- Email: [email protected]
Physico-chemical speciation and transformation reactions of particulate atmospheric nitrogen and sulphur compounds
Atn&wic Pnnted
Enuironmenr in Great
Vol. 18. No. 9, pp. 1829-1833,
0004~981184 t3.00 + 0.00 Pergamon Press Ltd.
1984
Britain.
PHYSICO-CHEMICAL SPECIATION AND TRANSFORMATION REACTIONS OF PARTICULATE ATMOSPHERIC NITROGEN AND SULPHUR COMPOUNDS ROY M. HARRISON* and W. T. STURGES Dept. of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, U.K. Abstract-Using X-ray powder diffraction it is possible to identify a number of discrete compounds of nitrogen and sulphur in atmospheric aerosols. The compounds observed include ammonia neutralization products of airborne acids, including mixed ammonium nitrate-sulphate salts. Products of hydrogen halide displacement by HNO, and H,SO, are identified both from size-differentiated ion balance studies and X-ray diffraction. A number of mixed metal--ammonium sulphate salts are attributed to reactions of ammonium sulphate subsequent to aerosol coagulation processes. Key word index: Sulphate, nitrate, X-ray diffraction, transformation
By far the major proportion of measurement data upon atmospheric aerosols has been concerned with determination of elemental or ionic components, rather than with the identification of specific chemical compounds. Indeed, the recognition of specific compounds (or physico-chemical speciation) is fraught with difficulties and no entirely satisfactory means of compound identification has yet been developed. Approaches to the physico-chemical speciation of atmospheric particles are generally based upon one or other of two types of method. The first type is based upon use of an electron microscope to analyse specific particles by measurement of X-ray emissions (SEM/XES). The major draw-backs of this approach are the semi-quantitative nature of the elemental analysis data, the lack of any information upon elements of low atomic number, and the small significance of conclusions drawn on the basis of analysis of only tens (or hundreds at most) of individual particles. The alternative approach, which we have favoured, is based upon use of X-ray powder diffraction (XRD) upon a bulk aerosol sample. By this means, major component crystalline compounds within the aerosol may be firmly identified on the basis of their diffraction patterns. The drawbacks of this approach are that amorphous and poorly crystalline components are not identified, and that although quantification of major components is possible (Davis, 1981) it is difficult and has not been achieved in this laboratory. The sensitivity of the method is limited and hence in the case of atmospheric aerosols, components are unlikely to be identified unless they comprise at least l-5 y0 by weight of the mixture. In earlier papers, we have shown the value of
*Author to whom correspondence
should be addressed.
reactions.
physi#~hemi~l speciation by XRD in elucidating the atmospheric chemistry of automotive lead (Biggins and Harrison, 1979a) and of atmospheric sulphates (Biggins and Harrison, 1979b). In this paper, we examine the available information upon particulatecompounds of nitrogen and sulphur with a view to understanding their atmospheric formation and transformation reactions.
EXPERIMENTAL Air samples were coIlected using either a low volume or high volume Andersen impactor with impaction surfaces of FE&Teflon and a polystyrene filter back-up (Delbag Luftfilter). Collected particles were removed from the FEP-Teflon film by transfer to adhesive tape (double-sided ‘Seilotape’), which was inserted directly into the powder diffractometer. The polystyrene back-up was dissolved in toluene,and the suspended particles filtered ontoa membrane filter which was analysed by XRD. Neither the adhesive tape nor membrane filter (Milhpore type HAWP) gave any significant diffraction lines. Samples were collected over periods of 7-14 days, and each size fraction typically contained approximately 50 mg total particles (Hi-V01 samples) or 4 mg (low volume samples). XRD analyses were carried out on a Philips PW 1050/1720 powder diffractometer with crystal mon~hromator and Cu K, radiation. Compounds were identified by d-spacing and intensity matching both with the JCPDS diffraction files and with standard compounds run on the same instrument, when available. The sampling sites used in this work were at HazIerigg (rural, near Lancaster), Leeds (urban), Lancaster (urban) and near Ellesmere Port (industrial area). In all, about 70 samples have been analysed and in most instances the reported compounds have been found in a number of samples.
