Reactions of chlorine radicals might play a role in aqueous aerosols where a core of inorganic components containing insulators such as SiO2 and dissolved HUmic-LIke Substances (HULIS) are present. Herein, we report conventional flash photolysis experiments performed to investigate the aqueous phase reactions of silica nanoparticles (NP) and humic acid (HA) with chlorine atoms, Cl, and dichloride radical anions, Cl2•−. Silica NP and HA may be taken as rough models for the inorganic core and HULIS contained in atmospheric particles, respectively. Both Cl and Cl2•− were observed to react with the deprotonated silanols on the NP surface with reaction rate constants, k ± σ, of (9 ± 6) × 107 M−1 s−1 and (7 ± 4) × 105 M−1 s−1, respectively. The reaction of Cl with the surface deprotonated silanols leads to the formation of SiO defects. HA are also observed to react with Cl and Cl2•− radicals, with reaction rate constants at pH 4 of (3 ± 2) × 1010 M−1 s−1 and (1.2 ± 0.3) × 109 M−1 s−1, respectively. The high values observed for these constants were discussed in terms of the multifunctional heterogeneous mixture of organic molecules conforming HA.