Characteristics of sulfur aerosol in Florida as determined by pixe analysis

CHARACTERISTICS OF SULFUR AEROSOL IN FLORIDA AS DETERMINED BY PIXE ANALYSIS M. S. AHLBERG, A. C. D. LFNIE and J. W. WINCHESTER Department of Oceanography. Florida State University, Tallahassee. FL 32306. USA. Ahstraet-The particle size distribution of sulfur has beeu determined in Florida during two seasons, July-August and December 1976. Approximately 120 cascade impactor samples from 10 widely distributed urban and nonurban sites in the state. each sample consisting of 6 separate particle size fractions from <0.25q to >4~ aerodynamic diameter, were analyzed by proton induced X-ray emission for elemental constituents in over 800 individual specimens, including blanks. Most of the sulfur occurred in a fine particle mode. c2 pm diameter. with lesser amounts in a coarse mode. r2 p. The mass median aerodynamic diameter, MMAD, of sulfur in the fine mode was found to be greater for samples collected under higher average relative humidity, r.h.. conditions than samples from lower humidities. The trend of MMAD with r.h. indicates that ammonium sulfate, rather than sulfuric acid, is more likely to be the principal chemical form of sulfur in the fine mode.

INTRODUCZHON

The atmospheric concentration of sulfur in particles of different size can be indicative of the principal natural and pollution sources, atmospheric transport and removal, chemical speciation and potential biological impacts of importance to man. Because sulfur is generally a reactive element under natural conditions, however, its concentration in discrete particle size ranges may not be as indicative of these aspects as may be the relationships between concentrations in different particle size ranges under various environmental conditions. In this paper (paper 1) we explore

some of these relationships using as a data base a series of samples collected in Florida during two periods in middle and late 1976. In a separate paper (paper 2) (Leslie et al., 1978) we examine certain geographic trends in the sulfur concentrations. EXPERIMENTAL Cascade impactors were operated during the periods 28 July-7 August and 5-18 December 1976, at 10 widely separated nonurban and urban sites for six 24 h intervals in each period. Tim sites were located as shown in the index map, Fig. 1. A discussion of the field program for sample collection and the specific samphng sites has been

KEY PN

JAX

KS l-PA AR PW MB MIA

Pensacola

Jscks,wil!e K,ssw-n~~~t Tamps Arcadm Pahoksa Marco Bsach Mtsmi

SAMPLING AEROSOL

SITES

FOR

SULFUR

WARACTERIZAT~ON

Fig. 1. Index map of Florida showing locations of sampling sites Samplers at PN, GV, JAX, KS, AR. PH and MB .were Iocated at smatl airports and at TL. TPA and MIA within the cities. 773

M

774

S. AHLFJ~RG.A. C. D. Lts~~r and J. W. WINCH~S~EH

I

1

qs,

m y

I

1

105-

lo,

200

1

I

TAMPA d,

STAGE

400 CHANNEL

600 NUMBER

I

I

5 AUG

76

1 (>4q)

I

800

Fig. 2. Example of X-ray spectrum from a PIXE analysis showmg charactenstic X-ray peaks of the elements resolved from background radiation.

given by Leslie et al. (1977). SIX particle size fractions, from >4pm to <0.25pm dia. were analyzed for S as well as for Cl. K. Ca. Ti. V. Fe. Zn, Br and Pb using proton induced X-ray emission, PlXE (Johansson YI al.. 1975). The X-ray spectra. exemplified by Fig. 2. were resolved by computer (Kaufmann rt al.. 1976).

RESULTS

Typical of the results of this investigation are the geometric mean particle size distributions for sulfur shown in Fig. 3 for Pensacola, Tampa and Marco Beach in the summer and December 1976 periods. The trends suggest the presence of a coarse particle mode. 2 2 pm dia. (stages I and 2) and a fine particle mode, 52 e (stages 3-6). The abundance of sulfur in the fine mode exceeds that in the coarse mode at all three locations. Dispersion of solids and liquids. e.g. sea spray, may be the predominant origm of coarse particle sulfur. Fine particle sulfur, on the other hand. is more likely to originate in the conversion of gaseous SO, to sulfuric acid and sulfates. Because of the geochemical independence of these two modes. we have considered their relationships separately. Fine particle sulfur. because of its link with SO1 emissions from pollution sources. is of principal interest. Empirically we judge the fine mode to be principally stages 3-6. The weather during the summer 1976 period was especially favorable for examining in detail the relationship between relative humidity and the particle size distribution of sulfur within the fine mode. It was typical for summertime over most of Florida with early morning fog. scattered afternoon showers and thundershowers. along with a generally southerly flow as a rule over the region. Relative humidity averages over the 24 h sampling intervals ranged from below 70% lo over 90% The range. therefore, covered the region on both sides of the critical humidity of 81~~

for the conversion of crystalline ammonium sulfate to aqueous solution droplets. In the discussion which follows it is important to note some details concerning a frontal passage or instability lines which developed during the sampling period. From 28 to 31 July the Bermuda high was centered well off the east coast of Florida with a narrow ridge extending westward through the central and south regions. Surface winds were southwesterly over the north and northwest. were variable over cen-

.---f+J

A-TF’A x_____p&

7/2 0 - 0/O?/ 76

1mpatcf

12/05-B/76

stags

Fig. 3. Particle size distributions of sulfur at sites PN. TPA and MB during summer and December 1976 periods. expressed as geometric mean. with its standard deviation. for each group of approx six samples taken in each period. Absolute calibration of concentrations in air is significant to a factor of 2.

