Methyl acetate reaction with OH and Cl: Reaction rates and products for a biodiesel analogue Vibeke F. Andersen, Elna J.K. Nilsson, Solvejg Jørgensen, Ole John Nielsen, Matthew S. Johnson * Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark article info Article history: Received 20 August 2008 In final form 23 February 2009 Available online 27 February 2009 abstract Relative rate experiments performed in a photochemical reactor at 293 ± 0.5 K and a total air pressure of 980 mbar were used to determine k(CH 3 C(O)OCH 3 + Cl) = (1.93 ± 0.27)  10 12 cm 3 molecule 1 s 1 and k(CH 3 C(O)OCH 3 + OH) = (3.18 ± 0.13)  10 13 cm 3 molecule 1 s 1 . The product branching ratio for the Cl-initiated oxidation of CH 3 C(O)OCH 3 was investigated at 293 ± 0.5 K and 980 mbar N 2 , air, or O 2 , in the presence and absence of NO. Products observed were CH 3 C(O)OCHO, CH 3 C(O)OH, HC(O)OH, CO, and CO 2 . The product branching ratios were dependent on NO, but not on the oxygen partial pressure. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The production and use of transportation biofuels has grown rapidly during the last decade due to the desire to reduce carbon dioxide emissions and dependence on oil. In addition to the carbon budget, biofuel production and use can involve emission of green- house gases such as N 2 O due to use of fertilizer [1], and the use of ethanol blends may increase the concentration of atmospheric pol- lutants including formaldehyde and acetaldehyde [2]. Production of biodiesel has been commercially important since the beginning of the 1990’s. From 2001 to 2006 the global production of biodiesel expanded six fold. In 2006, 73% of global biodiesel was produced in Europe with rapeseed and sunflower being the major sources [3,4]. Biodiesel consists of methyl esters derived from plant oils, which are long-chain fatty acids such as methyl oleate [5]. When used in engines, incomplete combustion leads to the formation of a variety of chemical compounds in the exhaust. In addition, an estimated 0.01–0.1% of all transportation fuel is emitted directly to the atmosphere [6]. Combustion products from biodiesel include shorter chained methyl esters, aldehydes, and methyl acrylates [7,8]. In order to assess the environmental impact of biofuels, life cy- cle analysis of the fuels must be carried out. The last step of the life cycle involves combustion of the fuel. In the present work, the atmospheric fate of a short chained methyl ester, used as an ana- logue to uncombusted biodiesel compounds, has been investi- gated. An important component of a compound’s environmental impact is its lifetime in the atmosphere. This can be estimated from the reaction rate of the compound with key oxidants. In ambient air the most important reactive species is the hydroxyl radical which is present at a concentration of approximately 1  10 6 radi- cals cm 3 [9]. Another atmospheric oxidant is the chlorine atom. The most important source of Cl is photolysis of chlorine containing species in sea salt aerosols [10]. Reaction mechanisms involving Cl are sim- ilar to those of the OH-radical, with rate constants 10–100 times larger than for the corresponding OH-reactions. Therefore, also due to their ease of production in the laboratory, it is useful to use Cl-atom initiated reactions as models for OH-radical initiated reactions. In the atmosphere, oxidation by Cl atoms may be impor- tant in the marine boundary layer where peak concentrations have been observed to be 10 3 –10 6 cm 3 [11]. In this study methyl acetate has been used as a model for bio- diesel. This short chained compound has a relatively high vapor pressure for a methyl ester, and is therefore feasible for study in the laboratory. The reactivity of methyl acetate with OH and Cl has been studied in the presence of varying concentrations of oxy- gen and in the presence and absence of NO x . For the Cl reaction the product distributions given various conditions have been determined. There are three previous studies of the rate constant for the reaction of methyl acetate with Cl published in the literature, and the details for these are given in Table 2. The product branch- ing ratio for the reaction has been determined in the presence and absence of NO with varying oxygen partial pressures [12], and the temperature dependence of this branching ratio was studied by Tyndall et al. in the temperature range 253–324 K, in the absence of NO, and at a total pressure of 910 mbar with varying oxygen par- tial pressures [13]. There are four previous studies of the rate constant for the reac- tion of methyl acetate with OH, which are summarized in Table 2. The product branching ratio of the OH-initiated oxidation of methyl acetate at 296 K has been determined in the presence of 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.02.066 * Corresponding author. Fax: +45 3532 0322. E-mail address: msj@kiku.dk (M.S. Johnson). Chemical Physics Letters 472 (2009) 23–29 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett