Measurement report: Seasonal Secondary and Biomass-Burning Aerosol Sources and Enhancements of Cloud Condensation Nuclei at a Central US Background Site

Abstract. Long-term measurements of aerosol chemistry are essential for understanding seasonal source variability and aerosol–climate interactions in continental environments. We present a multi-year (2021–2023) analysis of submicron aerosol composition at the U.S. Department of Energy Southern Great Plains (SGP) site using co-located Aerosol Chemical Speciation Monitor (ACSM), Interagency Monitoring of Protected Visual Environments (IMPROVE) network filter measurements, and supporting optical, size-distribution, and cloud condensation nuclei (CCN) observations. Organic aerosol (OA), nitrate (NO₃), ammonium (NH₄), sulfate (SO₄), and mineral dust exhibited pronounced seasonal cycles. NO₃ and NH₄ dominated winter aerosol mass, consistent with thermodynamic partitioning of ammonium nitrate (NH₄NO₃), while OA and SO₄ peaked in spring and summer, reflecting enhanced photochemistry and biogenic precursor emissions.

Positive Matrix Factorization with a rolling-window ME-2 framework resolved biomass-burning OA (BBOA), semi-volatile oxygenated OA (SVOOA), and low-volatility oxygenated OA (LVOOA). SVOOA and LVOOA together dominated annual OA mass, with strong summer maxima, while BBOA contributed episodically during fire events. We identified 168 biomass- burning episodes using BBOA spikes detected using rolling-median residual spike detection. Fire plumes increased aerosol absorption coefficient modestly but enhanced CCN number concentrations by up to a factor of four. HYSPLIT and satellite fire products indicate dominant contributions from Great Plains grassland and cropland burns, with additional forest and distant sources. These results provide a comprehensive characterization of aerosol seasonal behavior and biomass-burning impacts in the central United States, demonstrating the value of long-term, high-time-resolution mass-spectrometric observations.