Smog Chamber Studies of Temperature Effects in Photochemical Smog William P. L. Carter, Arthur M. Winer, Karen R. Darnall, and James N. Pitts, Jr.' Statewide Air Pollution Research Center, University of California, Riverside, Calif. 92521
Irradiations of selected hydrocarbon-NO,-air mixtures were carried out over the temperature range -3 to 60 "C in a 5800-L, thermostated evacuable environmental chamber. The federal air quality standard for ozone, 0.08 ppm (1-h average), was exceeded within 6 h of irradiation of a surrogate hydrocarbon-NO, mixture approximating an urban atmosphere, even a t 5 "C. In all three systems studied (propene, toluene, and surrogate), rates of ozone formation increased significantly with temperature. In propene-NO,-air and in toluene-NO,-air reaction mixtures, the amounts of hydrocarbon consumed and the approximate amounts of NO to NO2 conversion increased significantly with temperature. The nature of the 0 3 concentration-time profiles in high concentration propene-NO, experiments was significantly affected by temperature. In runs between 11 and 33 "C, [Os] peaked slightly before most of the initially present NO, was consumed as is usually the case. However, for T I11 "C, [O,] peaked significantly earlier and, for T I33 "C, significantly later than the time of NO, consumption. Furthermore, in a run a t T E 50 "C, [ 0 3 ] reached a maximum prior to complete NO, consumption, then decreased slightly, and finally slowly increased to a second maximum. The mechanistic implications of these observations are discussed.
It is well known that the formation of photochemical smog is influenced by a number of meteorological factors (1-5). Their natural fluctuations contribute to the observed variation in the frequency and the intensity of episodes in different geographic locations and at different times of the year. Among the most important factors from a fundamental gas-phase chemical kinetic standpoint is temperature. Unfortunately, although recently there has been significant progress in understanding the temperature dependences of elementary processes, to date no quantitative experiments concerning the overall temperature dependence of ozone production have been conducted. Such information could be useful to control officials and to atmospheric scientists alike, for example in testing the validity of complex kinetic-computer models of photochemical air pollution as applied to "real world" situations where temperature extremes exist. Previous studies concerning the effects of temperature on photochemical smog formation have been reviewed in the EPA air quality criteria documents for ozone and other photochemical oxidants ( 2 , 3 ) . I t was concluded that both laboratory and field studies verify the existence of a significant, positive temperature effect. Additionally, on the basis of an analysis of field data ( 4 ) ,in which a good correlation between maximum 1-h oxidant levels and daily maximum temperatures was observed, and on the basis of outdoor smog chamber studies performed in the winter and summer (3, 6 ) , it was suggested ( 4 , 7 ) that "below a cutoff temperature of approximately 55-60' F (15 "C) atmospheric reactions are not fast enough to yield oxidant/ozone at levels above 0.08 ppm when pollutants react for 1solar day." Substantial effects of seasonal changes of light intensity, duration, and spectral characteristics were predicted in computer model calculations by Nieboer et al. (8) and Bottenheim et al. (9).Indeed, Bottenheim et al. (9)also calculated that the effect of seasonal variations of temperature on overall smog chemistry was minor. However, their model did not include PAN (10, 11) and peroxynitric acid ( 1 2 , 1 3 ) decompo1094
Environmental Science & Technology
sitions, which are now known to be important and highly temperature dependent. Thus, it is highly probable that their model underpredicts temperature effects. Direct evidence that significant photochemical smog formation can occur a t low temperatures comes from ambient air measurements by the Colorado Department of Health (14). Over 0.2 ppm of 0 3 was observed near Denver on a clear March day in which the temperature did not exceed 52 O F . These results contradict the conclusion in the Air Quality Criteria Document for Ozone cited above ( 3 ) .Clearly, there is an important need for unambiguous data concerning the effect of temperature alone on photochemical smog formation. Few