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Specific determination of aerosol sulfuric acid in the presence of ammonium sulfate. A laboratory study
Discussions
1604
including several pollution sources. It would be necessary to program the method not only for punctual but for linear and superficial sources ofpollutants. One would then have to take into account the interaction of the pollutants, thus to use a more generai system of equations (as mentioned earlier). There are clearly other details which will have to be considered such as time delays, absorption, adsorption and chemical destruction of pollutants. Some of these would be considered as negative sources or sinks. One may hope that more of this kind of research will be undertaken in the future to allow more deterministic models to be established and to be used for the study of pollution control and prevention. Department of
~othemotic.~, Portland State Univ., Porr~and, OR 97207, U.S.A.
S. TAIJBER
REFERENCE
S. Tauber and J. Trau (1973) A vector partial differential equation model for air pollution. Atmospheric Environment 7.973-977.
AUTHORS REPLY We find thecomments by S. Tauber interesting. It was not our intent to give a thorough discourse on the definitions and properties of spline and chapeau functions. Comprehensive definitions of the two schemes have been discussed in between the two methods, as well as their striking similarities. The approximate modeling of diffusion is not difficult with either method ; diRtsion is not as troublesome as advection.
SPECIFIC DETERMINATION OF AEROSOL SULFURIC ACID IN THE PRESENCE OF AMMONIUM SULFATE. A LABORATORY STUDY* The authors have developed a commendable laboratory procedure for measuring sulfuric acid in the presence of ammonium sulfate. However, the casual reader should be aware, and perhaps the title and text should reflect more realisticallv. that NH,HSO, is undistineuishable from H,S04 by the described sample treatmen; Not only will NH,HSO, droplets probably remain liquid at humidities above 40”/, r.h. (or even lower, due to hysteresis effects) and be measured as H,SG,, but mixed aerosols of H,SO, and NH,HS04 can exist as heterogeneo~ mixtures of solid NH,HSO* within droplets containing both NH,HSO, and H2SOI down to very low humidities (Tang, 1976), clearly interfering with the reported method. Further complications may arise from the formation of the mixed NH,HSOI-(NH,)2S0, salt, letovicite. Unfortunately, the available equipment was unable to explore the possibility and consequence of on-filter topochemical reactions. In fact, one would like to have seen a mass balance experiment done to judge whether H,SO, was being neutralized to Na,SOI by portions of the glass fiber filter uncovered by the PDA-Br impregnant. This Na$O, would not, of
+ Dasgupta P. K., Lindquist G. L. and West P. W. (1979) Atmospheric
Environment
13,161-174.
considerably more depth and clarity in the cited references. We were particularly interested in the subtle differences Modeling of chemical reactions is difficult; we did not consider it feasible to begin such modeling attempts without first analyzing advection and diffusion. We agree that further analysis and modeling with both schemes, ~rticularly adaptable to the general non-linear concentration equation established by Tauber and Trau (1973), would be interesting, providing subgrid resolution of the concentration is not required. The utility of the two methods lie in their simplicity, speed, and higher-order accuracy than in more conventional finite difference schemes. However, they similarly suffer some of the problems of the simpler schemes, namely - the inability to resolve steep gradients (point sources) accurately without some numerical dispersion errors, manifested, for example, by unrealistic group velocities. Savannah
River Laboratory,
DARRELLW.
PEPPEH
Aiken, SC, U.S.A. CLIFFORDD. KERN
Lockheed Missile and Space Company. Sunnyvale, CA, U.S.A.
P. E.
Techniques Development Loboratory, NCIAA, Silver Spring, MD. U.S.A.
LONG, JR.
REFERENCES
Tauber S. and Trau J. (1973) Vector partial differential equation model for air pollution. Atmospheric Enoironmenf 7,973-977.
course, be analyzed by the “standard” pyrolysis-TSM method used for comparison. One would also like to have seen the presentation of a working device for atmospheric sampling, field results on ambient aerosols with accompanying precision data, recovery data for H,SO, in the presence of ambient particulates, and possibly comparisons of H,SOI atmospheric concentrations as determined by the present procedure and by other techniques. In view of the fact that even one of the authors (Dasgupta, 1979) has recently stated that analytical specificity for sulfuric acid can be achieved only when it is present alone or in external mixtures with NH,HSO* or (NH&SO, in the atmosphere (evidence for which is perilously slim), we agree that further refinement of the described procedure is certainly warranted. Br~k~ven rations ~~rotory, Deparrment of Energy and Environment. Lipton. NY 11973, U.S.A.
