in Groat Britain Atmosphe,icEnoitonment Vol 11, pp 391-395 PergamonPress 1977 Printed 1 ;'jl /i

SHORT COMMUNICATION

AN IMPROVED THIN-FILM SULPHATE TEST FOR SUBMICRON PARTICLES

(First receiued30 July 1976and in final form 4 October1976)

of testing sub- Abstract-Details are given of improvementsto the vapour-depositedthin-film method having a ,ri.i"" p".ti*lates fo-r sulphate.'The test is shown io be useful for sulphates of bisulphateand peroxydisulphatecommon ;;;;; iil -iO-'fvf dm-^3ar 20'C. With the exception ioiuUl" o*V-u" of carbon, nitrogen, phosphoroui and sulphur do not causeinterference.

ptmosphereduring INTRODUCTION causedby the exposureto the ambient

ilngs. 3I A reductionin the thicknessof the postcoatedbarium chloridefilm from 20 to 30 nm (Bigg et al., 1974)to 3-5 nm reducedthe amount of excessbarium chloride, with the result that recrystallizationseldom occurred' With giant particlesit may be necessaryto use.thicker layer-sin order io obtain completereaction, but it is clear that the smaller the excessof chloride the better the background'

limitations of the test.

THE SULPHATE TEST evaporatedsequentially from the sameevaporation source, witfi no breakin the vacuum betweenevaporations' Per-

than an alcoholic atmosphere.It was found also that bar- ium iodide could be used with good resultsas an alterna- tive reagentto . The reaction rings in this caseweie not as well defined,though recrystallizationwas less evident than with barium chloride, particularly when ethanolwas usedfor the developmentof the reaction'This in probably-whilst reflects the fact that barium iodide is soluble barium chloride is not. However, because the clarity ofthe reaction rings developedat 95/"r'h' using barium chloride was superior for particles of 100nm dia' or lessthis combination was preferred' prove the clarity of the reaction rings the test method was revised- as follows. 1. The method of precoating the screenswith barium RESULTS chloride prior to partiile collection was discardedin favour 1 the results of the reaction of 3-4nm of oostcoating. this was done because with precoated Fieure shows of barium chloride with a number of different ,.rd.n, t..ty.Lllization of the barium chloride was often thick-layers 391 ?l;::H :a..1

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SiO, developed at 957. r.h. for t h. The scale marked is 2 x 10-6 m Left to right-Top: (NH4)2SO4, H2SO4, Fer(SOa)3, FeSOo. Middle: NarSOa, K2SO4, MnSOo, CoSO*. Bottom: ZnSOn, MgSOa, CuSOn, Ti2(SO4)3,Ag2SOo.

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Fig Application of the sulphate test to particles produced from a variety of salts, with test conditions as for Fig. 1. The scale marked is 2 x 10-6m Left to right-Top: NH*NO., NaNO3, NarHPOo, Na.POa Middle: NH4HCO3, KHCO3, (NH4)rCO3, NarCO.. Bottom: NaHSO.. Na,SO3, NarS, CHTSO.H. 392 Short Communications 393

The only restriction upon application of this test to any sulphate is that the sulphate must have a solubility appre- ciably greater than that of barium sulphate For example, KrSOo particles gave a lesswell defined ring than the more soluble sulphates (Fig. 1) whilst CaSOn particles did not react at a1l with either barium chloride or iodide, devel- oped for a week at 95/.r.h. or in ethanol. At 20"C the concentration of aqueous SO'zo- in saturated solution is 3 3 1.0 x 10-6M dm for BaSOo, 1.2 x 10-2 M dm for 3 K2SOa (Linke and of the development atmosphere is repiaced by the vacuum CaSOo and 5.7 x 10-rM dm for of at in the electron microscope. In the casesof (NHo)2SOo and Seidell, 1965). Apparently the method development chemistry H2SOa the NH*CI or HCI produced by the reaction are 95Y"r.h. is not completely analogous to solution be of the order of voiatilized in the electron beam to give the very clear back- and requires the sulphate solubility to the of ground. However, in the cases of the Zn, Cu and Mg sul- 10-r M dm-3 for reaction to occur. Thus in case : x 3 M dm-3 phates the sparse and fine nature of the background crys- AgrSou Fig 1),for which tSO?-l 25 10 the reaction rlng tals create no problems in interpretation, since these crys- in saturated aqueous solution at 20"C, precipitate but to a mixed tals have no shadow and any small, unreacted particles is probably not due to BaSOo BaSOa/AgC1 precipitable formed because A^gCl, like should be shadowed. 'M ' BaSOo, has a low solubility (1.1 x 10 dm in at 20'C). Another possible limitation to this technique is that anions other than sulphate might produce reaction rings when postcoated with barium chloride. This possibility was investigafed by testing particles made from an arbitrary selection of soluble salts, some of which may be of atmos- 2, test of barium chloride which formed by recrystallization dur- pheric interest. The results are shown in Fig. with that ing exposure to the ambient atmosphere conditions similar to those of Fig. 1. It is apparent

