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F. E. LITTMAN, H. W . FORD, and N. ENDOW Stanford Research Institute, Pasadena, Calif.
Formation of Ozone in the Los Angeles Atmosphere
Oxidant formation in Los Angeles atmosphere is explained b y assumption that ozone is formed by nitrogen dioxide photolysis aided by rapid conversion of nitric oxide to nitrogen dioxide in presence of certain hydrocarbons. The quantitative aspects of such systems are under investigation
IN
W O R K I N G with reactions as complex as those occurring in Los Angeles smog, two approaches can be used-i.e., either a simplified system containing only a few components of known concentrations, or the outside air as a substrate for experimentation. While the former approach is obviously better suited for elucidation of reaction mechanisms, it is characterized by the inherent danger that reactions studied may not go at all, or may lead to products entirely different from those produced in the presence of multitudinous compounds normally present in the urban atmosphere. Thus, the best approach requires the use of both techniques. The work with outside air establishes the frameizork within M hich model reactions have to perform. and it is only by frequent reference to such a frammork that u e can avoid setting off a t a tangent. The work described in this paper represents primarily the first step in an attempt to characterize the reactions leading to the formation of oxidant in the LOS Angeles atmosphere. The composition of such oxidants is probably quite complex. Considerable evidence exists, however, that the bulk of this material consists of ozone (4). The characteristic diurnal changes in concentration, which parallel the amount of available
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sunlight, suggested that the oxidant \\'as produced by the action of light. This assumption \vas further borne out when it was demonstrated that an oxidant could be formed by irradiation of night air with artificial light. Other possible sources of oxidant had been examined previously. but neither electrical discharges nor transportation from the upper atmosphere could be substantiated experimentally. Photochemical formation seemed unlikely at first, because the Lzave lengths necessary for the dissociation of oxygen are not available a t sea level. The indirect formation of ozone, with a substance other than oxygen acting as light absorber. \vas then considered. Recent experimental evidence lends this h>-pothesis much credence. The daily fluctuations of oxidant concentration have been follo\zed for several years by means of a n automatic oxidant recorder a t the Standard Research Institute's Pasadena Laboratory (Figure 1) ( 3 ) . Typical oxidant curves (Figure 2) sho\v a characteristic pattern- lob7 concentrations at night increasing to a maximum around noon, and disappearing again at sunset. hlaximum concentrations vary greatly. \vith values as high as 60 p.p.h.m. occurring during smoggy weather. Concentrations at night seldom exceed 5 p.p.h.m. The variations from day to day are greater than those from month to month. h'evertheless, an annual cycle does exist. with high concentrations occurring more frequently from May through October than during the rest of the year. If: however, air is irradiated tvith artificial sunlight before entering the recorder, a totally different pattern results. The experimental arrangement used is shown in Figure 3. The air \vas passed through a 50-liter flask, irradiated with four H400 E-1 mercury vapor lights. The average residence time of the air was about 20 minutes. Since the mercury arcs gave off no radiation below about 3000 A. they were not capable of forming ozone directly by dissociation of oxygen. A typical oxidant pattern resulting from this arrangement is showm in Figure 4. The characteristic division between daytime and nighttime has
INDUSTRIAL AND ENGINEERING CHEMISTRY
disappeared, and oxidant concentrations of the same order of magnitude as those existing in daytime resulted from irradiation of night air. Continued monitoring of outside air shoived that the oxidant formed by irradiation of night air folloLved a daily and an annual cycle. The daily cycle frequently showed t\vo maxima! one around 7:00 P.M., the other about 8 : O O AM. High concentrations of oxidant could be formed regularly by irradiation of night air during fall and \\.inter, from September through March. During the summer months little. if any. oxidant was formed at night, despite the fact that, during that period high concentrations occurred in rhr daytime. Isolation of Oxidant Precursors
The ohservarion that oxidant could he formed experimentally by the action of light on polluted urban air led to an investigation of those impurities i n air \ \ hich, upon irradiation. formed oxi-
Figure 1 .
