditions, s u l f u r i c acid w i l l nucleate water a t a c t i v i t i e s i n the range lo-’ t o i.e. a t -6 p a r t i a l pressures i n the range 3 10 t o 3 t o r r corresponding roughly t o concentrations of 4 lom4 t o 4 lov3 ppm f o r one atmosphere t o t a l pressure. Such concentrations occur frequently near heavily populated area.
CONCLUSION Among the mo pollutants examined here, only sulfuric acid w i l l have an appreciable e f f e c t on the nucleation of water under atmospheric conditions. O t h e r pollutants having a weaker f r e e energy of mixing with water (such as W03, SOz, NH3) w i l l have no e f f e c t on t h i s nucleation.
271
REFERENCES
1 H. Reiss, D . I . Margolese and F.J. Schelling, J. Colloid Interface Sci., 56
(1976) 511 - 526. 2 D. Boulaud, G. Madelaine and D. Vigla, J . Chen. Phys., 66 (1977) 4854 -4860. 3 J . L . Katz, C . J . Scoppa, N.G. Kumar and P. Mirabel, J. Chm. Phys., 62 (1975) 448 465. 4 J . L . Katz, P. Mirabel, C . J . Scoppa and T.L. Virkler, J. Chm. Phys., 65 (1976) 382 392. 1704. 5 P. Mirabel and J . L . Katz, J. Chan. Phys., 67 (1977) 1697 6 H. Reiss, J. Chan. Phys., 18 (1950) 840 - 848, 7 P. Mirabel and J . L . Katz, J . Chem. Phys., 60 (1974) 1138 - 1144. 8 P. Mirabel and J.L. Clavelin, J. Aerosol Sci., i n press. 9 W.J. Shugard, R.H. Heist and H. Reiss, J. Chen. Phys., 61 (1974) 5298 5305.
-
-
-
-