Guaiacol-NO 3 SOA was generated in an environmental chamber (Fig. ) consisting of a 1.8 m Teflon bag (Ingeniven) suspended within an aluminum enclosure and surrounded by 24 UV lights (Sylvania Black lights, 40 W λ ~360 nm). The chamber was cleaned with deionized water and then flushed with zero-air (Aadco 737) for 48 h while the UV lights were on. SOA generation was carried out in the continuous flow mode at 294 K, in the dark, and under dry conditions (≲ 1% RH). The total flow rate through the environmental chamber was ~16 L min -1 , corresponding to an average residence time of ~112 min, calculated assuming plug flow. SOA was produced without seed particles to prevent their influence on the viscosity and hygroscopicity measurements. Organic vapors were added to the environmental chamber by continuously injecting a solution of 2 wt. % guaiacol (Sigma Aldrich, purity ≥ 99.9%) in ultrapure water (Merck Millipore, Milli-Q,) into a round bottom glass flask. The flask was heated to ~323 K to enhance vaporization. The resulting vapors were carried into the environmental chamber by continuously flushing the flask with zero-air. Prior to reaction, the concentration of guaiacol in the environmental chamber was ~50 ppb. NO 3 was generated within the environmental chamber by the thermal decomposition of dinitrogen pentoxide (N 2 O 5 ). N 2 O 5 was generated upstream of the environmental chamber by reacting O 3 and NO 2 in a flow reactor (Fig. ), following established methods. The flow reactor consisted of four large glass vessels coupled in series with an overall volume of 60 L. The average residence time within the flow reactor was ~39 minutes, assuming plug flow. The following reactions occurred within the flow reactor: