Chemical and Physical Studies of Secondary Organic Aerosol Formed from Beta-pinene Photooxidation

Atmospheric organic aerosols have a significant impact on climate and human health. However, our understanding of the physical and chemical properties of these aerosols is inadequate, thus their climate and health influences are poorly constrained. In this study, we investigated the secondary organic aerosol (SOA) formation from OH-initiated oxidation of -pinene. The majority of experiments were conducted in the York University smog chamber. The main objective was to identify the gas and particle phase products with an atmospheric pressure chemical ionization mass spectrometer (APCI-MS/MS). A wide variety of products were identified containing various functional groups including alcohol, aldehyde, carboxylic acid, ketone and nitrate.

Following the chemical composition characterization of products, the shape, phase state and density of generated particles were determined. Images from a scanning electron microscope (SEM) revealed that SOA particles from -pinene were commonly spherical in shape, and adopted an amorphous semi-solid/liquid state. Additionally, the density was determined for SOA particles generated from -pinene/OH, nopinone/OH and nopinone/NO3 experiments for the first time using a tapered element oscillating microbalance-scanning mobility particle sizer (TEOM-SMPS) method. Our results showed a correlation between the determined particle density and the particle chemical composition of the respective system. This demonstrates that changes in particle density can be indicative of the changes in chemical composition of particles. We also investigated the chemical aging of oxidation products by exposing them to additional OH radicals or ozone. The observed changes in chemical composition of products and additional SOA mass production during OH-induced aging were attributed to further oxidation of gas phase intermediate products.

The NOx dependence of SOA formation from -pinene photooxidation was investigated in the York University smog chamber and the Jlich Plant Atmosphere Chamber (JPAC). Consistent with previous NOx studies, SOA yields increased with increasing [NOx] at low-NOx conditions, whereas increasing [NOx] at high-NOx conditions suppressed the SOA yield. This increase was attributed to an increase of OH concentration. After removing the effect of [OH] on SOA yield in the JPAC, SOA yields only decreased with increasing [NOx]. Finally, the formation mechanisms of identified products were probed based on the information acquired throughout our study.