2609 Proceedings of the Combustion Institute, Volume 28, 2000/pp. 2609–2618 FORMATION OF POLYCYCLIC AROMATIC HYDROCARBONS AND THEIR RADICALS IN A NEARLY SOOTING PREMIXED BENZENE FLAME HENNING RICHTER, TIMOTHY G. BENISH, OLEG A. MAZYAR, WILLIAM H. GREEN and JACK B. HOWARD Department of Chemical Engineering Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA Polycyclic aromatic hydrocarbons (PAH) are associated with health hazardous effects, and combustion processes are major sources of their presence in atmospheric aerosols. In the present work, chemical reaction pathways of PAH formation have been investigated by means of the modeling of a nearly sooting, low-pressure, premixed, laminar, one-dimensional benzene/oxygen/argon flame (equivalence ratio   1.8, 30% argon, gas velocity at burner at 298 K v  50 cm s 1 , pressure  2.67 kPa). This flame has been investigated by Bittner and Howard using molecular-beam sampling coupled to mass spectrometry. More recently, Benish extended the set of available data for radicals up to 201 amu and for stable species up to 276 amu using nozzle-beam sampling followed by radical scavenging with dimethyl disulfide and subsequent analysis by gas chromatography–mass spectrometry. An existing kinetic model has been refined. Density functional theory computations were used to update the thermodynamic database, while transition state theory followed by a bimolecular quantum Rice-Ramsperger-Kassel analysis allowed for the deter- mination of kinetic data relevant for the present study. The reaction of phenylacetylene radicals with acetylene is shown to be limiting for the concentration of 1-naphthyl radicals, while naphthalene is formed mainly by self-combination of cyclopentadienyl. The insufficient consumption of PAH as well as acetylene beyond the reaction zone gives some evidence of the need of additional PAH growth pathways involving acetylene but thermodynamically more favorable than subsequent hydrogen-abstraction/acetylene addition reactions. A new pathway for acenaphthylene formation is suggested and consists of benzyne recombination followed by hydrogen attack and isomerization. Introduction In epidemiological studies, air pollution has been positively associated with death from lung cancer and cardiopulmonary disease [1]. Combustion pro- cesses are major sources of airborne species of health concern; and a possible explanation of the health hazardous effect of atmospheric aerosols is their association with polycyclic aromatic hydrocar- bons (PAH) [2]. Many PAH identified in aerosols have been found to be mutagenic [3], and their role as precursors of soot has been discussed [4,5]. Therefore, a better understanding of chemical re- action pathways leading to PAH formation is an es- sential issue in order to reduce their environmental impact and to improve combustion processes. Due to the complexity of such processes, the assessment of chemical reaction networks requires experiments in well-defined flow systems such as well-stirred re- actors, plug-flow reactors, or premixed flames. Such systems allow the use of numerical modeling, taking into account sets of chemical reactions sufficiently large to describe individual reaction steps. Flame structures of laminar, premixed, fuel-rich, low- pressure flames have been measured by means of molecular-beam sampling coupled to mass spec- trometry for different fuels such as acetylene [6] and benzene [6,7]. Molecular-beam sampling allows the measurement of radical intermediates, which is ex- tremely valuable for the testing of kinetic models, but, unfortunately, sooting conditions are prohibitive for this technique. Probe sampling and subsequent analysis by gas chromatography coupled to mass spectrometry (GC-MS) allowed the measurement of concentration profiles for stable species up to coro- nene (C 24 H 12 ) in premixed propane, acetylene, and benzene flames at reduced pressure [8], and up to pyrene (C 16 H 10 ) in premixed methane, ethane, and propane flames at atmospheric pressure [9]. Con- centration profiles of PAH up to ovalene (C 32 H 14 ) and for the fullerenes C 60 ,C 70 ,C 76 ,C 78 , and C 84 have been measured using gas and liquid chroma- tography (GC-MS and HPLC) [10]. Detailed kinetic models describing PAH growth have been devel- oped and tested for premixed methane [11], ethane [11], acetylene [12], and ethylene [12,13] flames. The present work focuses on the detailed descrip- tion of the first steps of the growth of PAH in a nearly sooting, low-pressure, premixed, laminar, one-dimen- sional benzene/oxygen/argon flame (equivalence