RESULTSAND DISCUSSION The compounds identified appear in Tables 1-3. Figure 1 shows selected diffractograms together with standard spectra for some of the com~nents identified. It has been noted in our XRD work that relative
1829
ROY M. HARRISON and W. T. STURGES
1830 Table
1. Products
of ammonia
neutralization
Size fraction
Compound
acids
Source
Site
<: 1 pm
WH,),SO,
of airborne
Various
this work; also Newman (1978); Brosset (1978)
NH,HSO,
Charlson et al. (1978); Newman (1978); Brosset (1978)
(NH&SO,.NH,HSO,
Charlson et al. (1978); Brosset (1978)
NH,NO, (orthorhombic)
0.62
pm
Leeds, Hazlerigg
this work
(NH,),S0,.2NH4N0,
0.4-2 pm
Leeds, Hazlerigg, Lancaster
this work Tani et al. (1983)
(NH,),S0,.3NH,NO,
0.7-2 pm
Leeds, Hazlerigg
this work Tani et al. (1983)
NH,Cl
<4pm
Various
this work
Table
2. Products
Compound
Size fraction
Na,SO, NaNO, NaNO,
2-10 pm 2-10 pm 2-5 pm
Na,SO,.NaNO,.H,O
l-2 pm
PbSO,
<2pm
Table
of halogen
3. Products
displacement
Source
Site Hazlerigg Hazlerigg Leeds Hazlerigg Leeds, __ Hazleriee Various”-
of reactions
Harrison Harrison
Size fraction
Site
Na,SO,.(NH,),S0,.4H,O
<2pm
CaSO,.(NH,),SO,.H,O
<2pm
Leeds, Ellesmere Port Various
PbSO,.(NH,),SO,
<2pm
Various
ZnS0,.(NH,),S0,.6H20
i 2.4 pm
Fe,(SO,),.3(NH,),SO,
<2pm
and Pio (1983~) and Pio (1983~)
this work this work Biggins and Harrison (1979a)
of aerosol
Compound
line intensities measured on our equipment often differ significantly from those in the diffraction files, and this should be borne in mind in evaluation of Fig. 1. One problem inherent in identification of components of atmospheric particles by X-ray diffraction is that substantial amounts of material are required to be collected, and it is not possible to state with certainty that a compound found on a filter or impaction substrate has been emitted into, or formed
reactions
sulphates Source this work
this work Tani et al. (1983) this work; Biggins and Harrison (1979a); O’Connor and Jaklevic (1981) Tani et al. (1983) O’Connor and Jaklevic (1981) Biggins and Harrison (1979b)
in the atmosphere, rather than produced on the filter by a chemical reaction occurring during or after air sampling. Perhaps the best evidence on this point comes from the finding by Biggins and Harrison (1980) of ammonium lead sulphate (PbSO,.(NH,),SO,) in street dust. The compound is not a primary pollutant and must be formed by reaction in the atmosphere (Biggins and Harrison, 1979a), rather than in the street dust as the dilution within the dust matrix (total lead
Physicochemical speciation and transformation reactions of atmospheric N and S compounds
Lancasjer
0.7-1.1
pm
Uni.
1.1-2.0
pm
Uni.
l.l-2.0um
1831
b
Leeds d
lW1Leeds
II
n
d AA---
lnce
(Ellesmere
.R..I
J
Port)
t
t
t
I t
I .. II I ,, t
45
40
35
30
25
I. I,
II
I&
I
II II.
