Tropospheric fate of allyl cyanide (CH_2=CHCH_2CN): Kinetics, reaction products and secondary organic aerosol formation

摘要

Allyl cyanide, CH2=CHCH2CN, is a volatile organic compound that is emitted by biogenic and anthropogenic sources. Daytime atmospheric degradation of CH2=CHCH2CN may be initiated by reaction with OH radicals, Cl atoms and O-3. In this work, we report the gas-phase rate coefficients for the reaction of these atmospheric oxidants with CH2=CHCH2CN (k(1), k(2) and k(3), respectively) determined in an atmospheric simulation chamber at 298 +/- 2 K and 760 +/- 5 Torr of air, in a NOx-free environment. A relative kinetic method was employed for OH and Cl kinetic studies, obtaining k(1) = (1.58 +/- 0.39) x 10(-11) cm(3) molecule(-1) s(-1) and k(2) = (2.26 +/- 0.30) x 10(-10) cm(3) molecule(-1) s(-1). For the O-3+CH2=CHCH2CN reaction, k(3) = (6.01 +/- 0.24) x 10(-19) cm(3) molecule(-1) s(-1) was obtained using an absolute kinetic method under pseudo-first order conditions (excess of allyl cyanide). In both cases, Fourier Transform Infrared (FTIR) spectroscopy was used for monitoring the loss of CH2=CHCH2CN relative to the loss of a reference compound and the loss of ozone, respectively. The estimated tropospheric lifetime considering all the homogeneous processes evaluated in this work is 16 h, with the major degradation route being the reaction with OH radicals. Gas-phase products detected in the IR spectrum have been identified by comparison with IR experimental spectra and/or theoretical spectra computed using density functional theory (DFT) calculations. In the Cl and O-3 reactions, among others, HC(O)CN, HC(O)CH2CN, HC(O) H, and CO were identified as products, indicating that these reactions proceed mainly by addition of the oxidant to the double bond of CH2=CHCH2CN. Even though allyl cyanide absorbs in the atmospheric IR window, due to its short lifetime it has a negligible contribution to the radiative forcing of climate. The formation of secondary organic aerosols (SOAs) has been investigated in the Cl reaction by a Fast Particle Mobility Sizer spectrometer. The determined SOA yield is lower than 6%, which could indicate a negligible impact on human health that could also be extrapolated to the OH reaction.