Environ. Sci. Technol. 2007, 41, 3972-3976
Contributions of Toluene and r-Pinene to SOA Formed in an Irradiated Toluene/r-Pinene/NOx/ Air Mixture: Comparison of Results Using 14C Content and SOA Organic Tracer Methods J O H N H . O F F E N B E R G , * ,† CHARLES W. LEWIS,† MICHAEL LEWANDOWSKI,† MOHAMMED JAOUI,‡ TADEUSZ E. KLEINDIENST,† AND EDWARD O. EDNEY† National Exposure Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, 27511, and Alion Science and Technology, Box 12313, Research Triangle Park, North Carolina, 27709
An organic tracer method, recently proposed for estimating individual contributions of toluene and R-pinene to secondary organic aerosol (SOA) formation, was evaluated by conducting a laboratory study where a binary hydrocarbon mixture, containing the anthropogenic aromatic hydrocarbon, toluene, and the biogenic monoterpene, R-pinene, was irradiated in air in the presence of NOx to form SOA. The contributions of toluene and R-pinene to the total SOA concentration, calculated using the organic tracer method, were compared with those obtained with a more direct 14C content method. In the study, SOA to SOC ratios of 2.07 ( 0.08 and 1.41 ( 0.04 were measured for toluene and R-pinene SOA, respectively. The individual tracerbased SOA contributions of 156 µg m-3 for toluene and 198 µg m-3 for R-pinene, which together accounted for 82% of the gravimetrically determined total SOA concentration, compared well with the 14C values of 182 and 230 µg m-3 measured for the respective SOA precursors. While there are uncertainties associated with the organic tracer method, largely due to the chemical complexity of SOA forming chemical mechanisms, the results of this study suggest the organic tracer method may serve as a useful tool for determining whether a precursor hydrocarbon is a major SOA contributor.
Introduction It is generally thought that biogenic monoterpenes, sesquiterpenes, and isoprene as well as anthropogenic aromatic hydrocarbons react with free radicals in the troposphere to form oxygenated compounds that can partition into the aerosol phase and/or undergo nucleation reactions to form secondary organic aerosol (SOA). SOA contains contributions from carbon, hydrogen, oxygen, and nitrogen, whereas the * Corresponding author phone: (919) 541-2915; fax: (919) 5411153; e-mail:
[email protected]. † U.S. Environmental Protection Agency. ‡ Alion Science and Technology. 3972
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 11, 2007
term secondary organic carbon (SOC) refers only to the contribution from carbon. The limited compositional data available suggest ambient SOA consists of a chemically complex mixture of multi-functional polar organic oxygenates, some of which may be oligomers. While a variety of methods, most of which are based on mass spectroscopic techniques, have been employed to identify and determine the concentrations of organic compounds in SOA in laboratory and ambient PM2.5 samples, the compositions have not been fully established, thus preventing a complete assessment of the contribution of SOA to ambient PM2.5. Such information is required to develop and evaluate air quality models to assess control strategies for reducing ambient concentrations of PM2.5. As part of ongoing efforts at the EPA National Exposure Research Laboratory to identify major sources of ambient PM2.5, we recently conducted a series of photochemical reaction chamber experiments and a field study to determine whether organic tracer compounds for SOA formation could be used to identify major SOA sources in a manner similar, in certain ways, to that employed successfully to determine the contributions of primary organic aerosol to ambient PM2.5 concentrations (1). To date, SOA tracer compounds for toluene, R-pinene, β-caryophyllene, and isoprene, each thought to be a significant SOA precursor, have been identified by individually irradiating the hydrocarbons in the presence of NOx in an atmosphere of air in a photochemical reaction chamber operated in the dynamic mode (2-5). After reaction with bis(trimethylsilyl)-trifluoroacetamide (BSTFA), the derivatized SOA samples were analyzed for polar compounds containing hydroxyl groups by positive chemical ionization gas chromatography-mass spectroscopy (GCMS). The sum of the mass concentrations of organic tracer compounds for each precursor was then divided by the gravimetrically determined SOA concentration yielding a tracer mass fraction. If it is assumed the tracer mass fractions of the laboratory samples and those of ambient PM2.5 samples are equal, which clearly is only an approximation, an estimate of the contribution of each