Complete elemental analysis of aerosols: PIXE, FAST, LIPM, and mass

Nuclear Instruments and Methods in Physics Research B3 (1984) 291-295 North-Holland, Amsterdam

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COMPLETE ELEMENTAL ANALYSIS OF AEROSOLS: PIXE, FAST, LIPM, AND MASS * Thomas A. CAHILL, Robert A. ELDRED, Danny SHADOAN, Patrick J. FEENEY, Bruce H. KUSKO and Yatsuda MATSUDA ** Cracker Nuclear Laboratory

University of California, Davis, California 95616, USA

Methods are described that allow additional information to be gained on aerosol parameters from samples suitable for PIXE analysis. In particular, these methods yield ways to directly compare the sum of all elemental species with total mass obtained by weighing or /3 gauging the same sample.

1. Introduction Traditional programs for monitoring atmospheric particles have evolved in response to both the needs of public and private agencies, the resources allocated to meet these needs, and the technology available within the resource constraints. Once a protocol has been accepted, it tends to become fixed in tradition and the law, and modifications are not easily made even when technology changes. Since the first perceived needs associated with atmospheric particles were based on health, protocols were developed to allow measurement that would (hopefully) correlate with how much material could be inhaled, Most particulate monitoring in the world remains at these levels, giving only the mass of particles in the air. This quantity, total suspended particulate mass (TSP), is usually measured by weighing a 500 cm* fiber filter, derived from a vacuum cleaner motor in a tin box that draws in air for 24 h on a one-day-in-six pattern. While this device, called a high volume filter, or Hi-vol, is quite inexpensive, weighing the filters is not easy, since even in a polluted urban atmosphere the loaded filter is only about 1.3% heavier than the blank filter. Humidity effects are serious, but through the “fiction” of equilibrating filters for 24 h at 50% relative humidity, consistent results can be obtained (table 1) [l]. Since the filters are not damaged by the gravimetric mass measurement, some chemical analyses can be made. In California, these include the organic fraction, nitrates, sulfates, and lead at selected sites, but occasionally more species are studied [l]. In all cases, material is removed from the glass fiber or quartz fiber filter media for such chemical tests. * Work supported in part under Environmental Protection Agency CR 808563. ** Present address: Radiation Center of Osaka Prefecture, Osaka 593, Japan. 0168-583X/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

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PIXE and other modem X-ray, nuclear, or optical techniques, are not well suited to such samples. The filters are thick, species are enmeshed deeply in fibers that, in themselves, contain many elemental species. PIXE does not need and cannot use the 500 cm* of ihy filter. Thus, new sampling methods have been developed that are well suited to PIXE’s capabilities [2]. In doing this, however, one risks giving information that is irrelevant to the air quality agency. They may want mass (TSP), and PIXE only sees element that make up about f of the typical mass. The agency wants sulfate; PIXE gives sulfur. Thus, wide utilization of PIXE requires the development of methodologies that allow one to quantitatively collect a sample suitable for PIXE but also able to give mass, optical opacity, or other parameters traditionally used by air quality agencies. In this report, we describe our attempts to bridge this gap through a suite of techniques that are all compatible with each other, yet give information that allows comparison with traditional methodologies.

2.

Mass

Measurements of mass on the thin membrane filters with diameters of 25 to 47 mm that are generated by modem particulate collection devices such as the dichotomous virtual impactor [3], stacked filter unit [4], or cyclone device [5] requires a whole different level of sensitivity than that common for Hi-vol filters. Filter blank masses now are around 20 to 40 mg, and loadings for a 24 h sample are about 1 mg. Thus, measurements must be made to better than 10 Fg for a 1% precision. Two methods are in use; gravimetric mass measurements using sensitive scales such as electrobalances, and thickness measurements made by scattering electrons, called j3 gauging. A comparison done by the US Environmental Protection Agency showed precisions of _+22 pg for weighing and +25 pg for /3 gauging [6]. IID. COMBINED TECHNIQUES

