Formation and Properties of Secondary Organic Aerosol Particles Generated by Ozonolysis of β-Caryophyllene with and without the Presence of Gas-phase Pyrene

Description

Sesquiterpenes are an important class of biogenic volatile organic compounds (BVOCs) with a high potential to form secondary organic aerosol (SOA). However, the formation and properties of SOA particles formed from the oxidation of sesquiterpene has received significantly less attention compared to other BVOCs, such as monoterpenes. Studies by our group, and others, have shown that at low relative humidity (RH) α-pinene SOA particles are highly viscous semi-solids with viscosity of ~10^8 Pa·s, characteristic of tars, and their evaporation rates are orders of magnitude slower than previously assumed. We have also shown that when hydrophobic organic vapors, like polyaromatic hydrocarbons (PAHs) are present during SOA formation and growth their presence significantly affects particle number concentrations, mass loadings, composition (i.e. fraction of non-volatile components, like oligomers), evaporation kinetics, and viscosity. We will present the results of a recent study focused on the formation and properties of SOA particles formed by ozonolysis of β-caryophyllene with and without the presence of gas-phase pyrene. We find that β-caryophyllene SOA particles formed with or without pyrene are composed of 80% of non-volatile products and are initially spherical. However, with time these particles coagulate to form aspherical particles, which in the case of “pure” β-caryophyllene SOA coalesce within 20 min, yielding viscosity of ~2 x 10^8 Pa·s. In contrast, coagulated β-caryophyllene SOA particles formed in the presence of pyrene do not coalesce, even after more than a day, yielding estimate of a viscosity greater than 3 x 10^10 Pa·s. Preliminary data indicate that at ~60% RH, 30% of the particle volume evaporates in 24 hours and coalescence rates become faster, such that particles formed in the presence of pyrene become spherical after 360 min, yielding a viscosity of 7 x 10^9 Pa·s. Moreover, we find that the presence of pyrene during β-caryophyllene SOA formation results in three-fold higher number concentrations and mass loadings, compared with “pure” β-caryophyllene SOA. We will discuss chemical processes responsible for the effects of PAHs on SOA formation and properties.