United States Environmental Protection Agency Environmental Sciences Research ^ Laboratory Research Triangle Park NC 27711 *^ Research and Development EPA-600/S3-80-095 Mar. 1981 Project Summary Aerosol Formation from Diesel Exhaust and SO2 A Chamber Study Auto exhaust emissions are one of the most omnipresent, man-made pollution sources. Especially within the past decade, a significant quantity of research has been devoted to the impact of the emissions of gasoline- fueled autos upon the health of man and the environment, the information on smog, and the composite levels of various pollutants. These investiga- tions have provided data upon which both federal and state control strate- gies are based. During this period, the use of light duty diesel-fueled autos in the United States was insignificant. Total on-highway diesel fuel usage was about 10% by volume of the amount of gasoline used (Office of Technology Assessment, 1977). With the recent escalation in the price of fuel for autos, there is an economic justification for light duty diesel-fueled autos. Several foreign auto makers and at least one domestic auto maker offer such vehicles. It is speculated that light duty diesels could comprise 25% of the new car fleet by 1985. The potential for such a dramatic increase of light duty diesel- fueled autos as well as development of medium duty diesels, necessitates that investigations anticipate the impact of such levels of diesel emis- sions upon urban pollution levels (both paniculate and gaseous), photochem- ical and catalytic oxidation processes responsible for significant rates of gas-to-particle conversion in the urban atmosphere, and the health of the urban population. As a result of the significant dif- ferences between diesel and gas- fueled auto emissions and the poten- tial for widespread use of diesels in the near future, smog chamber studies of a simulated diesel exhaust/urban system were performed by Calspan Corporation under Contract No. 68- 02-2987 from the U.S. Environmen- tal Protection Agency. The program, which was performed in Calspan's 600 m3 indoor smog chamber, was aimed at assessing the role of diesel emissions in future urban environ- ments. The experiments provided semi-quantitative data on aerosol formation and growth, chemical kinetics, and the evolution of muta- genicity in the diesel exhaust/SO2/ hydrocarbon system. This Project Summary was devel- oped by EPA's Environmental Sciences Research Laboratory. Research Triangle Park. NC. to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Experimental Results In recent years, a significant level of effort has been devoted to the study of the chemical and physical processes responsible for smog formation in urban environments. Experimental data pro- vided by field and laboratory measure- ments have allowed many of the more ------- important parameters to be identified and modeled. As a result, current photo- chemical kinetic models generally provide at least the qualitative trends observed experimentally. One of the primary differences be- tween the emissions of light duty diesels and gasoline-fueled, catalyst- equipped autos is the great quantity of diesel primary particulates (soot). One of the purposes of the program was to investigate the influence of the primary particulates of the diesel emissions upon the rate of gas-to-particle conver- sion. If the primary particulates do significantly influence the chemical reactions of the system, existing models will not provide an accurate assessment of the system. Condensible organics known to be mutagenic are also associated with the primary particulates of the diesel emis- sions. Another aspect of this program was to investigate the fate of these organics under various simulated conditions to determine if the mutagen- icity of the emissions is likely to be modified with exposure to an urban environment. This experimental study of the diesel emission/hydrocarbon system was performed to assess the physical and chemical nature of the system. Particu- lar interest was vested in the primary particulates of the diesel emissions and their influence upon the system. Overview Dilute diesel emission systems were studied in a 600 m3 indoor smog chamber under various conditions likely to be encountered in an urban environ- ment. The major emphases of the study were aerosol formation and growth, chemical kinetics, and the mutagenic activity of the diesel aerosol. Important parameters with respect to visibility degradation and the formation of new aerosol material included relative humidity, S02 concentration, HC/NOx ratio, irradiation interval, and the presence of primary diesel aerosol (primarily soot). Aerosol formation and growth in UV- irradiated diesel exhaust/SOz/propy- lene experiments were primarily attri- buted to sulfate aerosol formation. The rate of SO2 oxidation to form sulfate aerosol was 5% hr"1 maximum during the Oa production period. After the peak ozone production period, the first 3 to 6 hours of a 24-hour experiment, the ozone concentration remained relative- ly constant and the SO2 oxidation rate decreased to approximately 1% hr"1. These observations could be tentatively attributed to the S02/hydroxyl radical reaction during the initial