.lI
f
I
II1
I
t
, t
20
15
I
1
,g h
t loo28
Fig. 1. Selected X-ray diffractograms and standard spectra (below) as 26(CuKb) vs intensity. The phases identified by letters are: (a) NH,N03; (b) (NH&S0.,.2NHbNOB; (c) (NH,),S0,.3NH,N03; (d) NH&I; (e) NaNO,; (f) NaZSO,.NaNO,.HZO; (g) NaZS04.(NH&S04.4HZO; (h) CaSO,.(NH,),SO,.HZO; 6) (NH&S% (k) CaS0,.2Hz0; (1) a-Sic),; (m) NaCI; (n) possibly (j) PbS04.(NH4)2S04; PbBrC1.2(NH,),BrCI, or similar.
content c. 1000 mg kg-‘; mostly non-crystalline) is far too low to account for formation by reaction therein. Thus, although reaction on the filter or impaction substrate may occur, it can only serve to amplify a process which happens in the atmosphere as a result of aerosol coagulation processes. (a) Products acids
of ammonia
neutralization
of airborne
The compounds identified are listed in Table 1, which also includes relevant compounds reported by other workers. The fully neutralised salts (NH,),S04, NH,CI and NH,NO, are commonly observed. Our earlier ion balance studies (Harrison and Pio, 1983) have demonstrated the presence of these compounds in aerosols, as have measurements by other workers in the case of (NH,),SO~ We have not observed either NH.+HSO, or (NH&SO,.NH,HSO, in our work,
and attribute this to the high degree of aerosol neutralisation observed locally (Harrison and Pio, 1983)and the highly hygroscopic nature of NH,HSO, which renders it liquid at relative humidities > 31% and thus unamenable to XRD anafysis under the conditions which we use. We know of no other direct observations of NH,NO, in air, although its presence may be inferred from ion balance studies (Harrison and Pio, 1983) and from the observed equilibrium concentrations of NH, and HNO, in air (Stelson et al., 1979; Harrison and Pio, 1983b). The observed orthorhombic form of NH.,NOJ is stable from - 18°C to 32.3”C. The two mixed salts (NH,)2S04.2NH,N0, and (NH,),S04.3NH,N0, are of considerable interest as these demonstrate the presence of binary mixtures of ammonium nitrate and sulphate in the atmosphere. Stelson and Seinfeld (1982) and Bassett and Seinfeld (1983) have constructed a phase diagram for the
1832
ROY M. HARRISON and W. T. STURGES
N&NO,-(NH&SO,-HI0 system which indicates four stable phases: NH,NO,; (NH,),S0,.3NH,NO,; (NH,)2S0,.2NH,N03 and (NH&SO,: all four are shown in this work to exist in atmospheric aerosols. In a recent paper, Tani et al. (1983) report the latter three compounds as major phases in 0.3-l.Opm aerodynamic size range aerosols at a road-side site. NH,NO, was not detected, however. As noted above, we have previously inferred the existence of NH,Cl in air from ion balance studies (Harrison and Pio, 1983a). One interesting observation in this work was that, although NH,+Cl was observed in almost all samples, its abundance appeared to be far greater in winter than summer. This we attribute to the probably greater HCl source strength in winter than in summer, but primarily to the high volatility of NH,Cl, which shows a very strong temperature dependence (Marsh, 1983, private communication). Whilst (NH&SO, is restricted almost entirely to the very fine particle size range (< 1 pm), NH,NO,, the mixed nitrat~sulphate salts and ~rticularly NH4Cl show a distribution into larger sizes. This we attribute to condensation of NH,NO, and NH,Cl from the vapour phase onto particles over a range of sizes. This mechanism is not available for the far less volatile (NH,)2S0,. Wolff et al. (1982) report that 83% of nitrate in particles collected in the Detroit metropolitan area is > 2.5 pm in diameter, whereas sulphate is predominantly < 2.5 pm.
(The CaSO, is itself found in the atmosphere and may arise from the mineral gypsum, or from attack of CaCO, by H,SO,). In the case of automotive lead, reaction with ammonium sulphate is believed to be far the most important process for primary PbBrCl (Biggins and Harrison, 1979a). The major product, PbSO,.(NH,)$O, has been observed in both U.K. air and in the U.S. (O’Connor and Jaklevic, 1981; Tani et af., 1983) 2PbBrCl+ 2(NH,),SO,
-+ PbSO~.(NH~)~SO~ + PbBrCl.(NH,),BrCl.
