Temperature- and Humidity-Dependent Phase States of Secondary Organic Aerosols Geophysical Research Letters

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 f 44 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.

本研究报道了单萜衍生二次有机气溶胶（Secondary Organic Aerosols, SOAs）的黏度随温度与相对湿度（Relative Humidity, RH）的变化关系，以及干态SOA的玻璃化转变温度。研究采用合成的100纳米二聚体的凝聚时间尺度完成黏度测量。干态下随温度变化的SOA黏度与柠檬酸、煤焦油沥青及山梨醇的黏度相近。当干态黏度为106 Pa·s时，对应的温度介于14至36 ℃之间；外推得到的玻璃化转变温度则处于−10至20 ℃范围内（误差±10 ℃）。采用气溶胶化学组分监测仪（Aerosol Chemical Speciation Monitor）测得的质量碎片f44与黏度呈负相关关系。经增湿处理的Δ3-蒈烯与α-蒎烯SOA，在温度分别低于0 ℃与−5 ℃时，其黏度在所有亚饱和相对湿度条件下均超过106 Pa·s。针对这两类SOA，我们绘制了黏度为106 Pa·s的陡峭黏度等值线，其覆盖的温湿条件范围约为（−5 ℃, 100%）至（36 ℃, 0%）。在对流层低温环境下，组分差异进而导致的吸湿性差异，可改变增湿SOA的黏度等值线位置。