the pore. To utilize this graph for porosities other than the value of 0.111 for which it is prepared, a n equivalent d / D must be generated. This is done by converting the finishing diameter to the appropriate linear scale which leads to the result
(d/D)’ = 1 - (1 - d / D ) (e/eo)’” Recovery is found from Figure 5 by indexing the appropriate modified Stokes Number and diameter fraction, Nst’ and (d/D)’,respectively.
Conclusion Recoveries of particles by a membrane filter of the Nuclepore type, for the combined mechanisms of inertia and interception, are given in Figure 5. These values are derived from a computation of the slow flow of a viscous fluid to a pore of circular section and from the trajectories of particles through the resulting flow field. A means of applying the result, which is for a porosity of 0.111, to membranes of smaller porosity is suggested. A factor of evident importance, and one which because of its complexity has not been pursued here, is the distribution of pores in the membrane. The geometry adopted here, coaxial approach volume and pore, is appropriate to a membrane with uniformly spaced pores. In fact, Nuclepore filters have randomly distributed pores. Some deviations in recovery must be expected when the volume from which the approaching aerosol is drawn is neither cylindrical nor coaxial with the pore. Some comprehensive experimental work will be needed t o define the scale of this effect.
Nom e nc 1at ure d = particle diameter e = filterporosity
eo = filter porosity, reference value of 0.111 p = fluid dynamic pressure r = radial position t = time u = axial component of fluid velocity u = radial component of fluid velocity w = axial or radial component of particle velocity x = axial position D = porediameter Ns, = particle Stokes Number = psd2U/9qDe Nst’ = modified particle Stokes Number = (psd2U/qDeo) (e/eoP2 P = stream function Q = vorticity function U = aerosol approach velocity Greek Letters q = fluid dynamic viscosity
p = fluid mass density ps
= particle mass density
Literature Cited (1) ~, Fuchs. N. A.. “The Mechanics of Aerosols.” Macmillan. New York, N:Y., 1964. (2) Spumy, K . R., Pich, J., Collect. Czech. Chem. Commun., 28, 2886 (1963). (3) Spumy, K . R., Pich, J., ibid., 29,2276 (1964). (4) Twomey, S., Bull. Obs. Puy de Dome, 10,173 (1962). (5) Pich, J., Collect. Czech. Chem. Comm., 29,2223 (1964). (6) Happel, J., Brenner, H., “Slow Reynolds Number Hydrodynamics,” p 140, Prentice Hall, Englewood Cliffs, N.J., 1965.
Received f o r review J u n e 10, 1974. Accepted December 23, 1974. This program of work was started with the support of the Ministry of Environment, Ontario, and t h e Atkinson Charitable Foundation, and continued with the support of the Department of the Environment, Canada.
Formation of Photochemical Aerosol from Hydrocarbons Chemical Reactivity and Products Robert J. O’Brien,’v* John R. Holmes, and Albert H.Bockian California Air Resources Board, 9528 Telstar Ave., El Monte, Calif. 91731
The formation of photochemical aerosol-the haze associated with photochemical smog in polluted urban atmospheres-has been the subject of both study and speculation for more than 20 years. A totally satisfactory understanding of this phenomenon has not yet been achieved. Chemical analyses indicate that urban aerosol, a t least that portion of it that can be isolated on filters having submicronsize pores, is primarily inorganic in nature: metal oxides, salts-principally nitrates and sulfates-sulfuric acid, possibly absorbed nitric acid, and water. The organic portion, accounting for a t most 25-30% of the mass, is complex and variable in composition, insofar as its composition has been investigated. Despite representing a small percentage of the mass of urban aerosol, the organic portion is largely in the respirable range and 1 Present address, Department of Chemistry, Portland State University, Portland, Ore. 97207.
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hence may contribute to a potential health hazard. Moreover the organic portion contributes significantly to light scattering and visibility reduction. If one assumes that the observed correlation in time between photochemical activity and rapid reduction in local visibility (Figure 1) indicates the existence of photochemical aerosol (however it may be constituted), one is forced to look for a mechanism by which a relatively small amount of photochemical oxidation products can contribute disproportionately to the overall process of visibility reduction by aerosol formation. Haagen-Smit ( 1 ) pointed out that cyclic olefins a t high concentrations-e.g., cyclohexene and indene-formed aerosol when allowed to react with ozone. He postulated that these compounds underwent oxidative ring opening and, possibly, subsequent polymerization. A number of investigators a t the Stanford Research Institute looked into the problem during the fifties and early
~
Photochemical aerosol formation from organic compounds has been investigated in an environmental chamber. Various classes of compounds were irradiated with oxides of nitrogen, and light scattering caused by the aerosol was measured concurrently. Diolefins and cyclic olefins were very effective in reducing visibility while monoolefins, and aromatics produced only minimal amounts of lightscattering aerosol. Paraffins produced no measurable aeroH
PACIFIC
D1YLlBiiT
TIME
Typical diurnal visibility variation for a smoggy day in downtown Los Angeles Figure 1.