Continuous oxidant recorder
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Typical ozone concentration records
dant. The term "oxidant precursors" \vas coined to describe these materials, and a series of experiments was started to determine their physical and chemical properties. That light \vas responsible for the formation was demonstrated by using tlvo oxidant recorders-one Lvhich indicated the oxidant concentration in untreated air, and the other in irradiated air (Figure 5). Attempts were made to determine the region of active light by use of optical cut-off filters. The results, summarized as follows, indicate that primary light acceptor(s) absorb in a rather broad region, beginning near the short ivave end of the visible spectrum and extending to below 3600 .4.
Diagram of single-flaskprecursor recording
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Daily variations of ozone formed by constant irradiation of outside air
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Light envelope and flask Ivalls a'' window glass Soles glass X-ray lead glass Corning glass filter N o . 373
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Figure 6 shoivs the output o f t h e mercury vapor lamps and the regions of transmission for the various filtrrs. .4ttempts were then made to determine some of the chemical properties of the precursors by subjectinq the air (prior to irradiation) to the action of various scrubbers, in an effort LO remove the precursors These experiments in-
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5. Oxidant formed b y constant irradiation of outside air-Sept.
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VOL. 48, NO. 9
1954 SEPTEMBER 1956
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dicated that passage through a bed of activated carbon or through a combustion furnace of 760" C. removed or destroyed the precursors. On the other hand, when the incoming air was scrubbed counter-currently through packed columns 4 feet long, using 5% sodium hydroxide, the bulk of the oxidant precursors remained (Figure 7), although the concentration of acid gases such as carbon dioxide, sulfur dioxide, and nitrogen dioxide was greatly reduced by this procedure. Similarly, a 5% solution of semicarbazide buffered to a p H of 4 with phosphoric acid, had no effect (Figure 8), although this agent effectively removes carbonyl compounds such as aldehydes and ketones. A particle filter, capable of removing particulate material down to 0.2 p in diameter, also had no effect (Figure 9). Freeze-out traps were used next, in a n attempt to separate or trap the precursors. A trap packed with stainless steel helices and cooled in dry ice-acetone produced no significant reduction in the amount of oxidant formed upon irradiation of air passed through the trap (Figure 10). When a liquid-oxygen cooled trap was used in series with the dry ice trap, essentially complete retention of the oxidant precursors resulted (Figure 11). Upon removal of the coolant bath, the bulk of the precursors was released, as indicated on the same figure. Thus, this technique for the collection and concentration of the substances which yield oxidant on irradiation promised to make their analysis feasible and gave hope for their eventual identification. Numerous freeze-out collections were made and analyzed. The train finally
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Effect of a caustic scrubber on ozone formation
frared, ultraviolet, and mass spectrometric methods. After being collected in liquid-oxygen cooled traps, the samples were kept in liquid nitrogen. After connections Ivere made to the mass spectrometer, the noncondensable fraction was pumped off and the remaining sample \vas introduced into the inlet system by gradually warming the trap to room temperature. Samples for infrared and ultraviolet analyses $ v e x handled similarly, but the transfer was accomplished by freezing the sample over into the optical cell which was equipped with a cold finger. The results of these analyses indicated that the bulk of the material retained in the liquid-oxygen traps consisted of carbon dioxide and water. The balance of the sample was made up of organicss The general distribution of the mas. peaks resembled a gasoline mass spectrum with the high molecular end at-
used is shown schematically in Figure 12. The air was drawn through a semicarbazide column and a caustic scrubbing column. The stream was then splitone half passed through an irradiated flask and then into a series of traps; the other half passed through a dark flask on its way to the traps. By examining the contents of the traps. we hoped to obtain a differential analysis which would be easier to interpret. because irradiation should introduce a change which could be picked out from the general background common to the dark and irradiated samples. Various trap designs were tried, a few of which are shown in Figure 13. The over-all efficiency for both retention and desorption is compared in Table I. From this tabulation it appears that traps packed with stainless steel helices gave the best recovery of precursors. The samples were analyzed using in-
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Characteristics of optical filters and output of
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