JOSEPHFORREST ROGER L. TANNER
AUTHORS REPLY We appreciate the comments of Drs Forrest and Tanner on our paper. Whileit isentirely true that this method will detect an internal mixture of NHIHSOL and H,S04 which is capable ofexisting as a liquid aerosol even at a very low r.h. as H,SO+ we can hardly agree that the evidence for external mixtures is slim. In reality, no definitive data exist to determine whether ambient aerosols are predominantly external or internal mixtures. Virtually all microscopic
160.5
Discussion techniques of analysis of atmospheric particles rely on the assumption that individual atmospheric aerosol particles are ~hemi~lly pure species, and such techniques have been applied successfully. Several other analytical methods that rely on the growth characteristics of an individual particle with increasing r.h. also rely on this implicit assumption. Our present status of knowledge of atmospheric chemistry does not even allow one to reach a definitive conclusion if the bulk of the ammonium sulfates found in the atmosphere result from the partial or complete neutralization of H2SGI initially formed or if they result from the oxidation of dissolved SO, in a homogeneous aqueous phase system also confining NH,, a mechanism originally proposed by van den Heuvel and Mason (1963) and later extended by others(Scott and Hobbs, 1967; Healy et al., 1970 ; McKay, 1971; Friend, 1974). If the latter conclusion is closer to the truth, one has to conclude further that free sulfuric acid in the atmosphere must originate via an entirely different mechanism, in which case external rather than internal mixtures will be the mode rather than the exception. Laboratory-generated sulfuric acid aerosol is neutralized by ammonia as fast as the diffusion process permits the reaction to occur. Yet, free H,SO, has been shown tocoexist with free NH, in the atmosphereiJunge and Scheich, 1969). While the general trend is to rationalize this observation by invoking inadequate mixing of the air mass, a conscientiousinvestigator must admit that it raises a doubt as to whether laboratory-generated aerosol H,SO, is at all representativeof itscounterpart found in theatmosphere. Are atmospheric acid particles protected from ueutrali~tion by a thin, unreactive film as suggested by Junge and Scheich? Indeed, the intimate relationship between atmospheric acidity and combustion of fossil fuels makes this question too important to be summarily dismissed. Richards et ul. (1978) have reported the existence of a sulfur com~und in the Los Angeles atmosphere; this species behaves exactly the same as sulfuric acid in its volatilization profile by the low temperature volatilization technique, and reportedly it is not neutralized by NH,, while wet chemical teats indicate the presence of sulfate only. Marlow and Tanner f19?6), in their elegant work on size-related chemical speciation utilizing the diffusion battery noted that in at least one case the fine particle fraction was pure sulfuric acid aerosol, although the gross composition of the aerosol mass was more basic than ammonium bisulfate. In regard to the possible formation of Na2S04 on the PDA-Br impregnated glass fiber filters when they are used to sample H$O, aerosol, it has been shown previously (Thomas et al., 1976) that no Na$O, is formed and the recovery of H,SO, is quantitative. Also, the main attractiveness of this method is that the collected H,SO, is immobilized at the moment of impaction and thus on-filter reactions are minimized. It is impossible for us to tell a priori how much of a heterogeneous particle (containing solid and a solution phase) will react with these filters. We hope to publish the results obtained by our method in comparison to other t~hniqu~ as applied to actual ambient air studies at a future date. However, recent governmental decision that ambient H,SO, concentrations do not pose a health hazard has not facilitated our efforts in this area. In conclusion, we do not claim our method of determining sulfuric acid to be the ultimate and ideal. On the other hand.
we do not know of a better or even comparable method that does not involve sophisticated and expensive instrumentation. At the worst, even if ambient aerosols are proven at a later date to be largely internal mixtures, this procedure should still remain attractive as a sensitive and inexpensive method for plume studies, for which it was originally intended. Calfirnia Primate Research Center, Universify of Cali$ornia, Davis, Duois. CA 95616, U.S.A. 341, R & D Laboratory, Ethyl Corporution, Baron Rouge. LA 70821,
P. K. DASCUPTA
G. L. Lu~a~ursr
Box
U.S.A.
