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c 1983 Pergamon Press Lid

TECHNICAL NOTE

A COMPARISON OF OPTICAL AND AERODYNAMIC SIZES-II. AN IMPROVED METHOD FOR THE SIZING OF DELIQUESCENT

AEROSOLS TO ESTIMATE PULMONARY DEPOSITION

(First received 17 May 1982 and in finalform 6 October 1982)

Abstract-The utility and resolution of size measurements of deliquescent aerosols made for the purpose of estimating pulmonary deposition can be improved by humidifying the sample prior to sizing by either aerodynamic or optical instruments. This is due to two effects: inhaled deliquescent aerosols become droplets. and the opticat and aerodynamic properties of the spherical and homogeneous droplets are more uniform than those of dry particles. These effects are demonstrated with a monodisperse NaCl aerosol.

INTRODUCIION

In a previous paper we discussed the effects of inhomoge- neous particles on perceived optical and aerodynamic sizes (Harrison and Harrison, 1981). and demonstrated that meas- urements of the optical size distribution of dry particles could not be used to infer an aerodynamic size distribution to accuracy better than approximately a factor of two, or vice versa. The test aerosol was produced by the evaporation of NaC! solution droplets, a physical model for processes which are thought to produce atmospheric aerosols, and also a widely used technique for preparing test aerosols for inha- lation studies. We also remarked that the sizing of dry particles is an inappropriate measurement for the estimation of pulmonary deposition: the high relative humjdity en- countered in the trachea and nasopharynx causes deliquesc- ent aerosols to become solution droplets during inhalation.

Several problems are solved by sizing aerosols at a known high relative humidity (r.h.). The solution droplets are homogeneous and spherical, and so aerodynamic sizes can be reliably inferred from optical measurements, and vice versa. High relative humidities characterize the inhalation process. This is particularly important when comparing the effects of hydrophilic and non-hydrophilic aerosols or when working with mixtures of the two. At high relative humidities the resulting droplets are larger than were their dry nucleii, which extends the minimum sensitivity of sizing measurements. Finally, if the chemical composition of the aerosol is known, measured droplet sizes can be used to calculate a mass distribution of the deliquescent particles (Tang. 1976).

Variations in breathing behaviour and the external hu- midity produce variations in the saturation histories of inhaled aerosols. Droplet growth to sizes predicted by the relative humidity limit of 99.67”. set by vapor equilibrium over blood, does not occur in the upper airways: the growth rates are not sufficiently fast (at least for the droplets near 1 pn and larger) for the droplets to be at equilibrium at the time they enter the alveoli (Cocks and Fernando, 1982). In addition. if the ambieni air is warm and dry the molarity of salts and other species in the mucus on the surface of the nasopharynx and trachea may be higher than that predicted by equilibrium with an isotonic blood solution. Therefore, we cannot assign a single “correct” relative humidity, but values between 90 and 99% seem plausible, depending on the external temperature and humidity, inhalation rate and the position in the pulmonary system we wish to mode!. Further, the use of a measurement made at r.h. equilibrium to model a kinetic system requires consideration of growth rates, par- ticularly for dry particle diameters near one pm, where a r.h. lower than 90”,, may be justified.

METHOD

An inexpensive humidifier can be prepared by lining a 2 cm i.d. by 1 m tube with porous blotting paper, whose surface can be wetted with salt solutions to produce a known r.h. The tube is intentionally longer (and the flow rate lower) than the human trachea so the droplets may reach equilibrium, which can be checked a posteriori by varying the flow rate through the tube while observing the size distribution of the exiting droplets.

Figure I (taken from Tang et al., 1977) shows the effect of relative humidity on the droplet size for sodium chloride. The results are qualitatively similar for a!! deliquescent systems. When size measurements are made on aerosols of unknown composition we would clearly do best to mimic the human relative humidity. On the other hand, when working with prepared aerosols we are free to choose the relative humidity which best suits the intended measurement. A high relative humidity can be used to “magnify” the particles and improve the detection sensitivity: a low relative humidity, just above the deliquescent point, will reduce the sensitivity of the measurement to small variations in relative humidity.