Characteristics

775

of sulfur aerosol

Table 1. Mass median aerodynamic diameter. in pm, for sulfur in the small mode I2 pm. and corresponding averaged relative humidity. Date. a-29

29 -30

3&3

046 796

039 74 8 043 824 042 779

PN TL GV JAX KS

055 82.5 041 79 I

070 156 042 75 5

PH MB

027 75 I

0.32 72.8 045 711 I 040

31-I

I-?

041 7M9

040 756 04s 80.2 047 76 2 039 708 031 72 7 0.38 71 3 OS8 74.3 045 8?0 0.50 70.2

043 xv5

036 749 032 735

TPA AR

I

044 72 I

042 72. I

043 780 0.M 72.4

MIA

tral Fiorida, and were southeasterly in the south with some local sea breezes Intermittent nighttime fog and haze and afternoon to evening thunderstorms occurred throughout the state. From 1 to 4 August unsettled weather was observed over all but the extreme south of Florida. A weak Canadian cool front passed through the north and northwest regions and caused a wind shift from southwest to north and northeast. Instability lines formed over the north and moved south as far as Miami, a~m~nied by heavy thunderstorms and a switch to northerly air flow and heavy morning fog From 5 to 10 August Hurricane Belle was forming to the east of Miami, and a weak Pacific cool front affected the northwest region of the state. Whereas on 4 August a generally easterly flow was prevalent over the entire state, by 6 August the tropical depression had caused enhanced northeasterly flow over all except the northwest region. With these weather features in mind, let us examine the data of Table 1 which covers the period of frontal passage, 28 July-5 August. In the table we present the mass median aerodynamic diameter, MMAD in m for sulfur in the fine mode (stages 3-6) together with the average relative humidity in per cent for each 24 h sampling interval. The north Florida sites PN. TL, GV and JAX, exhibit a distinct increase in both relative humidity and fine mode sulfur MMAD from about 0.4 to 0.6 m on 2 August, but the central and south Florida sites, KS, TPA, AR, PH and MB show an increase in humidity and MMAD delayed until 3 August. On 4 August at TPA and AR a return to smaller MMAD was observed apparently corresponding to a decrease in relative humidity. We also noted the exact times of frontal passage across our north Florida sites: PN. 032 on 8fo3; TL, f8Z on 81’03; JAX OOZ on 8104; and GV. 04Z on 8p4. (In local EDT the times are 4 h earlier). Apparently the gradual passage of the front and its attendant humidity changes are associated with the MMAD increases we observe.

July 2-3

813 06il 930 069 1107 060 1135 045 764 040 746 039 6118 043 736 0.36 696

Au@usl 1976 3-4 058 74 I

0.56 8x7 083 91 I 0 56 868 055 830 0.51 75.2 055 81.3 067 758 050 68.5

4s

56

065 KO I 074 854 057 84

0% 7x x

67

78

89

0.54 75.6

057 70 1

058 73

I

038 71 M 044 75 1

0 39 705

063 73X

060 77 5

I

DISCUSSlON

It is of some interest to compare aerosol sulfur MMAD with that expected for its principal chemical compounds under the relative humidity conditions during sampling. On Fig. 4 we have plotted the fine mode sulfur MMAD vs average per cent relative humidity over the corresponding 24 h sampling intervals based on hourly airport weather data. In all cases, (except sites AR, PH and MB where observations at Sarasota, West Palm Beach and Fort Myers were used) the weather data were taken at or very near the sampiing sites. With very few exceptions, before frontal passage humidities were well below 81% and MMAD 0.3-0.5 p. After frontal passage MMAD were >OS m and 24 h average humidities ranged both above and betow 8to/, On Fig. 4 we also show the theoretical variation of MMAD with per cent relative humidity for H$O., and (NH,),SO,. based on Hiinel (1976) with the H$O, and dry (NH&SO, curves fit to the CO.5 pm points before frontal passage. The phase change at 81% from solid to aqueous (NH&SO., is experienced with rising humidity, but with falling humidity supersaturated solutions may persist well below this value. Consequently, the supersaturated curve is also shown. For H,SO,, which is liquid at all humidities, no phase change occurs The data points appear to conform to (IUH&SO~, rather than H#O, as the principal chemical form of sulfur. It seems sign&ant that there are no points near the H2S0, curve at high humidities. Of course, in view of the long 24 h sampling times, in which considerable diurnal humidity variation occurred, our ability to distinguish between the two forms is not as precise as if sampling were performed under constant humidity conditions. If sampling was restricted to shorter time intervals, with precise humidity recording at the sampler location or by using samplers activated by humidity sensing devices, greater precision in dis-

