Evolution of Particle-Bound Reactive Oxygen Species (ROS) During Photochemical Aging of Biomass Burning Emissions

 Understanding biomass burning emissions is critical because they represent a major source of atmospheric particulate matter, influencing air quality, climate, and public health. The chemical complexity and dynamic evolution of these particles during atmospheric aging pose significant challenges for predicting their environmental and health impacts. A key knowledge gap concerns the evolution of particle-bound reactive oxygen species (ROS) during aging, particularly short-lived ROS that are difficult to quantify using conventional offline methods.

In this study, we investigate the formation and transformation of particle-bound ROS in smoke generated from eucalyptus leaves under controlled photochemical aging. Atmospheric oxidation was simulated using a Rapid Aerosol Aging Device (RAAD), which enabled real-time monitoring of aerosol compositional changes and oxidative potential. A suite of instruments—including two Particle Into Nitroxide Quencher (PINQ) systems, a Scanning Mobility Particle Sizer (SMPS), gas monitors, Selected Ion Flow Tube mass spectrometry (SIFT), and a High-Resolution Aerosol Mass Spectrometer (HR-AMS)—was employed to characterize both physical and chemical transformations during aging. Relative humidity was maintained using an integrated humidification system, as it can significantly influence oxidation reactions and ROS formation.

Fresh smoke was first analyzed under dark, low-oxidant conditions to establish baseline properties. The aerosol was then subjected to RAAD-driven photochemical aging equivalent to 1–6 days of atmospheric OH exposure. The first PINQ measured initial particle-bound ROS levels, while the second PINQ quantified ROS after aging. SIFT provided measurements of key gas-phase species associated with oxidation chemistry, and HR-AMS supplied real-time information on chemical composition and mass-based size distribution. This integrated approach enabled continuous evaluation of ROS formation and transformation during simulated atmospheric aging, offering new insight into how biomass burning emissions develop enhanced oxidative potential over timescales of several days.

Dual-PINQ measurements revealed clear differences in particle-bound ROS before and after photochemical aging, demonstrating that aging processes substantially modify ROS levels compared to those measured immediately after burning. These findings highlight the importance of real-time techniques for detecting short-lived species that cannot be preserved through offline sampling. Overall, photochemical aging significantly increases the oxidative potential of biomass burning aerosols over short timescales, with implications for air quality assessment and human exposure during fire events.