Incomplete solid fuel burning as the major emission source of atmospheric phenols and environmentally persistent free radicals

Environmentally persistent free radicals (EPFRs) bonded to fine particle matters (PMs) are known to cause adverse health impacts. This study comprehensively addressed their emission sources and major precursors including phenols, based on field measurements and inventory estimation. The emission characteristics of phenols and EPFRs were obtained through direct measurement of PM2.5 and gaseous samples, collected from various real-world anthropogenic sources. The residential sector (coal and biomass combustion) was revealed to produce the highest level of EPFRs (0.66 to 3.21 × 1017 spins/g) and phenols (5.8 to 55.9 mg/g) per unit mass of emitted PM2.5 amidst all estimated samples, including power plant, various industrial processes, and transportation sources. Additionally, the emission of phenols strongly correlates with that of EPFRs, indicating an EPFR transition pathway during the pyrolysis process of lignin. Furthermore, the residential sector contributed approximately 80.6 % of atmospheric EPFRs and 99.9 % of phenols as primary emitted PM2.5 in mainland China in 2019. Much higher levels of primary phenols and EPFRs presented in agricultural regions, where biomass was commonly used as household fuel. The obtained results highlight that the pollution control of residential solid fuel burning can efficiently relieve the negative impacts associated with atmospheric phenols and EPFRs.