Correlations between gravimetry and light scattering photometry for atmospheric aerosols

Pergamon

Armosphmr

Enrwonmenr Vol. 28. No 5. pp. 935 938. 1994 Copyright ,i_‘, 1994 Elscwcr Science Ltd Pnntcd m Grca~ Brimin. All rights reserved 1352.2310/9456.00+0.&l

CORRELATIONS BETWEEN GRAVIMETRY AND LIGHT SCATTERING PHOTOMETRY FOR ATMOSPHERIC AEROSOLS A. GSF-Forschungszentrum

and J.

THOMAS

fiir Umwelt und Gesundheit Paul-Ehrlich-Str. 20, D-60596

schung,

GEBHART

GmbH, lnstitut fiir Biophysikalische Frankfurt am Main, Germany

Strahlenfor-

Abstract-Light scattering photometers are useful for in situ measurements of mass concentrations of environmental aerosols if certain requirements are fulfilled. For the determination of relative concentrations, the composition of the aerosol (particle size distribution, refractive index) has to be constant during the experiments. Absolute measurements of mass concentrations additionally require a calibration of the photometer in terms of gravimetric units. In the case of atmospheric aerosols special attention has to be paid to the relative humidity which has a strong effect on particle size. Preliminary measurements indicate that for atmospheric background aerosols with a dominating accumulation mode a fairly linear relationship between photometer response and mass (volume) concentration exists, a result, which is in agreement with theoretical predictions. Krx lord accumulation

indeu:

Light scattering photometer. mode, median refractive index.

mass

INTRODUCTION

Air quality standards for particulate matter in the atmosphere are usually based on gravimetry. Typical aerosol mass concentrations in the atmosphere cover the range from 10 to some hundred pg/m3. In normal polluted

areas

diurnal

averages

have

less

than

100 keg/m 3, hence sampling times of several hours are usually required; however, modern opto-electronic elements allow the construction of light scattering photometers which are suitable for “in situ” monitoring of aerosol mass concentrations down to a few pg/m3. It is therefore of general interest to correlate photometer responses with mass (volume) concentrations of atmospheric aerosols. Based on theoretical considerations the possibilities and limitations of light scattering photometry for concentration measurements of atmospheric aerosols are discussed. Two different photometers were used to carry out experiments with monodisperse test aerosols and to perform direct field measurements in the atmosphere. THEORETICAL

Light

scatrering

CONSIDERATIONS

w wlutne

(mass)

system.

In

the

presence

c,c

of many

f‘(d)S(d,

particles

m, r.)6d

I, is the illumination

intensity,

aerosol,

m, i., p)=-

S(d, WI, A)

(2)

p;d3

inside

which is the flux of scattered light S per unit mass concentration of aerosol. This function indicates whether or not the intensity of scattered light is proportional to the mass concentration of the measured aerosol. The general behaviour of Q(d,m, ,t, p) for a fixed m and d is illustrated in Fig. 1.

(1)

0

where

background

volume, c the number concentration and/(d) the size distribution function of the aerosol (d: particle diameter, m: refractive index, 1: wavelength). Usually for a certain optical sensor IO, V, and i are fixed. Equation (I) demonstrates the problem of photometric measurements of particle concentration: if the photometer response P varies, one cannot distinguish whether the number concentration c, the size distribution f(d) or the optical properties m of the material have changed. Using a test aerosol with a homogenous chemical composition the influence of the refractive index m can be eliminated so that only c andf(d) remain as variables. In this case a linear relationship P = const . c can be realized if either the function f(d) can be replaced by one particle size (monodisperse aerosol) or the function!(d) is constant during the measurements. In these cases the photometer response is linearly correlated with the number or mass concentration of an aerosol and a simple calibration procedure can be carried out. Using the electromagnetic theory of light scattering on spheres a specific scattering function Q(d, m, 1, p) can be calculated as: Q(d,

the sensing volume of the photometer the resulting light flux P collected by the detector is given by: P=lo

atmospheric

concentration

Let S(d, ttl, i) be the flux of light scattered by a single particle into the receiver aperture of an optical

concentration,

V, the sensing 935

936

A. THOMAS

Fig.

I. General

relationship scattering

between gravimetry photometry.

and

light

of d B i. light scattering is a surface effect (S-d’) and Q decreases proportionally to d-l. For particles with ti
tering

can

particle

size

be approximated

scattering

1‘I”“’ ,d01 92 ‘1143 10.i I

range

by dipole theory to d”. At about

(S h d’) d - i the

and Q increases proportional function Q runs through a maximum, which for dielectric spheres is additionally stressed by resonances. Only within this plateau light scattering is nearly proportional to particle volume. So it is necessary to adjust the wavelength to the properties of the aerosol. Consequently, for a fixed wavelength of an optical device the specific scattering function generally depends on the diameter, the refractive index and the density of the particles. Rayleigh

and J. GEEHART

of‘ air

Sensitive aerosol photometers can detect Rayleigh scattering of different gases (Charlson et al., 1967). Therefore, in such instruments the straylight background for the detection of particulate matter in the atmosphere is given by the Rayleigh scattering on gas molecules. For randomly oriented gas molecules the mean polarizability pm of a molecule is connected with the macroscopic refractive index trig of the gas according to Born (1965): Cg.pm=(nl;-

partlcle Fig. 2. Rayleigh

scattering refractive

VOLUME

d, pm

of air in relation index )?I = I .5.

MASS-SIZE

DISTRIBUTIONS

ENI’IRONhlENTAL

AEROSOLS

to spheres

with

OF

The particle size distribution of atmospheric aerosols is influenced by local particle sources like traffic or industrial emissions and by the weather situation. Relatively stable conditions exist for aged background aerosols where the accumulation mode in the size range between 0.1 and 1 ltrn dominates. With a combined system of a laser aerosol spectrometer (LAS-X) and a differential mobility analyser (DMA), Brand (1989) measured particle size distributions in a primary residential area of Frankfurt (Main). Samples were taken about 10 m above ground at a distance of about 50 m from a main traffic road. The average volume size distribution derived from his measurements during January 1989 is shown in Fig. 3. It can be seen that for visible and near infra-red light the accumulation mode of this urban aerosol nearly coincides with the plateau of the specific scattering

I)

where cg is the number concentration of the molecules. Applying dipole theory for light scattering by gas molecules one finally obtains for the contribution of a gas volume V, to light scattering

Volume Concentratk3n

lpm’ cm ’ pm ‘I

‘E+03J-

l&02'

./

,'-."'.

\ \

sg=vgI,~(~(I+cos’o)

I 3

diameter

(4)

lE+Ol

I

\... :z

Cg

where I0 is the illumination intensity and fI the scattering angle. In Fig. 2 Rayleigh scattering of air is compared with light scattering by single spherical particles with refractive index WI= 1.5. As can be seen 1 mm’ of air scatters about the same amount of light as a transparent 0.22 pm particle. In terms of mass concentration the scattering equivalent of air corresponds to about 6 pg/m3 particulate matter consisting of 0.22 itrn particles.

/

1tz+oo

. lE-01

!

lE-021.

..:ti-1 0.01

1-e: 0.1 Panicle Diameter

Fig. 3. Average volume size distribution particles at Frankfurt ‘Main City

: I

: :::r l(

bml

of ambient aerosol in January 1989.

Gravimetry and light sealttering photometry function of Fig. I. Therefore for atmospheric background aerosols a linear relationship between photometer response and mass (volume) concentration is found (Charlson er al., 1967). With respect to the sensitivity of aerosol photometers for the atmospheric environment it is interesting to note that particles within the size range of the accumulation mode are optically most active. With a combined DMA/LASX-system, Brand (1989) also measured the median refractive index of aerosol particles contained in the accumulation mode of the background aerosol in the Frankfurt area. For this purpose during a period of four weeks monodisperse fractions ot particles in the diameter range from 0.1 to 0.5 pm were separated from the atmospheric environment by means of the DMA and classified in the LAS-X. The median refractive indices of the aerosol particles found during these measurements cover the range from NaCl (m= 1.54) to DEHS-oil ())I= 1.45) with a weighted average of m= 1.52.

931

rated this instrument with monodisperse test aerosols of different optical properties. Experimental results are shown in Fig. 4 where the photometer signal per mass or volume concentration of aerosol is plotted vs the geometrical particle diameter. As can be seen different optical properties and densities of the particle material lead to different curves but the results generally confirm the course of the specific scattering function as indicated in Fig. 1. Aerosols with different chemical composition usually require different calibrations. But in some cases where a higher density p of the material is accompanied by an increased flux of scattered light a compensation effect occurs which reduces the influence of the optical constants no upon the specific scattering function. Good examples for such a compensation effect are mineral particles (high density, transparent) in comparison to coal particles (low density, absorbing). FIELD

LABORATORY

INVESTIGATIONS

Experimental

With an instantaneous reading dust photometer (Tyndallometer TM digital /tP, H. Hund GmbH, Wetzlar) specific scattering functions have been investigated experimentally. The portable, batterydriven instrument measures the light scattered by the aerosol particles under a mean scattering angle of 70’. The wavelength of light is 940 nm. Its open measuring chamber is self-ventilating and continuously filled with aerosol. The daylight is absorbed by a filter in front of the detector. Armbruster ef al. (1984) calibI