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Table 1 Comparison of gravimetric analyses

Area1 density Filter area Filter mass Flow rate Volume of air (24 hrs.) Mass of deposit a’ Ratio of deposit to blank filter

Hi-vol Glass fiber

Hi-vol W-41 paper

Stacked filter unit Coarse or fine stage nuclepore

Stacked filter unit Fine stage GA-1 or fluoropore

30 mg/cm’ 500 cm2 15000 mg 1400 l/m

8 mg/cm2 500 cm2 4000 mg 1400 l/m

1 mg/cm2 17 cm2 17 mg 10 l/m

3 mg/cm2 17 cm2 51 mg 10 l/m

2000 m’

200 m3

14 m3

14m3

200 mg

200 mg

0.48 mg

0.48 mg

5%

3%

1%

1.3%

‘) Assumes a total particulate density of 100 pg/m3 of which l/3 is larger than 15 pm, l/3 between 3 and 15 pm and l/3 is smaller than 3 pm.

Careful control of blank values in a solid quality assurance protocol, and mitigation of electrostatic effects through deionizing techniques [7], can obtain precisions down to f5 pg (81. The cost of such a program is not very great, but one gains both an ability to give air quality agency numbers they need and the ability to reduce PIXE elemental values into fractional mass units (ppm). This allows direct comparisons with known source compositions, and addresses a very important unmet need at most agencies - finding the source of their sometimes excessive mass loadings. We feel that developing this capability is the single most important act a PIXE laboratory can take to gain wider acceptability by air quality agencies (fig. 1) [8].

3. Very light element data Once one can measure both the mass of the sample and the mass of all elements sodium and heavier, the

obvious question of the nature of the “missing” mass of very light elements, hydrogen through fluorine, arises. From the point of view of air quality agencies, information on these elements is extremely interesting. The ability to measure nitrogen allows one to address the role of nitrates, an important industrial and automotive effluent linked to both health and acidic rainfall. The ability to measure carbon addresses serious questions involving the role of diesel particles, and forest burning practices, and industrial organic emission. In these areas, ion beam techniques have an ally in that the unreliability and lack of precision of existing methodologies, especially for nitrates, are a well known problem. Artifact formation of nitrates raises observed values, by conversion of gaseous NO, to NO,, while acids in the filters will destroy nitrate and reduce observed values. This mass presents opportunities for new techniques that use chemically inert filters of low surface area to mass ratios, reducing artifacts. However, most analyses done by ion beams are in vacuum, and so results will

Fig. 1. Filter handling system. Sizing pests are performed on the 8.0 pg nuclepore filters to ensure a cut point of 2.5 pm at 10 lpm.

Least square fit slope i 1.01 r 1 0.91

Sample:

Fine

aerosols

Concentration

fsurn of

ti,e

all

Fig. 2. The correspondence

elements

t”g,m-J

measured

between c

by PlXE andfAST

(all) and g.m.

not correlate directly with traditional methods. Sulfur, however, correlated at the 0.99 correlation level with SOqE/3, and is widely accepted as equivalent in ambient samples [9]. A very simple technique, laser integrated plate method (LIPM) measurements of total filter optical absorption, yields a parameter closely linked with carbon soot [lo]. In this technique, one uses a radiometer or photomultiplier to measure the light absorbed by a filter. Since in most cases, carbon soot dominates absorption, the measurement is sometimes referred to as a measure of optically absorbing carbon particles. In fact, the relationship between soot mass and optical absorption varies by a factor of 4 depending on soot morphology from various sources. The relative opacity is, however, quite consistent at a given site. Tbe measurement itself is quick and precise, with its high precision obscuring its shaky absolute basis. As a measurement of absorbing power, the measurements are on sound footing. The important role of soot carbon in global heating and cooling, arctic hare, urban pollution from diesels and rural pollution from agricultural buming and forest fires illustrates the value of this parame-

Hydrogen

Fine

Fine

Carbon 20

I

-

stage

stage

Carbon by d-scattering --- Soot carbon

1

Fig. 3(a,b) IID. COMBINED

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G&Ii et al. / Complete elemetsfaf analysis of aerosols Fine stage

Sulfur

1.0

>o*El -

7

Fine

Stage,

< 2.5mictons 7

d

10

13

16

19

22

25

26 Nov.

Olec.

