LAUSR

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 (approximately 36 °C, 0%). Differences in composition and thus hygroscopicity can shift humidified viscosity isopleths for SOAs at cold tropospheric temperatures.