On the performance of the Goetz aerosol spectrometer

Attttospheric E~u~ro~~~~~fPergamon Press 1973. Vol. 7, pp. 1003-1011. Printed in Great Britain.

DISCUSSIONS ON THE

PERFORMANCE

AEROSOL

OF THE GOETZ

SPECTROMETER*

A PROJECT very similar to the discussed paper with a slightly different aerosol spectrometer was done 5 years ago (HOKVATH,1967, 1968) and a brief summary of the results obtained then will be given. The centrifuge was built at the First Physics institute of the University of Vienna. Since the machines used for building it were different the dimensions were also different (e.g. the channel dimensions were 9.8 x 59 mm instead of IO.6 x 6.35 mm). The inlet batBe had a shape as shown in FIG. I and was never removed from the centrifuge. The operating conditions of the instrument were 18~revmin-’ with I.0 mm jet orifices at the end of the channel. The method of calibration used by H~KVATH(1967. 196X) is essentially the same as in the discussed paper. The Iatex particles usually avaiiabie were used for calibration purposes. The concentration of the deposited particles on the foil was determined by counting and with a photometer. The concentration of monodisperse particfes usually varied with channel length with a characteristics cutoff. Only for some streamlines (approx. along the middle of the channel) the curve was horizontal. FIGURE 2 shows a measured concentration profile for 0.796 nm particles. The most constant concentration along the channel is achieved for the curve with $J = 5.5. Fortunately for all calibrations the curves with 4 = 5.5 were horizontal.

FIG. I. Inlet hatllc of the Aerosol Spectrometer used at the University of Vienna. By numerical integration of the experimentally determined concentration profiles a function W(R,d) for a fixed value d, (FIG. 2) could be derived, being the probability of finding a particle per unit area of size d at the location R on the depositing foil. Several of these curves are shown in FIG. 3. If n(R.#) particles of size d are found at locations by the coordinates (R&j then W(R,d) is obtained by

For different streamlines (different value of 4) a different W(R.d) is obtained. Since all later measurements were performed along a line characterized by 4 = 55 the dependence of W(R,d) on C$is not explicitly mentioned. The shape of the cutoff is due only to the varying size of the monodispersed latex particles. It must be mentioned that all measurements were taken with the inlet baffle in position. The location of the cutoff shows very little change with varying rev min-‘, since an increase in rotation generates a higher centrifugal force as well as a higher flow rate, which almost compensate each other for slight changes in rev min-’ which may occur during operation. The concentration of monodisperse particles is almost horizontal if a proper starting point is used. A perfect cutoff exists. Measurements and calculations by STOEBER and ZESSACK(1964) showed that no horizontal concentration may exist although a perfect cutoff was observed. Despite the general opinion, even in this case an evaluation is possible if a simple recurrence relation is used (HORVATR,1968)

1004

Discussions

f%& 2. Top- Foil geometry of the GAS. The h&al channel bottom forms an Achimedian spiraf, when the foil is flattened. The location of a point P on the foil can be given by the two numbers R and 4. Below-Concentration profile of monodispersed 0.796 pm particles on the foil of the Aerosol Spectrometer.

The concentration ofparticleson the Foil at locarlon R: C{R) depends on the size distribution n(J) and the prohnbiiity of deposition W(R,J): d(R)

C(R) =

s dR

n(d)W(R,

d)Ad.

(1)

For this and the following it is assumed. that the particle ~ouutin~s are p~rforrn~d along a line qh= const. (see FIG. 2). ~ifferenti~ition with respect to R gives

d=lX6

,005

pm

tn

5

10

15

20

25

30

35

45mm

40

R

FIG. 3. Probabilities W(R,d) of finding a particle of size d at location R per unit area of the depositing foil. For an ideal instrument the curves should be horizontal with a sudden drop. The S-shaped cut 0% seems to be due only to the SD. of the latex particles.

fbf

3.

/m 10

15

20

Pasitionon foil

25

.5 -

10

Fur&

2.0

2.5

30

dia.