hours of the experiments. The hydroxyl radical concentration is high during the first few hours of ozone production and the gas phase SOa/hydroxyl reaction pre- dominates. The effect of the primary diesel aerosol on new particle formation was investigated by conducting experiments with and without prefiltered diesel exhaust. The presence of diesel aerosols precluded the formation of new aerosol particles; instead, the existing aerosols grow in diameter by the condensation of nuclei on them. The presence of the primary diesel aerosols appeared to increase the rate of S02 oxidation and the amount of sulfate formed via unknown heterogeneous or catalytic reactions. Increases in aerosol sulfate of from 25 to 75 percent were attributed to the presence of the primary diesel aerosol in contrast to what was attributed to the SO2/hydroxyl radical gas phase reaction. In most irradiated experiments, the production of organic and nitrate aerosols represented minimal contribu- tions to aerosol formation and growth. Nitrate aerosols (<1000 //g/m3) did represent a substantial fraction of the total aerosol growth in an experiment in which NH3 was added to the system. The mutagenic activity of the diesel aerosol was observed to be affected by simulated atmospheric exposure. Containment of dilute diesel emissions under non-irradiated conditions resulted in an increase in the mutagenic activity of the diesel aerosol. While irra- diation alone appeared to have no effect upon the mutagenic activity, photochemically reactive systems exhibited decreases in mutagenic activity of the diesel aerosols with irradiation. At least a portion of the observed decrease in mutagenicity was attributed to the presence of O3 (a product of the photochemistry). Conclusions For UV-irradiated tests, the activity of the diesel emission/SOz/propylene system (with respect to aerosol forma- tion and growth) had three distinct levels which correlated well with the rate of change of ozone concentration with time. Although new particle forma tion was frequently associated with th initiation of irradiation, the rate c increase in aerosol volume remaine low until substantial amounts of nitre gen dioxide were formed by the oxida tion of nitric oxide. The ozon concentration was very low during thi period (<10 ppb). Following an increas in the concentration of nitrogen dioxide the ozone concentration began t increase. This period of increasing ozone concentration represented thi most active portion of the experimen for aerosol growth. Sincethere general ly were sufficient particles to serve a sites for condensation of the nev matter, few new particles were formed The final level of system activit occurred after the ozone peak, whicl generally coincided with the depletioi of propylene (the predominant reactivi hydrocarbon species). Subsequent t< the ozone peak, there was a gradual bu persistent decay in the ozone concen tration. Little further aerosol growth oi visibility degradation was associatec with this period. Aerosol formation and growth in the diesel emission/SO2/propylene system was primarily due to sulfate formation. Prior to the period of rapidly increasing ozone (during the decay of nitric oxide), S02 oxidation rates (Rso2)on the order oj 0.1 % hr"1 were observed. S02 oxidatidl rates were determined from measured increases in aerosol sulfate with time and the concentration of available S02. During the period of increasing ozone concentration, Rso2 generally approached 5% hr"1. Subsequent to the formation of the ozone peak, Rso2 decreased to a low value (<1 % hr"1) and occasionally was undetectable due to losses of sulfate aerosols in excess of the rate of formation of sulfate. Such aerosol losses were due to sedimenta- tion and diffusion to the chamber walls. The fact that sulfate formation was greatly reduced, even though ozone and SO2 continued to coexist, provided evidence that the SOz-ozone reaction was not the primary path for sulfate formation during the period of high Rso2. Comparisons of aerosol sulfur as deter- mined by 1C (soluble sulfate) and XRF (total elemental sulfur) demonstrated good agreement, indicating that the concentration of sulfonated organics was small with respect to total sulfates. The primary particulates of the diesel emissions were principally composed of elemental carbon and various condensible organic compounds. ------- Analyses of the primary particulates of diluted diesel emissions revealed an elemental to organic carbon ratio of about 5 to 1 . One of the major effects of the primary particulates was to increase the mass mediandiameter(MMD)of the aerosol (e.g., 0.3 to 0.4 /urn) through a redistribution of mass versus particle size. As a result, the presence of primary particulates yielded an increased effec- tiveness of the aerosol to scatter light. The increased MMD might be indicative of an increasing fraction of material deposited deep within the human lungs during respiration, as a larger portion of the individual aerosol particles falls within the respirable size range (0.5 to 3 fjm diameter). Greater quantities of aerosol sulfate were consistently produced by irradi- ated systems containing the primary diesel aerosol than by comparable systems which did not contain the