Short term effect of rainfall on elemental composition and size distribution of aerosols in north Florida

Atmospheric Environment. Vol. 14, pp. 1421-1426. 0 Pergamon PressLtd. 1980.Printed in GreatBritain.

OCG-6981/80/1201-1421

SO2.00/0

SHORT TERM EFFECT OF RAINFALL ON ELEMENTAL COMPOSITION AND SIZE DISTRIBUTION OF AEROSOLS IN NORTH FLORIDA S. TANAKA,*

M. DARZI

and J. W.

WINCHESTER

Department of Oceanography, Florida State University, Tallahassee, FL 32306, U.S.A. (First

received

17 December

1979 and

in,jinalfirm

15 February 1980)

Abstract -

Aerosol and rain samples were collected during winter 1978-79 in Tallahassee, Florida and analyzed by particle induced X-ray emission (PIXE) to determine 12-15 elements from Al to Pb. The aerosol sampling was carried out before, during, and after rainfall episodes using time sequence filter samplers (streakers) and cascade impactors to investigate the variation of aerosol composition with rainfall. Al, Si, Ca, Fe, Mn, and Cl, predominantly large particle aerosol constituents (> 1 pm), were strongly affected by rain, their before/during rainfall concentration ratio being greater than 4, whereas K, Zn, Br, Pb and S, predominantly in small particle fractions ( < 1.0 pm), had ratios of less than 2. The degree of apparent removal followed the order Cl > Al, Si, Mn > Ca > Fe >> K > S, Pb, Zn, Br. Differences in the speed of recovery after rain for elements, to their before rainfall values were also observed. The recovery of soil-derived elements such as Al, Si, Fe, and Mn was delayed, probably because of the wetting of the soil by rain, while S, Zn, Pb, and Br, emitted from anthropogenic sources, recovered more rapidly. The enrichments (fractionation factors) in rain of Ca, K, Zn, Cu, S, Br, and Pb normalized to Fe, relative to particulate matter in the air, were 13. 7.0. 4.3. 3.4, 2.9. 0.96. and 0.36. resmctivelv. Hinh enrichments of K. Zn. Cu. and S suggest that these elements might be & relatively gr~aterconcenirati& than Fe at higher’alti;udes and be transported from moredistant sources, since their apparent extent ofremoval is less near ground level compared to soil derived elements.

SAMPLING

INTRODUCTION

Investigations of chemical composition and size distribution of atmospheric aerosols have generally been concerned with long term behavior, averaging over possibly large fluctuations that may occur over periods of a few hours due to the difficulty in obtaining large enough aerosol samples for chemical analysis. In order to investigate changes in aerosol characteristics due to rainfall on an hourly time scale, a highly sensitive method of elemental analysis, particle induced X-ray emission (PIXE) (Johansson et al., 1975) was used along with two types of aerosol samplers, streakers (Nelson et al., 1976) and cascade impactors (Mitchell and Pilcher, 1959), specifically designed for this method (Van Grieken et al., 1976; Lawson and Winchester, 1978). PIXE is a simultaneous, multi-element technique, providing rapid non-destructive analysis of small samples at the nanogram level. Streakers were used to obtain two-hour time resolution of total elemental concentrations, while elemental size distributions were obtained using six stage cascade impactors before, during, and after rainfalls. The methods of rainwater analysis and some elemental composition relationships in rainwater have been reported elsewhere (Tanaka et al., 198Oa, b).

* Present address: Department of Applied Chemistry, Keio University, Yokohama 223, Japan.

AND

ANALYSIS

Aerosols and rain samples were collected on the roof of the Oceanography-Statistics Building on the Florida State University campus, Tallahassee, Florida, which is located in a relatively unpolluted region of southeastern U.S.A. having only light industry. Three week-long sampling periods, which included rainy days, were used and are shown, together with rainfall periods and amounts, in Fig. 1. The streaker sampler, with a 0.1 cm* sucking orifice moving along a strip of 0.4pm pore diameter Nuclepore filter at a rate of 1.0 mm h-‘, operates continuously for up to one week at an air flow rate of 0.8 P min-‘. The filter strips were then analyzed in 2 mm steps, giving two hour time resolution of aerosol concentrations. The six stage, single orifice, Battelle type cascade impactor produces size fractions of > 4, 4-2,2-l, l-0.5, and 0.5-0.25 pm aerodynamic particle diameters (pmad) on stages l-5, respectively, with particles less than 0.25 pm being caught by a 0.4 pm pore diameter Nuclepore filter on stage 6. Samples of 6 to 24 h, at an air flow rate of 1.0 f min- l, were obtained by impactors before, during, and after rain periods. The rain samples were collected in polyethylene bottles for l-4 h intervals during each rainfall episode. The aerosol samples were analyzed, with no further preparation, by PIXE during bombardments of a few minutes by a 5 MeV proton beam. The X-rays emitted were detected by a Si(Li) detector and the spectra were

