Absolute determination of chemical kinetic rate constants by optical tracking the reaction on the second timescale using cavity-enhanced absorption spectroscopy

We report a new spectroscopic platform coupled to an atmospheric simulation chamber for the direct determination of chemical rate constants with high accuracy at a second time-scale resolution. These developed analytical instruments consist of an incoherent broadband cavity enhanced absorption spectrometer using a red light emitting diode (LED) emitting at similar to 662 nm (LED-IBBCEAS) associated with a multipass cell direct absorption spectrometer (MPC-DAS) coupled to an external cavity quantum cascade laser (EC-QCL) operating in the mid-infrared region at approximately 8 mu m (EC-QCL-MPC-DAS). Spectrometers were employed to investigate the NO3-initiated oxidation of four selected volatile organic compounds (VOCs) for the determination of the corresponding rate constants with a dynamic range of 5 orders of magnitude (from 10(-11) to 10(-16) cm(3) molecule(-1) s(-1)). Rate constants of (6.5 +/- 0.5) x 10(-15), (7.0 +/- 0.4) x 10(-13), and (5.8 +/- 0.5) x 10(-16) cm(3) molecule(-1) s(-1) for propanal, isoprene and formaldehyde, respectively, were directly determined by fitting the measured concentration-time profiles of NO3 and VOCs (measured using a proton transfer reaction time-of-flight mass spectrometer, PTR-ToF-MS) to chemical models based on the FACSIMILE simulation software (version 4.2.50) at 760 torr and 293 +/- 2 K. The obtained rate constants are in good agreement with the most recent recommendations of the IUPAC (International Union of Pure and Applied Chemistry). In addition, a rate constant of (2.60 +/- 0.30) x 10(-11) cm(3) molecule(-1) s(-1) for the oxidation of 2-methoxyphenol by NO3 radicals was first determined using the absolute kinetic method. Compared to the mostly used indirect relative rate method, the rate constant uncertainty is reduced from similar to 20% to similar to 12%. The results demonstrated the high potential of using modern spectroscopic techniques to directly determine the chemical reaction rate constants.