Determination of Particulate Lead Content in Air Results of Tests in City Traffic BARBARA J. TUFTS Cloud Physics Laboratory, Deparfment of Meteorology, The University of Chicago, Chicago, 111.
b Individual particles which contain lead, in both soluble and insoluble form, are identified by a micro spot test on membrane fiiters. The testing reagent is an alcoholic solution of tetrahydroxyquinone, which forms a red precipitate with lead. The light microscope reveals the reactions of the lead-containing particles as discrete spots, which may be counted and sized. Calculations are given for determining the original size of the particles. Air samples collected in several heavily traveled sections of Chicago showed that the lead particle concentration varied with the time of d a y and the average trafwspeed. Knowledge of the particle size spectrum permits estimation of the respiratory retention of the lead and also provides a rapid means of spot-checking potential danger spots in industries using lead and lead products to ascertain the actual inhalation hazard.
R
ECENT discoveries in atmospheric
chemistry include those of new particulate species and a new understanding of the interrelation of these chemical constituents with each other and the atmosphere a t large. Also many new techniques of collection and analysis have been developed. This paper describes the development of an analytical method for lead compounds in particulate form, which might also lend itself to an evaluation of their physiological effects. The common use of gasoline containing tetraethyllead as an antiknock agent prorides a plentiful source of lead compounds. Lead is well known to be a cumulative poison, with a rated maximum allowable concentration for the industrial 8-hour working day of 150 y per cubic meter. Recent work (6) has shown that the amount of lead compounds discharged into the atmosphere under all driving conditions depends to some degree on the speed and acceleration of the engine. Also a correlation between the size of the particles and the engine speed was shown. Research (3, 4) on lung retention as a function of particle size indicates that ranges from 1 to 3 microns are retained almost completely.
238
ANALYTICAL CHEMISTRY
Most of those smaller appear to be exhaled again and the larger sizes are trapped in the upper respiratory passages. Accordingly, a method for determining concentration and sizes of leadcontaining particles, applicable in areas of lead contamination, n as deyiscd and supplements methods iyhich give only the total mass of lead present. This study combines t n o previous techniques used for atmospheric sampling and testing. The method for the identification of lead of Amdur and Silvermen ( 1 ) involves the use of an alcoholic solution of tetmhydroxyquinone, which gives a deeply colored red precipitate nith lead. In their method, the sample is collected on filter paper and buffered reagent is applied to the area. The color developed is compared nith a chart in nhich knon-n amounts of lead are present. The lead content is indicated by the depth of color in the standard spots. The sampling method is bawd on the xork of Lodge (6) who used XUipore fltcrs (hlillipore Filter Co., Watertown 72, Mass.) for collection, and floated them on the appropriate reagent to cause a specific reaction of the collected particles. The Millipore filter n-as made transparent with immersion oil and examined visually for the sites of such reactions with a light microscope. EXPERIMENTAL
The reagent used was a saturated solution of tetrah3.droxlciuiiione (K & K Laboratories, Long Island City 1, S. Y.) in 50% aqueous ethrl alcohol The pure free phenol must be used. More strongly ethanolic solutions will distort the filters. The sample t o be tested was placed for 5 minutes on a piece of blotting paper well soaked Ivith the reagent. Washing mas not necessary. Tests made with and v-ithout a buffer showed its effects here to be negligible. The reagent is relatively unstable and must be freshly made each day. After reacting, the membrane filter was transferred to a glass elide and dried in a desiccator or on the barely r a r m surface of a hot plate. The dry sample was mounted in a drop of im-
mersion oil and examined a t moderate magnification with a light microscope. Soluble lead compounds react without further treatment. Samples of insoluble lead compounds were exposed to fumes of hydrofluoric acid for 1 hour in a covered plastic dish before reaction. To prevent inhibition of the reaction of soluble particles by the hydrofluoric acid, the final method neutralized the acid remaining in the filter by inverting it over a fern drops of ammonium h j droxide for about 3 minutes before reaction. The colored precipitate caused by the reaction is a very stable clear light rrd which does not fade on standing. The color developed by the insoluble particles may be a darker red, but is distinctive. The pH of this solution is stable a t 4.5 without buffering (this may vary from lot to lot of the reagent and should be checked with each new supply of the phenol and adjustment made if necessary). Under these conditions, calcium, silver, ferrous, and ferric ions gave no reaction. Barium ion will occasionally give a greenish yellow reaction spot which is readily distinguishahle from that of the lead ion. Type HA hlillipore filters, with grid marks, were used. Blank tests showed the lead in the filter proper to be too uniformly distributed to interfere. The lead in the marking ink shows as 3 general pink color along the grid lines, but this does not interfere with particulate tests. The method was reproducible over about 25 laboratory samples, and is sensitive to individual particle sizes of the order of tenths of microns. In counting, the diameter of a circle of equal projected area was taken as the diameter of soluble lead particles. The diameter of the undissolved nucleus was similarly measured in the insoluble particles. In field testing the method, an aerosol standard filter holder for h4illipore filters was used. This was clamped to the outside mirror support of a car, and connected to the windshield wiper vacuum sirstem. A flowmeter in the line showed sampling rates to be about 26 liters per minute. RESULTS
In previous methods using the micro spot test technique on Millipore filters and gelatin, a definite size relationship existed between the original particle and the reaction site (8-10). The
’7
APRII
23. 1957
25
Figure 1. Reaction of soluble particles with tetrahydroxyquinone Line of this and other photomicrograph. is 50
-i?max--
290,000
...
