CURRENT RESEARCH Variations in Chemistry of Airborne Particulate Material with Particle Size and Time Paul T. Cunningham,' Stanley A. Johnson, and Ralph T. Yang C h e m i c a l Engineering Division, Argonne National Laboratory, 9700 South Cass Ave., Argonne, I II. 60439
w Samples of airborne particulate material are classified with respect t o particle size and time during their collection by inertial impaction. Infrared spectroscopic analysis is used to measure the major chemical constituents of the samples. Initial results clearly show variations in the chemistry of airborne particulates as a function of particle size and time. These results support recent theories on the origin of particulates t h a t have been based on size distribution measurements and may be important in assessing the potential health hazard of particulates and in evaluating the appropriateness of control techniques.
It is generally acknowledged t h a t a more complete understanding of the variation in particle composition as a function of particle size and time is essential to proper evaluation of' the potential health hazard posed by airborne particulate material and to the selection of appropriate control practices required to reduce the level of airborne particulate to acceptable levels. Such information could also provide insight into the relative importance of various mechanisms t h a t have been proposed for formation of particulates from gaseous constituents of the atmosphere. The group of papers describing the results of the 1969 Pasadena Smog Aerosol Study (Hidy, 1952) has focused attention on the need for further study o f t h e chemistry of size- and t ime-classified particulates. Whitby and coworkers (Whitby et al., 1972a.b; Husar et al.. 1972) have examined the mutlimodal nature of aerosol size distributions. The variation of t h e size distribution with time, found in these studies, is compelling evidence that in photochemical smog most of the mass of particles smaller t h a n one micron arises from condensation and coagulation of photochemical reaction products. whereas most of t h e mass of particles larger t h a n a few microns originate from refloatation or other mechanical sources. With such a model for particle origin, one would expect short-time sampling to show t h a t particles less t h a n about 1 p would be very much alike in their chemistry and would reflect to some extent the chemistry of the atmosphere in which they were formed. On t h e other hand. for particles greater than a few microns, one would expect considerable variation in t h e chemistry. thus reflecting a wide variety of sources. Most of the d a t a on the chemical composition of airborne particulate material have been obtained from filtration samples with a typical collection period of about a day. These d a t a are useful in establishing long-term trends but do not reveal the detailed variation in chemistry and elemental content as a function of particle size
* To whom correspondence should be addressed
a n d time. A glimpse of the structure that is present is provided by the spectrographic work of Lee et al. (1972), which shows the variation in trace-metal content with particle size for samples collected by the Sational Air Surveillance Cascade Impaction Network, and the electron spectroscopy (ESCA) of Novakov e t al. (1972). which shows the variation in chemical states of sulfur and nitrogen with both particle size and time. This paper describes a methodology for the collection. handling, and analysis of airborne particulate material as a function of particle size and time and reports on some early results. Size and time classification are achieved during t h e collection process through the use of a Lundgren impactor. Samples are analyzed using infrared spectroscopic techniques. Data obtained clearly show t h a t airborne particulate material collected at Argonne National Laboratory contains ammonium sulfate. ammonium halide. carbonate, silicate and silica, nitrate, hydrocarbon. and chemisorbed nitrogen dioxide (surface nitrate). and that the composition does indeed vary with particle size and time. An attempt has been made to interpret some of the d a t a in terms of existing theories on particle origin,
Experimental Procedures When the procedures are considered t h a t might be used for collection and analysis of airborne particulates classified as to particle size and time, it is essential to be fully aware that even modest classification of the sample with respect to time and particle size greatly increases the number of analyses that must be made. At the risk of being pedantic, we point out t h a t sampling with threehour time resolution in three size ranges simultaneously a t three collection sites will create 72 samples per day and that meaningful correlation of the analytical results obtained with local meteorological variables, emission sources, and other parameters may require several consecutive days of sampling. Therefore, rapid techniques must be developed if results are desired on a reasonable time scale. Size and time classification of the samples were achieved during collection by the use of a four-stage Lundgren inertial impactor (Lundgren. 1967: Lundgren, 1951) obtained from Environmental Research Corp. The Lundgren impactor functions according to basic principles common to all such devices (Ranz and Wong. 1952; Ranz, 1956) but has the unique feature of using rotating drums about 38 m m in diameter as the impaction surface in each stage. To facilitate sample handling and subsequent analysis. samples were collected on a 0.05-mm-thick Mylar film stretched over the surface of each drum. The impactor was operated at the manufacturer's recommended flow rate of 6.8 m3hrr1 thereby collecting samples for which the mass-median-diameters have nominal values of 12, 3, 1.2. and 0.3 p on stages I through IV. respectively. Volume 8, Number 2, February 1974
