These results highlight that SOA generated by NO 3 reactions with phenols could be drastically more viscous than other types of SOA. The difference between the viscosities of guaiacol-NO 3 SOA and the other types of SOA can be explained, at least in part, by the low hygroscopicity parameter for guaiacol-NO 3 , as discussed above. The differences may also be related to difference in molecular weights of the SOA components. Viscosity is known to correlate with molecular weight, and guaiacol-NO3 SOA contains large molecular weight dimers, based on previous mass spectrometry studies. 7,117