We found that both phases of Kamloops and Kelowna BBOA samples had viscosities ≥2.5×10 As BrC reacts with ozone, it can be "bleached" and lose its absorptive properties, changing its impact on the climate. This process is slowed down when BBOA/BrC is highly viscous, as it takes longer for ozone to diffuse into the aerosol. The experiments here predict that BBOA in the field will be more viscous than reported by Schnitzler et al. and Gregson et al. at RH values of 50% and below (Figure ). Under conditions where fresh lab-generated BBOA would be a liquid, the field sample BBOA would be a solid, and the lifetimes of processes like ozonolysis would be even longer than predicted in Schnitzler et al. and in Gregson et al. This framework can be extended and applied to other reactions in BBOA that have diffusion-limited kinetics.