131
rotated several degrees and the next sample was collected. In this way, continuous unattended sampling with threehour time resolution can be attained over a three-day period. After sample collection. the Mylar film was removed from each impaction d r u m . The size- and time-resolved samples, which appear as bands of impacted material on the Mylar films, were separated by cutting the film into strips. The samples were removed from the Mylar strips. mixed thoroughly with 0.2 gram of potassium bromide, and pressed into 13-mm-diameter pellets for subsequent infrared spectrometric analysis using standard procedures (Kendall. 1966). The mass of each sample was obtained by weighing the Mylar strip before and after sample removal. Infrared spectra of the potassium bromide pellets containing the samples were obtained usiiig a Digilab Model FTS-14 Fourier Transform Spectrometer. Typically, satisfactory spectra covering the range from 400-3600 c m - l with 8 c m - l resolution were obtained in less than five minutes.
Table I. Size Distribution of Particulate M a w Nominal particle d i a m e t e r , ,,m
I m p a c t o r stage
I II Ill IV After filter
Wt. collected,
mgh
of total w t . collected
0.46 1.63 3.10 9.86 11.87
1.7 6.1 11.5 36.6 44.1
12.0 3.0 1.2 0.3
' Average particulate b u r d e n 64.5 pg/m3. Accumulated weight over a three.day s a m p l i n g period.
During the early portion of this study, time resolution was achieved by using a continuous drum drive mechanism which provided 21 hr of continuous sample collection. Subsequently it was determined t h a t sample handling and analysis were greatly simplified without loss in time resolution if the drums were rotated in a stepwise fashion. In this stepwise mode of operation. the first sample was collected with the drums stationary for a preset period (usually 1-3 hr), after which the drums were rapidly
a
_ _ _ I -
AMBIENT AIR TEMPERATURE, "C
1
-
31 5
0
STAGEE
5t
f
ABSORBANCE STAGE IU NO; (1384crn-I) rn C O , (1435crn-')
0
STAGE
E
o
NO; (1384 crn-'1
0
NH; (1400 cm-')
o SO; (I I I O cm-'1
.5_L---'
'
TIME Figure 1. Composite diagram showing selected meteorological parameters and experimental results for a three-day sampling period from March 20 through 23, 1973
132
Environmental Science & Technology
/--
-
-
-
-
~
85",
d3
dl 03
,
b2
w 95-
04
w 0
'
1 e2
z
el
65a2
2
b3
c
bl
+ 43 0 i
w 0-
25-
'5u
I c
c
30C.d
'5""
23
-
~~
F-ECJEIUCY
5--
__ _ cnr
75-
~-
3000
530
2500
20c0
Results and Discussion The results reported here were obtained using samples collected at Argonne National Laboratory, located about 45 km southwest of downtown Chicago, Ill. The area surrounding the laboratory can be characterized as being in transition from a rural to a suburban character and without a significant air pollution problem. Most of the samples described here were collected 32 meters above ground level over a three-day period during the early spring of 1973. Data on the size distribution of particulate mass are presented in Table I and in Figure 1. Table I shows accumulated total mass, whereas Figure 1 shows the variation of mass with time for impaction stages 11. 111, and IV. These data are in general agreement with those obtained by the National Air Surveillance Cascade Impactor Network (Lee and Goranson, 1972) but indicate less mass in the size range greater than 3.0 p. Differences in the particle bounce characteristics of the impactors used or the effect of sampling height (Sehmel, 1973) on the observed size distribution are, experimental variables that could have reduced our observed large-particle mass. Infrared spectra were obtained for the samples collected in stages 11, 111, and IV when the sample mass was greater than 10 pg. Selected spectra representative of those obtained are shown in Figures 2 and 3. The observed bands. for which we have made assignments, are listed in Table I1 together with their assignments. a cross reference to the figures, and appropriate comments. The assignment of the observed bands to the various vibrational modes qf the species present was, for the most part, straightforward based on direct comparison with known spectra. Thus. the presence of ammonium sulfate as the major compound present in stage IV samples (Figure 2) appears to be unambiguous. Carbonate. nitrate, silica and silicate, and hydrocarbon are clearly present in varying amounts in the stage I1 and I11 samples. The time variation of these substances and the significance of their presence in the various size ranges are considered below. There are a number of bands for which definite assignment is not yet possible. A series of bands at 1190, 1140. 1120. 670. 627, and 600 c m - l show the same temporal variation in intensity and resemble bands due to various forms of phosphate. Because of the complexity of phosphate spectra and their sensitivity to crystal structure and associated cations (Chapman and Thirlwell. 1964; Pustinger et al., 1939), assignment of these bands to specific phosphates is not possible. These bands appear in the spectra of daytime particulate and are most intense in the spectra of stage 111 samples.