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" .;, w i ' '': (1\':. ! L., \/ ';.! ",i ":rt i} .l Fig.3. rhe sulphatetest applied ," o2fito.i,jiliso?:i saltmixtures containing sulphate fllT*t 394 Short Communications some form of reaction occurred in most cases.This is con- ticles (Jungeand Manson, 1961;Bigg, 1975).The clear sistent with the fact that in all cases the barium salt of backgroundsand clarity of the reaction rings are typical each anion is less soluble than barium chloride (Linke and of the resultsobtained for atmosphericsamples. Seidell, 1965), though the general spread of crystals over each surface (Fig. 2) may simply be due to recrystallization of mixed salts composed of barium chloride plus the salt CONCLUSION from the particles. In any event the "reaction products" This improved version of the test described by Bigg et have spread over the screens in ways quite unlike the al. (1974)should prove to be of use in caseswhere the characteristic barium sulphate rings, so that there is no size distribution of particles containing sulphate rs difficulty in distinguishing the positive test results of Fig. required, or in caseswhere the sampleis too small to be 1 from the test results in Fig. 2. tested by other methods. The results here indicate that Salts tested but not included in 2 Fig were NaHSO4, interferencesto the test should be a problem onlv when NH4HSO4, NarSrOr, (NH4)2SrO8 and NaNOr. The the barium salt of the interferinganion is as insoiubleas bisulphates gave positive sulphate reactions upon use of barium sulphate,or when there is interferencethrouqh the extended development times, indicating that the second formation of an insolublemetal chloride.It was Jhown ionization of sulphuric (K, - ') just acid 10 is sufficient that sulphate could be detectedin particulatescomposed to allow the sulphate peroxydisulphates test to work The of binary salt mixturesdown to levelsas low as 1 jmole gave positive results on some occasions, which is consistent I sulphate,though the failure of the test on CaSOoindi- with their tendency for decomposition to sulphate. The cated that the usefulnessof the test does not extend to NaNO2 gave a result similar to that for NaNO. sulphatesless soluble than - 10-l M dm-3. Finally,it was An additional series of tests involved introducins a sul- found that bisulphate and peroxydisulphatemay phate not be at levels of 1,2, l0 or 2Omole /" into solutions distinguishedfrom sulphate. of the salts tested in Fig 2, generating an aerosol and Work will be continuedto examinethe possibilitvof testing the particles collected for sulphate. The tests usingthe areaof the barium sulphatereaction ring to grve showed (Fig. 3) that the sulphate test was not inhibited an estimateof the amount of sulphatepresent in eachpar- by an excess of any of the salts tested, and that in the ticle. cases of these particles composed of binary mixtures the sulphate was detected in all casesat 10mol I and in most Acknowled.gements-The cases at 2 mole /". author wishesto thank Dr. E. K. The final experiment performed was to carry out the Bigg for his encouragementand suggestions,and Mrs. R. sulphate test on particuiate samples collected from the at- Taylor for preparing the microscopescreens and assistrng mosphere. Figure 4 shows the test results for particles col- with much of the laboratory work. lected at ground level from urban air and for stratospheric particles collected at 17 km The number of positive iesults in each case is in accord with the conclusion that sulDhate Diuisionof CloudPhysics, G. P. Aynns is a common constituent of particles in urban air iCharlson CSIRO,Sydney, et al ,1974) and the major constituent of stratospheric par- Australia

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!: r o 4);t'; 'l .e..':,'- / ... Fig. 4. Application of the sulphate test to particles collected from the atmosphere. Test conditions as for Fig. ll the icale marked is 2 x l0-6m. Top: Particles from the Sydney urban aerosol. Bottom: Stratospheric particles, collected at 17 km over Australia. Short Communications

REFERENCES background aerosol: optical detectionin the St. Louts region.Atmo spheric Enoir onment 8, 1.257-1267. Bigg E. K. (1975) Stratospheric particles. J. Atmos. Sci. JungeC. E. and MansonJ. E. (1961)Stratospheric aerosol 32,9r0F91'1. studies.,L geophys.Res. 66, 2163-2182. Bigg E. K., Ono A. and Williams J. A. (1974)Chemical Kellog W. W,, CadleR. D., Allen E. R., LazrusA. L. and tests for individual submicron aerosol particles.Atmos- Martell E. A. (1912) The sulphur cycle. Science 175, pheric Enuironment8, l'13. 587-595. CadleR. D.11974)Discussion of : Chemicaltests for indivi- Linke W. F. and SeidellA, (1965) of Inorganic dual submicron particles, AtmosphericEnuironment 8, and Metal Inorganic Compounds.Amer. Chem. Soc., 869. Washington D.C. CharlsonR. J., Vanderpool A. H., Covert D. S.,Waggoner Winkler P. (1975) Chemical analysis of Aitken particles A. P. and Ahlquist N. C. (1974) HrSO4/(NH4)2SO4 over the Atlantic ocean.Geonhvs. Res Lett. 2. 4548. ERROR