Generation of polar organic aerosols from combustion processes

Transcription

1 Generation of polar organic aerosols from combustion processes Lynn M. Hildemann and Liya E. Yu Civil and Environmental Engineering Department Stanford University, Stanford, California, U.S.A edu Abstract The characteristics of combustion processes are examined to gain insights into what factors influence the generation of polar vs nonpolar organics during combustion. The fuel type as well as the temperature and efficiency of the combustion process are influential. For highly-efficient combustion processes, thermal processing tends to convert the organics initially released via devolatilization to more nonpolar products. Under less efficient combustion conditions, the organic emissions tend to retain more of the compositional characteristics of the fuel being burned. 1 Introduction Aerosols containing highly-polar organics are of particular interest from an atmospheric standpoint. The light-scattering properties of atmospheric aerosols are greatly influenced by their hygroscopicity, that is, their tendancy to take up water at elevated relative humidities. Like many inorganic aerosols, moderately-polar organic aerosols are expected to have limited solubility in water; because of this, they will exhibit deliquescence, where an abrupt uptake of water occurs once a certain relative humidity is reached. However, because polar organics in the atmosphere span a wide range of solubilities, there are also much more polar organic compounds

2 936 Air Pollution which are highly soluble in water ; these compounds should take up water across the entire range of relative humidities. Therefore, it is of interest to better understand the circumstances under which certain types of combustion sources may generate significant quantities of highly-polar organic aerosols. This paper will utilize measurements of the polarity characteristics of organic emissions from coal under highly-controlled combustion conditions, along with more limited measurements of organic aerosol characteristics in ambient samples and in emissions from other combustion sources, to evaluate under what conditions the primary organic aerosol emissions from combustion processes contain a significant proportion of polar compounds. 2 Background Some evidence indicates that organic aerosols can play a significant role in water uptake. Comparing biomass smoke emissions with crude oil combustion emissions, it has been reported^ that % of the biomass particles were active as cloud condensation nuclei, as compared with <10% of the crude oil smoke particles while the ionic contents of these two combustion aerosols were not dramatically different, the biomass emissions were found to contain a much larger fraction of polar organics. Ambient aerosol measurements in a marine environment indicated that ~63% of the cloud condensation nuclei were attributable to organic aerosol mass concentrations^. A more recent analysis offieldmeasurements from a remote desert location found that the amount of water taken up by ambient aerosols at elevated relative humidities was up to twice what was expected based on the hygroscopic characteristics of the inorganic compounds present?. While organic aerosol components can be formed in the atmosphere via secondary chemical reactions involving gas-phase precursors, there is also a significant proportion of organic aerosol that originates from primary source emissions, especially in polluted urban areas where local source contributions are substantial. For example, in downtown Los Angeles, secondary organic aerosols have been estimated to constitute just 5-38% of the total fine organic mass^; the bulk of the rest of the organic mass has been attributed to local primary source emissions. Current analytical methods limit our ability to fully evaluate the polarity of organic aerosol emissions from various combustion sources. The extraction methodologies typically used for gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS) analyses use solvents

3 Air Pollution 937 that effectively dissolve nonpolar organics but are much less effective for highly-polar species. While derivatization methodologies allow conversion of certain polar functional groups to elutable analogues, the derivatization procedures most commonly used are highly effective for widely-spaced carboxyl groups, but much less efficient at derivatizing hydroxyl groups or multiple polar substituents in close proximity. Because of this, only a subset of the total organic mass has been amenable to analysis via GC and GC/MS techniques. However, one cannot conclude that all the non-eluting organics are highly polar, because high molecular weight nonpolar organics would also fail to elute using a standard analytical protocol. Thus, at most one can place an upper bound on the polar organics present, by comparing the organic carbon measured using a thermal evolution/combustion technique with the eluted organics measured by GC. For 24-hr fine organic aerosol samples collected in the Los Angeles basin*, on average, 52% of the organic mass over a year-long period was found to consist of neutral organic compounds which eluted chromatographically without derivatization. Another 21% of the fine particulate organics consisted of somewhat polar compounds which were soluble in a benzene/isopropanol mixture, and were modified to an elutable form using a derivatization procedure. The remaining 27% of organic aerosol either was too polar to be extracted, resistant to derivatization, or of too high a molecular weight to elute. 3 Findings 3.1 Effect of Combustion Conditions A simplified picture of the combustion process for the hydrocarbons present in a solid or liquid fuel might consist of three stages. In the first stage, heating of the fuel causes devolatilization, where substances like organic compounds in the fuel volatilize into the gas phase. At the next stage, called secondary pyrolysis, thermal alteration of the volatilized organics occurs. Because of the timescales involved for diffusion of oxygen to the fuel, this initial thermal processing occurs in an environment that is quite fuel rich. Finally, during oxidative pyrolysis, additional thermal processing occurs in the presence of higher oxygen concentrations. Two coals with quite different organic compositions were combusted in a laboratory-scalefiuidizedbed reactor, varying the temperature and the residence time in order to examine differing extents of secondary pyrolysis.