280.000
290000
Figure 2. Reaction of insoluble lead particles, treated with hydrofluoric acid, with tetrahydroxyquinone
3:30 P.M
> PM
’.M.
6.00P.M.
!E
Figure 5. Particle cancentratian and vehicle concentration for 1 0-minute sampling intewals
Figure 3. Reaction of soluble lead porticles, treated with hydrofluoric acid and ammanium hydroxide, with tetrahydroxyquinone
Figure 4. Sample collected from tailpipe of car, reacted with tetrahydraxyquinone, after treatment with hydrofluoric acid and ammonium hydroxide
method of establishing this relationship has been described by Lodge (7). This proved to be true in this case also. However, soluble and insoluble lead compounds differed in this respect. Both types of compounds were examined to provide models for studies of industrial contamination. The example of a soluble salt u-as lead nitrate; that of an insoluble salt, lead monoxide. For calibration purposes, the complctely water-soluble and completely water-insoluble substances were used t o cover the total range of possible growth characteristics found in natural atmospheres. The two types differed in appearance, so it was usually possible to distinguish between them. The soluble particles (Figure l ) , untreated, yielded a linear relationship: dh = 8.2d,. The estimated geometric standard deviation of the constant aas 1.02-that is, approximately, the value is expected t o be accurate within
+2%-where
dr and d, are reaction
site and particle diameter, respectively. The insoluble particles (Figure 2 ) treated with hj-drofluoric acid only, obeyed a higher order law: dn = 0.98 dp*.”. This simplified statistical treab ment does not yield precise information on the variance of the, constants in this type of expression; however, it is estimated that they are accurate within about 10%. After the acid and ammonia treatment, the insoluble particle equation remained unchanged, but the soluble particle rzlat,ionship (Figure 3) was changed to dh = 1.40 d?, v i t h a geometric standard deviation of 1.015 for the constant. It was now very difficult to distinguish visually between the two types but because the size relationships of the two classes of compounds were now similar, it became less important. Some samples werc collected a t the tailpipe of a car and examined for lead VOL 31, NO. 2, FEBRUARY 1959
239
*,[
APRIL 25, 1957
160,000 156,000
110,ooc 134,000
Ioo,ooc
sqooo I10,OOO
80,000 m
100,000
I
r
70,000
cn W -1
9
& 60,000
total )articles
aQ
i
I
porticles LI/- il/
90,000
0.5/-5.2/
80,000~ CI
50,000
70.WO-
cn a W
w u)
0a 40,000
U
60,000-
2
2 50.000-
30,000
v)
$ W
20,000
40,0001
l0,000
00 RM. 3:OO RM.
4.w-I
P.M
TIME Figure 6. Particle concentration and vehicle concentration for 20-minute sampling intervals
230PM
300PM 330PM400PM
TIME
content in relation to other debris resulting from combustion of gasoline (Figure 4). An average of 47, lead compounds in the total particles collected ivas found. FIELD TEST METHOD
The method was applied to the study of leaded particulates from auto eshaust under field conditions. Samples were taken on Chicago streets, usually during a time \Then traffic could be expected to increase to a peak and then diminish. The objectives were to test the method under field conditions, to check actual levels of concentration a t heavily traveled points, and to correlate, if possible, particle size TI ith 240
ANALYTICAL CHEMISTRY
Figure 7. Particle, mass, and vehicle concentrations for 1O-minute sampling intervals
engine speed. The work of Hirschler ( 5 ) indicated that in city stop-and-go driving, large amounts of lead are built up in the engine exhaust system, and given off in large particles, particularly as the car starts after a halt. Further, in the first few miles of high speed or country driving, these deposits are blown out of the system, and thereafter during the high speed driving, the particles are given off uniformly and in small sizes. The first site was a busy intersection with stop signs a t each corner, so that each vehicle was forced to come to a complete halt before proceeding. The
sampling station was just north of the intersection, on the east side of the street, and the sampling intake pointing west, across the stream of northsouth traffic. The intake was about 3 feet from the nearest line of vehicles and about 3 feet above the ground. The wind was from the southwest. Samples were taken a t IO-minute intervals from about 3:OO to 6:OO PXthat is, during the rush hour. The sampling site was not in a restricted traffic zone, so trucks and buses were also passing the sampler. Results are shown in Figure 5. The second site mas on C. S. Highn a y 66,west of the city of Chicago. This area has a posted speed limit of 45
miles per hour, though inany vehicles seemed t o be exceeding this limit. The sampler was placed north and downwind of the highnay ivith the collection head pointing slightly south of west. It was about 5 feet above the road level and 10 feet from the nearest vehicles. The wind )vas from the southwest. Samples were taken a t 20-minute intervals from about 2:OO to 4:OO P.u. Trucks and buses vere also included in this survey. Results ale shown in Figure 6. The third sample n a s taken on a street in a small shopping district. The two-lane street reduced the total number of cars passing the sampler, r et there was a nearly constant stream bf traffic. The sampling head \?as on the south side of the street, pointing
