Effect of Heterogeneous Uptake of Ammonia on Properties and Transformations of Secondary Organic Aerosols Particles

Particle phase and morphology can greatly impact the properties and transformations of secondary organic aerosol (SOA). For example, the rates of heterogeneous reactions strongly depend on the particle phase and whether the condensed-phase reacting substances are on the particle surface or inside the particle, behind a protective shield. 

We will present the results of a recent study, in which we characterized the properties and transformations of size-selected SOA particles generated by dark ozonolysis of α-pinene particles as a result of ammonia exposure at different relative humidity (RH).

We show that at low RH (<5% RH) the uptake of ammonia by α-pinene SOA particles is consistent with surface adsorption by highly viscous semi-solids, in agreement with recent studies. As a result we find that smaller particles, with higher surface-to-volume ratios, exhibit the largest changes in particle density and fraction of reactive products.

We find that after ~4 hours of ammonia exposure, SOA particles developed a solid “crust” that imposes diffusion limitations on gas-particle mass transfer, including SOA evaporation. We will present the comparison of room-temperature evaporation kinetics of size-selected SOA particles before and after ammonia exposure. Moreover, we find that ammonia-induced changes particle morphology greatly impact coalescence times of SOA particles, which has large implication on recently proposed method to determine SOA viscosity based on their coalescence time. We compare the coalescence rates of reacted and un-reacted SOA particles as function of particle size and ammonia exposure. Using recently developed approach for real time shape-based particles separation; we quantify the fraction of aspherical particles and follow the temporal evolution of their shape.

In contrast, at high RH (>95%RH) all characterized particles are spherical, have orders of magnitude lower viscosity, and contain significantly higher fraction of reactive products.