Light scattering in continuous d a t a , oxidant a n d carbon monoxide 2-hr averages
sixties ( 2 ) .They reported that olefins could be induced to form aerosol under simulated atmospheric conditions and that SO2 enhanced the formation of aerosol. Similarly, Stevenson et al. ( 3 ) reported enhancement of aerosol formation by SOz, particularly for lower-molecular-weight olefins. More recently, Wilson and Levy ( 4 ) have reported that the degree of enhancement by SO2 is dependent on the relative humidity. Ripperton and Jeffries ( 5 ) have studied the formation of organic aerosol that occurs when a-pinene undergoes ozonolysis in the gas phase. Large amounts of aerosol are formed. They postulated that, since the kinetics of the reaction between ozone and a-pinene appear to be more than first order in hydrocarbon concentration, the aerosol was formed a t least in part by radical polymerization of the olefin into higher molecular weight products. Groblicki and Nebel (6) have investigated aerosol formation from NOx-HC and NOx-HC-S02 systems. They found the aerosol to consist largely of sulfates except when very reactive hydrocarbons, such as a-pinene or cyclohexene, were employed. They suggested that the reactive nature of these compounds was related to their possession of two oxidized sites resulting from ring rupture. They also concluded that ozone is chiefly responsible for aerosol formation in the NOx-HC-S02 systems, and that initial NOx concentration has little effect, provided some minimum amount is present. Recent work of Whitby and coworkers (7, 8) has demonstrated that the condensational growth of ambient aerosols (presumably via photochemical process) results in preferential accumulation of aerosol particles in the critical size range 0.1-1 pm. It is this range of particle diameters that is primarily responsible for the scattering of visible light (visibility impairment) and which is most important from a health standpoint (lung penetration). The nature of the gaseous precursors of these condensable species is only partially understood. Inorganic nitrates and sulfates are known to be formed via photochemical processes. In addition, various purely organic compounds such as some olefins and aromatics form aero-
~~~
sol. More detailed study of the reactivity of diolefins was carried out. Inhibition of aerosol formation by higher levels of NO2 was observed. Product analysis revealed that the major aerosol constituents were mono- and dicarboxylic acids and short chain polymers incorporating organic nitrate groups. A reaction scheme accounting for aerosol formation from reactive olefins is proposed.
sol when irradiated with oxides of nitrogen under simulated atmospheric conditions. In forested regions with relatively high emissions of terpenes, natural organic aerosols have been observed. In California, considerable effort has been expanded in the past in determining the inorganic constituents of the atmospheric particulate. This includes the measurement of various metallic elements as well as sulfate, nitrate, and ammonium ions. Data obtained by the National Air Surveillance Network in 1968 (9) in the South Coast (Los Angeles) Basin of California indicate that on a basin-wide average, amounts of sulfate, nitrate, and benzene solubles are roughly comparable. These data are tabulated in the following paper (IO). Values of the benzene-soluble fraction are, however, a n underestimation of the total organic contribution to the mass loading because much of the organic particulate matter is insoluble in benzene. Strictly from an emission-control point of view, reduction of SOZ and NO, emissions will ultimately reduce the sulfate and nitrate particulate burden of the atmosphere. The strategy for reduction of the organic aerosol is not so clearly defined, however, since the identity of the gaseous precursors is not known with certainty. In simulated smog experiments some classes of organic compounds have been found to be efficient aerosol formers. However, the extent to which these compounds take part in actual atmospheric processes has not been demonstrated. In view of the uncertainty of the effect of present hydrocarbon emission control strategies on reducing organic aerosol precursors, we have been primarily interested in the relationship between hydrocarbons and aerosol formation. Our approach, in the beginning, was to look a t the problem from a more or less phenomenologi’cal point of view, asking, primarily, what kinds of hydrocarbon can be made to form aerosol in the laboratory under simulated atmospheric conditions. Subsequently, we began to look in more detail a t both the chemistry and physics of the aerosol formation process. The initial studies were carried out in an environmental chamber. Subsequent studies involved work in a longpath infrared (LPIR) apparatus and analysis of atmospheric aerosol samples. This paper deals with the results of laboratory investigations into the chemical formation of photochemical aerosol. The following paper (10) describes the separation and analysis of atmospheric aerosol. Some observations on the physical process of aerosol formation will be covered in a paper to be published subsequently. Experim en ta 1
