Impacts of Phase State and Water Content on Secondary Organic Aerosol Formation and Partitioning

Principal Investigator(s):
Manabu Shiraiwa, Regents of the University of California, Irvine

Co-Investigator(s):
James Smith, University of California, Irvine
Annmarie Carlton, University of California, Irvine
Sergey Nizkorodov, University of California, Irvine

Secondary organic aerosols (SOA) account for a major fraction of particulate matter in the atmosphere, affecting climate, air quality, and public health. SOA formation and evolution are highly complex processes involving both chemical reactions and transport of molecules in air, at particle surfaces, and within particles. The state of the matter, or “phase state,” comprising SOA can vary from a liquid, over an amorphous semi-solid, to a glassy solid, depending on chemical composition, water content, relative humidity (RH), and temperature. The occurrence of glassy and amorphous semi-solid states can pose limitations on the rate of transport of molecules, affecting gas-particle interactions and challenging the treatment of SOA in atmospheric models. There is still a lack of understanding of the impacts of phase state and water content on SOA processes. The first objective of this project is to improve fundamental understanding of the interplay of the phase state of particles and water content on the evolution of SOA mass and particle size distribution. We will analyze the existing literature on RH effects on SOA formation and perform additional targeted laboratory experiments using state-of-the-art instrumentation. Data from lab experiments performed at different RH will be analyzed using a novel framework developed by the PI, called the “molecular corridor approach,” in order to estimate two key parameters related to particle phase state: volatility and glass transition temperature. In addition the evolution of particle size and composition will be modeled using the kinetic multi-layer model of gas-particle interaction in aerosols and clouds (KM-GAP). We will also analyze field data of SOA chemical composition and concentrations of key gases from four recent DOE-funded campaigns (HI-SCALE, GoAmazon2014/15, NPFS, BAECC). Using the molecular corridor approach and KM-GAP, our analysis will focus on quantifying the limitations on the rates of reactive and non-reactive (reversible) uptake of organics, ammonia and amines into particles. The second objective of this proposal is to develop a model representation and evaluate the effects of phase state and water content on formation, growth, and chemical transformation of SOA in a regional climate and air quality model (e.g., WRF-Chem) and to compare model predictions with ARM field measurements. Spatial distribution and temporal evolution of aerosol water content, glass transition temperature and SOA phase state will be modeled for campaign-long simulations. The impacts of phase state and water content on SOA lifecycle will be evaluated in a regional scale with the ultimate goal of reducing the uncertainty of SOA representation in regional climate and air quality predictions.