Key toxicity enhancement effect of aqueous-phase secondary formation: Insights from hourly measurements during haze events

Haze events pose substantial health risks, yet the link between the chemical composition of particulate matter (PM) and the exacerbated health impacts during such episodes remains unclear. This study conducted hourly offline measurements of the chemical composition and oxidative potential (OP) of water-soluble fractions (WSF) of PM2.5 during three haze episodes in the North China Plain (NCP). Results revealed that water-soluble inorganic ions were the primary contributors to the increase in WSF mass (60.8 %), while water-soluble organic carbon (WSOC) was the key driver of OP enhancement, accounting for 78.7 % of OP per unit WSF mass (OPm). Molecular characterization via excitation-emission matrix spectroscopy (EEM) and high-resolution mass spectrometry (Orbitrap) identified highly oxygenated humic-like substances (HO-HULIS) as the major contributors to OPm (43 %). Notably, secondary organic aerosol (SOA)-related HO-HULIS, including highly oxygenated and unsaturated compounds, oxygenated/nitro polycyclic aromatic hydrocarbons (o/n-ConA), and oxygenated/nitro polyphenols (o/n-Poly), were identified as key toxic components. Source apportionment (PMF) analysis indicated that secondary organic aerosols (SOA), particularly those formed through aqueous-phase reaction, contributed 64.8 % of OPm, underscoring the critical role of aqueous-phase SOA in health risk enhancement during haze events.