Abstract
Abstract. The atmospheric fate of a series of saturated alcohols (SAs) was evaluated through kinetic and reaction product studies with the main atmospheric oxidants. These SAs are alcohols that could be used as fuel additives. Rate coefficients (in cm3 molecule−1 s−1) measured at ∼298 K and atmospheric pressure (720±20 Torr) were as follows: k1 ((E)-4-methylcyclohexanol + Cl) = (3.70±0.16) ×10-10, k2 ((E)-4-methylcyclohexanol + OH) = (1.87±0.14) ×10-11, k3 ((E)-4-methylcyclohexanol + NO3) = (2.69±0.37) ×10-15, k4 (3,3-dimethyl-1-butanol + Cl) = (2.69±0.16) ×10-10, k5 (3,3-dimethyl-1-butanol + OH) = (5.33±0.16) ×10-12, k6 (3,3-dimethyl-2-butanol + Cl) = (1.21±0.07) ×10-10, and k7 (3,3-dimethyl-2-butanol + OH) = (10.50±0.25) ×10-12. The main products detected in the reaction of SAs with Cl atoms (in the absence/presence of NOx), OH radicals, and NO3 radicals were (E)-4-methylcyclohexanone for the reactions of (E)-4-methylcyclohexanol, 3,3-dimethylbutanal for the reactions of 3,3-dimethyl-1-butanol, and 3,3-dimethyl-2-butanone for the reactions of 3,3-dimethyl-2-butanol. Other products such as formaldehyde, 2,2-dimethylpropanal, and acetone have also been identified in the reactions of Cl atoms and OH radicals with 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol. In addition, the molar yields of the reaction products were estimated. The products detected indicate a hydrogen atom abstraction mechanism at different sites on the carbon chain of alcohol in the case of Cl reactions and a predominant site in the case of OH and NO3 reactions, confirming the predictions of structure–activity relationship (SAR) methods. Tropospheric lifetimes (τ) of these SAs have been calculated using the experimental rate coefficients. Lifetimes are in the range of 0.6–2 d for OH reactions, 7–13 d for NO3 radical reactions, and 1–3 months for Cl atoms. In coastal areas, the lifetime due to the reaction with Cl decreases to hours. The calculated global tropospheric lifetimes, and the polyfunctional compounds detected as reaction products in this work, imply that SAs could contribute to the formation of ozone and nitrated compounds at local, regional, and even global scales. Therefore, the use of saturated alcohols as additives in diesel blends should be considered with caution.
Highlights
A multitude of scientific studies on combustion emissions confirm that fossil fuels, especially diesel fuel, are the substances mainly responsible for air pollution
– The kinetic and product study support that (1) the atmospheric degradation mechanism for SAs, and possibly for other unstudied saturated alcohols, proceeds by the abstraction of a hydrogen atom bonded to a carbon rather than a hydrogen atom bonded to the oxygen atom of the alcohol group; and (2) the reaction mechanism in the H atom abstraction process depends on the oxidant
Cl atoms abstract any type of hydrogen (α, β, δ) from SAs with a high percentage compared to the OH and the NO3 radicals
Summary
A multitude of scientific studies on combustion emissions confirm that fossil fuels, especially diesel fuel, are the substances mainly responsible for air pollution. Ethanol, and isopropanol are some of the main alcohols detected in urban areas such as Osaka and Sao Paulo cities (Nguyen et al, 2001) with concentrations of 5.8– 8.2 and 34.1–176.3 ppbv, respectively Other alcohols, such as (E)-4-methylcyclohexanol, have been identified in exhaust gas emissions resulting from burning fuel blends containing 7 % v/v (B7) and 20 % v/v (B20) of soy bean/palm biodiesel (84 %/16 %) (Lopes et al, 2014). In the present work, studies into the kinetics and the products of gas-phase reactions of some SAs, such as (E)-4-methylcyclohexanol (4MCHexOH), 3,3dimethyl-1-butanol (3,3DM1ButOH), and 3,3-dimethyl-2butanol (3,3DM2ButOH) with Cl atoms and with OH and NO3 radicals have been carried out These SAs have been chosen for their potential use as biofuels and because their reactivity has not yet been evaluated. Our work will permit us to complete the kinetic and mechanism database, to improve our knowledge of the atmospheric chemistry of higher alcohols, and to assess their environmental chemical impact in the case of their future use as biofuels
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