It is convenient to use a saturated solution for the wa!I of the humidifying tube, which eliminates concern over the change in relative humidity in the tube as water is evaporated from it’s surface. Tables listing equilibrium relative humi-

Y 3 Q EXPERIMENTAL

53 - CALCULATED

5

tl

2

30 40 50 60 70 El0 90 too

% RELATIVE HUMIDITY

Fig. 1. Conden~tiona! growth and evaporation of NaCl aerosols in a moist atmosphere (figure courtesy of Dr. I. N. Tang et at, 1977). The vertica! line at approximately 77 ‘%, r.h. is the deliquescent transition: values to the left are produced by hysteresis upon drying and should not

be used for droplet growth calculations.

895

896 Technical note

drtres over various saturated solutions can be found m standard handbooks of chemistry and physics.

To illustrate the utility of pre-treating aerosols with an humidifier, Fig. 2 shows two size distributions taken with an optical single particle counter beforeand after humidification to 80 *‘(, relative humidity. The test aerosol was prepared by nebulizrng a 0.1 I“, NaCl solution in a Bergland-Liu single particle generator* through a IO-/cm orifice at 200 kHz.

15oor

OPTICFlL DIAMETER, MICROUETERS

Fig. 2. Optical size distributions taken with a Royce 220 optical single particle counter and pulse height analyser for IO logarithmic channels on the abscissa. The curve to the left is the perceived optrcal size distribution of monmodis- perse (by mass) dry NaCl particles with a calculated dtameter of 0.9 ltm. The curve to the right is the optical size distribution of the same aerosol after humidification to

80” r.h. 0

* Model 3050. manufactured by Thermo-Systems Inst., St. Paul. MN, USA.

? Current address Battelle-Pacific Northwest Research Laboratories, Richland, WA. U.S.A. Publication costs were supported by U.S. DOE and U.S. EPA under contract DC- AC06-76RL0 and Interagency Agreement IAG No. EPA-82- D-X0533.

Although the test aerosol was monodisperse by mass, tt showed a rather broad optical size distribution. This result is expected from our previous work,and suggests that about I;2 of the observed variability in optical and aerodynamic sizes of these particles is simply associated with the optical variability of irregular dry salt particles, of constant mass. Also as expected the droplet size distribution is much narrower, and within the limit of the resolution of the pulse height analyser, the size is accurately predicted by the dehquescence curve.

The results of Fig. 2 confirm the utility of prehumiditi- cation. With an inlet relative humidity of less than 20”,,. no variation was seen in the droplet sizes produced by the tube,at flows less than lO/min~ ‘. This flow is adequate for most optical sizing instruments, and is near that needed by most impactors. cyclones and dichotomous separators used for the estimation of pulmonary deposition. Larger flows can be accomodated by lengthening the tube or operating more than one in parallel. 1 urge that such instruments be modified to incorporate this improvement.

Care must be taken to ensure that the relative humidity is not altered by the sizing instrument. Optical particle counters and single stage dichotomous separators have short inlet residence times and little diabatic heat transport. This technique may not be usable with multistage impactors.

Enovonmenral and Enyineeriny

Scirncrs Proyram (FC-05) C’nirersir~ of’ Washington Seutrlr, WA 98195. U.S.A.

LEE HARRISON+

REFERENCES

Cocks A. T. and Fernando R. P. (1982) The growth of sulfate aerosols in the human airways. J. Aerosol Sci. 13, 9919.

Harrison L. and Harrison H. (1981) A comparison of optical and aerodynamic sizes: the effects of inhomogeneous particles. Atmospheric Environmenf 16, 273-300.

Tang 1. N. (1976) Phase transformation and growth ofaerosol particles composed of mixed salts. J. Aeroso/ Sci. 7, 361-371.

Tang 1. N. rr al. (1977) Aerosol growth studies-II. Preparation and growth measurements of monodisperse salt aerosols. J. Aerosol Sci. 8, 149-159.