The compound ZnSO,.(NH,),S0,.6H,O is believed to have been the major water-soluble aerosol component in the Donora, Pa. pollution episode (Hemeon, 1955), as well as having been reported more recently in U.S. air (O’Connor and Jaklevic, 1981). CONCLUSIONS
Particulate ammonium salts of airborne acids are readily identified in air by XRD methods, and include mixed salts of ammonium nitrate and sulphate. These can undergo further reactions subsequent to aerosol coagulation processes, and some of the resultant compounds are identifiable in the case of reactions of ammonium sulphate. Sulphuric and nitric acids may displace HCl (or HBr) from halide aerosols with resultant formation of the corresponding sulphate or nitrate salt.
(b) Products of halogen displacement reactions We have previously noted the presence of Na,SO, and NaNO, in marine aerosol, in a size range which led us to infer formation by displacement of HCl from NaCl by the less volatile H,SO, or HNO, (Harrison and Pio, 1983a). Displacement of HCl by reaction of NaCl with H,SO, has also been inferred by Hitchcock et al. (1980) on the basis of aerosol composition measurements. In this work, we have further identified the mixed salt Na,SO,.NaNO,.H,O which we presume to arise from attack of the two acids upon marine NaCl. The existence of PbSO, from attack of H,SO, or NH,HSO, upon automotive PbBrCl has been reported previousty (Biggins and Harrison, 1979a,c). (c) Products
ofreactions with
aerosol suiphates
Since sulphates commonly dominate the submicrometer fraction of aerosols, it is not surprising that chemical reaction products resulting from the coagulation of sulphates with other submicrometer aerosols can be found in the atmosphere. Some of the compounds believed to arise from such reaction processes are listed in Table 3. These compounds occur frequently in air samples and may be formed in such processes as: ZCaSO, + (NH~)~SO~ -+ 2CaSO~(NH~)~SO~
~C~now~e~~e~n~s-The authors are grateful to the following for assistance‘in arranging sampling site facilities: Dr. A. Clarke and Dr. M. Willison, University of Leeds; Mr. A. Slater, Associated Octel Co. Ltd.; Mr. T. Davies, C.E.G.5. and Mr. Dennison, Beadle, Lancaster Town Hall.
REFERENCES
Bassett M. and Seinfeld J. H. (1983) Atmospheric equilibrium model of sulphate and nitrate aerosols. Atmospheric Environment 17,2231-2252. Biggins P. D. E. and Harrison R. M. (1979a) Atmospheric chemistry of automotive lead. Envir. Sri. Technol. 13,
558-65. Biggins P. D. E. and Harrison R. M. (1979b) Characterisation and classification of atmospheric sulphates. J. Air Pollut. Control Ass. 29, 838-40. Biggins P. D. E. and Harrison R. M. (1979c)The identifi~tion of specific chemical compounds in size-fractionated atmospheric particles collected at roadside sites. Atmospheric Enuironment 13, 121s.16. Biggins P. D. E. and Harrison R. M. (1980) Chemical speciation of lead compounds in street dusts. Enuir. Sci.