1 IC00 500
FREQUENCY. cm-
cm
Figure 2. Infrared spectrum of a stage I V sample collected between 04 00 and 07 00 of the last day of sampling Total sample weight, 276 p g Labeled absorption bands are described in Table II
1500
Figure 3. Infrared spectrum of a stage I1 sample collected between 19:OO and 22:OO of the last night of sampling. Total sample weight, 54 p g . Labeled absorption bands are described in Table I I
A weak band at 1720 c m - l observed in some stage IV samples is tentatively assigned to the v 4 (bending mode) V g (lattice libration) combination band for "4ion in ammonium halide. Hornig and coworkers (Wagner and Hornig, 1950; Plumb and Hornig, 1953; Vedder and Hornig, 1961) have shown that this combination band is observed only when the NH4- ion does not rotate freely in the crystal lattice as in the case of KH4C1, NH4Br, and NH4F. We observe this band only when the ratio of XH4absorption to S 0 4 2 - absorption is significantly greater than that for (NH*)*S04-i.e., when there appears to be excess NH4-. Elemental analysis, which has confirmed the presence of C1 and Br in these samples, tends to support this assignment.
+
Table II. Listing of Assigned lnfared Bands Observed i n Particulate Samples Assignment Frequency, c m -1
3140 3020 2920 2860 2800 1768 1720 1620 1435 1400 1384 1360 1190 1140 1120
1110 1035 980 880 840 800 780 728 670 627 620 600 470
Species
N H4N HqHydrocarbon (C-H) Hydrocarbon (C-H) N HqNos- (bulk) NHd+(halide) Hp 0
co32-
~ 0 ~ 3 - b
0
~
a1
a2 el
e2 03 dl
a4 f 91
N HqNOa- (surface) NOa- (bulk) ~
Designation i n Figs. 2 and 3
*
3
-
sop SiOt-
c03'NOa- (bulk) SiOd4-
PO43-b
sod?-
Pods- *
Si04?-
Overlappedc kl kl bl hl b2
sod*-
POr3-b
C
kl
~ 0 ~ 3 - b
Si04qCO32-
a5
92
d3 h2
h3 93 k2 k2 b3 k2 h4
'I S p e p o s c o p i c designation of vibrational m o t i o n a f t e r H e r z b e r g (1964). ' A s s i g n m e n t u n c e r t a i n . This b a n d is overlapped in Ftgure 2 by bands c and d5.