776

M. S. AHLBERG. A.

C. D. LESLIEand

J. W. WINCHESTER

OMMAD.before

lMMAD,after

0.3

0.5

frontal passage frontal passage

0.7

0.9

MMAD, Sulfur Small Mode(pm) Fig. 4. Plot of mass median aerodynamic diameter of sulfur m ~2 w particles (impactor stages 3-6) against r.h. averaged over the 24 h sampling periods.

tinguishing these two chemical forms may be achieved. Alternatively, the humidity of the air before sampling could be controlled The MMAD values were calculated using concentrations from all the impactor stages 3-6, in which sulfur is most abundant in stages 4 and 5 (Fig. 3). The method, therefore, does not reveal possible differences in the chemical form of sulfur which may exist

0

Sat

35%r.h.

0

Sat

98%r.h.

V V at 35 % r. h. v

Vat

98%r.h.

in the different fine mode particle size ranges. With more extensive data of this kind. such as mentioned above. an extension of the method may provide evidence for such differences. Figure 5 reports laboratory measurements of aerosol-humidity relationships for ambient sulfur and vanadium (Ahlberg et a/.. 1977). Two impactors. one equipped with a horizontal humidification tube halffilled with water simultaneously sampled air in the laboratory. One sample represented the particle size distributions of the elements in air at its ambient relative humidity of 35%, and the other sample represented distributions at a relative humidity of over 9%; because of passage of the air over water in the tube. Sulfur showed substantial growth of stage 6 particles ( <0.25 q) whereas vanadium did not, indicatmg a relatively greater hygroscopic character for S in finest particles The calculated MMAD values for S were 0.34 pm before and 0.58 m after humidifkation, in agreement with the data of Fig. 4. It should be noted that this evidence also indicates that S and V must be constituents of different particles; the humidification-controlled sampling technique, therefore, appears to be useful in distinguishing trace element associations with different populations of particles. Acknowledgements-We are indebted to Stephen L. Cohn for assistance with meteorologleal analysis. The study was supported in Part by the Florida Sulfur Oxides Study, Inc., by the National institutes of Health. by the Federal Interagency Energy/Environment Research and Development Program through EPA grant R803887,and by the National Science Foundation for accelerator facilities support. We also acknowledge technical support of William E. Wilson. ESRL, EPA, Director of and Scott Rheingrover

I

6

5 4 lmpactar

3 Stage

2

I

Fig. 5. Particle size distributions of sulfur and vanadium m Tallahassee laboratory air before and after humidification pretreatment.

Project MISTT. The conclusions reached do not necessarily represent endorsement by FSOS. Inc.

Characteristics REFERENCES

Ahlberg M. S.. Leslie A. C. D. and Winchester J. W. (1977) Environmental and occupational health analyses using proton induced X-ray emission. In Proceedings of Symposium on Electron Microscopy and X-Ray Applications to Environmental and Occupational Health Analyses, Denver. Colorado. Ann Arbor Science Publishers. Ann Arbor, Michigan. Hiinel G. (1976) The properties of atmospheric aerosol particles as functions of the relative humidity at thermodynamic equilibrium with the surroundmg moist air. Adc. Geophw

19. 73-188.

Johansson ?.. B.. Van Grieken R. E.. Nelson J. W. and Winchester J. W. (1975) Elemental trace analysis of small samples by proton induced X-ray emisston. Analyt. Chrm. 41. 855-860.

of sulfur aerosol

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Kaufmann H. C.. Akselsson K. R. and Courtney W. J. (1976) REX: a comnuter oroaram for PIXE spectrum . .resolution of aerosols. Advances in X-Ray Analysis. (Edited by R. W. Gould. C. S. Barrett, J. B. Newkirk and C. 0. Ruud) Vol. 19. pp. 355-366. Kendall Hunt, Dubuque. Iowa. Leslie A. C. D.. Ahlberg M S.. Winchester J. W. and Nelson J. W. (1978) Aerosol characterization for sulfur oxide health effects assessment. Armospheric Encironmenr 12, 729-733. Leslie A. C. D.. Ahlberg M. S., Winchester J. W. and Nelson J. W. (1977) Aerosol characterization for sulfate health eRects assessment in Florida. In Trace Subsrances in Encironmental Heulrh-XI. (Edited by D. D. Hemphill) Universtty of Missouri Press, Columbia (in press).