Characterization of secondary organic aerosol formation and evolution from coal combustion utilizing an oxidation flow reactor

Coal combustion (CC) represents an important contributor of the atmospheric primary particulate matter and drives a large fraction of secondary organic aerosol (SOA) formation in cold season. However, the evolution of composition and physicochemical properties of SOA from CC remains poorly understood. This study uses an oxidation flow reactor to investigate SOA formation and aging from fresh CC emissions to over 7.2 equivalent photochemical aging days (7.2 eq. days). Compared to fresh emissions, the initial aging stage (1.2–1.5 eq. days) shows a slight increase in the H/C (hydrogen-to-carbon ratio, 1.74–1.85) and a decrease in hydrocarbon fragments (C_xH_y⁺) above m/z 60, whereas the moderate (2.1–3.6 eq. days) and highly aging stages (7.2 eq. days) are characterized by an increasing fraction of C_xH_yO₂⁺ from 13% to 29%, implying a continuous transformation of the molecular composition toward more oxidized species. Stage-dependent evolution of OA components is revealed by positive matrix factorization (PMF). Two primary organic aerosol (POA) factors dominate OA during fresh emissions, accounting for 95% of the total mass but decline rapidly with aging. The least oxidized SOA reaches its maximum contribution (63%–90%) at ∼1.2 eq. days, followed by a decrease. The moderately oxidized SOA then dominates OA during 2.1–3.6 eq. days, contributing 49%–76%, while the highly oxidized SOA accounts for an increasing proportion as aging proceeds. These findings elucidate distinct transformation pathways of OA and provide theoretical support for refining regional CC pollution mitigation strategies in China.