Department of Chemistry. Environmental Sciences Institute, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
P. w. WEST
REFERENCES
Dasgupta P. K. (1979) Personal communication. Friend J. p. (1974) Formation of atmospheric aerosols bv aas to particle conversions. AIChE Symi. Ser. 70, 267-270. Healy T. V., McKay H. A. C., Pi&earn A, and Scargill D, (1970) Ammonia and ammonium sulfate in the trowsphere over the United Kingdom. J. geophys. Res.’ 75, 2317-2321. Junge C. E. and Scheich G. (1969) Studien zur bestimmung des sauregehaltes von aerosolteilchen. Atmospheric Environment 3,423-44 1. Marlow W. H. and Tanner R. L. (1976) Di~us~on ~mpiing method for ambient aerosol size discrimination with chemical composition determination. Analyt. Chem. 48, 1PP9-2001. McKay H. A. C. ( 1971)The atmospheric oxidation of sulphur dioxide in water dropiets in the presence of ammonia, Atmospheric Environment 5, 7-14. Richards L. W., Johnson K. R. and Shepard L. S. (1978) Sulfate Aerosol study. Report No. AM~8~.13FR, Rockwell Intemationat, Atom& International Divisiou, Air Monitoring Center, Newbury Park, CA. Scott W. D. and Hobbs P. V. (1967) The formation of sulfate in water droplets. J. atmos. Sci. 24, 54-57. Tang I. N. (1976) Phase transformation and growthofaerosol particles composed of mixed salts. J. aerosol Sci. 7, 361-371. Thomas R. L., Dharmarajan V., Lundquist G. L. and West P. W. Measurement of sulfuric acid aerosol, sulfur trioxide and the total sulfate content of ambient air. Andyt. Chem. 48,639~641.
Van den Heuvel A. P. and Mason B. J. (1963) The formation of ammonium sulphate in water droplets exposed to gaseous sulphur dioxide and ammonia. Q. Jl R. Met. Sot. llP, 271-215.
1604
including several pollution sources. It would be necessary to program the method not only for punctual but for linear and superficial sources ofpollutants. One would then have to take into account the interaction of the pollutants, thus to use a more generai system of equations (as mentioned earlier). There are clearly other details which will have to be considered such as time delays, absorption, adsorption and chemical destruction of pollutants. Some of these would be considered as negative sources or sinks. One may hope that more of this kind of research will be undertaken in the future to allow more deterministic models to be established and to be used for the study of pollution control and prevention. Department of
~othemotic.~, Portland State Univ., Porr~and, OR 97207, U.S.A.
S. TAIJBER
REFERENCE
S. Tauber and J. Trau (1973) A vector partial differential equation model for air pollution. Atmospheric Environment 7.973-977.
AUTHORS REPLY We find thecomments by S. Tauber interesting. It was not our intent to give a thorough discourse on the definitions and properties of spline and chapeau functions. Comprehensive definitions of the two schemes have been discussed in between the two methods, as well as their striking similarities. The approximate modeling of diffusion is not difficult with either method ; diRtsion is not as troublesome as advection.
SPECIFIC DETERMINATION OF AEROSOL SULFURIC ACID IN THE PRESENCE OF AMMONIUM SULFATE. A LABORATORY STUDY* The authors have developed a commendable laboratory procedure for measuring sulfuric acid in the presence of ammonium sulfate. However, the casual reader should be aware, and perhaps the title and text should reflect more realisticallv. that NH,HSO, is undistineuishable from H,S04 by the described sample treatmen; Not only will NH,HSO, droplets probably remain liquid at humidities above 40”/, r.h. (or even lower, due to hysteresis effects) and be measured as H,SG,, but mixed aerosols of H,SO, and NH,HS04 can exist as heterogeneo~ mixtures of solid NH,HSO* within droplets containing both NH,HSO, and H2SOI down to very low humidities (Tang, 1976), clearly interfering with the reported method. Further complications may arise from the formation of the mixed NH,HSOI-(NH,)2S0, salt, letovicite. Unfortunately, the available equipment was unable to explore the possibility and consequence of on-filter topochemical reactions. In fact, one would like to have seen a mass balance experiment done to judge whether H,SO, was being neutralized to Na,SOI by portions of the glass fiber filter uncovered by the PDA-Br impregnant. This Na$O, would not, of
+ Dasgupta P. K., Lindquist G. L. and West P. W. (1979) Atmospheric
Environment
13,161-174.
