Stability of α-Pinene and <scp>d</scp>-Limonene Ozonolysis Secondary Organic Aerosol Compounds Toward Hydrolysis and Hydration
Cynthia Wong, Daniel Vite, Sergey A. Nizkorodov
Abstract
Secondary organic aerosol (SOA), formed through the gas-phase oxidation of volatile organic compounds (VOCs), can reside in the atmosphere for several days and sometimes for weeks. The formation of SOA takes place rapidly, often within hours after VOC emissions, and SOA can then undergo much slower physical and chemical processes throughout its lifetime in the atmosphere. Water, in the form of water vapor, aerosol liquid water, and cloud and fogwater, can strongly impact the composition of SOA by altering formation and short-term aging mechanisms; however, less is known about how water impacts long-term SOA aging. The goal of this work is to systematically explore the effects of water on the chemical composition of α-pinene and d-limonene SOA during long-term aging processes. SOA samples were generated in an oxidation flow reactor and collected on foil substrates. Samples were aged in the presence of water vapor at 97% relative humidity for 7–14 days or an aqueous solution for 1–2 days, then analyzed with direct infusion electrospray ionization high-resolution mass spectrometry to gain insight into the chemical composition of SOA before and after aging. The patterns of peak intensities and observed molecular formulas were examined for evidence of water-driven chemistry. We found that chemical composition of the SOA did change during long-term exposure to water vapor and liquid water, but the extent of the change was surprisingly small. This indicates that the exposure to water is not a strong driver of long-term aging processes compared to other mechanisms of aging for the monoterpene SOA studied.