Kinetics of the reactions of NO3 radical with alkanes

Abstract

The rate coefficients for the reactions of NO₃ radicals with methane (CH₄), ethane (C₂H₆), propane (C₃H₈), n-butane (n-C₄H₁₀), iso-butane (iso-C₄H₁₀), 2,3-dimethylbutane (C₆H₁₄), cyclopentane (C₅H₁₀) and cyclohexane (C₆H₁₂) at atmosphere pressure (1000 ± 5 hPa) and room temperature (298 ± 1.5 K) were measured using an absolute method. Careful attention was paid to the role of secondary reactions and impurities. The upper limits of rate coefficients for methane and ethane at 298 K are <4 × 10⁻²⁰ and <5 × 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹, respectively. The rate coefficients at 298 K for propane, n-butane, iso-butane, 2,3-dimethybutane, cyclopentane and cyclohexane are, (9.2 ± 2.9) × 10⁻¹⁸, (1.5 ± 0.4) × 10⁻¹⁷, (8.2 ± 2.2) × 10⁻¹⁷, (5.8 ± 2.4) × 10⁻¹⁶, (1.5 ± 0.6) × 10⁻¹⁶ and (1.3 ± 0.4) × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹, respectively. Rate coefficients for the reactions of NO₃ radical with two deuterated n-butanes (butane-D10 and butane-1,1,1,4,4,4-D6) are also reported. We show that the rate coefficients for NO₃ reactions correlate with the enthalpy change for the reaction, thereby suggesting that the mechanism for NO₃ reactions with alkanes is through H atom abstraction. The measured rate coefficients are compared with available literature values. This study increases the number of available rate coefficients for the reactions of NO₃ with alkanes and sets significantly lower upper limits for reaction of NO₃ with ethane and methane. The atmospheric significance of our reported rate coefficients is briefly discussed.