System to measure relative rate constants of semivolatile organic compounds with hydroxyl radicals

A system to determine rate constants for the reaction of the hydroxyl radical with semivolatile organic compounds under tropospheric conditions is described. The system incorporates on-line mass spectrometric detection to improve experimental time resolution and to minimize sampling volume. The reaction apparatus is heated to provide detectable vapor phase levels of semivolatile compounds without sample preconcentration. Temperature control of the system also allows investigation of the temperature dependence of OH-organic reactions. OH radicals are generated by the photolysis of O-3 in the presence of water vapor to avoid the wall effects associated with the more commonly used alkyl nitrite photolysis. The temperature dependence of the OH-toluene reaction is investigated from 276 to 397 K in 1 atm of He and from 296 to 363 K in 1 atm of N-2/O-2. In He, the OH-toluene reaction rate constant at 298 K is (5.9 +/- 1.5) x 10(-12) cm(3) s(-1), and in N-2/O-2, the rate constant at 298 K is (5.8 +/- 1.5) x 10(-12) cm(3) S-1. Rate constants for the reaction of OH with 4,4'-dichlorobiphenyl from 329 to 366 K are also reported. Over this temperature range, the 4,4'-dichlorobiphenyl reaction with OH is only weakly temperature dependent. From the experimentally determined temperature dependence, the OH rate constant at 298 K of 4,4'dichlorobiphenyl is (2.0 +/- 0.5) x 10(-12) cm(3) s(-1).