data concerning temperature effects are available from smog chamber studies of photochemical air pollution, since such experiments have traditionally been conducted over a relatively narrow range of temperature, typically -25-35 "C. This has resulted from the experimental complexities involved in accurate temperature regulation of the large environmental chambers required for such studies. On the other hand, analysis of air quality and outdoor smog chamber data is not, unfortunately, an unambiguous method for determining temperature effects, since conditions of lower temperature are often also associated with conditions of reduced light intensity or duration, which of course has a significant impact on smog formation (1-4,8, 9). In order to address this problem, a 5800-L evacuable environmental chamber (15,16) in our laboratories was designed and constructed with the capability for a wide range of temperature control (e.g., -20 to +lo0 "C). This capability already has been utilized in kinetic studies of the temperature-sensitive species peroxynitric acid (12, 13). This evacuable chamber-solar simulator facility is currently being used in a program to determine the effects of temperature and other physical parameters such as UV spectral distribution ( 1 7 ) on photochemical smog formation processes. In this paper three categories of hydrocarbons are treated: (a) mixtures designed to resemble as closely as possible the composition and concentration ranges occurring in a polluted urban airshed [in our studies, these consist of sub-parts per million concentrations of oxides of nitrogen (NO,) and a "surrogate" hydrocarbon mixture consisting of 12 alkanes, olefins, and aromatics designed to represent hydrocarbon emissions from all sources in the Los Angeles air basin (18)l;(b) mixtures consisting of sub-parts per million concentrations of NO, and the single hydrocarbons propene or toluene; (c) mixtures consisting of NO, and propene in concentration ranges greater than ambient. Studies of high concentration systems are useful because chamber effects relating to unknown radical sources, known to be important in ambient concentration smog chamber studies (19-21), are minimized by increasing the relative importance of known homogeneous radical sources. This also allows tests of detailed models under a wider variety of conditions, and allows a larger number of trace intermediates and products to be detected.
Experimental Experimental Facilities and Methods. The experimental facilities and methods employed in these smog chamber experiments are discussed in detail elsewhere (15,16,22-24) and are only briefly described here. The evacuable thermostated chamber (15, 1 6 ) consists of a 5800-L cylindrical aluminum alloy cell coated on the inside with Teflon and equipped with
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quartz end windows (UV cutoff -200 nm). Photolyses were performed using a 25 000 W solar simulator ( 2 2 ) producing a collumated beam designed to minimize direct irradiation of the chamber walls. The light was filtered by a 0.25-in. Pyrex pane to cut off radiation below -290 nm. The temperature control system was designed to regulate the temperature of the chamber walls to f0.5 "C over the -20 to +lo0 "C range. In this system ethylene glycol is heated or cooled by external heat exchangers and then circulated by means of a 1.5 hp pump through channels welded to the chamber exterior. A YSI Model 71A temperature controller diverts the circulating fluid through the heat exchangers in response to a thermocouple signal. A 10-kW electrical heater powers the exchanger used for the heating cycle, and a 7.5-ton two-stage refrigeration unit is used for the cooling cycle. The chamber temperature can be taken from ambient to either -20 "C or +lo0 'C in less than 4 h. The f0.5 "C control of the fluid temperature provided by this system results in regulation of the chamber air temperature to better than f0.2 "C due to the large heat capacity of the chamber. The chamber walls are insulated with 1 in. of fiberglass insulation and 2 in. of polyurethane foam and are covered with an aluminum sheath. Further details of this system and its performance specifications are reported elsewhere (15, 16). Prior to each experiment, the chamber was evacuated to a t least Torr After we filled the chamber with purified matrix air (23) a t the desired temperature and relative humidity, reactants