1421

1422

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DATE

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NOVEMBER

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DECEMBER

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1979

Fig. 1. Times of aerosol sampling by streakers (Sl, S2, S3) and impactors (It to HO),of rainfall fm, mm precipitation), and of rain samples collected (\i) in Tallahassee, Florida.

then resolved by a computer program (Kaufmann et al., 1977). The rain samples were filtered by 0.41.lm pore diameter Nuclepore filters and then concentrated by evaporation. An ahquot of this concentrate was then dropped on Mylar film and allowed to dry. Both this soluble material and the insoluble material on the filter were also analyzed by PIXE (Tanaka et al., 1980b). In impactor samples and most streaker samples, 12 elements, Al, Si, S, Cl, K, Ca, Mn, Fe, Cu, Zn, Br, and Pb, were detected, and 15 elements, Al, Si, S, K, Ca, V, Cr, Ti, Mn, Fe, Ni, Cu, Zn, Br, and Pb, were detected in the rain samples.

Fig. 2. Size distribution for Ca and Fe before (impactor 6), during (average of impactors 7 and g), after (impactor 9) rainfall episodes. Approximate aerodynamic diameter ranges of particles collected by the impactor stages are > 4,4-2,2- 1. l&OS, 0.5-0.25, and ~0.25 itrn for stages 1-6, respectively.

impactor stages. The before/during rainfall ratios on stage 1, 8.9 for Ca and 6.9 for Fe, were higher than other stages, indicating greater susceptibility of large particles to removal by rain. Figure 2 shows that the recovery of Ca and Fe concentrations after rainfah

ii

RESULTS AND DISCUSSION

In order to investigate the effect of removal by rain on elemental size distributions, aerosol impactor sampies were collected before, during, and after rainfalls in three sampling periods between November 1978 and January 1979. Previous investigations of aerosol composition in north Florida (Johansson et al., 1976) have indicated that particle size distributions of many of the elements investigated here are relatively insensitive to airmass history. Consequently, we interpret changes mainly in terms of the effect of rainfall on particle removal from the atmosphere. The results of the third sampling period (see Figs. 2 and 3) show the typical extent to which Ca, Fe, Sand Pb in aerosols are removed by rain. A total of 14 mm of rain fell from 8:00 p.m., January 11 to 5 :00 a.m., January 13. Impactor sample 6 was collected before the rain, samples 7 and 8 during the rain, and sample 9 after the rain, as shown in Fig. 1. The soil-derived elements Ca and Fe, which were prin~palIy found in coarse particles (> 1 pm), showed large decreases in concentrations during rainfall for all

S

i

Pb

I

I

!

t u

/

L --..‘.-4’

;._.;y--_

A~__

654321

_.-..

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Fig. 3. Size distribution for Sand Pb before (impactor 6) and during (average of impactors 7 and 8) rainfall episodes. See caption to Fig. 2.

1423

Short term etfect of rainfall on elemental composition differed according to particle size, as shown by the after/during rainfall ratios. The concentrations of the large particles on stage 1 increased immediately after rainfall to nearly their before rainfall concentrations, whereas small particle concentrations on stages 3 and 4 did not increase or even continued to decrease. S and Pb, emitted mainly from anthropogenic sources and found in the smaller particles, had before/during rainfall ratios for stages 4, 5, and 6 of less than 2, as shown in Fig. 3, while their stage 1 and 2 ratios were 2-4, not as large as for Ca and Fe. (A comparison of after rainfall data for S and Pb with those before or during, as was made for Ca and Fe, is not as meaningful because of their anthropogenic sources and the effect of variations near the sampling site and on the weekend.) Table 1 shows the concentrations of fine ( < 1 pmad, stages 4, 5, and 6) and coarse (> 1 pmad, stages 1, 2, and 3) particles for Al, Si, Cl, Mn, Ca, Fe, K, Zn, Br, Pb, and S, before (impactor sample 6) and during (impactor samples 7 and 8) rain episodes, and their before/ during rainfall ratios. The approximate mass median aerodynamic diameter (MMAD) for each element is calculated from the before rain sample, assuming a 10 pm maximum particle diameter for stage 1, and is also shown in Table 1. Al, Si, Mn, and Cl, produced largely by dispersion of soil dust or sea spray, were only found as large particles with MMAD of 6.0, 6.4, 6.2, and 5.0 pm, respectively. As shown in Table 1, their before/during rainfall ratios are greater than 10, indicating efficient removal. Ca and Fe, also predominantly coarse particles, with MMAD of 6.2/1m for Ca and 5.7 pm for Fe, had somewhat smaller before/during rainfall ratios for the coarse mode of 8.3 and 4.8, respectively. Fine Ca and Fe were found only on stage 4 and their before/during