Environmental Chamber. The initial work of determining the aerosol-forming properties of a number of hydrocarbons was carried out in a n environmental chamber. This l100-ft3 glass-walled reactor was filled with ambient air that was passed through charcoal and permanganate beds and a final filter, and measured amounts of the deVolume 9, Number 6, June 1975 569
sired reactants were added. Internal uv irradiation was supplied by a total of 72 96-in. black-light fluorescent tubes arranged in 36-lamp banks. These lights caused an increase in temperature from ambient (22” C) to 43“ C over a period of about 2 hr. During this period, the light intensity decreased from a kl of about 1.0 to 0.6 min-l. Under uv irradiation, the chamber generated 0 3 a t a slow iate even when purged and filled with purified (liquid) air containing less than 1 ppb reactive hydrocarbon. In spite of these deficiencies, no additional condensation nuclei or light-scattering aerosol was observed without the addition of suitable hydrocarbons, so the chamber was deemed acceptable for these initial investigations into the aerosol formation mechanism for reactive hydrocarbons. The system was equipped with standard air monitoring instrumentation: continous analyzers for NO and NO2 (colorimetric Saltzman), hydrocarbon (flame ionization detector), carbon monoxide (nondispersive infrared absorbance), oxidant (neutral buffered KI), and ozone (ethylene chemiluminescence). Aerosol formation was monitored with an integrating nephelometer, based on the design of Alquist and Charlson ( I I ) . The nephelometer measures, on a continuous basis, the light scattered by the aerosol in the chamber. This provides a parameter, bScat, the scattering part of the extinction coefficient, proportional to the amount of aerosol formed in the light-scattering range. In addition, bSat can be related to equivalent atmospheric visibility assuming uniform spatial distribution of aerosol a t the same concentration with the same particle size distribution and refractive index. The nephelometer was calibrated with filtered air ( L t = 0.23 x m - l ) and Freon-12 (b,,,, = 3.6 X 10-4m-1). Nuclei concentration was determined with a G.E. Model CN condensation nuclei counter. In some experiments aerosol mass loadings were determined by collection on 2-in. glass-fiber filters, as well as with a Thermo Systems particle mass monitor. Agreement between these two methods was reasonably good. Aerosol for chemical analysis was recovered from the filters by solvent extraction. A few runs were made in the dark with 0 3 produced a t the rate of 2.4 ppm h r - l by an ozone generator (Alpine Engineering Co., Model FC) which stood at one end of the chamber in front of a mixing fan. During these runs, the fan was operated a t a low speed. During the irradiated runs, the fan was not operated, but some degree of mixing still occurred owing to convection along the thermal gradients between the lamps and the walls. Extrapolating the results of chamber experiments to the atmosphere is always a difficult problem, particularly in the case of aerosol studies. The atmosphere is essentially an infinite medium; there are no wall surfaces to modify the many homogeneous or heterogeneous chemical reactions which occur. On the other hand, the atmosphere normally contains a number of submicron-sized particles which can either provide chemically active surfaces upon which certain reactions might be expected t o occur or serve as condensation nuclei, thus altering the physical process of aerosol formation. The air used in these chamber experiments, however, had been filtered and was relatively free of such particulate matter. Background condensation nuclei counts were generally 2 x lo4 cm-3, or less. Background mass loadings were less than 25 pg m-3. The following two factors should be taken into consideration: Wall Reactions. These can alter the course of gasphase reactions. In theory, a t least, they should be negligible in a reactor the size of the environmental chamber. This has not been demonstrated experimentally, however. Wall Deposition. Experiments by Wilson et al. (12), 570
Environmental Science 8 Technology
100
/eo
;’.. , .-’/
1RR401A11OM
Figure 2.