were added and allowed to mix for a t least 30 min. During the course of a run, sampling consumed approximately 2% of the reaction mixture per hour; the chamber pressure was maintained by the addition of purified air (at room temperature) from a Teflon bag outside the chamber. Absolute light intensity within the chamber was determined periodically using NO2 actinometry (25). Relative spectral distributions were obtained with a double monochromatorphotomultiplier system located a t the far end of the chamber facing the solar simulator through the chamber end windows. Temperature was monitored using thermocouples, pressure with a Validyne gauge, and relative humidity with a Brady array (16). Methods and reliabilities for monitoring reactants and products are described in detail elsewhere (15,16,24).Ozone, NO, N o r , and NO, were monitored by chemiluminescence methods, and CO. organics, and peroxyacetyl nitrate (PAN) and organic nitrates by gas chromatography. Known interferences by PAN and organic nitrates on commercial chemiluminescence NO, analyzers (26) were corrected for by subtracting the chromatographically determined PAN and organic nitrate concentrations from the NO2 readings. Reactants a n d Conditions Employed. Five different reactant mixtures were irradiated at a variety of temperatures. Specific initial concentrations and temperature ranges employed were: (a) propene -0.5 ppm, NO, -0.6 ppm, T = 16-29 "C, (b) propene -1 ppm, NO, -0.5 ppm, T = 10-29 "C, (c) propene -10 ppm, NO, -6 ppm, T = 3-59 "C, (d) toluene -1 ppm, NO, -0.5 ppm, T = 8-53 "C, and (e) "surrogate" mixture -2.5 ppmC, nonmethane hydrocarbons (consisting of -40 ppb of ei,hene, -80 ppb of ethane, -40 ppb of acetylene, -15 ppb of propene, -17 ppb of n-butane, -12 ppb of cis-2-butene, -110 ppb of 2,3-dimethylbutane, -10 ppb of 2-methylbutene-2, -20 ppb of toluene, and -60 ppb of mxylene), methane -2.5 ppm, NO, -0.25 ppm, T = 5-34 "C. The initial concentrations, temperatures, humidities, and light intensities (as measured by h l , the NO2 photolysis rate constant) for these runs are given in Table I. Resulta and Discussion
Ozone Formation in "Ambient" Concentration Runs. Table I gives the times of the ozone maxima (if attained),the
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TIME (min) Figure 1. Selected experimental concentration-time profiles for T = 16.1 "C, run no. 193
maximum 0 3 concentrations observed, and the 0 : )concentrations observed a t selected times prior to the maximum for runs a t ambient concentrations. Clearly, the rate of ozone formation, as indicated by the times of the 0:jmaxima or the 2- or 6-h 0 3 concentrations, increases significantly with temperature. Despite this, O:]formation occurred at the lowest temperatures, and in every case, even at temperatures as low as 5 "C, the ozone concentrations observed after 6 h of irradiation exceeded the federal air quality standard for oxidant a t 0.08 ppm. Thus, while these experiments confirm the previous evidence of a significant and positive temperature effect for photochemical smog formation, they do not support the assertion in the revised air quality criteria document for ozone (4)that oxidant concentrations are unlikely to exceed the federal standard a t temperatures below -15 "C. While extrapolation of our smog chamber results to the atmosphere must be done with caution, the observation of high ozone levels (exceeding a first-stage alert in California) on bright and cold ( T 5 11 "C) days in Denver ( 1 4 ) is consistent with our chamber data. Ozone Formation a n d NO, Consumption in Higher Concentration Runs. Table I1 gives times and levels of the ozone maxima and minima, the times of probable complete NO2 consumption, and the times of the PAN maxima observed in the high concentration propene runs. In addition, Figures 1-3 show the concentration-time profiles for 0 3 , NO2 " 0 3 , and PAN for experiments performed a t 16,33,and 51 "C, respectively. It can be seen that for T ? 16 "C the times of the 0:iand PAN maxima and of estimated NO, consumption decrease with increasing temperature, while the ozone maximum concentration is relatively unaffected h y temperature for T ? 11 "C. On the other hand, the maximum 0 : ) observed in the 3 "C runs was significantly lower, and occurred earlier than in the higher temperature runs. In addition to affecting the rates of O:,formation, temperature was found to affect the shapes of the O:
