their direct climate effects. While transported in the atmosphere, BB-POA can undergo various aging processes, such as heterogeneous oxidation reactions that can impact the chemical composition of the brown carbon. Ozonolysis of BB-POA causes a decrease in the absorbance of brown carbon (i.e., whitening) of the BB-POA, thus changing the particle's radiative properties from a warming to a cooling climate effect. showed that the whitening of BB-POA depends on the viscosity of the BB-POA, with whitening rates strongly decreasing with increasing particle viscosity. The strong contrast in viscosity between BB-POA and phenolic BB-SOA (Fig. ) could have considerable implications for the whitening rates of BB-POA in the atmosphere. Since BB-POA and phenolic BB-SOA coexist in biomass burning plumes, BB-POA particles could become coated with or internally mixed with highly viscous phenolic BB-SOA material. In this scenario, the high-viscosity phenolic BB-SOA would slow the uptake of O3, thereby slowing down the rate of whitening (Fig. ).