Formation of Nitrosyl Chloride from Salt Particles in Air William H. Schroeder' and Paul Urone2* Department of Chemistry, University of Colorado, Boulder, Colo. 80302
Investigations of the reaction of sodium chloride particles with 0.1-1.5% mixtures of nitrogen dioxide and sulfur dioxide in air showed that nitrogen dioxide reacted with sodium chloride to form nitrosyl chloride (NOCl), a corrosive gas. The reaction was rapid but surface area dependent. It proceeded equally on dry or moist particles. Free energy calculations indicate a favorable free energy change and a large equilibrium constant. It is suggested that this reaction is of importance in studying the chemical and physical interactions of halide aerosols in an atmosphere containing nitrogen dioxide.
During some infrared studies of the products of the photochemical reactions of sulfur dioxide with nitrogen dioxide and various hydrocarbons at 0.1-1.5% in air ( I ) , some concern was felt for the possible adsorption or reaction of the mixtures on the sodium chloride windows of the infrared gas sample cell. A number of experiments were designed to test this possibility. When gaseous mixtures of nitrogen dioxide in air were introduced into a 2.5 x 10 cm infrared gas sample cell containing small amounts of powdered sodium chloride, a new strong absorption band occurred in the vicinity of the nitrogen dioxide absorption band. This new peak was assigned to nitrosyl chloride (NOCl), a major gaseous product from the heterogeneous reaction of NO2 with sodium chloride as follows:
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Figure 1 depicts the gas phase infrared absorption spectrum of a mixture initially containing 0.17~SO2 and 1.570 NO2 in dry air after it has been expanded into an evacuated gas sample cell containing approximately 1 gram of crystalline sodium chloride with a size distribution falling between 40-60 mesh. There is a pronounced reduction in the intensities of N204 and NO2 peaks in the presence of sodium chloride, and the appearance of a new intense doublet absorption band a t 1786 and 1805 wavenumbers. Several other new absorption peaks of lesser intensity also appear in Figure 1 and these are located at 595, 795. 2135, and at about 3525 c m - l . The intensity of the sulfur dioxide peak at 1360 cm-I is not significantly reduced in the presence of the sodium chloride. This was confirmed by carrying out a quantitative experiment involving a 1.07~ SO2-air mixture in the presence of 1.0 gram of moist sodium chloride (1.00 gram dry NaCl 30 mg H20). The reaction of nitrogen dioxide with sodium chloride is of considerable interest beyond the scope of the original investigation, since it has been suggested (2, ,5') as a possi-
+
Present address. Technology Development Branch, Air Pollution Control Directorate. Environmental Protection Service. E n vironment Canada, Otta&-aK1A OH3. Present address. Department of Environmental Engineering Sciences. University of Florida. Gainesville. Fla. 32601. 756
Environmental Science & Technology
ble pathway for the production of free chlorine in the atmosphere. Chlorine could result from photochemical decomposition of the nitrosyl chloride possibly formed through physicochemical interaction (i.e., physical adsorption followed by chemical reaction) of ambient NO2 with salt particles suspended in the air, as from sea spray for example. Although a considerable amount of work has been published on the possible synergistic effects of sulfur dioxide and salt aerosols (NaC1) on man and animals ( 4 , 5 ) , very little if any work has been reported for nitrogen oxides-salt aerosols effects (6). This is unfortunate, because earlier studies showed very little reaction between sulfur dioxide and salt particles at relative humidities less than 75% (I, 7 ) . Some quantitative data of the KOz-NaCl system were obtained and are summarized in Figures 2 and 3. These figures graphically present the infrared absorbance values of the nitrogen dioxide and nitrosyl chloride peaks as a function of residence time, particle size, and moisture. At "negative residence times" the absorbance value of the nitrogen dioxide peak is established for the gaseous mixture in an infrared gas sample cell not containing sodium chloride cqstals. No peak for nitrosyl chloride is present under these conditions. At positive residence times the absorbance values for both the nitrogen dioxide peak and the nitrosyl chloride peak were monitored after the gas mixture was introduced (at time zero) into a similar infrared gas sample cell containing the salt particles. This procedure is equivalent to introducing the sodium chloride (at time zero) into the gas sample cell containing the NO2 -air mixture, and is much more convenient to carry out experimentally. The NaCl for these experiments was dried at 110°C for at least 1 hr prior to use (at room temperature), and otherwise stored in a desiccator. A mixture of 0.10% SO2 and 1.50% N02, used in connection with other studies, was employed in the experiments illustrated in Figure 2. Figure 3 shows the NO2 reaction when SO2 was absent. The band center for the asymmetric stretching vibration of the nitrosyl chloride molecule was experimentally determined to be 1796 cm-1 and 1801 cm-I on two different occasions. Both numbers are in good agreement with the value of 1799 cm-I reported by other investigators for this molecule in the gas phase (8, 9 ) . Figures 2 and 3 show that at a constant initial nitrogen dioxide concentration the extent of the heterogeneous reaction between NO2 and NaC1, which leads to the production of NOCI, depends upon the amount of sodium chloride present as well as the surface area of the salt particles. In none of the investigations conducted did the reaction go to completion. The reason for this behavior is believed to be a surface-controlled phenomenon. One possible explanation might be that the forward reaction stops when the NaCl entities exposed at the surfaces of the particles have all been converted to SaNOs. Under the conditions studied, the conversion of NOz(N204) to NOCl is relatively fast, as illustrated by the observation that no residual N204 absorption band was found in the spectral region between 700 and 800 cm-I after a period of less than 3 min had elapsed from the time of introduction of