Technol. 14, 336-339. Brosset C. (1978) Water soluble sulphur compounds in aerosols. Atmospheric Environment 12, 25-38. Charlson R. J., Covert D. S., Larson T. V. and Waggoner A. P. (1978) Chemistry protxrties of trooosoheric sulphur aerosoIs. Atmospherk- Entkonment 12;391-53. _ Davis B. L. (19811._Quantitative analvsis of crvstalline and amorphous airborne particulates in Provo-Orem vicinity, Utah. Atmospheric environment l&613418. Harrison R. M. and Pio C. A. (1983a) Major ion composition
Physico-chemical speciation and transformation and chemical associations of inorganic atmospheric aerosois. Envir. Sci. Technol. 17, 167-114. Harrison R. M. and Pio C. A. (1983b) An investi~tion of the atmospheric HNO,-NH,-NH&NO, equilibrium relationship in a cool, humid climate. ?elius 33B, 155-159. Harrison R. M. and Pio C. A. (1983~1 Size differentiated composition of atmospheric aerosols ‘of both marine and polluted continental origin, Atmospheric Environment 17, 1733-1738. Hemeon W. C. L. (1955) The estimation of health hazards from air pollution. A. ‘M. A. Arch. id. Hith 11, 3974. Hitchcock D. R., Spiller L. R. and Wilson W. E. (1980) Sulphuric acid aerosols and HCl release in coastal atmospheres. Evidence for rapid formation of sulphuric acid particulates. Atmospheric Environment 14, 165-182. Newman L. (1978) Techniques for determining the chemical composition of aerosol sulfur compounds. Atmospheric ~~v~ro~~~t 12, 113-125. O’Connor B. H. and Jaklevic J. M. (198 1)Characterization of ambient aerosol particulate samples from the St. Louis area
reactions of atmospheric N and S compounds
1833
by X-ray powder diffractometry. Atmospheric Environment 15, 1681-1690. Stelson A. W., Friedlander S. K. and Seinfeld J. H. (1979) A note on the equilibrium relationship between ammonia and nitric acid and particulate ammonium nitrate. Atmospheric Environment 13, 369-37 1. Stelson A. W. and Seinfeld J. H. (1982) Thermodynamic predictions of the water activity, NH,NO, dissociation constant, density and refractive index for the NH~NO~-(NH~)~SO~-H~O system at 2S’C. A&~Iosp~erie Env~ro~~~r 16;2507-25i4. Tani B., Siegel S., Johnson S. A. and Kumar R. (1983) X-ray diffraction identification of atmospheric aerosols in the 0.3-1.0 pm aerodynamic size range. Atmospheric Environment 17,2271-2283. Wolff G. T., Ferman M. A., Kelly N. A., Stroup D. P. and Ruthkovsky M. S. (1982) The relationship between the chemical com~sition of fine particles and visibility in the Detroit metropolitan area. .r. Air Poftut. Conrrol Ass. 32, 1216-1220.
Enuironmenr in Great
Vol. 18. No. 9, pp. 1829-1833,
0004~981184 t3.00 + 0.00 Pergamon Press Ltd.
1984
Britain.
PHYSICO-CHEMICAL SPECIATION AND TRANSFORMATION REACTIONS OF PARTICULATE ATMOSPHERIC NITROGEN AND SULPHUR COMPOUNDS ROY M. HARRISON* and W. T. STURGES Dept. of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, U.K. Abstract-Using X-ray powder diffraction it is possible to identify a number of discrete compounds of nitrogen and sulphur in atmospheric aerosols. The compounds observed include ammonia neutralization products of airborne acids, including mixed ammonium nitrate-sulphate salts. Products of hydrogen halide displacement by HNO, and H,SO, are identified both from size-differentiated ion balance studies and X-ray diffraction. A number of mixed metal--ammonium sulphate salts are attributed to reactions of ammonium sulphate subsequent to aerosol coagulation processes. Key word index: Sulphate, nitrate, X-ray diffraction, transformation
By far the major proportion of measurement data upon atmospheric aerosols has been concerned with determination of elemental or ionic components, rather than with the identification of specific chemical compounds. Indeed, the recognition of specific compounds (or physico-chemical speciation) is fraught with difficulties and no entirely satisfactory means of compound identification has yet been developed. Approaches to the physico-chemical speciation of atmospheric particles are generally based upon one or other of two types of method. The first type is based upon use of an electron microscope to analyse specific particles by measurement of X-ray emissions (SEM/XES). The major draw-backs of this approach are the semi-quantitative nature of the elemental analysis data, the lack of any information upon elements of low atomic number, and the small significance of conclusions drawn on the basis of analysis of only tens (or hundreds at most) of individual particles. The alternative approach, which we have favoured, is based upon use of X-ray powder diffraction (XRD) upon a bulk aerosol sample. By this means, major component crystalline compounds within the aerosol may be firmly identified on the basis of their diffraction patterns. The drawbacks of this approach are that amorphous and poorly crystalline components are not identified, and that although quantification of major components is possible (Davis, 1981) it is difficult and has not been achieved in this laboratory. The sensitivity of the method is limited and hence in the case of atmospheric aerosols, components are unlikely to be identified unless they comprise at least l-5 y0 by weight of the mixture. In earlier papers, we have shown the value of
*Author to whom correspondence
should be addressed.