4 938 Air Pollution Gravity flow column chromatography was used to characterize the polarity distribution of the organic aerosols emitted from this coal combustion process^. By starting with nonpolar solvents and progressing towards more and more polar solvents, the polarity composition of the organic emissions can be characterized as a function of the extent of pyrolysis. As the pyrolysis conditions became more severe, the amount of coal tar organics eluting in the most polar solvents (methanol and tetrahydrofuran) dropped from 37% to 14-16% for Dietz coal, and from 30% to <20% for Pittsburgh No. 8 coal^. Correspondingly, the fraction of organics eluting in the most nonpolar solvents (heptane and toluene) increased from 48% to 72% for Dietz coal, and from 54% to 65-72% for the Pittsburgh No. 8 coal. Dietz coal has a much higher oxygen content than the Pittsburgh No. 8 coal, and is known to contain oxygen mainly in the form of hydroxyl and carboxylic substituents, which are highly polar. Thus, it is not surprising that at the early stages of secondary coal pyrolysis, when not much thermal processing has occurred, the Dietz coal emits a larger fraction of highly-polar organics. By the late stages of secondary pyrolysis, the two coals emit organics that have become quite similar in their polarity distributions, with a large fraction of relatively-nonpolar compounds. This results from the greater influence of thermal processing in modifying the initially volatilized organic compounds to less-polar analogues. It has been hypothesized that significant cleavage of polar substituents from the parent aromatic ring structures occurs during secondary pyrolysis, thereby causing organics that originally eluted with the highly-polar solvents to elute much earlier, with the nonpolar solvents. Further analyses of the coal tar extracts using high-performance liquid chromatography** and GC^ have revealed trends that support the importance of this thermal processing pathway. Thus, the extent of pyrolysis, that is the efficiency of the combustion process, determines the extent to which the emissions resemble the polarity characteristics of the fuels being burned. 3.2 Effect of Fuel Type Some limited information is also available regarding the emissions of polar organic aerosols from other primary combustion sources^. By comparing the total organic carbon present in an aerosol sample (as determined via a thermal combustion/evolution technique) with the organic mass eluting via GC with and without derivatization, an estimate can be obtained of how much of the organic matter was moderately polar. One can also examine what fraction of the organic aerosol did not elute, as an upperbound measurement of what portion may have been more highly polar.

5 Air Pollution 939 Table 1 summarizes these organic characteristics for a number of different sources of combustion emissions. The numbers indicating what percent of the organics eluted are somewhat approximate, because a conversion factor must be chosen to convert organic carbon mass to total organic mass; here, a factor of 1.2 was used. Table 1. Characteristics of Organic Aerosols in Combustion Emissions % of organics % of eluted organics Combustion source (no. of samples) that eluted that were neutral Boiler, no. 2 fuel oil (3) Catalyst-equipped automobiles (1) Noncatalyst automobiles (1) Diesel trucks (1) Home appliances, nat'l. gas (1) Fireplace, pine wood (1) Fireplace, oak wood (1) Fireplace, synthetic log (1) Cigarette smoke (2) Hamburger charbroiling (2) Hamburger frying (1) 57-67% >100% >100% 92% 94% 51% 45% 77% 81-87% 27-43% 60% -100% 99% 97% 98% 87% 99% 49% -100% % 76-81% 87% Greater than 90% of the fine organic aerosol emitted from motor vehicles and from natural gas combustion was elutable via GC, and 904-% of this eluted mass was considered neutral (meaning that it eluted without needing derivatization). This indicates that the bulk of the organic aerosol emitted from these fossil fuels under well-controlled combustion conditions was nonpolar. Even though a smaller fraction of the organic aerosol emissions from the combustion of no. 2 fuel oil was elutable, all of the eluted mass from this fossil fuel-fired combustion source also was nonpolar. In contrast, the organic aerosol emissions from sources involving combustion of biomass spanned a broader range of polarities. Cigarette combustion of tobacco generated a much largerfractionof elutable organics (81-87%) than burning natural wood in afireplace (45-51%). This suggests that thermal processing of the biomass contained in a cigarette is more thorough, since a much larger portion of the organic mass emitted consisted of lower molecular weight, relatively nonpolar (that is, elutable) compounds. Almost half of the elutable organics from combustion of oak wood consisted of moderately polar species (where deri vatization was necessary for the compounds to elute), whereas >90% of the elutable organics in tobacco smoke and in pine wood emissions were neutral.