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Figure 1 . Infrared absorption spectrum of 0.1% SO2 and 1 . 5 % NO2 in d r y air introduced into a gas cell containing 1 gram of 40-60 mesh sodium chloride
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the gas sample into the infrared cell containing 5 grams of NaCl(40-60 mesh). Figure 3 reveals that the resultant nitrosyl chloride and nitrogen dioxide concentrations do not depend on whether the sodium chloride is wet or dry. The sodium chloride employed for this experiment was initially dried a t 110°C for about 8 hr. The moist NaCl was prepared by adding to the dried material slowly, while a t the same time distributing as evenly as possible, an arbitrary amount of water corresponding to a ratio of 1 mole of H20 for every 10 moles of sodium chloride. A relatively small, but apparently real, decrease in the measured absorbance value for the NOCl peak with increasing residence time was consistently observed in these experiments. This may be due to the occurrence of some decomposition of NOCl or perhaps a shift in the relative distribution of reactants and products as the gas sample cell and its contents gradually warmed u p inside the sam-
ple compartment of the infrared spectrophotometer. Since the infrared molar absorptivities for NOCl and NO2 would not be expected to be identical, the slopes of these lines (absorbance vs. residence time) need not necessarily be the same. Robbins et al. (10) and Cadle and Robbins (11)reported data on the reaction of NO2 with NaCl over a wide range of humidities. Gas concentrations ranged from 0.1-100 ppm and the concentrations of aerosol from 10-25,000 ug/m3. Under their experimental conditions they suggest a reaction route involving as a first step the hydrolysis of NO2 to form “ 0 3 vapor which then interacts with sodium chloride to produce HC1 as the major gaseous product. In our studies, no indication of HC1 was found in the infrared spectra obtained ( 1 ) . Instead, NOCl was identified as being the major gaseous reaction product. It is considered possible that in the investigations conducted by Cadle and co-workers-at least a t relative humidities less than about 7570-nitrosyl chloride was initially formed as an important intermediate, but which-because of its reactivity-was short lived and hence could easily have gone undetected. In addition to being photodissociated a t wavelengths below about 7600 A (12), NOCl is known to be readily decomposed by water to produce H N 0 3 , ” 0 2 , NO, and HCl(13, 14). Robbins et al. ( I O ) state that if NO2 and NaCl aerosol were to react directly to produce KOC1 and NaN03 a t very low concentrations, the reaction rate might be expected to be slow. In their experiments they found that “equilibrium fractions of NaN03” were produced quickly ( < I O min) when NaCl aerosol and NO2 were mixed in air at various relative humidities. They went on to say that the equilibrium was not affected greatly by changes in temperature or in absolute concentration of reactants a t constant mole ratios. From tabulated values of thermodynamic properties (15-173. we have calculated a value of A GO298 = -4.16 kcal mole-1 for the reaction as written in Equation 1. This would correspond to a value of the equilibrium constant K = 1.1 x 103. Unless inhibited, the reaction would be expected to go to completion because of the large, favorable, and free energy change involved. In these studies the rate of the NOz-NaCl reaction proceeded rapidly until limited by the amount of surface area available (Figures 2,3). Although the concentration of nitrogen dioxide used was several orders of magnitude greater than that in the atmosphere, if we recognize that atmospheric halide and sea salt aerosols are complex mixtures, the favorable free energy of the reaction makes it reasonable to predict that it can occur a t the surface of halide particles suspended in air in the presence of nitrogen dioxide. If formed, the nitrosyl chloride can remain adsorbed to the surface to react further, or it can be released to the surrounding air. The implications to atmospheric chemistry is obvious, and the possibility of the existence of such a reaction should be a matter of further investigation.
Literature Cited (1) Schroeder. W. H., P h D Thesis, University of Colorado, 1971. ( 2 ) Altshuller, A. P.. Sartrych ur Tellus, 10, 487 (1958). (3) Katz, M.. in “Air Pollution,” Vol. 11, Stern, A. C.. Ed., p 99,
Academic Press, New York, N.Y., 1968. (4) Amdur, M . 0.. Underhill, D., Arch. Enuiron. Health, 16, 460-8 (1968). (5) “Air Quality Criteria For Sulfur Oxides.” ibid., 1969. (6) “Air Quality Criteria For Nitrogen Oxides,” U.S. Department of Health, Education and Welfare, Washington, D.C., 1971. (-7)Urone, P.. Lutsep, H., Noyes, C. M., Parcher, J . F., Enciron. sei. Techno/., 2,611 (1968).
