Aerosol Acidity Across Seasons: Unveiling Dynamic Shifts in Aerosol-Cloud Processes from Regional and Urban Atmospheres

Principal Investigator: Chris Hennigan, University of Maryland Baltimore County

Co-Investigators: Akua Asa-Awuku (University of Maryland College Park), Peter DeCarlo and Ben Nault (Johns Hopkins University)

Aerosol physics and chemistry are fundamental properties necessary to describe microphysical aerosol-cloud processes that affect the Earth’s energy balance and water-cycle. Yet, much of the considered aerosol-cloud chemistry relies on our knowledge of the composition of solid and liquid particles.  To simultaneously improve our understanding of aerosol and cloud processes, it is critical that we consider parameters that encompass the complexity of aerosol and cloud processes.  Aerosol pH affects numerous aerosol and droplet processes, including the gas-particle partitioning of many semi-volatile species, which impacts the aerosol size distribution, mass concentration, and hygroscopic properties.  Therefore, aerosol pH directly affects Earth’s climate system; however, state-of-the-art global models struggle to predict pH.  We will perform research that addresses critical knowledge gaps in multiphase chemistry and aerosol processes on urban-rural gradients.  We will measure aerosol- and gas-phase composition in and around Baltimore, MD, during the DoE ARM Coast-Urban-Rural Atmospheric Gradient Experiment (CoURAGE).  Specifically, we will measure aerosol chemical composition, gas-phase ammonia, and gas-particle partitioning of carboxylic acids via simultaneous measurements in each phase.  We will model aerosol liquid water content (ALWC), aerosol pH, and organic acid phase partitioning using thermodynamic equilibrium models constrained by total (gas and particle) ammonia measurements.  We will model organic aerosol factors giving detailed insight into primary and secondary organic aerosol sources and their controlling factors using positive matrix factorization. Finally, we will conduct closure studies of measured cloud condensation nuclei (CCN) concentrations by applying κ- Köhler theory to the size and composition measurements. The proposed research adds significant value to the CoURAGE deployment as our measurement and modeling products will become part of the data repository available to all CoURAGE scientists and the broader scientific community.    

This research will significantly advance knowledge of aerosol sources, formation, and processing, ultimately improving our predictive capabilities in these areas. ALWC and particle acidity play dominant roles in aerosol composition, growth, and aging processes. Our measurements will enable more nuanced characterization of the processes connecting aerosol chemistry with their optical properties, which will be measured as part of the ARM deployment. The optical properties of brown carbon, a key contributor to aerosol absorption in many environments, depend strongly on aerosol pH. We will contribute calculations of aerosol pH on the same timescale as ALWC, providing a necessary component for the analysis of brown carbon radiative forcing, which is currently missing from the planned ARM deployment. The proposed work will provide detailed insight into SOA formation and the formation and partitioning of highly abundant carboxylic acids, a major contributor to SOA mass.  The κ-Köhler analysis will provide critical insight into the sources and processes influencing particle hygroscopicity and CCN abundance at the ARM site. This work will augment the ARM deployment through enhanced characterization of aerosol processes along the urban-suburban-rural gradient. Finally, the proposed measurements are planned for the spring-to-summer transition, an understudied time period in the eastern U.S. when dynamic changes in agricultural and biogenic emissions are expected to produce widely varying – yet highly uncertain – impacts on aerosol-cloud processes.