Table 1. Comparison

ratios were 4.8 and 2.2, respectively. K, Zn, Br, Pb, and S, found mainly as small particles, with MMAD of 0.54, 0.60, 0.52, 0.41, and 0.30, respectively, had total before/during rainfall ratios of 2 to 6 times lower than for soil elements, indicating a much smaller degree of removal. The large difference in the before/during rainfall ratios between these two element groups for the coarse mode is due to a difference in the mass distribution of that mode. From 60 to 80% of the Al, Si, Cl, Mn, Ca, and Fe was found on stage 1, but less than 20% of K, Zn, Br, Pb, and S (cf. Figs 2 and 3). Similar reasoning also accounts for the difference in the fine mode ratios. Figure 4 clearly shows this dependency of removal on particle size, in agreement with previous observations (Gatz, 1976). Differences in extent of removal were also observed among elements with similar size distributions. For example, the removal effect was greater for Ca than for Fe, as can be seen in Fig. 2, suggesting a dependency on the solubility of the element (Tanaka et al., 1980b). Aerosol Cl is also highly soluble and exhibits a high before/during rainfall ratio of 17. The greater apparent removal of Al and Si relative to Ca cannot, however, be explained by solubility differences since Ca is more soluble. Instead there may be a suppression of dust-forming activity of their source, the soil, during rain. Although Ca and Fe are also considered to be soil-derived elements, their lesser removal relative to Al and Si suggests that they may have mineralogically distinct sources influenced differently by rain; Kowalczyk et al. (1977) report analogous distinctions for urban areas. In order to help understand the compositional variation of aerosols in the atmosphere before, during, and after rainfall, the elemental ratios relative to Fe for January rain episodes are given in Table 2. The ratios

of elemental concentrations and ratios of before* and duringt Tallahassee, Florida, January 1979 Concentration, ng mw3

Element

MMADj (pm)

Al Si Cl Mn Ca Fe K Zn Br Pb S

6.0 6.4 6.2 5.0 6.2 5.7 0.54 0.60 0.52 0.41 0.30

Concentration ratio

Fine5

Coarse]/

Before During

Before During

NDll ND ND ND 16 9.2 55 4.8 64 321 1420

ND 7.5 ND 17 3.3 4.2 29 4.9 51 195 848

189 505 273 ND 515 193 61 5.3 52 179 96

rainfall episodes,

15 40 16 1.6 62 40 22 1.5 25 98 23

Before/during Fine

Coarse

-

13 13 17 11 8.3 4.8 2.8 3.5 2.1 1.8 4.2

4.8 2.2 1.9 1.0 1.3 1.6 1.7

* Impactor sample 6. t Average of impactor samples 7 and 8. t Mass median aerodynamic diameter calculated from impactor sample 6 data. 4 Fine particles (< 1 pmad), impactor stages 4-6. oarse particles (> 1 pmad), impactor stages l-3. !I :D = not detected.

