We also measured the hygroscopicity of the guaiacol-NO 3 SOA to better understand the high viscosities of this SOA type. For comparison purposes, we also measured the hygroscopicity of guaiacol-OH SOA. The results suggest that phenolic SOA generated by NO 3 during the night can have a much higher viscosity than phenolic SOA generated by OH during the day, and the differences can be explained, at least partially, by the low hygroscopicity of the guaiacol-NO 3 SOA.