primary diesel aerosol. While the actual quantity of increased sulfate was difficult to assess (due to few duplicate experiments), increases in aerosol sulfate of from 25 to 70 percent were attributed to the primary particulates. This apparent increase in sulfate formation may have been due to hetero- geneous reactions on the surface of the carbonaceous particles or may have been associated with the high molecu- lar weight organics known to be carried by the primary aerosols of the diesel exhaust. No significant sulfate forma- tion was observed in the non-irradiated diesel exhaust/SOa/hydrocarbon system at low humidity. However, at high humidities (—92%) SO2 oxidation fates as high as 1.5% hr"1 were observed under dark conditions. To minimize error in the measurement of calibrated at the beginning of each experiment using carrier air with same RH as that to be encountered in the experiment. In irradiated diesel emission/SO2/ hydrocarbon systems, aerosol nitrate concentrations of about 1 pg/m" (approximately the blank level of the filter.) and gaseous ammonia concen- trations of less than 10 ppb were typically observed. With the introduction of ozone at low RH (<80%) into the system, the concentration of aerosol nitrate appeared to increase temporarily. At high humidity (94%), significant aerosol nitrate was formed (~30 pg/m3) and persisted with the introduction of ozone. Aerosol nitrate represented a substantial fraction of total aerosol growth in only one experiment (where 10 ppm of NH3 had been added to the chamber). The reaction of NH3 with gaseous nitric acid to produce the more stable ammonium nitrate apparently was responsible for the presence of a significant concentration of aerosol nitrate (in excess of 1000 fjg/m3) in this particu- lar experiment. Recommendations This program was wide-ranging in scope and as a result, survey-type in nature. The goals of the program were to identify potential problem areas associated with diesel emissions in an urban atmosphere to isolate the specific parameters responsible. The most significant trends in the tests of the diesel emission/SOa/ hydrocarbon system were associated with primary particulates. The primary particulates were apparently respon- sible for an increase in the amount of sulfate formed during irradiated tests and during high humidity conditions in the absence of irradiation. Further experiments should be performed to determine the expected increase in sulfate in contrast with the loading of primary particulates. Experiments should also be performed to identify the component of the primary particulates responsible for the increased sulfate formation (i.e., the carbon or the organics). The experiments performed in this program concentrated on simple systems (i.e., the addition of propylene and butane to diluted diesel emissions) to simulate the diesel emission/urban atmosphere. This system was chosen initially to aid in modeling the observed photochemistry. Only a limited number of experiments were performed with a more representative mixture of diesel and gasoline-fueled engine emissions. While the photochemical reactivities of the simple and more complex systems are similar, there are definite differ- ences in specific aspects of the system. To assure optimum relevance of experi- mental data, future experiments should be performed with the more representa- tive diesel/gasoline-fueled engine emission system. The potential for nitrates to contribute substantially to aerosol formation and growth in the presence of high NHs concentrations has been demonstrated. Further investigations are needed to determine the role of NH3 and liquid water in promoting the formation of aerosol nitrate from an apparently abundant source of gaseous nitrates. This program began to explore the possible effects of age and atmospheric exposure upon the mutagenicity of diesel aerosols. There are many unan- swered questions pertaining to the mutagenicity of diesel aerosols and aerosols in general. The questions concern: the chemical species respon- sible for the mutagenicity of diesel emissions; the effects of irradiation upon specific mutagenic compounds; the overall lifetime of mutagenic materials in the atmosphere; mechan- isms for the elimination of mutagenic compounds in the atmosphere; potential mechanisms by which mutagenic compounds may be formed by atmospheric reactions; and, the mutagenicity of gaseous constituents of diesel emissions. With the potential for an increasing atmospheric burden of mutagenic aerosols to adversely affect the public health, basic studies should be performed to determine the lifetimes and possible formation mechanisms of mutagenic compounds in the atmosphere. This Project Summary was authored by Staff of the Center for Environmental Research Information, USEPA, Cincinnati. OH 45268. H. M. Barnes is the EPA Project Officer (see below). The complete report, entitled "Aerosol Formation from Diesel Exhaust andS02," (Order No. PB81-121 147; Cost: $11.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Sciences Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 > U.S. GOVERNMENT FWNTINO OFFICE: Ml -757-012/7009 ------- United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use S300 "« i9°OQ329 ------- |