In Fig. , we have also included the viscosity of other SOA types determined with the poke-flow technique and reported previously. All of the other SOA types were generated using OH or O 3 as the oxidant, i.e. major daytime oxidants. The viscosity of guaiacol-NO 3 SOA is drastically higher than the other types of SOA at ≳ 40% RH. For example, at 50% RH, the viscosity of guaiacol-NO 3 SOA is 2-6 orders of magnitude higher than that of other SOAs investigated.