reactions.
physi#~hemi~l speciation by XRD in elucidating the atmospheric chemistry of automotive lead (Biggins and Harrison, 1979a) and of atmospheric sulphates (Biggins and Harrison, 1979b). In this paper, we examine the available information upon particulatecompounds of nitrogen and sulphur with a view to understanding their atmospheric formation and transformation reactions.
EXPERIMENTAL Air samples were coIlected using either a low volume or high volume Andersen impactor with impaction surfaces of FE&Teflon and a polystyrene filter back-up (Delbag Luftfilter). Collected particles were removed from the FEP-Teflon film by transfer to adhesive tape (double-sided ‘Seilotape’), which was inserted directly into the powder diffractometer. The polystyrene back-up was dissolved in toluene,and the suspended particles filtered ontoa membrane filter which was analysed by XRD. Neither the adhesive tape nor membrane filter (Milhpore type HAWP) gave any significant diffraction lines. Samples were collected over periods of 7-14 days, and each size fraction typically contained approximately 50 mg total particles (Hi-V01 samples) or 4 mg (low volume samples). XRD analyses were carried out on a Philips PW 1050/1720 powder diffractometer with crystal mon~hromator and Cu K, radiation. Compounds were identified by d-spacing and intensity matching both with the JCPDS diffraction files and with standard compounds run on the same instrument, when available. The sampling sites used in this work were at HazIerigg (rural, near Lancaster), Leeds (urban), Lancaster (urban) and near Ellesmere Port (industrial area). In all, about 70 samples have been analysed and in most instances the reported compounds have been found in a number of samples.
RESULTSAND DISCUSSION The compounds identified appear in Tables 1-3. Figure 1 shows selected diffractograms together with standard spectra for some of the com~nents identified. It has been noted in our XRD work that relative
1829
ROY M. HARRISON and W. T. STURGES
1830 Table
1. Products
of ammonia
neutralization
Size fraction
Compound
acids
Source
Site
<: 1 pm
WH,),SO,
of airborne
Various
this work; also Newman (1978); Brosset (1978)
NH,HSO,
Charlson et al. (1978); Newman (1978); Brosset (1978)
(NH&SO,.NH,HSO,
Charlson et al. (1978); Brosset (1978)
NH,NO, (orthorhombic)
0.62
pm
Leeds, Hazlerigg
this work
(NH,),S0,.2NH4N0,
0.4-2 pm
Leeds, Hazlerigg, Lancaster
this work Tani et al. (1983)
(NH,),S0,.3NH,NO,
0.7-2 pm
Leeds, Hazlerigg
this work Tani et al. (1983)
NH,Cl
<4pm
Various
this work
Table
2. Products
Compound
Size fraction
Na,SO, NaNO, NaNO,
2-10 pm 2-10 pm 2-5 pm
Na,SO,.NaNO,.H,O
l-2 pm
PbSO,
<2pm
Table
of halogen
3. Products
displacement
Source
Site Hazlerigg Hazlerigg Leeds Hazlerigg Leeds, __ Hazleriee Various”-
of reactions
Harrison Harrison
Size fraction
Site
Na,SO,.(NH,),S0,.4H,O
<2pm
CaSO,.(NH,),SO,.H,O
<2pm
Leeds, Ellesmere Port Various
PbSO,.(NH,),SO,
<2pm
Various
ZnS0,.(NH,),S0,.6H20
i 2.4 pm
Fe,(SO,),.3(NH,),SO,
<2pm
and Pio (1983~) and Pio (1983~)
this work this work Biggins and Harrison (1979a)
of aerosol
Compound
line intensities measured on our equipment often differ significantly from those in the diffraction files, and this should be borne in mind in evaluation of Fig. 1. One problem inherent in identification of components of atmospheric particles by X-ray diffraction is that substantial amounts of material are required to be collected, and it is not possible to state with certainty that a compound found on a filter or impaction substrate has been emitted into, or formed
reactions
sulphates Source this work
this work Tani et al. (1983) this work; Biggins and Harrison (1979a); O’Connor and Jaklevic (1981) Tani et al. (1983) O’Connor and Jaklevic (1981) Biggins and Harrison (1979b)
in the atmosphere, rather than produced on the filter by a chemical reaction occurring during or after air sampling. Perhaps the best evidence on this point comes from the finding by Biggins and Harrison (1980) of ammonium lead sulphate (PbSO,.(NH,),SO,) in street dust. The compound is not a primary pollutant and must be formed by reaction in the atmosphere (Biggins and Harrison, 1979a), rather than in the street dust as the dilution within the dust matrix (total lead
Physicochemical speciation and transformation reactions of atmospheric N and S compounds
Lancasjer
0.7-1.1
pm
Uni.
