UC Riverside

Atmospheric secondary aerosol (SA) is comprised of an array of low volatility organic and inorganic compounds that are produced from atmospheric chemical reactions. It contributes significantly to overall aerosol burden and, consequently, plays an important role in air quality and regional and global climate change. Secondary organic aerosol (SOA), which makes up a substantial fraction of SA, forms when reaction of volatile organic compounds (VOCs) with gas-phase oxidants (e.g., O3, OH radical, and NO3 radical) produces less-volatile functionalized compounds followed by nucleation and/or gas-particle partitioning (gasSOA). An alternative pathway is when water-soluble organics dissolve in the aqueous phase (aerosol water or cloud droplets) and are subsequently oxidized (aqSOA). Most traditional investigations of SA formation in the laboratory have been carried out in environmental smog chambers. For the past decade or so, oxidation flow reactors (OFRs) have been increasingly used to study the formation and evolution of secondary aerosol in the atmosphere and have become valuable tools for improving the accuracy of model simulations and for depicting and accelerating realistic atmospheric chemistry. However, the pathways and mechanisms of SOA formation via aqueous-phase chemistry in aqueous aerosols and clouds have received comparatively little attention compared with that involving only gas-phase chemistry. In this thesis research, driven by rapid development of OFR techniques and the increasing appreciation of their wide application, a number of studies were carried out to understand the formation and evolution of SA formation under both gas- and aqueous-phase oxidation. This includes the designs of two newly-built all-Teflon reactors – the Particle Formation Accelerator (PFA) and the Accelerated Production and Processing of Aerosols (APPA) reactor, intended for the study of gas-phase atmospheric chemistry and aqueous secondary aerosol (aqSA) formation. Characterization tests of the two reactors have been performed. Computational fluid dynamics (CFD) simulations were combined with experimental determination of the residence time distribution (RTD) to optimize the flow tube design and the atmospheric relevance of the measurements. Studies of SA produced via gas-phase oxidation and subsequent partitioning (gasSA) were carried out in the PFA and APPA reactors operating in low RH mode, for which no liquid water was initially present. These studies include gasSA yields from various precursors in a laboratory environment and ambient observation in Riverside. The potential applications of measurement of SA formation though aqueous phase oxidation in the APPA reactor will be presented in the second part. Several studies of aqSA formation using the APPA reactor are presented, including developing a description of the relative gas phase and aqueous phase yields from different precursors, quantifying the sensitivity of the aqSA to several parameters (droplet surface area concentration, droplet pH, temperature, and particle composition) and exploring brown carbon formation from aqueous SA and droplet evaporation during one or more field studies.