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A series of experiments was conducted in the CESAM (French acronym forExperimental Multiphasic Atmospheric Simulation Chamber) simulation chamber toinvestigate the evolution of the physical and chemical properties ofsecondary organic aerosols (SOAs) during different forcings. The presentexperiments represent a first attempt to comprehensively investigate theinfluence of oxidative processing, photochemistry, and diurnal temperaturecycling upon SOA properties. SOAs generated from the ozonolysis of α-pinene were exposed under dry conditions (< 1% relativehumidity) to (1) elevated ozone concentrations, (2) light (under controlledtemperature conditions) or (3) light and heat (6 °C light-inducedtemperature increase), and the resultant changes in SOA optical properties(i.e. absorption and scattering), hygroscopicity and chemical compositionwere measured using a suite of instrumentation interfaced to the CESAMchamber. The complex refractive index (CRI) was derived from integratednephelometer measurements of 525 nm wavelength, using Mie scatteringcalculations and measured number size distributions. The particle sizegrowth factor (GF) was measured with a hygroscopic tandem differentialmobility analyzer (H-TDMA). An aerosol mass spectrometer (AMS) was used forthe determination of the f44 / f43 and O : C ratio of the particlesbulk. No change in SOA size or chemical composition was observed during O3and light exposure at constant temperature; in addition, GF and CRI of theSOA remained constant with forcing. On the contrary, illumination of SOAs inthe absence of temperature control led to an increase in the real part ofthe CRI from 1.35 (±0.03) to 1.49 (±0.03), an increase of theGF from 1.04 (±0.02) to 1.14 (±0.02) and an increase of thef44 / f43 ratio from 1.73 (±0.03) to 2.23 (±0.03). Thesimulation of the experiments using the master chemical mechanism (MCM) andthe Generator for Explicit Chemistry and Kinetics of Organics in theAtmosphere (GECKO-A) shows that these changes resulted from the evaporationof semi-volatile and less oxidized SOA species induced by the relativelyminor increases in temperature (~ 6 °C). Thesesurprising results suggest that α-pinene–O3 SOA properties maybe governed more by local temperature fluctuations than by oxidativeprocessing and photochemistry

Aging of secondary organic aerosol generated from the ozonolysis of α-pinene: effects of ozone, light and temperature