6 940 Air Pollution It is not surprising that the organic characteristics of the synthetic log emissions are intermediate between those of the fossil fuel sources and the natural wood sources. Like other fuels burned in a fireplace, the combustion process for a synthetic log is rather inefficient-, so the organic emissions will retain many of the characteristics of the parent fuel. A synthetic log consists of a mixture of petroleum waxes and sawdust, so the organic compounds emitted will contain long-chain, nonpolar compounds representative of petroleum waxes along with semivolatile biomass organics of more variable composition. Finally, meat cooking is included in Table 1 as an example of a source where volatilization is expected be a dominant contributor to the organic aerosol emissions. One would expect, especially for the case of frying, the polarity characteristics of the organic aerosol emissions to closely resemble what would be found in the fats present in the uncooked meat. 4 Discussion Due to the less-than perfect mixing present in a typical combustion environment, the organic aerosols emitted will represent a range of combustion conditions some compounds will be only slightly altered from what is present in the fuel source, while others will have undergone significant alterations that may have included ring cleavage, polymerization, aromatization, and removal of substituent groups. Thus, fuels like gasoline, no. 2 fuel oil, and synthetic logs (which contain a large proportion of petroleum waxes) will tend to generate relatively nonpolar organic aerosols, regardless of the combustion conditions. In contrast, processes like woodburning and meat cooking will generate emissions that depend greatly on the combustion conditions: since such combustion processes typically occur with a great excess of air and less-than ideal mixing between the "fuel" and the oxygen, the emissions include a substantial fraction of volatilized organics which have undergone little or no thermal alteration. Observations consistant with this picture have been reported for biomass burning. In wood combustion, dehydroabietic acid is recognized as a thermally altered version of the resin present in coniferous wood, and unaltered resins have also been observed in the emissions^. In addition, biomass combustion emissions contain odd-numbered n-alkanes, which are characteristic of plant waxes. Since biomass burning is a low-temperature, inefficient combustion process, it is not surprising that woodburning generates organic aerosol emissions characteristic of the wax and resin components present in the fuel, which have been volatilized with only limited thermal processing.

7 Air Pollution 941 Thus, inefficient, low temperature combustion processes involving fuels containing substantial amounts of polar organic compounds are expected to generate organic aerosols with a high potential for hygroscopic behavior. Because of this, combustion sources like biomass burning may represent a significant primary source of organic cloud condensation nuclei. References 1. Hildemann, L.M., A study of the origin of atmospheric organic aerosols, Ph.D. thesis, California Institute of Technology, Pasadena, CA, Hildemann, L.M., Mazurek, M.A., Cass, G.R., and Simoneit, B.R.T., Quantitative characterization of urban sources of organic aerosol by high-resolution gas chromatography, Environmental Science & Technology, 25, pp , Hildemann, L.M., Cass, G.R., Mazurek, M.A., and Simoneit, B.R.T., Mathematical modeling of urban organic aerosol: properties measured by high-resolution gas chromatography, Environmental Science & Technology, 27, pp , Hildemann, L.M., Mazurek, M.A., Cass, G.R., and Simoneit, B.R.T., Seasonal trends in Los Angeles ambient organic aerosol observed by high-resolution gas chromatography, Aerosol Science & Technology, 20, pp , Novakov, T., and Penner, J.E., Large contribution of organic aerosols to cloud-condensation-nuclei concentrations, Nature, 365, pp , Rogers, C.V., Hudson, J.G., Zielinska, B., Tanner, R.L., Hallet, J., and Watson, J.G., Cloud condensation nuclei from biomass burning, Global Biomass Burning Atmospheric, Climatic and Biospheric Implications, MIT Press, Cambridge, MA, Saxena, P., Hildemann, L.M., McMurry, P.H., Seinfeld, J.H., Organics alter hygroscopic behavior of atmospheric particles, Journal of Geophysical Research, 100, pp. 18,755-18,770, Saxena, P., and Hildemann, L.M., Water-soluble organics in atmospheric particles: a critical review of the literature and application of thermodynamics to identify candidate compounds, Journal of Atmospheric Chemistry, 24, pp , 1996.

8 942 Air Pollution 9. Simoneit, B.R.T., Rogge, W.F., Mazurek, M.A., Standley, L.J., Hildemann, L.M., and Cass, G.R., Lignin pyrolysis products, lignans, and resin acids as specific tracers of plant classes in emissions from biomass combustion, Environmental Science & Technology, 27, pp , Yu, L.E., Hildemann, L.M., DaDamio, J., and Niksa, S., Characterization of coal tar organics via gravity flow column chromatography, Fuel, 77, pp , Yu, L.E., Hildemann, L.M., and Niksa, S., Trends in aromatic ring number distributions of coal tars during secondary pyrolysis, Energy & Fuels, 12, pp , Yu, L.E., Hildemann, L.M., and Niksa, S., Characteristics of nitrogen-containing aromatic compounds in coal tars during secondary pyrolysis, Fuel, 78, pp , 1999.