1.1-2.0
pm
Uni.
l.l-2.0um
1831
b
Leeds d
lW1Leeds
II
n
d AA---
lnce
(Ellesmere
.R..I
J
Port)
t
t
t
I t
I .. II I ,, t
45
40
35
30
25
I. I,
II
I&
I
II II.
.lI
f
I
II1
I
t
, t
20
15
I
1
,g h
t loo28
Fig. 1. Selected X-ray diffractograms and standard spectra (below) as 26(CuKb) vs intensity. The phases identified by letters are: (a) NH,N03; (b) (NH&S0.,.2NHbNOB; (c) (NH,),S0,.3NH,N03; (d) NH&I; (e) NaNO,; (f) NaZSO,.NaNO,.HZO; (g) NaZS04.(NH&S04.4HZO; (h) CaSO,.(NH,),SO,.HZO; 6) (NH&S% (k) CaS0,.2Hz0; (1) a-Sic),; (m) NaCI; (n) possibly (j) PbS04.(NH4)2S04; PbBrC1.2(NH,),BrCI, or similar.
content c. 1000 mg kg-‘; mostly non-crystalline) is far too low to account for formation by reaction therein. Thus, although reaction on the filter or impaction substrate may occur, it can only serve to amplify a process which happens in the atmosphere as a result of aerosol coagulation processes. (a) Products acids
of ammonia
neutralization
of airborne
The compounds identified are listed in Table 1, which also includes relevant compounds reported by other workers. The fully neutralised salts (NH,),S04, NH,CI and NH,NO, are commonly observed. Our earlier ion balance studies (Harrison and Pio, 1983) have demonstrated the presence of these compounds in aerosols, as have measurements by other workers in the case of (NH,),SO~ We have not observed either NH.+HSO, or (NH&SO,.NH,HSO, in our work,
and attribute this to the high degree of aerosol neutralisation observed locally (Harrison and Pio, 1983)and the highly hygroscopic nature of NH,HSO, which renders it liquid at relative humidities > 31% and thus unamenable to XRD anafysis under the conditions which we use. We know of no other direct observations of NH,NO, in air, although its presence may be inferred from ion balance studies (Harrison and Pio, 1983) and from the observed equilibrium concentrations of NH, and HNO, in air (Stelson et al., 1979; Harrison and Pio, 1983b). The observed orthorhombic form of NH.,NOJ is stable from - 18°C to 32.3”C. The two mixed salts (NH,)2S04.2NH,N0, and (NH,),S04.3NH,N0, are of considerable interest as these demonstrate the presence of binary mixtures of ammonium nitrate and sulphate in the atmosphere. Stelson and Seinfeld (1982) and Bassett and Seinfeld (1983) have constructed a phase diagram for the
1832
ROY M. HARRISON and W. T. STURGES
N&NO,-(NH&SO,-HI0 system which indicates four stable phases: NH,NO,; (NH,),S0,.3NH,NO,; (NH,)2S0,.2NH,N03 and (NH&SO,: all four are shown in this work to exist in atmospheric aerosols. In a recent paper, Tani et al. (1983) report the latter three compounds as major phases in 0.3-l.Opm aerodynamic size range aerosols at a road-side site. NH,NO, was not detected, however. As noted above, we have previously inferred the existence of NH,Cl in air from ion balance studies (Harrison and Pio, 1983a). One interesting observation in this work was that, although NH,+Cl was observed in almost all samples, its abundance appeared to be far greater in winter than summer. This we attribute to the probably greater HCl source strength in winter than in summer, but primarily to the high volatility of NH,Cl, which shows a very strong temperature dependence (Marsh, 1983, private communication). Whilst (NH&SO, is restricted