Volume 8, Number 2, February 1974
133
The strong sharp feature a t 1384 c m - l is of special interest. This feature usually dominates the spectra of stages I1 and I11 samples and is frequently prominent in stage IV samples. A similar sharp band has been observed by Blanco and coworkers (Blanco and Hoidale. 1968; Hoidale and Blanco, 1969; Blanco and McIntyre, 1972) and assigned to sodium nitrate and possibly potassium nitrate. Such a n assignment is not consistent with the known spectra of these or other common nitrates in KBr pellets as recorded in this laboratory and elsewhere (Ferraro. 1960). As part of a study of the acidic and basic properties of hydroxylated metal oxide surfaces, Boehm (1971) has observed a similar sharp band which he attributed to chemisorbed nitrate ion formed on the hydroxylated surface of powdered Ti02 and A1203 t h a t had been exposed to N O z . Using various mixtures of finely divided silica (sea sand), ammonium sulfate. alumina, calcite, and carbon black to simulate the airborne particulate samples, we obtained spectra before and after exposure to nitrogen dioxide. In all cases the spectra of the mixtures obtained after exposure to S O 2 contained the strong sharp 1384 c m - I band. On the basis of this observation. and the sharpness of the feature, which is consistent with a loosely bound surface species, we have assigned the 1384 c m - l band to surface nitrate ion formed through the chemisorption of atmospheric S O 2 on the particulate surface. The difference in the chemistry of the submicron particles collected in stage IV compared with that of the larger particles is clearly evident from the spectra shown in Figures 2 and 3. The only spectral feature common to both size ranges is the 1384 cm-1 band discussed above. Temporal variations in particulate chemistry within a size range have been more quantitative than qualitative for the samples we have examined. Figure lf shows the variation in the infrared absorbance for some of the more prominent bands. The most desirable units for the ordinate of such a plot would be Concentration. but the relationship between absorbance and concentration is not so straightforward as one might hope (Kendall. 1966). Preliminary studies of the effect of particle size and concentration on the absorbance of ammonium sulfate in potassium bromide pellets have shown that samples with smaller particles have greater absorbance than samples of equal mass composed of larger particles and that for both size ranges positive deviations from Beer's law occur with increasing total sample weight. The details of these effects are not yet well enough understood to permit accurate interpretation of the spectra in terms of the concentration of the absorbing species, It was also noted in the ammonium sulfate samples that the ratio of the ammonium ion absorbance at 1400 cm-1 to the sulfate ion absorbance at 1110 c m - I increased with increasing ammonium sulfate concentration. The range observed for this ratio is shown by the shaded horizontal region in Figure lg. Data points on this figure show the value of this ratio for the stage IV sample. The higher values of the ratio observed during the later part of the sampling period correspond to those samples for which the 1720 c m - l combination band. identified above with the presence of ammonium halide, was observed. It is difficult to identify any definite correlations in these relatively short-term d a t a but some general tendencies are noted. The total mass on each stage seems to follow the diurnal variation in relative humidity except during the morning of the second full day of sampling when the wind was beginning to veer into the southeast. Other factors. such as stability, no doubt also have a n effect. Stage-111-adsorbed nitrate appears to peak during the 134
Environmental Science & Technology