A Latex pxticles prticles

y-1.05

0 011 droplets

~=0.912

0 FeZ03-prllcles

I

In a preliminary field study intercomparisons between gravimetry and photometry were carried out using the photometer TM-E (H. Hund GmbH, Wetzlar). The optical arrangement and the measuring chamber of the instrument are shown in Fig. 5. Particles traversing the sensing area are illuminated by a collimated beam of monochromatic light of 880 nm wavelength. Light scattered by the particles in the angular range between 25 and 5@ is collected by a receiver optic and directed to a photodetector. An aerosol stream of I m3/h is continuously drawn through the measuring chamber. To prevent particle deposition on optical elements the aerosol is surrounded by a clean-air-jacket. The inlet system

n=l.575

qlc,3

103 -t

n=l.U3

>

I III,

1

2

5 PARTICLE

Fig. 4. Response

set-up

7s3.2 g/cm3

(mass

0.5

q/cm3

MEASUREMENTS

of the TM

digital

LIP with monodisperse

0.2 DIAMETER

0.5

1

2

in pm

test aerosols

of different

chemical

composition.

5

938

A. THOMAS

and J. GEBHART )Aerosol

1 ma/h

Photo-detector

Luminescence-diode (A = 880 nm)

Fig

5. Optical

arrangement

and measuring

chamber

for

the photometer

TM-E.

for the aerosol sampling was taken from the FH 62 I /Iradiometer (VDI-Richtlinie 2463 Blatt 5). AI a sampling rate of 1.m3/h the sampling efficiency of this inlet system can be characterized by a 50% cut-off diameter of 7 pm. The TM-E photometer delivers an electrical current as an analog signal which is fed to an analog-digital-converter (ADC) and then stored in a P.C. as function of time. For the in situ calibration of the photometer in gravimetric units a special sampler (“Hund sampler”) has been constructed which uses the same inlet system and the same flow rate as the photometer. Gravimetric samples were collected on glass-fiber-filters (Schleicher & Schiill No. 9).

RESULTS Figure

6 shows

mass

concentration

pier”.

The

residential

the

measurements area

photometer

measured about

were 8 m

response with

the

performed

above

ground

vs

the

“Hund-samin an urban and

50 m

from a main traffic road. A good linear correlation was found between photometric and gravimetric measurements expect for one experimental run, where the relative humidity exceeded 70% (m, Fig. 6). In all other cases the relative humidity in the atmosphere was below 60% and no systematic influence could be observed, The increased photometer response at high relative humidities is caused by the uptake of water by the aerosol particles (Hinel and Lehmann, 1981). Since the measurements summarized in Fig. 6 have been performed with urban aerosols the following conclusions may be drawn: as long as the differential mass size distribution of the atmospheric aerosol (dominating accumulation mode) approximately coincides with the course of the specific scattering function Q(d, m, i, p) and the optical properties of the aerosol can be described by a median refractive index a nearly linear relationship between photometry and gravimetry can be found. In the presence of coarser size fractions and high relative humidities, however, additional information is required in order to estab-

Fig. 6. Photometer response as a function of the mass concentration by means of the “Hund sampler”. Measurements made in a residential area In Frankfurt (Main).

lish a correlation between photometer signals mass concentrations of atmospheric aerosols.

and

REFERENCES Armbruster L.. Breuer H.. Gebhart J. and Neulinger G. (1984) Photometric determination of respirable dust fincentration without elutriation of coarse particles. Part. Characr. 1. 966101 Born M. (1965) Oprik. Springer, Berlin. Brand P. (I 989) Forschungsund Entwicklungsarbelten zum Aufbau eines mobilen Mebstandes zur Charakterisierung von Umweltaerosolen. Dissertation. Universitat Frankfurt am Main. Charlson R. J., Horvath H. and Pueschel R. F. (1967) The direct measurement of atmospheric light scattering coefficient for studies of visibility and pollution. Atmospheric Enrironmenr 1. 469-478 Hanel G. and Lehmann M. (1981) Equilibrium size of aerosol particles and relative humidity: new experimental data from various aerosol types and their treatment for cloud physics application. Conrrih. Atmos. Phys. 54. 57-71 Verein Deutscher Ingenieure. Richtlinie 2463. Blatt 5 (1987) Messen von Partikeln, Messen der Massenkonzentration (lmmision). Filterverlahren. Automatisiertes Filtergerlt FH 62 1. VDI-Verlag, Dusseldorf.