The reaction of Cl atoms, in the presence of Cl2 and O2, with sub-micron squalane particles is used as a model system to explore how surface hydrogen abstraction reactions initiate chain reactions that rapidly transform the chemical composition of an organic particle. The heterogeneous reaction is measured in a photochemical flow tube reactor in which chlorine atoms are produced by the photolysis of Cl2 at 365 nm. By monitoring the heterogeneous reaction, using a vacuum ultraviolet photoionization aerosol mass spectrometer, the effective reactive uptake coefficient and the distributions of both oxygenated and chlorinated reaction products are measured and found to depend sensitively upon O2, Cl2, and Cl concentrations in the flow reactor. In the absence of O2, the effective reactive uptake coefficient monotonically increases with Cl2 concentration to a value of ∼3, clearly indicating the presence of secondary chain chemistry occurring in the condensed phase. The effective uptake coefficient decreases with increasing O2 approaching a diffusion corrected value of 0.65 ± 0.07, when 20% of the total nitrogen flow rate in the reactor is replaced with O2. Using a kinetic model it is found that the amount of secondary chemistry and the product distributions in the aerosol phase are controlled by the competitive reaction rates of O2 and Cl2 with alkyl radicals. The role that a heterogeneous pathway might play in the reaction of alkyl radicals with O2 and Cl2 is investigated within a reasonable range of reaction parameters. These results show, more generally, that for heterogeneous reactions involving secondary chain chemistry, time and radical concentration are not interchangeable kinetic quantities, but rather the observed reaction rate and product formation chemistry depends sensitively upon the concentrations and time evolution of radical initiators and those species that propagate or terminate free radical chain reactions.