Homogeneous aerosol formation by the chlorine atom initiated oxidation of toluene

The photolysis of Cl-2 molecules in the presence of toluene and oxygen, at levels of similar to 10(14) radicals/cm(-3), initiates a sequence of chemic al reactions that rapidly produce an aerosol. Size distributions of the aer osol particles are examined, using a scanning mobility particle sizer, as a function of time, photolysis energy, the initial concentrations of toluene and chlorine, and of added NO and HO2. The number of particles and the vol ume of aerosol both exhibit a steep nonlinear increase as the initial chlor ine atom level is raised. Surprisingly, the number of particles displays a strong inverse dependence on the initial toluene concentration, whereas the aerosol volume remains nearly unaffected by toluene level. Kinetic measure ments of particle formation made using a flow reactor reveal an incubation period after initiation of the Cl + C6H5CH3 reaction, followed by steep inc reases in particle number and volume. The particle number rapidly reaches a plateau, whereas the aerosol volume continues to increase with time. The e arliest observed particles are unexpectedly large, with mean diameters as h igh as 100 nm; a continuous growth from <10 nm is generally not observed. B oth NO and HO2 suppress aerosol formation. These observations prompt us to postulate a mechanism whereby a minor reaction channel between chlorine ato ms and benzylperoxy radicals to produce a Criegee intermediate controls the number of critical nuclei. This rate-limiting step is followed by rapid co ndensation of semivolatile compounds onto the nuclei. Because the aerosol v olume can represent 10%, or more, of the toluene consumed, this necessarily includes products from the major oxidation pathways. As part of this work, we report 295 K rate constants of k(4) = (8 +/- 2) x 10(-12) cm(3) s(-1) f or the benzylperoxy self-reaction, and k(6) (2.7 +/- 0.5) x 10(-11) cm(3) s (-1) for its reaction with NO.