Source-oriented pollution characteristics and decay kinetics of environmentally persistent free radicals in PM2.5 and PM10

Environmental persistent free radicals (EPFRs) are emerging pollutants in atmospheric particulate matter (PM) with significant health implications. This study investigated the pollution characteristics, decay kinetics, and potential health risks of EPFRs in PM_2.5 and PM₁₀ collected from various sources and ambient air in Islamabad, Pakistan. Solid fuel combustion, including biomass, waste and residential/industrial coal burning, were identified as major sources of atmospheric EPFRs in Islamabad. Meanwhile, mineral particles from stone crushing exhibited inherently higher ·OH generation compared to other sources, highlighting their significant contribution to the oxidative potential of ambient dust. The decay lifetimes of EPFRs ranged in 43.5–63.0 days for PM_2.5 and 37.2–67.8 days for PM₁₀ in ambient air, and 38.4–51.6 days for PM_2.5 and 40.2–53.4 days for PM₁₀ from source emissions, respectively, closely resembling those of semiquinone-type radicals. For PM samples that were more prone to produce EPFRs under simulated sunlight irradiation, the associated EPFRs were more reactive and decayed faster, while EPFRs in those less prone to generate EPFRs generally exhibited slower decay. This source-oriented investigation of EPFRs in PM thereby provides critical insights for developing targeted PM control strategies to mitigate their health risks.