almost entirely to the very fine particle size range (< 1 pm), NH,NO,, the mixed nitrat~sulphate salts and ~rticularly NH4Cl show a distribution into larger sizes. This we attribute to condensation of NH,NO, and NH,Cl from the vapour phase onto particles over a range of sizes. This mechanism is not available for the far less volatile (NH,)2S0,. Wolff et al. (1982) report that 83% of nitrate in particles collected in the Detroit metropolitan area is > 2.5 pm in diameter, whereas sulphate is predominantly < 2.5 pm.
(The CaSO, is itself found in the atmosphere and may arise from the mineral gypsum, or from attack of CaCO, by H,SO,). In the case of automotive lead, reaction with ammonium sulphate is believed to be far the most important process for primary PbBrCl (Biggins and Harrison, 1979a). The major product, PbSO,.(NH,)$O, has been observed in both U.K. air and in the U.S. (O’Connor and Jaklevic, 1981; Tani et af., 1983) 2PbBrCl+ 2(NH,),SO,
-+ PbSO~.(NH~)~SO~ + PbBrCl.(NH,),BrCl.
The compound ZnSO,.(NH,),S0,.6H,O is believed to have been the major water-soluble aerosol component in the Donora, Pa. pollution episode (Hemeon, 1955), as well as having been reported more recently in U.S. air (O’Connor and Jaklevic, 1981). CONCLUSIONS
Particulate ammonium salts of airborne acids are readily identified in air by XRD methods, and include mixed salts of ammonium nitrate and sulphate. These can undergo further reactions subsequent to aerosol coagulation processes, and some of the resultant compounds are identifiable in the case of reactions of ammonium sulphate. Sulphuric and nitric acids may displace HCl (or HBr) from halide aerosols with resultant formation of the corresponding sulphate or nitrate salt.
(b) Products of halogen displacement reactions We have previously noted the presence of Na,SO, and NaNO, in marine aerosol, in a size range which led us to infer formation by displacement of HCl from NaCl by the less volatile H,SO, or HNO, (Harrison and Pio, 1983a). Displacement of HCl by reaction of NaCl with H,SO, has also been inferred by Hitchcock et al. (1980) on the basis of aerosol composition measurements. In this work, we have further identified the mixed salt Na,SO,.NaNO,.H,O which we presume to arise from attack of the two acids upon marine NaCl. The existence of PbSO, from attack of H,SO, or NH,HSO, upon automotive PbBrCl has been reported previousty (Biggins and Harrison, 1979a,c). (c) Products
ofreactions with
aerosol suiphates
Since sulphates commonly dominate the submicrometer fraction of aerosols, it is not surprising that chemical reaction products resulting from the coagulation of sulphates with other submicrometer aerosols can be found in the atmosphere. Some of the compounds believed to arise from such reaction processes are listed in Table 3. These compounds occur frequently in air samples and may be formed in such processes as: ZCaSO, + (NH~)~SO~ -+ 2CaSO~(NH~)~SO~
~C~now~e~~e~n~s-The authors are grateful to the following for assistance‘in arranging sampling site facilities: Dr. A. Clarke and Dr. M. Willison, University of Leeds; Mr. A. Slater, Associated Octel Co. Ltd.; Mr. T. Davies, C.E.G.5. and Mr. Dennison, Beadle, Lancaster Town Hall.
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