early morning whereas stage IV adsorbed nitrate is very erratic. It should be noted t h a t for other samples collected during periods when the wind was southerly, that is from a direction where there is very little development and motor traffic, adsorbed nitrate is not observed in stage IV samples. Stage I11 carbonate and silicate (not shown in Figure 1) are quite erratic. Stage IV sulfate and ammonium follow the general trend of the sample weight. The results we have obtained appear to be completely consistent with the particle origin model proposed by Whitby (Whitby et al., 1972133. The variety of materials found in samples of particles greater than one micron diameter and the erratic variation in their concentration with time suggest that they are of diverse origin. The most striking feature of the submicron particles is their chemical uniformity. Two principal mechanisms for the formation of ammonium sulfate are reported in the literature. In one scheme. sulfur dioxide is oxidized to sulfate in the presence of ammonia and water to form ammonium sulfate. The kinetics of this reaction have been studied (McKay. 1971: Englund and Beery. 1971). Ammonium sulfate can also be formed by the photochemical oxidation of sulfur dioxide in the presence of nitric oxide and unsaturated hydrocarbon and subsequent reaction with ammonia present in the air. This latter reaction has been studied by Groblicki and Kebel (Tuesday, 1971) and, although they did not study the kinetics directly, a n approximate reaction rate in the presence of propylene can be obtained from their d a t a and is comparable to the rate observed for the ammonia reaction. Our d a t a suggest that the heterogeneous mechanism involving ammonia is more important in our locale than the photochemical mechanism. since sulfate correlates more closely with relative humidity than with solar radiation. The sampling and analysis techniques described herein are rapid enough to permit examination of time- and sizeclassified samples obtained from a variety of locations over an extended period of time. Careful selection of future sampling sites will permit more detailed evaluation of the various mechanisms that have been proposed for the formation of particles in the atmosphere. The methodology described here may also prove useful in evaluating the local effects of specific primary sources. Acknouledgment
The authors wish to acknowledge the technical assistance of G. T . Reedy and H. Moses. S. D. Gabelnick and V. A . Maroni participated in many useful discussions. L i t e r a t u r e Cited Blanco. A , .J.. Holidale. G . B.. r l t m o s Enciron..,2, 327-30 (1968). Blanco. A . .J.. SlcIntyre. R. G.. ibid., 6 , 557-62 (1972). Boehm. H . P., Discuss. Farads?, Soc., 52,264-75 (1971). Chapman, A . C., Thirlwell, L. E., Spectrochirn. i l c t a , 20, 937-47 i1964 ) . Englund. H. 31.. Beery, LV, T., "Proceedings of t h e Second, International Clean Air Congress. Fyashington. D.C .. 1970." Paper No. C P 7C. Academic. S e w York. N.T.. 1971. Ferraro. .J. R.. J . .Wo/. S p e c t r u s t . . 1,99-105 (1960). Hid>-. G . SI.,E d . "Aerosols and Atmospheric Chemistry." pp 189-344. Academic. S e w York. S . Y . . 1972 Herzberg. G.. "Molecular Spectra and Molecular Structure 11. Infrared and R a m a n Spectra of Polyatomic Molecules." Van S o s trand. S e w York, S . Y . . 1964. Hoidale. G. 8.. Blanco. A . J.. Pure A p p l . Geophys. 74, 151-64 (1969). Husar. R. B.. lt-hitby. K . T.. Liu. B. Ti. H.. J . C'o//oid fnterface Sci.. 39 ( I ) .211-24 (1972) (also reprinted in Hidy. 197?). Kendall. D. N..E d . . "Auulied Infrared Soectroscouv. .. ou . . 136-51. Reinhold. S e w York. S.;-.k.. 1966. Lee. R. E.. Goranson. S.S..Eniiron. Sci. Techno!.. 6 ( 1 2 ) . 101924 (1972)
Lee. R . E.. Goranson. S.S., Enrione. R. E.. Morgan, G. B.. ibid.. 6 (12) 1025-30 (1972). Lundgren. D. A , . A t m o s . Enciron.. 3 ( 8 ) . 645-51 (1971). Lundgren, D . A,, J . .-lirPol/ut. Contr. Ass.. l i , 225-8 (1967). McKay. H . A . C.. ibid.. p p 7-14. J . CoiNovakov. T.. Mueller. P. K.. Alcocer. A . E.. Otvos. ,J. W,, loid Interface Sei.. 38 (1).225-34 (1972) (also reprinted in Hidy. 1972). Plumb, R. C., Hornig, D. F., J . Chem. Ph\s., 23 is), 947-53 (1953). Pustinger, .J. V.. Cave. U'.T.. Nielsen. M . L.. Spectrochim. Acta. 11,909-25 11959). Ranz. W . E . . "Principles of Inertial Impaction." Engineering Research Bull. S o . 66. Penn S t a t e Univ.. 1956. Ranz. R. E.. CVong. .J. B., Ind. Eng. C h e m . . 14 ( 6 ) . 1371-81 (1952).
Sehmel. G . A , . Paper No. 73-162 presented at the 66th .i\nnual Meeting of' the Air Pollution Control Ass.. Chicago. Ill.. .June 1973. Tuesday. C . S.. E d . . "Chemical Reactions in C r h a n Atmospheres." pp 241-67. American Elsevier. S e w York. S . Y . . 1971. Vedder. LV..Hornig. D . F.. ./. Chrrn. P h p . . 3.5 ( 5 ) .1560-8 (1961). Wagner. E. L.. Hornig. D . F.. ibid , 1X ( 3 ) . 296-300 (1%0). Whitby. K. T.. Husar. R. B.. Liu. B. Y. H . . J C'oiioid Intcrfacc3 Sci.. 3 9 (11,177-204 (1972a) (also reprinted in Hidy. 19721. Whitby. K . T.. Liu, B. Y. H.. Husar. R . B.. Barsic. S . ,J., ibid.. p p 136-64 (1972b) (also reprinted in Hidy. 19721.
