The above analysis also did not consider the internal mixing of phenolic-NO 3 SOA with other types of aerosol. Mixing of phenolic-NO 3 SOA with other types of aerosol may lead to lower viscosities than illustrated here if they form a single phase. On the other hand, phenolic-NO 3 SOA may form a separate phase when mixed with other types of aerosols due to differences in polarity and hygroscopicity. Studies are needed to determine if phenolic SOA generated by NO 3 reactions will mix with other types of organic aerosol. The particles employed in our study ranged from 50 to 100 µm in diameter. Finite size effects can significantly reduce the viscosities of SOA when the diameter is less than or equal to approximately 100 nm. Consequently, we expect our findings to be relevant to particles larger than approximately 100 nm, but experiments are needed to confirm this hypothesis.