FIG. 4.(a) Assumed probabilities of deposition (proportional to particle concentration of monodispersed particles) for testing the recurrence formula (3); (b) Size distributions after 1.2.3 and 4 recursion operations. The convergence is very rapid, n5 n6 arealmost identical with n, which satisfies equation (I) within the allowed error margin.

1006 If this formula

Discussions is written

as:

dC(R)

__ dR

= q(d).

W(R, d). ;

+

?I-,(.x)-

aWR, xl

dx,

aR

a recursion relation for calculation n,(d) from hi_,(d) is established. It can be proved relation will give a convergence at least as good as a geometric sequence as long as W’(R, d)

---.m.T(d aR

1 ,

AR

- 4) < 1.

that this recurrence

(4)

To show the usefulness of this formula a numerical calculation has been performed. The probability of deposition was assumed to vary with deposition length as in Stiiber’s measurements and is shown in FIG. 4a. The size distributions obtained by the different iteration steps from an assumed particle concentration on the foil C(R) are shown in RG. 4b. The convergence is very good. After the fifth iteration step no more change in the curve occurs. The fifth size distribution was substituted in equation (1) and the original and calculated C(R) are in perfect agreement. The deposition lengths of monodisperse particles can be determined very accurately and well reproducible. Thus one is tempted to conclude that the aerosol spectrometer can be used as an accurate means for determining the particle concentration of the aerosol. Still. there are some inadequacies which are to be taken into account before a measurement is made. In most publications (c.g. Gor 7~. PKI INING and KALALI. 1961) a light microscope is used to determine the number of the deposited particles on the foil of the spectrometer. The smallest particle size of atmospheric aerosols that can definitely be seen with a microscope was determined to be approx. 0.27 pm dia. (HORVATH, 1967). It is not certain that smaller particles will be seen. If an aerosol has a size distributions such that n(r) increases with decreasing particle size, smaller and smaller percentages of the particles below 0,27/irn will be seen; thus a maximum is measured, although it does not exist. A highly accurate determination of the air flow through the instrument is not possible; thus the particle number per volume of air will be inaccurate. The size distribution of the particles in the airborne state is obtained by means of the calibration curve and a numerical differentiation of the counted concentration function on the foil. dCIR)/dR: it is well known that the differentiation of an empirically determined function is one of the least precise operations of numerical mathematics (e.g. STIEFEL. 1961). In order to obtain reasonable results directly from the counted particles numbers a high counting accuracy would be necessary, and about I 000000 particles would have to be counted at one location on the foil. Since usually 40~1000 particles are counted per field of view it is not possible to use the data points obtained directly for the differentiation procedure, because the scatter would be too large and the results unusable. Instead the data points must be plotted in a graph and a smooth curve drawn through the points. This curve is then differentiated numerically and the size distribution in the airborne state calculated with one of the formulas. Many years of experience show that it is necessary to read the coordinates of the smooth curve to at least three significant figures; (i.e. @I per cent). otherwise the numerical differentiation gives unrealistic results. Since there are different opinions on what a smooth curve through scattering data points should look like. the result of a measurement will depend on who drew the curve. Application of the least square method was also considered. but assumptions on the curve had to be made (e.g. Polynomial* Gauss curve, Exponential etc.). Experience showed that the result using this method is biased. Thus”human interpretation” turned out to be the most feasible way. Since this interpretation can be done in many ways an experiment was done to study the possible variations. A routine run of the Aerosol spectrometer, measuring the atmospheric aerosol at Seattle. was selected for this purpose. The particle concentration on the foil was obtained by counting 4Ot&lOOO particles per field of view. Thirty data points were available and were plotted on graph paper. Copies of the paper were sent to 20 scientists who were more or less familiar with the GAS with the request that they draw a smooth curve through the data points. From these curves the raw data were obtained by interpolation with a magnifier. The results of this experiment discourage use of the GAS. Although the interpolated values for ditferent people lie within 10 per cent the corresponding points in the size distribution cmve differ by a factor of two. Even results which were obtained by integrating over the calculated size distribution (such as total particle number. particle surface or volume) showed ditferences of ;I factor of 1.9. Detailed results of this study Hill be published soon. Since the recursion relation (3) also uses the first derivative of C(R) the same problems occur if this formula is used. In conclusion the following could be said: Both the centrifuge reported earlier (HORVATH. 1967. 1968) and the centrifuge of the discussed paper show similar behaviour with respect to the deposit of monodispersed particles. The high accuracy with respect to particle size obtained during calibration with monodis-

