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Viscosity of monoterpene‐derived secondary organic aerosols (SOAs) as a function of temperature and relative humidity (RH), and dry SOA glass transition temperatures are reported. Viscosity was measured using coalescence time scales of synthesized 100 nm dimers. Dry temperature‐dependent SOA viscosity was similar to that of citric acid, coal tar pitch, and sorbitol. The temperature where dry viscosity was 10⁶ Pa·s varied between 14 and 36 °C and extrapolated glass transition varied between −10 and 20 °C (±10 °C). Mass fragment f₄₄ obtained with an Aerosol Chemical Speciation Monitor was anticorrelated with viscosity. Viscosity of humidified Δ³‐carene and α‐pinene SOAs exceeded 10⁶ Pa·s for all subsaturated RHs at temperatures <0 and –5 °C, respectively. Steep viscosity isopleths at 10⁶ Pa·s were traced for these across (temperature, RH) conditions ranging from (approximately −5 °C, 100%) and …