Fine +coarse 13 11 17 11 8.1 4.4 2.3 1.6 1.5 1.7 1.7

1424

S. TANAKA.M. DAR-LX and J. W. WINCHESTER

Fig. 4. Dependence of washout concentratjon ratio, beFore/ during, a measure of removal by rain, on MMAD (mass median aerodynamic diameter) calculated from impactor 6 data. of Al, Cl, Mn, Si, and Ca relative to Fe greatly decreased during rainfall due to a greater removal of Cl and Ca than of Fe or to a greater source suppression by rain, such as may be the case for Al, Si, and Mn. On the other hand, the ratios for K, S, Pb, Zn, and Br more than doubled due to a smaller decrease of their concentrations during rainfall than for Fe. The before/ during rainfall concentrations normalized to Fe can indicate the extent of the removal effect for each element, although the variation of these ratios also depends on the activity of sources during rainfall. From Table 2, the removal order for the elements is seen to be Cl > Al, Si, Mn > Ca > Fe >> K > S, Pb, Zn, Br. The after/before rainfall ratios normalize to Fe give the extent of recovery for each element after rainfall. This ratio was high for S, Zn, K, Pb, and Br, whereas the recovery ratios of Al and Si were less than 1.0,

Table 2. Variation _ Ratio*

_.

of elemental

~on~ntrations Tallahassee, _. ____-_.__ _I_-__ Before raint

_____ _~.__-...-.__.-~ Cl/Fe t.4 Al/Fe Si/Fe Mn/Fe Ca/Fe K/Fe S/Fe Pb/Fe &/Fe B;/Fe

0.94 2.5 0.084 2.6 0.57 1.5 2.5 0.050 0.57

samples

relative to Fe in the air for before, during and after rainfall, Florida, January 1979

During rain:

After rain!

0.36 0.34 1.1 0.036 1.5 1.2 20 6.6 0.14 1.7

1.7 0.56 2.3 0.08 1 4.6 0.89 15 3.3 0.66 0.084

__..______---_-

* Elemental ratios in order of decreasing + Impactor sample 6. : Average of impactor 6 Impactor sample 9.

indicating delayed recoveries for these elements. The recovery of elemental concentrations after rainfall should depend on the sources for each element. Elements emitted mainly from anthropogenic sources, such as S, Zn, Pb, and Br, could continue to be supplied from sources during and after rain episodes, but the elements derived from dispersion of soil, such as Al and Si, may be suppressed even after a rainfall due to the wetting of the soil. From Table 2, the recovery order for the elements observed is S, Ca, Zn, K > Pb, Br, Cl > Fe, Mn > Si, Al. Although Ca is considered to be largely soil derived, its high recovery ratio implies a distinct source process or mineralogy in Tallahassee, as mentioned previously. Figures 5 and 6 show the time variation of the concentrations of Pb and Br, Ca and Fe, and of the weight ratios of Br/Pb, Ca/Fe for each two hour interval in the third sampling period, from 1500, January 10 to 1500, January 17, 1979. Figure 5 indicates that the variations of Br and Pb are very similar, with concentration peaks occurring in the morning and evening (January 11, 15, and 16), corresponding to periods of high motor vehicle use. The correlation coefficients between Br and Pb before, during, and after (I and II, corres~nding to the change of impactor samples from I9 to 110, respectively) rain periods were very high, r = 0.96 (n = 14), I = 0.90 (n = 18) and r = 0.98 (n = 52), respectively. The concentration ranges of Br and Pb were also large, 10 to 500ngm-3 for Br and 40 to 2000ngmW3 for Pb, consistent with their production by local sources. The sampling site is surrounded by a large parking area which can be expected to strongly influence these concentrations. Large concentration decreases for Br and Pb were not observed during the rain period on January 12 while a concentration peak was observed in the evening as for non-rainy days, both facts indicating a small degree of removal and the same for both elements, the Br to Pb ratios before, during, and after (I) rain periods being constant, 0.18 k 0.08 (n

7 and 8.

removal

-.-.

ratios

--

(before/during

Removal ratio Before/during 3.9 2.8 2.3 2.3 1.7 0.48 0.38 0.38 0.36 0.34 rainfall).

Recovery ratio After/before 1.2 0.6 0.9 1.0 1.8 1.6 2.0 1.3 1.7

1.2

Short term effect of rainfall on elemental composition

Fig. 5. Time variation of Br and Pb concentrations and Br/ Pb ratio before, during and after rainfall episodes, TaUahassee, Florida, January 1979.Average values of time steps are given numerically near lines of lengths of the averaging intervals. Times are hours EST. After rainfall episodes I and II correspond to the change of impactor samples from I9 to I IO, respectively.

Dote

Jonuory

1979

Fig. 6. Time variation of Ca and Fe concentrations and Ca/ Fe ratio before, during and after rainfaIl episodes, Tallahassee, Florida, January 1979. See caption to Fig. 5.

1425

= 14), 0.17 5 0.08 (n = 18), and 0.08 f 0.10 (n = 14), respectively. The concentrations of Br and Pb appear to depend primarily on the strengths of their sources. The lower concentrations observed after rainfall on January 13 and 14 can be expIained by the lower amount of driving on the weekend, amounting for recovery ratios that are lower than for S, Zn, and K, given in Table 2. The variations of Ca and Fe concentrations in Fig. 6 are similar except during rainfall. Concentration peaks for both Ca and Fe were usually observed in the moving and evening, the lowest concentrations occurring from midnight to early morning on non-rainy days (January 11,15, and 16). The behavior of Ca and Fe on non-rainy days was quite similar to that of Pb and Br, with concentration peaks occurring at the same time, implying increased soil dispersion for Ca and Fe, perhaps because of moving motor vehicles. The correlation coefficients between Ca and Pb before and after (I and II) rain periods were high, r = 0.74 (n = 14) and r = 0.68 (n = 51), respectively. The behavior of Ca and Fe during the rain period from 2000 h, January 11 to 0700 h, January 13 was very different from that before and after the rain. The concentration peaks observed on non-rainy days did not occur on January 12, and a large decrease of both concentrations was seen. The removal of Ca was especially marked, its concentration decreasing immediately when the rain began to fall and not being detected four times during the rain period as shown in Fig. 5. The elemental ratios of Ca to Fe before, during, and after (I) rain periods were 2.5 &- 1.2 (n = 14), 1.2 + 1.0 (n = 14), and 3.9 f 3.0 (n = 13), respectively, with a removal ratio of 2.1 and a recovery ratio of 1.6, agreeing with the impactor results in Table 2. The correlation coefficient for Ca and Pb during the rain period was Iow, r = 0.24 (n = 14), due to the differences in extent of washout. Table 3 lists the weight ratios of elements to Fe for aerosols and rainwater during rain episodes. The ratios for aerosols, A, are from streaker data, while the ratios for the rain, R, are from the PIXE analysis of the soluble and insoluble material in the rainwater (Tanaka et al., 198Ob). The enrichment of each element in rain can be calculated from these normalized concentrations. As shown in Table 3, the average enrichments for Ca, K, Zn, Cu, S, Br, and, Pb in seven different rainfalls were 13, 7.0, 4.3, 3.4, 2.9, 0.96, and 0.36, respectively, an enrichment order similar to that reported by Muhlbaier (1978). The high enrichment of Ca is due to its greater removal effect while the low enrichments of Pb and Br are due to their lower removal effects. Although the removal of K, Zn, Cu, and S should be lower than that of Fe, as shown in Tables 1 and 2, these elements were found to be enriched in rain relative to Fe. This can be expected for elements existing in the gas phase as well as in particles, such as sulfur dioxide in the case of S, due to a gas absorption by rain. The difference of enrichment between Br and Pb. which have similar washout

S. TANAKA,M. DARZI and J. W. WINCHESTER

1426

Table 3. Comparison of elemental concentrations relative to Fe in aerosols and rainwater during rain episodes

~____._. .--

Rain sample no.

Ratio*

S/Fe

K/Fe

Ca/Fe

Q/Fe

November 7

10

November 29

12

R A R/A R

50.4 t3.9 3.6 21.6 12.7 2.2

11.9 1.27 9.4 6.57 0.94 7.0

13.9 1.87 7.4 14.4 2.19 6.6

January 11

19

20

January 11

21

8.60 2.18 3.9 23.1 5.35 4.3 29.1 3.84 7.6

19.0 1.76 11

January 11

64.3 10.9 5.9 57.1 28.3 2.0

Date, 1978- 79

R:A R R?A R RTA R A R/A R A R/A R

24

January 12

25

January 12

R;a Average & S.D.

R/A

47.1 30.2 1.6 37.7 30.4 1.2 45.7 12.9 3.5 2.9+

17.5 2.32 7.5 12.6 1.38 9.1 7.03-

1.6

Br/Fe

Pb/Fe

0.81 0.35 2.3 0.64 _~

1.39 0.28 5.0 1.69

0.74 0.94 0.79 0.75 0.53 I .4

2.30 7.87 0.29 1.67 2.77 0.60

0.80 0.20 4,o 1.36 0.61 1.3,

0.59 0.62 0.95

2.07 4.92 0.42

21.4 71 .

