Surprising formation of p-cymene in the oxidation of α-pinene in air by the atmospheric oxidants OH, O₃, and NO₃

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, C ₆H₄(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.