considerably more depth and clarity in the cited references. We were particularly interested in the subtle differences Modeling of chemical reactions is difficult; we did not consider it feasible to begin such modeling attempts without first analyzing advection and diffusion. We agree that further analysis and modeling with both schemes, ~rticularly adaptable to the general non-linear concentration equation established by Tauber and Trau (1973), would be interesting, providing subgrid resolution of the concentration is not required. The utility of the two methods lie in their simplicity, speed, and higher-order accuracy than in more conventional finite difference schemes. However, they similarly suffer some of the problems of the simpler schemes, namely - the inability to resolve steep gradients (point sources) accurately without some numerical dispersion errors, manifested, for example, by unrealistic group velocities. Savannah
River Laboratory,
DARRELLW.
PEPPEH
Aiken, SC, U.S.A. CLIFFORDD. KERN
Lockheed Missile and Space Company. Sunnyvale, CA, U.S.A.
P. E.
Techniques Development Loboratory, NCIAA, Silver Spring, MD. U.S.A.
LONG, JR.
REFERENCES
Tauber S. and Trau J. (1973) Vector partial differential equation model for air pollution. Atmospheric Enoironmenf 7,973-977.
course, be analyzed by the “standard” pyrolysis-TSM method used for comparison. One would also like to have seen the presentation of a working device for atmospheric sampling, field results on ambient aerosols with accompanying precision data, recovery data for H,SO, in the presence of ambient particulates, and possibly comparisons of H,SOI atmospheric concentrations as determined by the present procedure and by other techniques. In view of the fact that even one of the authors (Dasgupta, 1979) has recently stated that analytical specificity for sulfuric acid can be achieved only when it is present alone or in external mixtures with NH,HSO* or (NH&SO, in the atmosphere (evidence for which is perilously slim), we agree that further refinement of the described procedure is certainly warranted. Br~k~ven rations ~~rotory, Deparrment of Energy and Environment. Lipton. NY 11973, U.S.A.
JOSEPHFORREST ROGER L. TANNER
AUTHORS REPLY We appreciate the comments of Drs Forrest and Tanner on our paper. Whileit isentirely true that this method will detect an internal mixture of NHIHSOL and H,S04 which is capable ofexisting as a liquid aerosol even at a very low r.h. as H,SO+ we can hardly agree that the evidence for external mixtures is slim. In reality, no definitive data exist to determine whether ambient aerosols are predominantly external or internal mixtures. Virtually all microscopic
160.5
Discussion techniques of analysis of atmospheric particles rely on the assumption that individual atmospheric aerosol particles are ~hemi~lly pure species, and such techniques have been applied successfully. Several other analytical methods that rely on the growth characteristics of an individual particle with increasing r.h. also rely on this implicit assumption. Our present status of knowledge of atmospheric chemistry does not even allow one to reach a definitive conclusion if the bulk of the ammonium sulfates found in the atmosphere result from the partial or complete neutralization of H2SGI initially formed or if they result from the oxidation of dissolved SO, in a homogeneous aqueous phase system also confining NH,, a mechanism originally proposed by van den Heuvel and Mason (1963) and later extended by others(Scott and Hobbs, 1967; Healy et al., 1970 ; McKay, 1971; Friend, 1974). If the latter conclusion is closer to the truth, one has to conclude further that free sulfuric acid in the atmosphere must originate via an entirely different mechanism, in which case external rather than internal mixtures will be the mode rather than the exception. Laboratory-generated sulfuric acid aerosol is neutralized by ammonia as fast as the diffusion process permits the reaction to occur. Yet, free H,SO, has been shown tocoexist with free NH, in the atmosphereiJunge and Scheich, 1969). While the general trend is to rationalize this observation by invoking inadequate mixing of the air mass, a conscientiousinvestigator must admit that it raises a doubt as to whether laboratory-generated aerosol H,SO, is at all representativeof itscounterpart found in theatmosphere. Are atmospheric acid particles protected from ueutrali~tion by a thin, unreactive film as suggested by Junge and Scheich? Indeed, the intimate relationship between atmospheric acidity and combustion of fossil fuels makes this question too important to be summarily dismissed. Richards et ul. (1978) have reported the existence of a sulfur com~und in the Los Angeles atmosphere; this species behaves exactly the same as sulfuric acid in its volatilization profile by the low temperature volatilization technique, and reportedly it is not neutralized by NH,, while wet chemical teats indicate the presence of sulfate only. Marlow and Tanner f19?6), in their elegant work on size-related chemical speciation utilizing the diffusion battery noted that in at least one case the fine particle fraction was pure sulfuric acid aerosol, although the gross composition of the aerosol mass was more basic than ammonium bisulfate. In regard to the possible formation of Na2S04 on the PDA-Br impregnated glass fiber filters when they are used to sample H$O, aerosol, it has been shown previously (Thomas et al., 1976) that no Na$O, is formed and the recovery of H,SO, is quantitative. Also, the main attractiveness of this method is that the collected H,SO, is immobilized at the moment of impaction and thus on-filter reactions are minimized. It is impossible for us to tell a priori how much of a heterogeneous particle (containing solid and a solution phase) will react with these filters. We hope to publish the results obtained by our method in comparison to other t~hniqu~ as applied to actual ambient air studies at a future date. However, recent governmental decision that ambient H,SO, concentrations do not pose a health hazard has not facilitated our efforts in this area. In conclusion, we do not claim our method of determining sulfuric acid to be the ultimate and ideal. On the other hand.
we do not know of a better or even comparable method that does not involve sophisticated and expensive instrumentation. At the worst, even if ambient aerosols are proven at a later date to be largely internal mixtures, this procedure should still remain attractive as a sensitive and inexpensive method for plume studies, for which it was originally intended. Calfirnia Primate Research Center, Universify of Cali$ornia, Davis, Duois. CA 95616, U.S.A. 341, R & D Laboratory, Ethyl Corporution, Baron Rouge. LA 70821,
P. K. DASCUPTA
G. L. Lu~a~ursr
Box
U.S.A.
Department of Chemistry. Environmental Sciences Institute, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
P. w. WEST
REFERENCES
Dasgupta P. K. (1979) Personal communication. Friend J. p. (1974) Formation of atmospheric aerosols bv aas to particle conversions. AIChE Symi. Ser. 70, 267-270. Healy T. V., McKay H. A. C., Pi&earn A, and Scargill D, (1970) Ammonia and ammonium sulfate in the trowsphere over the United Kingdom. J. geophys. Res.’ 75, 2317-2321. Junge C. E. and Scheich G. (1969) Studien zur bestimmung des sauregehaltes von aerosolteilchen. Atmospheric Environment 3,423-44 1. Marlow W. H. and Tanner R. L. (1976) Di~us~on ~mpiing method for ambient aerosol size discrimination with chemical composition determination. Analyt. Chem. 48, 1PP9-2001. McKay H. A. C. ( 1971)The atmospheric oxidation of sulphur dioxide in water dropiets in the presence of ammonia, Atmospheric Environment 5, 7-14. Richards L. W., Johnson K. R. and Shepard L. S. (1978) Sulfate Aerosol study. Report No. AM~8~.13FR, Rockwell Intemationat, Atom& International Divisiou, Air Monitoring Center, Newbury Park, CA. Scott W. D. and Hobbs P. V. (1967) The formation of sulfate in water droplets. J. atmos. Sci. 24, 54-57. Tang I. N. (1976) Phase transformation and growthofaerosol particles composed of mixed salts. J. aerosol Sci. 7, 361-371. Thomas R. L., Dharmarajan V., Lundquist G. L. and West P. W. Measurement of sulfuric acid aerosol, sulfur trioxide and the total sulfate content of ambient air. Andyt. Chem. 48,639~641.
Van den Heuvel A. P. and Mason B. J. (1963) The formation of ammonium sulphate in water droplets exposed to gaseous sulphur dioxide and ammonia. Q. Jl R. Met. Sot. llP, 271-215.