precursor to the organic fraction of ambient PM2.5 samples can be obtained by dividing the sum of the organic tracer compound mass concentrations in the ambient sample by the laboratory-determined mass fraction. This procedure has been employed to analyze ambient PM2.5 samples collected in Research Triangle Park, North Carolina during 2003 (1). The tracer data were consistent with toluene, R-pinene, β-caryophyllene, and isoprene all contributing to significant SOA formation, with relatively high levels of SOA occurring during the summer, but decreasing significantly during the colder months. While the tracer-based SOA results from this single field study seem reasonable, additional evaluations of this method are required. In the present study, we have evaluated the SOA tracer method by conducting a laboratory study where a binary hydrocarbon mixture, consisting of the anthropogenic aromatic hydrocarbon, toluene, and the biogenic monoterpene, R-pinene, was irradiated in the presence of NOx to form SOA. The contributions of toluene and R-pinene to the total SOA contribution, calculated using the organic tracer method, were compared with those obtained with a 14C method. Over the past two decades, 14C measurements have been made by a number of research groups to determine the anthropogenic and biogenic contributions to the organic fraction of ambient PM2.5 (6-11). Here, the 14C technique was used to measure the fraction of modern carbon in the toluene/R-pinene SOA. Using this fraction, the contributions 10.1021/es070089+ CCC: $37.00
2007 American Chemical Society Published on Web 05/04/2007
TABLE 1. Initial Concentrations and Steady-State Temperatures and Relative Humidities for Hydrocarbon/NOx Photooxidation Experiments (Initial NOx Was >95% NO) precursors toluene toluene toluene R-pinene R-pinene R-pinene toluene/R-pinene toluene/R-pinene toluene/R-pinene
temp RH toluene propene r-pinene NOx (°C) (%) (ppm C) (ppm C) (ppm C) (ppmv) 28.4 28.7 28.4 25.6 25.5 25.7 25.8 25.6 25.5
29.9 30.0 30.0 30.5 30.5 30.4 30.4 30.4 30.5
9.7 10.1 9.1
10.7 10.7 10.4
1.0 1.0 1.0 2.3 1.9 2.4 2.3 2.3 2.3
0.37 0.38 0.38 0.42 0.42 0.42 0.42 0.40 0.40
of (a) the anthropogenic compound toluene, which consists nearly entirely of old carbon with a modern fraction of carbon near zero, and (b) the biogenic hydrocarbon R-pinene, composed mostly of new carbon with a modern fraction of about one, were calculated. In the present study, the 14Cbased contributions serve as a standard to which the tracerbased results are compared.
Experimental Section The laboratory study consisted of conducting toluene/NOx, R-pinene/NOx, and toluene/R-pinene/NOx photooxidation experiments in an atmosphere of clean air. Propylene, which does not contribute to SOA formation, was added to the toluene/NOx system to increase its reactivity. In this study, the toluene/NOx and R-pinene/NOx experiments were carried out to measure the tracer to SOA ratio and the [SOA] to [SOC] ratio for the two precursors, with the SOA concentration measured in µg m-3 and the SOC concentration expressed in µg C m-3. The SOA filter samples, collected during the toluene/R-pinene/NOx photooxidation experiment, were analyzed for 14C content and concentrations of toluene and R-pinene tracer compounds. The photooxidation experiments were conducted in a 14.5 m3 irradiation chamber operated as a continuous stirred tank reactor, which has been described in detail in previous publications (3, 4, 12). By operating the chamber in the flow mode at a residence time of 6 h, a mixture containing highly oxidized constant composition SOA is generated, which permits large volumes to be collected for chemical analyses. Prior to their introduction into the chamber, the hydrocarbons and NO were mixed in a 4 L inlet manifold. The hydrocarbons were introduced into the manifold by passing air through impingers containing individual neat liquids, whereas NO was injected from a gas cylinder with a mass flow controller. Inlet and chamber concentrations of reactant hydrocarbons were measured by gas chromatography with flame ionization detection (Hewlett-Packard, model 5890 GC) using a 30-m-long, DB-1 megabore column operated isothermally at a temperature of 115 °C. NO and NOx were measured with a TECO model 42C (Franklin, MA) oxides of nitrogen chemiluminescent analyzer, and O3 was monitored with a conventional ozone monitor (Bendix model 8002, Lewisburg, WV). Ammonium sulfate seed aerosol, generated by nebulizing a 0.5 mg L-1 aqueous solution, was passed through a 85Kr neutralizer (TSI, model 3077, Shoreville, MN) into the irradiation chamber and was maintained at a concentration of