Discussions

1007

persed particles does not automatically give high accuracy with polydispersed aerosols. For atmospheric aerosols the error is about a factor of two. The use of a light microscope may simulate a maximum in the size distribution which actually does not exist. I, Physikalisches d. Universitht Austria

H. HORVATH

Institut Wien

REFERENCES GOI.TZ, A., PREININ(;0. and KALAI T. (1961) The l~~ct~lstability of natural and urban aerosol. Geof: pur. Appf. 50,67-80. HORVATH H. (1967) A comparison of natural and urban aerosol distribution measured with the aerosolspectrometer. Environ. Sri. Trchnol. 1,651-65X HORVATH H. (1967) The measurement of atmospheric air pollution with the Aerosol Spectrometer. Anrtual Meeting of the Pacijc Northwest International Section, Air Pollution Control Association, Salem. Oregon. November 1967. HOHVA.I.H H. ( 19681 A calibration of the XI-osol spectrometer. 8rh Confi,rencc~ of Mr,thods irl Air Po//[r/iorr and Zndustrial Hygeiw. Los Angeles California, February 1968. STM~L E. (1967) Eitqti/zruny in diu rzujnerischc Murhert~u~~k, p. 127. Teubner, Stuttgart. STEER W. and ZESSACK U. (1964) Zur Theorie einer konischen Aerosolzentrifuge. Stauh 24,295-305.

COMMENT ON ESTIMATION OF PHYSIOLOGICAL POTENTIAL FROM CHEMICAL REACTIVITY OF

SMOG SYMPTOM HYDROCARBONS*

I HAVE read this paper, several times, in order to try to find the answers to three questions. These three questions are: What do Yeung and Phillips have to sav that Heuss and Glasson did not sav? What is the proper interpretation of the computed c&relation cokfficients? What does the paper add’to our understanding of photochemical smog, and what additional studies does it suggest? The paper does not.provide new experimental information, but performs further analysis from data of Ht+uss and GLASSON(196X). Yeung and Phillips say that previous correlational studies “overlooked” the dependence of eye irritation on both the rate of production of biologically active species, and the nature of the species. But in their FIG. 4, Heuss and Glasson show the effect of hydrocarbon structure on eye irritation reactivity. They comment that “there is little or no statistically significant difference (in eye irritation reactivity) among compounds within a given structural class.” Yeung and Phillips propose that relative severity of eye irritation (E.I.R.) is approximately equal to relative chemical reactivity (R.C.R.) multiplied by a biological potency factor. Ethylene is the substance whose eye irritation and chemical reactivity is used as the referent. The authors then hypothesize that the factor differs among chemical classes according to the nature of the lachrymator produced by photooxidation. Discarding PAN as not likely to be found in the presence of nitric oxide, they suggest that three products, benzaldehyde, formaldehyde and acrolein are the suspected agents. When three different levels of formaldehyde yield, and multi product reactions are included, ten different numerical values 01 the potency factor based on five different parameters are hypothesized. When the hydrocarbons are grouped by class. the mean values of eye irritation (E.I.R.), and chemical reactivitv (R.C.R.) are used to compute the numerical value of the parameters. The sums and averages which consiitute the second set of five numerical estimates give good approximations to the relationship. This is one new thing that the paper presents. So far, the authors make a fair case for chemical reactivity within groups being a multiplicative factor, with the biological potency of the characteristic product of that group being the second term. However, in numerical columns 4, 5, 6 and 7 of TABLE 2, we have up to three different numerical estimates for the biological potency of groups If), and despite diligent effort, the distinctiveness of these eludes me. The last column of TABLE 2 (No. 8, according to my count) shows a structural group