19.8 1.20 17

0.25 0.096 2.6 0.39 0.15 2.6

2.36 5.35 0.44

5.9 46

18.9 0.88 22

0.93 0.20 4.7

2.21 0.53 4.2

0.63 1.0 0.63 0.68 0.50 1.4

1.29

2.8 32

14.3

0.92 0.18 5.1 0.46 0.14 3.3

0.22 6.5

0.92 0.84 1.1

0.74 0.19 3.9

0.84 1.s2 0.46

1.s7 5.97 0.31 1.64 11.0 0.15

12.8 .-

13+ 2.2

_.

3.4+ 6.6

1.42

4.3 t 1.2

1.4

ratios, suggests a gaseous phase for some of the Br. K, ‘Zn, and Cu may be transported from distant sources, and thus be abundant at altitudes of rain formation while their removal effect is lower than Fe at ground level, as has been reported previously (Peirson et ai., 1973). Therefore, high enrichments in rain are not only due to efficient removal but also to greater concentrations at higher altitudes. The very high enrichment of Ca may be due to both effects. Enrichments in rain of soil elements, such as Al and Si, couid not be calculated because their concentrations in the air during rainfall were usually below detection limit. However, these enrichment values might be expected to be lower than those of K, Zn, and CU owing to their soil source. Acknowlerigeinents - We are grateful for technical assistance from Margaret Dancy, Charles Donahue, Carole Lockridge, and Scott Rheingrover, and for the encouragement of Professor Y. Hashimoto, Keio University, to undertake this study. This work was carried out as part of the Acid Precipitation Experiment with financial assistance from the U.S. Environmental Protection Agency through the National Center for Atmospheric Research. Additional support by a predoctoral traineeship from the National Institute of Environmental Health Science for one of us (M.D.) is acknowledged. REFERENCES Gatz D. F. (1976) Wet deposition estimation using scavenging ratios. .l. Great Lakes Res. 2, 21-32. Johansson T. B., Van Grieken R. E., Netson J. W. and

4.38 0.29

0.96 & 0.36 + 0.36 0.14

* Elemental concentrations relative to Fe, R-in the rain, soluble -t insoluble components, A-in samples. t Fe concentrations in ng ml-’ (R) and ng m- 3 (A).

concentration

-.._

Zn!Fe

Fe c0nc.t ~__._._ 12.4 913 17.0 344

3.7 53 h.1 45 _..

aerosols from streaker

Winchester J. W. (1975) Elemental trace analysis of small samples by proton induced X-ray emission. An&y{. Chem. 47, 855-859. Johansson T. B., Van Grieken R. E. and Winchester J. W. (1976) Elemental abundance variation with particle size in north Florida aerosols, Z. geophys. Res. 81, 1039-1046. Kaufmann H. C., Akselsson K. R. and Courtney W. J. (1977) Rex: a computer program for PIXE. Nucl. Instrum. Method 142,251-251. Kowalczyk G. S., Choquette C. E. and Gordon G. E. (1977) Chemical element balances and identification of air pollution sources in Washington, D.C. Atmospheric Emironment 12, 1143-1153. Lawson D. R. and Winchester J. W. (1978) Sulfur and crustal reference elements in nonurban aerosols from Squaw Mountain, Colorado. Encir. Sci. Techno/. 12, 716.-721. Mitchell R. I. and Pilcher J. M. (1959) Improved cascade impactor for measuring aerosol particle sizes. Ind. Enqng Chem. 51, 1039-1042. Muhlbaier, J. L. (1978) The chemistry of precipitation near the Chalk Point Power Plant. Ph.D. thesis, University of

Maryland. Nelson J. W., Jensen B., Desaedeleer G. G., Akselsson K. R. and Winchester J. W. (1976) Automatic time sequence filter

sampiing of aerosols ‘for rapid rnu~t~-element gnalysis by proton induced X-ray emission. Adz:. X-r@!: Anaf. 19, 403-413. Peirson D. H., Cawse P. A., Salmon L. and Cambray R. S. (1973) Trace elements in the atmospheric environment Nature 241, 252-256.

Tanaka S., Darzi M. and Winchester J. W. (19SOa)Sulfur and associated elements and acidity in continental and marine rain from north Florida. J. geophys. Res. (in press). Tanaka S., Darzi M. and Winchester J. W. (19SOb)Elemental analysis of soluble and insoluble fractions of rain and surface water by particle induced X-ray emission. TO be published.