Effect of Decreased Temperature on the Evaporation of α-Pinene Secondary Organic Aerosol Particles

The interplay of volatility distribution and particle viscosity governs the gas-to-particle partitioning dynamics of atmospheric secondary organic aerosol (SOA) constituents. Temperature-induced shifts in both particle volatility distribution and viscosity can influence the evaporation behavior of atmospheric SOA particles. However, knowledge of particle evaporation at low temperatures is still limited. Here, we combined isothermal evaporation measurements and process modeling to explore the evaporation of alpha-pinene ozonolysis (alpha pinO(3)) and photo oxidation (alpha pinOH) SOA particles under a series of relative humidities (RHs) at two different temperatures. Experimental results revealed that the particle evaporation was hindered at low temperature in agreement with the temperature dependence of the effective saturation vapor concentration and the possible temperature impact on the particle viscosity. Both alpha pinO(3) and alpha pinOH SOA particles showed similar evaporation rates at 80% RH when particles were in a liquid state. However, they showed different evaporation behaviors when present in a semi-solid phase state at low RH. Furthermore, using the evaporation measurement data in model simulations, we could derive particle volatility distributions, enthalpies of vaporization, and composition-dependent viscosities for both the investigated SOA systems. The observed particle size change was well reproduced by process models using the fitted particle volatility distribution and viscosity. Although the observed evaporation rate of alpha-pinene SOA particles at an intermediate RH is similar to high RH (80% RH), the simulation results showed that the evaporation of organic compounds from viscous particles shifts from a liquid-like process at the initial stage to a kinetic-limited one at the later stage of evaporation.