Receiced for recieic, Auguht Xi. 1973 rlcccptc~! October 26. 1 W i . Work performed under t h e auhpic'e,\ of t h c A t o m i c Enere). C'rimmi sion
Hydrolysis of Polyurethane Foam Waste Lee R. Mahoney,' Steven A. Weiner, and Fred C. Ferris Chemistry Department, Scientific R e s e a r c h Staff, Ford Motor Co., P. 0. Box 2053, Dearborn. M i c h . 48121
w The reaction of polyurethane foam with superheated water has been investigated as a method for the volume reduction and possible material recovery from the increasingly large amounts of low-density scrap foam generated from junk car shredders. Upon reaction for 15 min with superheated water a t 200"C, the low-density foam is converted to a liquid more dense than water. A 65-8570 theoretical yield of toluene diamines and a 90% yield of liquid polypropylene oxide are isolatable from the liquid. A systematic kinetic study revealed t h a t the rates of formation of the toluene diamines, TDA, and polypropylene oxide in the temperature range 160-190°C are given by the pseudofirst-order rate expression
where
0
[NH-C-]
I1
represents the total amounts of unreacted urea and polyether-urethane linkages. The temperature dependence of the rate constant k' is given by the Arrhenius expression
log h'
=
12.7 -
29,000
4.6 T ( O K )
It is concluded t h a t the hydrolysis process represents a viable solution to the solid waste disposal problems associated with scrap polyurethane foam materials. A late 1960 model automobile contains nearly the same percentage by weight of plastic materials as does solid municipal waste, namely 2-3%. Although a good deal of discussion and some research have been directed toward the problems associated with the solid waste management of plastics in municipal waste (Warner et al., 1970b), no attention has been given to the unique problems which To whom correspondence should he addressed
may be involved in the disposal of waste plastic materials generated from the recycling of junk vehicles. In contrast to the plastic materials in municipal waste which are ultimately disposed of in some 20,000 sites in this country (Warner et al., 1970a), a high percentage of the plastics in automotive products will be transported and concentrated at approximately 100 sites in this country-i.e.. junk automobile shredders (Dean et al., 1973). These giant machines shred entire automobiles 50,000-200,000 units per year, into fist-size fragments. The ferrous and nonferrous metals are recovered and recycled while the nonmetallic fraction containing concentrated plastic materials are rejected and disposed of by landfill. This laboratory has carried out a n analysis of the quantities of plastic and polymeric materials to be generated annually as scrap from junk vehicles in the period 1971 through 1981 (Mahoney and Weiner, 1972). The results of the analysis revealed t h a t the weight (volume) of polyurethane foam material generated per average junk car will increase from 2.2 lb (0.032 yd3) in 1972-73 to 11 Ib (0.16 yd3) in 1976-77 to 23.4 Ib (0.345 yd3) in 1980-81. Carshredding experiments on late model cars confirm these projections and reveal that greater than 95% of this foam material is collected in pieces larger than %-in. in the airfraction collector a t the auto shredder (Dean et al., 1973). In urban areas, the disposal by landfill of this low-density material, which is resistant to biodegradation (Darby and Kaplan, 1968), will represent a n increasing cost item for the junk car processor and is thus an impediment to the economical collection and recycling of junk automobiles. A method of volume reduction and disposal of this waste foam material should be inexpensive. ecologically sound and technically simple enough to be carried out a t the auto shredder site. The present work describes the results of a systematic study of a hydrolysis method which potentially satisfies all of these requirements and which, in addition, offers the possibility of recovery of valuable materials from the hydrolysis products,
Materials and Methods
Foam Utilized for Study. Unless otherwise stated, all experiments were performed on samples of high-resiliency Volume 8, Number 2, February 1974
135
