Hygroscopicity of Organic Aerosol from Wildfire Emissions
NAGENDRA RAPARTHI, Anthony S. Wexler, Ann M. Dillner, University of California, Davis
Abstract Number: 148
Working Group: Aerosols, Clouds and Climate
Abstract
In recent years, a significant decline in inorganic particulate matter (PM) concentrations has been observed in the United States, primarily due to increasingly stringent air quality regulations. In contrast, organic matter (OM) has become a more dominant and complex component of fine particulate (PM2.5) emissions. Due to the vast chemical diversity of OM, comprising thousands of compounds with varying functionalities, its hygroscopic behavior remains insufficiently characterized. This limited understanding is particularly concerning in the context of wildfires, which have become increasingly frequent and intense. Wildfire-emitted organic aerosols not only threaten human health but also influence Earth’s radiative balance and cloud microphysics. Yet, despite their atmospheric relevance, the hygroscopic properties of these organic aerosols remain poorly characterized. This study investigates the hygroscopicity of organic aerosols originating from both local and long-range transported wildfires using our recently developed filter-based water uptake method at three relative humidities (~84%, ~90%, and ~97%). PM2.5 samples were collected on Teflon filters via the Interagency Monitoring of Protected Visual Environments (IMPROVE) network during wildfire events in Yosemite National Park (YNP), California, and during long-range smoke transport episodes at the Proctor Maple Research Facility (PMRF), Vermont, in 2023. Four samples representing three distinct wildfire phases in YNP and four samples representing two transport phases at PMRF were analyzed. Organic functional groups were characterized using Fourier-transform infrared spectroscopy (FT-IR), and inorganic constituents were quantified using energy-dispersive X-ray fluorescence (ED-XRF) and ion chromatography (IC), following IMPROVE protocols. The inorganic contribution to overall PM2.5 hygroscopicity was estimated using the Extended Aerosol Inorganics Model (E-AIM), allowing organic contributions to be isolated and quantified. Results revealed that OM hygroscopicity varies significantly with stages of wildfire and atmospheric aging, highlighting the influence of combustion and chemical evolution on particle water uptake, thereby affecting atmospheric processes and climate models.