4

7

IO

13

16

19

Fig. 3. Observational results for (a) hydrogen, (b) carbon (by FAST and LIPM), (c) oxygen (fine aerosols), and (d) sulfur (fine aerosols); for sulfate equivaient, multiply X 3.

ter. It is correlated with traditional “smoke shade” or “coefficient of haze” measurements. Finally, one can go to nuclear methods for analysis of very light elements. Nuclear reaction methods exist for any given element, but scattering, and especially forward alpha scattering techniques (FAST), allows a wide range of elements to be seen in a single PIXE-compatible sample. In FAST, the forward angle allows one to see hydrogen, so that all elements can be seen by a combination of FAST plus PIXE. Using the teflon filters, we have correlated the sum of all elements, H through U, to mass as measured gravimetrically on a vacuum dried sample. The vacuum drying removes, in dry California conditions, around 30% RH, (11 * 2)% of the deposit mass. The resultant correlation is shown in fig. 2. Note that the statistics alone limited the best possible correlation about 0.95. As an example of the use of all these techniques, me~~ements were made in the Sacramento Valley during periods of agricultural burning ill]. Other important sources were transported sulfates and nitrates from the San Francisco region, local wind blown dust, and weak but local anthropogenic sources (fig. 3). The almost overwhelming amount of information contained

in such studies allows entirely new directions in air quality research, since traditional parameters such as mass have not been loss but new diagnostic information is also available. Recent studies include tracing pollutant transport in the Arctic and resulted in a complete suite of measurements on 10 Arctic flights from Norway, the North Pole, to Alaska. Interpretation of these samples will hopefully provide some resolution of the sources of Arctic haze and its impacts on polar ice conditions. To do such measurements by non-ion beam techniques is very difficult, and no other suite of techniques gets so much information from a single filter, and nondestructively, too.

References [l] California Air Quality Data, California Air Resources Board. 121 T.A. Cabill, Nucl. Instr. and Meth. 181 (1981) 473. [3] RK. Stevens and T.G. Dzubay, EPA report 600/2-78-112 (1978). [4] T.A. Cahill, L.L. Ashbaugh, J.B. Barone, R.A. Eldred, P.J. Feeney, R.G. Flocchini, C. Goodart, D.J. Shadoan and G.W. Wolfe, J. Air Poll. Control Assoc. 27 (1977) 675.

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T.A. CahilI et al. / Complete elemental analysis of aerosols [5] T.T. Mercer, Aerosol Technology in Hazard Evaluation (Academic Press, New York, 1973). 16) W.J. Courtney, R.W. Shaw, T.G. Dzubay, Environ. Sci. Technol. 16 (1982) 236. [7] D.R. Engelbrecht, T.A. Cahill and P.J. Feeney, J. Air Poll. Control Assoc. 30 (1980) 391. [8] P.J. Feeney, T.A. Cahill and J. Ohvera, J. Air Poll. Control Assoc. (to be published). [9] E.S. Macias, J.O. Zwicker, J.R. Ouimette, S.V. Hering,

[lo] [ll]

SK. Friedlander, T.A. Cahill, G.A. Kuhlmey and L.W. Richards, Atmos. Environ. 15 (1981) 1971. H.E. Gerber and E.E. Hindman, Light Absorption by Aerosol Particles (Spectrum Press, Hampton VA, 1982). T.A. Cahill, Y. Matsuda and B.H. Kusko, The Nature of Sacramento Valfey Aerosols during Agricultural Burning, 1st meeting of Amer. Assoc. for Aerosol Research, Santa Monica, CA (1982).

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