----
T l l L (mln I
Typical pattern for generation of photochemical aero-
sol Each variable plotted as percent of maximum. Maxima given in parentheses
confirmed in this laboratory, indicate that the rate of mixing in the chamber has a pronounced effect on the rate a t which aerosol appears to form and on the amount formed. Presumably, mixing speeds up the processes of agglomeration and/or deposition of the aerosol on the chamber walls. Similarly, when internal uv irradiation is used, thermal gradients produce convection currents within the chamber. This would be expected to lead to aerosol impaction on the chamber walls. Long-Path Infrared System. The long-path infrared system (LPIR) is similar to that described by Pitts ( I 3 ) , except that a 90-liter Pyrex reaction cell is surrounded by a 400-W bank of fluorescent lamps (black light and sunlight). The system is evacuable to torr. The interior metal surfaces, excepting the mirrors, have been coated with a nonporous fluorocarbon resin. The spectrophotometer is a Perkin-Elmer Model 521 equipped with scale expansion which, in effect, allows the maximum optical path length of the cell to be increased electronically from 40-800 meters. Temperature and dew point within the reaction cell are monitored continuously. External forcedair cooling maintains the cell within 2°C of ambient temperature throughout a run. Reactions proceed somewhat more slowly in this system than in the environmental chamber since the intensity of ultraviolet radiation is appreciably less (k1 = 0.2 min-l compared to about 0.6 min-l in the chamber). In general, however, one can assume that the same reactions occur in both systems a t roughly the same relative rates. The LPIR runs were made up with reactants measured out in gas pipets on a small vacuum line and introduced into the evacuated cell. Hydrocarbon-free air (Matheson Air Zero) was then added to 1 atm total pressure. In the “wet” runs, filtered deionized water was measured as a liquid and evaporated into the evacuated cell. Infrared Spectra of Aerosol. Aerosol samples from a number of chamber runs were collected on prewashed glass fiber filters and extracted with ethanol. The extracts were dried by gentle heating, weighed, and redissolved. These solutions were used to prepare ultramicro (1.5 mm diam., 1.0 mm thick) potassium bromide pellets from which infrared spectra were obtained. Materials. Hydrocarbons used as reactants in these studies were Phillips Petroleum, Reseqrch Grade, or Aldrich Chemical Co., Purified Grade. Before use, olefins were stored in the dark at 0°C. Purity was checked by gas chromatography and found to be greater than 99% in all cases. Nitric oxide was Matheson C.P. Grade (99.0%). Nitrogen dioxide was Matheson Standard Grade (99.5%). Elemental analyses were performed by Schwartzkopf Lab-
Table I. Aerosol Formation from Selected Hydrocarbons [Each hydrocarbon (2.0 ppm) irradiated with 1.0 pprn nitric oxide and 70% R H measured at 22"C] Hydrocarbon
Glutaraldehyde Ethyl benzene Mesitylene 2,6-0ctadiene 1-Octene trans-4-Octene 5-Methyl-1-hexene 2,6-Dimethylheptane 1-Heptene o-Xylene 1,5-Hexadiene Cyclohexene 2-Methyl-1,5-hexadiene 1,6-Heptadiene 1,7-0ctadiene a.Pinene
Maximum light scattering, bscat X l o 4 m-1
0 1
1 1
1 1 1
1 1 8 40 90 110 160 180 180
oratories, Woodside, New York, N.Y. Molecular-weight determination was by osmometry.
Results and Discussion Typical Reaction Profile. Figure 2 gives a reaction profile illustrating typical changes in concentration of a number of parameters during t h e course of a n aerosol-forming chamber experiment. With olefins, the onset of aerosol formation was always delayed until oxidation of XO to NO2 was complete. Following the NO2 maximum, ozone appearance, a n increase in the nuclei count a n d the beginning of nephelometer light scattering occurred. For those runs where NO2 rather than N O was t h e starting material there was no induction period and aerosol formation began when the lights were turned on. These results suggest that aerosol formation is associated with a reaction between ozone a n d t h e precursor olefin. Hydrocarbon Dependence. The data summarized in Table I illustrate the aerosol-forming behavior of a representative group of hydrocarbons. Aerosol formation from these hydrocarbons was measured under a standardized set of conditions. In all cases, the initial hydrocarbon concentration was 2.0 ppm, the initial NO concentration was 1.0 ppm, a n d the initial relative humidity was 70% a t 22" C. Ultraviolet radiation was supplied until aerosol light scattering, as measured by the nephelometer, reached a maximum. All experiments were run without a mixing fan. From these data i t is apparent t h a t olefins with two reactive sites per molecule produce on the order of 100 times more light-scattering aerosol than the corresponding olefins of the same carbon number with only one reactive site (cf. cyclohexene and 1-hexene; 1,6-heptadiene and 1-heptene). Beyond this. we found t h a t only olefins of carbon number greater than five form appreciable amounts of light-scattering aerosol under these conditions. The effect of chain length is illustrated in the case of octadiene: a t 1 ppm the 1,Y-isomer forms large amounts of light-scattering aerosol (visibility
