title: Surprising formation of p-cymene in the oxidation of α-pinene in air by the atmospheric oxidants OH, O3, and NO3.
abstract: Anthropogenic sources release into the troposphere a wide range of volatile organic compounds (VOCs) including aromatic hydrocarbons, whose major sources are believed to be combustion and the evaporation of fossil fuels. An important question is whether there are other sources of aromatics in air. We report here the formation of p-cymene [1-methyl-4-(1-methylethyl) benzene, C6H4(CH3)(C3H7)] from the oxidation of α-pinene by OH, O3, and NO3 at 1 atm in air and 298 K at low (<5%) and high (70%) relative humidities (RH). Loss of α-pinene and the generation of p-cymene were measured using GC-MS. The fractional yields of p-cymene relative to the loss of α-pinene, Δ [p-cymeme]/Δ [α-pinene], were measured to range from (1.6±0.2)×10(-5) for the O3 reaction to (3.0±0.3)×10(-4) for the NO3 reaction in the absence of added water vapor. The yields for the OH and O3 reactions increased by a factor of 4-8 at 70% RH (uncertainties are ±2s). The highest yields at 70% RH for the OH and O3 reactions, ∼15 times higher than for dry conditions, were observed if the walls of the Teflon reaction chamber had been previously exposed to H2SO4 formed from the OH oxidation of SO2. Possible mechanisms of the conversion of α-pinene to p-cymene and the potential importance in the atmosphere are discussed.
authors.affiliation: Department of Chemistry, University of California Irvine, Irvine, California 92697-2025, United States.
authors.forename: Aline
authors.initials: A
authors.lastname: Gratien
authors.forename: Stanley N
authors.initials: SN
authors.lastname: Johnson
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Ezell
authors.forename: Matthew L
authors.initials: ML
authors.lastname: Dawson
authors.forename: Raffeal
authors.initials: R
authors.lastname: Bennett
authors.forename: Barbara J
authors.initials: BJ
authors.lastname: Finlayson-Pitts
mesh_terms: D000393:Air Pollutants / Q000032:analysis / Q000138:chemical synthesis*; D001272:Atmosphere / Q000737:chemistry; D000081008:Bicyclic Monoterpenes; D000081007:Cymenes; D017665:Hydroxyl Radical / Q000737:chemistry*; D039821:Monoterpenes / Q000032:analysis / Q000138:chemical synthesis* / Q000737:chemistry*; D009589:Nitrogen Oxides / Q000737:chemistry*; D016877:Oxidants / Q000737:chemistry; D010126:Ozone / Q000737:chemistry*; D055549:Volatile Organic Compounds / Q000138:chemical synthesis / Q000737:chemistry
chemical_list: D000393:Air Pollutants; D000081008:Bicyclic Monoterpenes; D000081007:Cymenes; D039821:Monoterpenes; D009589:Nitrogen Oxides; D016877:Oxidants; D055549:Volatile Organic Compounds; C031701:nitrogen trioxide; C007210:4-cymene; D017665:Hydroxyl Radical; D010126:Ozone; C005451:alpha-pinene