Assessing the Drivers of Isoprene SOA: Laboratory studies, field observations and modeling

Principal Investigator(s):
Joel Thornton, University of Washington

Rainer Volkamer, University of Colorado, Boulder

Jerome Fast, Pacific Northwest National Laboratory
Manish Shrivastava, Pacific Northwest National Laboratory
Rahul Zaveri, Pacific Northwest National Laboratory
John Shilling, Pacific Northwest National Laboratory

Atmospheric aerosol particles, small liquid or solid particles in air, exert a strong influence on Earth’s energy balance, by directly interacting with solar radiation and affecting the optical properties and lifetime of clouds, thereby indirectly affecting transmission of solar radiation through the atmosphere. Describing the sources and variability of aerosol particle abundance over time represents an ongoing challenge in determining the net impact of anthropogenic activities on climate. A potentially important natural source of aerosol particle mass that can be modulated by anthropogenic activities associated with energy production is through secondary organic aerosol (SOA) from isoprene, a hydrocarbon emitted by vegetation. More carbon is emitted to the atmosphere in the form of isoprene than any other non-methane hydrocarbon. Its contribution to SOA, even at low efficiency, can be globally significant. Isoprene photochemistry produces epoxy diols (IEPOX) and hydroxy hydroperoxides (ISOPOOH) which partition to aqueous aerosol or clouds and undergo subsequent chemistry to form aqueous SOA (aqSOA). These same compounds also react in the gas-phase to lower volatility products which form SOA, even in the absence of an aqueous phase. Each of these processes is sensitive to perturbations of atmospheric composition by anthropogenic activities such as energy production (i.e., NOx, SO2 emissions). These perturbations likely feedback on to aerosol-climate effects, but significant knowledge gaps limit our ability to accurately describe their impact. This work will directly improve model parameterizations of isoprene-derived SOA for use in box, regional, and global models.

We propose to conduct: (1) a detailed analysis of the ambient isoprene oxidation products and SOA measured aboard the G-1 aircraft during the DOE ARM Hi-SCALE field campaign; (2) a series of targeted laboratory experiments to measure the solubility rules of isoprene-SOA precursors that affect the rate of SOA formation and SOA properties from aqueous pathways. We will use atmospherically relevant aerosol components, systematically vary the mass fraction of inorganic mixtures, and study mixed inorganic/organic systems in both bulk solution and in actual aerosols to develop a database of structure-activity relationships for IEPOX reactive uptake, SOA yield and volatility. These measurements are accompanied by quantum calculations, and by simulation chamber experiments that employ the most advanced set of gas and aerosol composition measurement capabilities currently available to characterize how gas and multiphase reactions modify volatility distributions, and the evolution of aerosol size distributions.

Expected Outcomes: The analysis and experimental results will be used to construct a highly constrained molecular aqueous SOA (aqSOA) module for isoprene under low oxidant concentrations representative of regional and global scales, for use in computer models to predict and evaluate isoprene-SOA formation using data from DoE-ARM/ASR field campaigns such as Hi-SCALE.

Connection with DoE-ARM/ASR: The DOE ARM campaigns TCAP, CARES, and Hi-SCALE, are characterized by extreme differences in predicted SOA mass from aqueous processes and oxidant fields. In Hi-SCALE, substantial concentrations of the sum of ISOPOOH and IEPOX were measured, along with an entire suite of other lower volatility isoprene oxidation products. These ARM datasets are therefore particularly well suited to evaluate model performance over a wide range of conditions. These ASR-funded process studies and data analyses we propose will bring the model parameterizations to new levels of fidelity in their treatment of gas and aqueous phase processes.