Volume 8.Number 8.August 1974
757
( 8 ) Burns, W. G., Bernstein, H. J.. J . C h e m . Phys., 18, 1669
(1950). (9) Eberhardt, W. H.. Burke, T. G.. ibid.. 20.529 (1952). (IO) Robbins, R. C., Cadle, R. D., Eckhardt, D. L., J Meteorol, 16.53 (1959). (11) Cadle, R. D., Robbins, R. C., Discuss. Faraday S O C ,30, 155 (1960). (12) Calvert, J. G., Pitts, J. N., Jr., “Photochemistry,” p 230, Wiles & Sons. New York, N.Y., 1966. (13) Cotton, F . A . , Wilkinson, G., “Advanced Inorganic Chemistry.” p 355, Interscience, New York, Y.Y., 1966. (14) Beckman. L. J., Fessler, W. A , , Kise, M . A , , Chem ReL 48, (3), 319 (1951). (15) U.S. Department of Commerce, Xational Bureau of Stan~
dards. Circ. 500-Part I. “Selected Values of Chemical Thermodynamic Properties,” 1952. (16) Stull, D. R., Westrum, E . F., Jr., Sinke, G . C., “The Chemical Thermodynamics of Organic Compounds,” pp 224, 232, Wiley & Sons, New York, N.Y., 1969. (17) Latimer, W. M.. “The Oxidation States of the Elements and Their Potentials in Aqueous Solutions,” 2nd ed., Prentice-Hall, New York, N.Y., 1952.
Receiced for recieu Jul? 20, 1973. Accepted April 22, 1974. This Lcork U Q S made possible b3 grants from the National Science Foundation (GP-34238)and the Air Pollution Control Office. EPA (AP-00357-07).
Aerosol Filtration by Means of Nuclepore Filters Filter Pore Clogging Kvetoslav R. Spurny* Institute of Aerobiology, 5949 Grafschaft, Germany
Jarmila Havlova J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia
James
P. Lodge, Jr., Evelyn R . Ackerman, and David C. Sheesley
National Center for Atmospheric Research, Boulder, Colo. 80303
Boris Wilder Institute for Medical Research and Occupational Health, Zagreb, Yugoslavia
Experimental Experiments and results are described to help clarify the filtration kinetics and pore clogging mechanisms during filtration or sampling of aerosols with Nuclepore filters (NPF). Monodisperse latex aerosols (MLA) of different sizes were used to clog the filter. The curves of pressure drop, L p , and collection efficiencies, E, as functions of time, t , have very similar shapes; they are thus an aid to identifying the clogging mechanisms and to determining the equations L p = f ( t ) and E = F ( t ) . (A subsequent paper will deal with the equations.) Electron microscopy makes the NPF surfaces visible a t the different clogging phases and helps one to imagine how particles of different sizes settle inside and outside the pores. In previous papers (1-6), the filtration properties of clean NPFs were studied and equations given to describe the dependence of the filter pressure drop, Lp, on the gas filtration face velocity, the gas pressure, and the gas temperature. In addition, the dependence of the filter collection efficiency, E, on the previous parameters, as well as on the aerosol particle radius r, was presented, verified, and discussed. Although some preliminary considerations and measurements were also presented in those previous papers ( I , 4, 7), NPF clogging had not been investigated systematically. In this communication the equipment, measurement technique, and experimental results of such an investigation are described and discussed. Mathematical description of the clogging theory and comparison with experimental results will be the subject of a subsequent communication. 758
Environmental Science & Technology
Filters. American analytical NPFs (Kuclepore Corp., Pleasanton, Calif.) with pore diameters from 0.5-0.8 pm were used for the investigation. Structural properties (such as pore radius, porosity, thickness) were measured according to methods described previously (3, 7). Aerosols. Monodisperse latex aerosols (MLA) were used to produce clogging. Suspensions of monodisperse latex (Dow Chemical Co., USA and Serva, Heidelberg, Germany) with particle sizes, 2 r, of 0.088 pm, 0.268 pm, 0.312 pm, and 0.796 pm were used for aerosol generation. The aerosols were generated by atomizing a dilute water suspension of the particles in a glass nebulizer (Jouan, Paris; similar to the U.S.-made DeVilbiss nebulizer); the suspensions were diluted from their original concentrations (10%) according to the statistical theory of dilution ( 8 ) using nebulizer characteristics determined by Mercer et al. (9). Suspensions of the particles in bidistilled and filtered water with concentrations from 0.01-0.2% were the most practical according the particle size. The aerosol concentration (cme3 or pg m - 3 and so forth) and the number of aggregates of ML particles were measured by electron microscopy after the aerosol had been sampled in a thermal precipitator (Casella). Table I shows some of the results. Agreement with the theory of Raabe ( 8 ) for the smallest particles ( r = 0.044 pm) was not satisfactory; use of much lower concentrations brought no improvement. Probably agglomerates are already present in the water suspension (Table I). Equipment. Measurement of E during the clogging process was accomplished with practically the same equip-
