AAAS

Aerosol Acidity Across Seasons

12 March 2025 - 30 November 2025

Lead Scientist: Christopher Hennigan

Observatory: AMF (ARM Mobile Facility 1) - CRG

An ARM mobile facility will be deployed near Baltimore, Maryland for the Coast-Urban-Rural Atmospheric Gradient Experiment (CoURAGE) field campaign in 2024-2025. CoURAGE seeks to characterize spatial gradients in surface-atmosphere exchange, aerosol and cloud processes, atmospheric composition, precipitation, and radiation that occur in and around a coastal urban area.

We will deploy a suite of instrumentation to measure aerosol- and gas-phase composition in and around Baltimore to support the CoURAGE campaign. Specifically, we will measure aerosol chemical composition with a time-of-flight aerosol mass spectrometer (ToF-AMS), 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 + particle) ammonia measurements. We will model organic aerosol factors, giving detailed insight into primary and secondary organic aerosol (POA and SOA, respectively) 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.

This research will 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.